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Industrial radiography - Wikipedia the free encyclopedia
Industrial radiography
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Industrial Radiography is the use of ionizing radiation to view objects in a way that cannot be seen otherwise It is not to be confused with
the use of ionizing radiation to change or modify objects radiographys purpose is strictly viewing Industrial radiography has grown out of
engineering and is a major element of nondestructive testing It is a method of inspecting materials for hidden flaws by using the ability of
short X-rays and Gamma rays to penetrate various materials
Contents
[hide]
1 History
2 Applications
21 Inspection of products
22 Airport security
23 Non-intrusive Cargo Scanning (aka Non-Intrusive Inspection - NII)
3 Sources
31 X-ray sources
32 Radioisotope sources
321 Torch design of radiographic cameras
322 Cable based design of radiographic cameras
33 Contrast agents
34 Neutrons
4 Safety
5 See also
6 References
7 External links
[edit] History
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Radiography started in 1895 with the discovery of X-rays (later also called Roumlntgen rays after the man who first described their properties in
detail) a type of electromagnetic radiation Soon after the discovery of X-rays radioactivity was discovered By using radioactive sources such
as radium far higher photon energies could be obtained than those from normal X-ray machines Soon these found various applications with
one of the earliest users being Loughborough University[1]
from helping to fit shoes more lasting medical uses and the examination of non-
living objects X-rays and gamma-rays were put to use very early before the dangers of ionising radiation were discovered After World War
II new isotopes such as caesium-137 iridium-192 and cobalt-60 became available for industrial radiography and the use of radium and radon
decreased
[edit] Applications
[edit] Inspection of products
GemX-160 - Portable Wireless
Controlled Battery Powered X-ray
Generator for use in Non
Destructive Testing and Security
Gamma radiation sources most commonly Iridium-192 and Cobalt-60 are used to inspect a variety of materials The vast majority of
radiography concerns the testing and grading of welds on pressurized piping pressure vessels high-capacity storage containers pipelines and
some structural welds Other tested materials include concrete (locating rebar or conduit) welders test coupons machined parts plate metal
or pipewall (locating anomalies due to corrosion or mechanical damage) Non-metal components such as ceramics used in the aerospace
industries are also regularly tested Theoretically industrial radiographers could radiograph any solid flat material (walls ceilings floors
square or rectangular containers) or any hollow cylindrical or spherical object
For purposes of inspection including weld inspection there exist several exposure arrangements
First there is the panoramic one of the four single wall exposuresingle wall view (SWESWV) arrangements This exposure is created when
the radiographer places the source of radiation at the center of a sphere cone or cylinder (including tanks vessels and piping) Depending
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upon client requirements the radiographer would then place film cassettes on the outside of the surface to be examined This exposure
arrangement is ideal - when properly arranged and exposed all portions of all exposed film will be of the same approximate density It also has
the advantage of taking less time than other arrangements since the source must only penetrate the total wall thickness (WT) once and must
only travel the radius of the inspection item not its full diameter The major disadvantage of the panoramic is that it may be impractical to
reach the center of the item (enclosed pipe) or the source may be too weak to perform in this arrangement (large vessels or tanks)
The second SWESWV arrangement is an interior placement of the source in an enclosed inspection item without having the source centered
up The source does not come in direct contact with the item but is placed a distance away depending on client requirements The third is an
exterior placement with similar characteristics The fourth is reserved for flat objects such as plate metal and is also radiographed without the
source coming in direct contact with the item In each case the radiographic film is located on the opposite side of the inspection item from the
source In all four cases only one wall is exposed and only one wall is viewed on the radiograph
Of the other exposure arrangements only the contact shot has the source located on the inspection item This type of radiograph exposes both
walls but only resolves the image on the wall nearest the film This exposure arrangement takes more time than a panoramic as the source
must penetrate the WT twice and travel the entire outside diameter of the pipe or vessel to reach the film on the opposite side This is a double
wall exposuresingle wall view DWESWV arrangement Another is the superimposure (wherein the source is placed on one side of the item
not in direct contact with it with the film on the opposite side) This arrangement is usually reserved for very small diameter piping or parts
The last DWESWV exposure arrangement is the elliptical in which the source is offset from the plane of the inspection item (usually a weld
in pipe) and the elliptical image of the weld furthest from the source is cast onto the film
The beam of radiation must be directed to the middle of the section under examination and must be normal to the material surface at that point
except in special techniques where known defects are best revealed by a different alignment of the beam The length of weld under
examination for each exposure shall be such that the thickness of the material at the diagnostic extremities measured in the direction of the
incident beam does not exceed the actual thickness at that point by more than 6 The specimen to be inspected is placed between the source
of radiation and the detecting device usually the film in a light tight holder or cassette and the radiation is allowed to penetrate the part for the
required length of time to be adequately recorded Lead is often placed behind the film to reduce the back scattered radiation which can lead
to the film becoming over exposed
The result is a two-dimensional projection of the part onto the film producing a latent image of varying densities according to the amount of
radiation reaching each area It is known as a radiograph as distinct from a photograph produced by light Because film is cumulative in its
response (the exposure increasing as it absorbs more radiation) relatively weak radiation can be detected by prolonging the exposure until the
film can record an image that will be visible after development The radiograph is examined as a negative without printing as a positive as in
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photography This is because in printing some of the detail is always lost and no useful purpose is served
Before commencing a radiographic examination it is always advisable to examine the component with ones own eyes to eliminate any
possible external defects If the surface of a weld is too irregular it may be desirable to grind it to obtain a smooth finish but this is likely to be
limited to those cases in which the surface irregularities (which will be visible on the radiograph) may make detecting internal defects difficult
After this visual examination the operator will have a clear idea of the possibilities of access to the two faces of the weld which is important
both for the setting up of the equipment and for the choice of the most appropriate technique
[edit] Airport security
Both hold luggage and carry-on hand luggage are normally examined by X-ray machines using X-ray radiography See airport security for
more details
[edit] Non-intrusive Cargo Scanning (aka Non-Intrusive Inspection - NII)
Gamma-ray Image of intermodal cargo container
with stowaways
Gamma Radiography and High-Energy X-ray radiography are currently used to scan intermodal freight cargo containers in US and other
countries Also research is being done on adapting other types of radiography like Dual-Energy X-ray Radiography or Muon Radiography for
scanning intermodal cargo containers
[edit] Sources
[edit] X-ray sources
A high energy X-ray machine can be used It is often important to use a high accelerating voltage to provide the electrons with a very high
energy This is because in a braking radiation source the maximum photon energy is determined by the energy of the charged particles
[edit] Radioisotope sources
These have the advantage that they do not need a supply of electrical power to function but they do have the disadvantage that they can not be
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turned off Also it is difficult using radioactivity to create a small and compact source that offers the photon flux possible with a normal sealed
X-ray tube One of the leading makers of radiographic equipment is the Source Production amp Equipment Co Inc [1]
It might be possible to use Cs-137 as a photon source for radiography but this isotope is always diluted with inactive caesium isotopes This
makes it difficult to get a physically small source and a large volume of the source makes it impossible to capture fine details in a radiographic
examination
Both cobalt-60 and caesium-137 have only a few gamma energies which makes them close to monochromatic The photon energy of cobalt-
60 is higher than that of caesium-137 which allows cobalt sources to be used to examine thicker sections of metals than those that could be
examined with Cs-137 Iridium-192 has a lower photon energy than cobalt-60 and its gamma spectrum is complex (many lines of very
different energies) but this can be an advantage as this can give better contrast for the final photographs
It has been known for many years that an inactive iridium or cobalt metal object can be machined to size In the case of cobalt it is common to
alloy it with nickel to improve the mechanical properties In the case of iridium a thin wire or rod could be used These precursor materials can
then be placed in stainless steel containers that have been leak tested before being converted into radioactive sources These objects can be
processed by neutron activation to form gamma emitting radioisotopes The stainless steel has only a small ability to be activated and the small
activity due to 55Fe and 63Ni are unlikely to pose a problem in the final application because these isotopes are beta emitters which have very
weak gamma emission The 59Fe which might form has a short half life so by allowing a cobalt source to stand for a year much of this isotope
will decay away
The source is often a very small object which must be transported to the work site in a shielded container It is normal to place the film in
industrial radiography clear the area where the work is to be done add shielding (collimators) to reduce the size of the controlled area before
exposing the radioactive source A series of different designs have been developed for radiographic cameras Rather than the camera being
a device that accepts photons to record a picture the camera in industrial radiography is the radioactive photon source
[edit] Torch design of radiographic cameras
One design is best thought of as being like a torch The radioactive source is placed inside a shielded box a hinge allows part of the shielding
to be opened exposing the source allowing photons to exit the radiography camera
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This torch-type camera uses a hinge The radioactive source is
in red the shielding is bluegreen and the gamma rays are
yellow
Another design for a torch is where the source is placed in a metal wheel which can turn inside the camera to move between the expose and
storage positions
This torch-type camera uses a wheel design The radioactive
source is in red and the gamma rays are yellow
[edit] Cable based design of radiographic cameras
One group of designs use a radioactive source which connects to a drive cable contained shielded exposure device In one design of equipment
the source is stored in a block of lead or depleted uranium shielding that has a S shaped tube-like hole through the block In the safe position
the source is in the centre of the block and is attached to a metal wire that extends in both directions to use the source a guide tube is attached
to one side of the device while a drive cable is attached to the other end of the short cable Using a hand operated winch the source is then
pushed out of the shield and along the source guide tube to the tip of the tube to expose the film then cranked back into its fully-shielded
position
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A diagram of the S shaped hole through a metal
block the source is stored at point A and is driven
out on a cable through a hole to point B It often goes
a long way along a guide tube to where it is needed
[edit] Contrast agents
Defects such as delaminations and planar cracks are difficult to detect using radiography which is why penetrants are often used to enhance
the contrast in the detection of such defects Penetrants used include silver nitrate zinc iodide chloroform and diiodomethane Choice of the
penetrant is determined by the ease with which it can penetrate the cracks and also with which it can be removed Diiodomethane has the
advantages of high opacity ease of penetration and ease of removal because it evaporates relatively quickly However it can cause skin burns
[edit] Neutrons
In some rare cases radiography is done with neutrons This type of radiography is called neutron radiography (NR Nray N-Ray) or neutron
imaging Neutron radiography can see very different things than X-rays because neutrons can pass with ease through lead and steel but are
stopped by plastics water and oils Neutron sources include radioactive (241AmBe and Cf) sources electrically driven D-T reactions in
vacuum tubes and conventional critical nuclear reactors It might be possible to use a neutron amplifier to increase the neutron flux[2]
Since the amount of radiation emerging from the opposite side of the material can be detected and measured variations in this amount (or
intensity) of radiation are used to determine thickness or composition of material Penetrating radiations are those restricted to that part of the
electromagnetic spectrum of wavelength less than about 10 nanometres
[edit] Safety
Industrial radiographers are in many locations required by governing authorities to use certain types of safety equipment and to work in pairs
Depending on location industrial radiographers may have been required to obtain permits licences andor undertake special training Prior to
conducting any testing the nearby area should always first be cleared of all other persons and measures taken to ensure that people do not
accidentally enter into an area that may expose them to a large dose of radiation
The safety equipment usually includes four basic items a radiation survey meter (such as a GeigerMueller counter) an alarming dosimeter or
rate meter a gas-charged dosimeter and a film badge or thermoluminescent dosimeter (TLD) The easiest way to remember what each of these
items does is to compare them to gauges on an automobile
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The survey meter could be compared to the speedometer as it measures the speed or rate at which radiation is being picked up When
properly calibrated used and maintained it allows the radiographer to see the current exposure to radiation at the meter It can usually be set
for different intensities and is used to prevent the radiographer from being overexposed to the radioactive source as well as for verifying the
boundary that radiographers are required to maintain around the exposed source during radiographic operations
The alarming dosimeter could be most closely compared with the tachometer as it alarms when the radiographer redlines or is exposed to
too much radiation When properly calibrated activated and worn on the radiographers person it will emit an alarm when the meter measures
a radiation level in excess of a preset threshold This device is intended to prevent the radiographer from inadvertently walking up on an
exposed source
The gas-charged dosimeter is like a trip meter in that it measures the total radiation received but can be reset It is designed to help the
radiographer measure hisher total periodic dose of radiation When properly calibrated recharged and worn on the radiographers person it
can tell the radiographer at a glance how much radiation to which the device has been exposed since it was last recharged Radiographers in
many states are required to log their radiation exposures and generate an exposure report In many countries personal dosimeters are not
required to be used by radiographers as the dose rates they show are not always correctly recorded
The film badge or TLD is more like a cars odometer It is actually a specialized piece of radiographic film in a rugged container It is meant to
measure the radiographers total exposure over time (usually a month) and is used by regulating authorities to monitor the total exposure of
certified radiographers in a certain jurisdiction At the end of the month the film badge is turned in and is processed A report of the
radiographers total dose is generated and is kept on file
When these safety devices are properly calibrated maintained and used it is virtually impossible for a radiographer to be injured by a
radioactive overexposure Sadly the elimination of just one of these devices can jeopardize the safety of the radiographer and all those who are
nearby Without the survey meter the radiation received may be just below the threshold of the rate alarm and it may be several hours before
the radiographer checks the dosimeter and up to a month or more before the film badge is developed to detect a low intensity overexposure
Without the rate alarm one radiographer may inadvertently walk up on the source exposed by the other radiographer Without the dosimeter
the radiographer may be unaware of an overexposure or even a radiation burn which may take weeks to result in noticeable injury And
without the film badge the radiographer is deprived of an important tool designed to protect him or her from the effects of a long-term
overexposure to occupationally-obtained radiation and thus may suffer long-term health problems as a result
There are three ways a radiographer will ensure they are not exposed to higher than required levels of radiation time distance shielding The
less time that a person is exposed to radiation the lower their dose will be The further a person is from a radioactive source the lower the level
of radiation they receive this is largely due to the inverse square law Lastly the more a radioactive source is shielded by either better or
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greater amounts of shielding the lower the levels of radiation that will escape from the testing area The most commanly used shielding
materials in use are sand lead (sheets or shot) steel spent (non-radioactive uranium) tungsten and in suitable situations water
Industrial radiography appears to have one of the worst safety profiles of the radiation professions possibly because there are many operators
using strong gamma sources (gt 2 Ci) in remote sites with little supervision when compared with workers within the nuclear industry or within
hospitals[3] Due to the levels of radiation present whilst they are working many radiographers are also required to work late at night when
there are few other people present as most industrial radiography is carried out in the open rather than in purpose built exposure booths or
rooms Fatigue carelessness and lack of proper training are the three most comman factors attributed to industrial radiography accidents Many
of the lost source accidents commented on by the International Atomic Energy Agency involve radiography equipment Lost source
accidents have the potential to cause a considerable loss of human life One scenario is that a passerby finds the radiography source and not
knowing what it is takes it home[4] The person shortly afterwards becomes ill and dies as a result of the radiation dose The source remains in
their home where it continues to irradiate other members of the household[5] Such an event occurred in March 1984 in Casablanca
(Mohammedia) which is part of Morocco This is related to the more famous Goiacircnia accident where a related chain of events caused
members of the public to be exposed to radiation sources Also see List of civilian radiation accidents
[edit] See also
Radiographic testing
Collimator
Industrial CT scanning
[edit] References
1 ^ httpwwwlboroacuklibrarynewSpotlighton-Archivehtml Loughborough University Library - Spotlight Archive [Accessed 22 October 2008]
[edit] External links
NISTs XAAMDI X-Ray Attenuation and Absorption for Materials of Dosimetric Interest Database
NISTs XCOM Photon Cross Sections Database
NISTs FAST Attenuation and Scattering Tables
A lost industrial radiography source event
UN information on the security of industrial sources
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Radiation Production for Industrial Radiography
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Production of Radiation for Industrial Radiography
Industrial radiography uses two sources of radiation X-radiation and Gamma radiation X-rays and Gamma rays differ only in their source of origin X-rays are produced by an X-ray generator and Gamma radiation is the product of radioactive atoms An in depth discussion on radiation production can be found in other areas of this site but will be reviewed briefly in the following sections
Production of X-Rays There are two different atomic processes that can produce X-ray photons One process produces Bremsstrahlung radiation and the other produces K-shell or characteristic emission Both processes involve a change in the energy state of electrons X-rays are generated when an electron is accelerated and then made to rapidly decelerate usually due to interaction with other atomic particles
In an X-ray system a large amount of electric current is passed through a tungsten filament which heats the filament to several thousand degrees centigrade to create a source of free electrons A large electrical potential is established between the filament (the cathode) and a target (the anode) The cathode and anode are enclosed in a vacuum tube to prevent the filament from burning up and to prevent arcing between the cathode and anode The electrical potential between the cathode and the anode pulls electrons from the cathode and accelerates them as they are attracted towards the anode or target which is usually made of tungsten X-rays are generated when free electrons give up some of their energy when they interact with the electrons or nucleus of an atom The interaction of the electrons in the target results in the emission of a continuous Bremsstrahlung spectrum and also characteristic X-rays from the target material
Production of Gamma Rays Gamma radiation is the product of radioactive atoms Depending upon the ratio of neutrons to protons within its nucleus an isotope of a particular element may be stable or unstable Over time the nuclei of unstable isotopes spontaneously disintegrate or transform in a process known as radioactive decay Various types of radiation may be emitted from the nucleus andor its surrounding electrons when an atom experiences radioactive decay Nuclides which undergo radioactive decay are called radionuclides Any material which contains measurable amounts of one or more radionuclides is a radioactive material
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There are many naturally occurring radioactive materials but manmade radioactive isotopes or radioisotopes are used for industrial radiography Man-made sources are produced by introducing an extra neutron to atoms of the source material For example Cobalt-60 is produced by bombarding a sample of Cobalt-59 with an excess of neutrons in a nuclear reactor The Cobalt-59 atoms absorb some of the neutrons and increase their atomic weight by one to produce the radioisotope Cobalt-60 This process is known as activation As a material rids itself of atomic particles to return to a balance state energy is released in the form of Gamma rays and sometimes alpha or beta particles
Physical size of isotope materials will very slightly between manufacturer but generally an isotope is a pellet that measures 15 mm x 15 mm Depending on the activity (curies) desired a pellet or pellets are loaded into a stainless steel capsule and sealed Unlike X-ray tubes radioactive sources provide a continual source of radiation that cannot be turned off Once radioactive decay starts it continues until all of the atoms have reached a stable state The radioisotope can only be shielded to prevent exposure to the radiation In industrial radiography the instruments that are used to shield the radioisotope so that they can be safely handled and used are commonly called cameras or exposure devices Exposure devices will be discussed later in more detail
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Radiation Decay and Half-life
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radioactive Decay and Half-Life
As mentioned previously radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation As a radioisotope atom decays to a more stable atom it emits radiation only once To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step However once the atom reaches a stable configuration no more radiation is given off For this reason radioactive sources become weaker with time As more and more unstable atoms become stable atoms less radiation is produced and eventually the material will become non-radioactive
The decay of radioactive elements occurs at a fixed rate The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material For example a source will have an intensity of 100 when new At one half-life its intensity will be cut to 50 of the original intensity At two half-lives it will have an intensity of 25 of a new source After ten half-lives less than one-thousandth of the original activity will remain Although the half-life pattern is the same for every radioisotope the length of a half-life is different For example Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days
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Radiation Activity and Intensity
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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Exposure Limits
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Industrial radiography - Wikipedia the free encyclopedia
Radiography started in 1895 with the discovery of X-rays (later also called Roumlntgen rays after the man who first described their properties in
detail) a type of electromagnetic radiation Soon after the discovery of X-rays radioactivity was discovered By using radioactive sources such
as radium far higher photon energies could be obtained than those from normal X-ray machines Soon these found various applications with
one of the earliest users being Loughborough University[1]
from helping to fit shoes more lasting medical uses and the examination of non-
living objects X-rays and gamma-rays were put to use very early before the dangers of ionising radiation were discovered After World War
II new isotopes such as caesium-137 iridium-192 and cobalt-60 became available for industrial radiography and the use of radium and radon
decreased
[edit] Applications
[edit] Inspection of products
GemX-160 - Portable Wireless
Controlled Battery Powered X-ray
Generator for use in Non
Destructive Testing and Security
Gamma radiation sources most commonly Iridium-192 and Cobalt-60 are used to inspect a variety of materials The vast majority of
radiography concerns the testing and grading of welds on pressurized piping pressure vessels high-capacity storage containers pipelines and
some structural welds Other tested materials include concrete (locating rebar or conduit) welders test coupons machined parts plate metal
or pipewall (locating anomalies due to corrosion or mechanical damage) Non-metal components such as ceramics used in the aerospace
industries are also regularly tested Theoretically industrial radiographers could radiograph any solid flat material (walls ceilings floors
square or rectangular containers) or any hollow cylindrical or spherical object
For purposes of inspection including weld inspection there exist several exposure arrangements
First there is the panoramic one of the four single wall exposuresingle wall view (SWESWV) arrangements This exposure is created when
the radiographer places the source of radiation at the center of a sphere cone or cylinder (including tanks vessels and piping) Depending
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upon client requirements the radiographer would then place film cassettes on the outside of the surface to be examined This exposure
arrangement is ideal - when properly arranged and exposed all portions of all exposed film will be of the same approximate density It also has
the advantage of taking less time than other arrangements since the source must only penetrate the total wall thickness (WT) once and must
only travel the radius of the inspection item not its full diameter The major disadvantage of the panoramic is that it may be impractical to
reach the center of the item (enclosed pipe) or the source may be too weak to perform in this arrangement (large vessels or tanks)
The second SWESWV arrangement is an interior placement of the source in an enclosed inspection item without having the source centered
up The source does not come in direct contact with the item but is placed a distance away depending on client requirements The third is an
exterior placement with similar characteristics The fourth is reserved for flat objects such as plate metal and is also radiographed without the
source coming in direct contact with the item In each case the radiographic film is located on the opposite side of the inspection item from the
source In all four cases only one wall is exposed and only one wall is viewed on the radiograph
Of the other exposure arrangements only the contact shot has the source located on the inspection item This type of radiograph exposes both
walls but only resolves the image on the wall nearest the film This exposure arrangement takes more time than a panoramic as the source
must penetrate the WT twice and travel the entire outside diameter of the pipe or vessel to reach the film on the opposite side This is a double
wall exposuresingle wall view DWESWV arrangement Another is the superimposure (wherein the source is placed on one side of the item
not in direct contact with it with the film on the opposite side) This arrangement is usually reserved for very small diameter piping or parts
The last DWESWV exposure arrangement is the elliptical in which the source is offset from the plane of the inspection item (usually a weld
in pipe) and the elliptical image of the weld furthest from the source is cast onto the film
The beam of radiation must be directed to the middle of the section under examination and must be normal to the material surface at that point
except in special techniques where known defects are best revealed by a different alignment of the beam The length of weld under
examination for each exposure shall be such that the thickness of the material at the diagnostic extremities measured in the direction of the
incident beam does not exceed the actual thickness at that point by more than 6 The specimen to be inspected is placed between the source
of radiation and the detecting device usually the film in a light tight holder or cassette and the radiation is allowed to penetrate the part for the
required length of time to be adequately recorded Lead is often placed behind the film to reduce the back scattered radiation which can lead
to the film becoming over exposed
The result is a two-dimensional projection of the part onto the film producing a latent image of varying densities according to the amount of
radiation reaching each area It is known as a radiograph as distinct from a photograph produced by light Because film is cumulative in its
response (the exposure increasing as it absorbs more radiation) relatively weak radiation can be detected by prolonging the exposure until the
film can record an image that will be visible after development The radiograph is examined as a negative without printing as a positive as in
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photography This is because in printing some of the detail is always lost and no useful purpose is served
Before commencing a radiographic examination it is always advisable to examine the component with ones own eyes to eliminate any
possible external defects If the surface of a weld is too irregular it may be desirable to grind it to obtain a smooth finish but this is likely to be
limited to those cases in which the surface irregularities (which will be visible on the radiograph) may make detecting internal defects difficult
After this visual examination the operator will have a clear idea of the possibilities of access to the two faces of the weld which is important
both for the setting up of the equipment and for the choice of the most appropriate technique
[edit] Airport security
Both hold luggage and carry-on hand luggage are normally examined by X-ray machines using X-ray radiography See airport security for
more details
[edit] Non-intrusive Cargo Scanning (aka Non-Intrusive Inspection - NII)
Gamma-ray Image of intermodal cargo container
with stowaways
Gamma Radiography and High-Energy X-ray radiography are currently used to scan intermodal freight cargo containers in US and other
countries Also research is being done on adapting other types of radiography like Dual-Energy X-ray Radiography or Muon Radiography for
scanning intermodal cargo containers
[edit] Sources
[edit] X-ray sources
A high energy X-ray machine can be used It is often important to use a high accelerating voltage to provide the electrons with a very high
energy This is because in a braking radiation source the maximum photon energy is determined by the energy of the charged particles
[edit] Radioisotope sources
These have the advantage that they do not need a supply of electrical power to function but they do have the disadvantage that they can not be
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turned off Also it is difficult using radioactivity to create a small and compact source that offers the photon flux possible with a normal sealed
X-ray tube One of the leading makers of radiographic equipment is the Source Production amp Equipment Co Inc [1]
It might be possible to use Cs-137 as a photon source for radiography but this isotope is always diluted with inactive caesium isotopes This
makes it difficult to get a physically small source and a large volume of the source makes it impossible to capture fine details in a radiographic
examination
Both cobalt-60 and caesium-137 have only a few gamma energies which makes them close to monochromatic The photon energy of cobalt-
60 is higher than that of caesium-137 which allows cobalt sources to be used to examine thicker sections of metals than those that could be
examined with Cs-137 Iridium-192 has a lower photon energy than cobalt-60 and its gamma spectrum is complex (many lines of very
different energies) but this can be an advantage as this can give better contrast for the final photographs
It has been known for many years that an inactive iridium or cobalt metal object can be machined to size In the case of cobalt it is common to
alloy it with nickel to improve the mechanical properties In the case of iridium a thin wire or rod could be used These precursor materials can
then be placed in stainless steel containers that have been leak tested before being converted into radioactive sources These objects can be
processed by neutron activation to form gamma emitting radioisotopes The stainless steel has only a small ability to be activated and the small
activity due to 55Fe and 63Ni are unlikely to pose a problem in the final application because these isotopes are beta emitters which have very
weak gamma emission The 59Fe which might form has a short half life so by allowing a cobalt source to stand for a year much of this isotope
will decay away
The source is often a very small object which must be transported to the work site in a shielded container It is normal to place the film in
industrial radiography clear the area where the work is to be done add shielding (collimators) to reduce the size of the controlled area before
exposing the radioactive source A series of different designs have been developed for radiographic cameras Rather than the camera being
a device that accepts photons to record a picture the camera in industrial radiography is the radioactive photon source
[edit] Torch design of radiographic cameras
One design is best thought of as being like a torch The radioactive source is placed inside a shielded box a hinge allows part of the shielding
to be opened exposing the source allowing photons to exit the radiography camera
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This torch-type camera uses a hinge The radioactive source is
in red the shielding is bluegreen and the gamma rays are
yellow
Another design for a torch is where the source is placed in a metal wheel which can turn inside the camera to move between the expose and
storage positions
This torch-type camera uses a wheel design The radioactive
source is in red and the gamma rays are yellow
[edit] Cable based design of radiographic cameras
One group of designs use a radioactive source which connects to a drive cable contained shielded exposure device In one design of equipment
the source is stored in a block of lead or depleted uranium shielding that has a S shaped tube-like hole through the block In the safe position
the source is in the centre of the block and is attached to a metal wire that extends in both directions to use the source a guide tube is attached
to one side of the device while a drive cable is attached to the other end of the short cable Using a hand operated winch the source is then
pushed out of the shield and along the source guide tube to the tip of the tube to expose the film then cranked back into its fully-shielded
position
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A diagram of the S shaped hole through a metal
block the source is stored at point A and is driven
out on a cable through a hole to point B It often goes
a long way along a guide tube to where it is needed
[edit] Contrast agents
Defects such as delaminations and planar cracks are difficult to detect using radiography which is why penetrants are often used to enhance
the contrast in the detection of such defects Penetrants used include silver nitrate zinc iodide chloroform and diiodomethane Choice of the
penetrant is determined by the ease with which it can penetrate the cracks and also with which it can be removed Diiodomethane has the
advantages of high opacity ease of penetration and ease of removal because it evaporates relatively quickly However it can cause skin burns
[edit] Neutrons
In some rare cases radiography is done with neutrons This type of radiography is called neutron radiography (NR Nray N-Ray) or neutron
imaging Neutron radiography can see very different things than X-rays because neutrons can pass with ease through lead and steel but are
stopped by plastics water and oils Neutron sources include radioactive (241AmBe and Cf) sources electrically driven D-T reactions in
vacuum tubes and conventional critical nuclear reactors It might be possible to use a neutron amplifier to increase the neutron flux[2]
Since the amount of radiation emerging from the opposite side of the material can be detected and measured variations in this amount (or
intensity) of radiation are used to determine thickness or composition of material Penetrating radiations are those restricted to that part of the
electromagnetic spectrum of wavelength less than about 10 nanometres
[edit] Safety
Industrial radiographers are in many locations required by governing authorities to use certain types of safety equipment and to work in pairs
Depending on location industrial radiographers may have been required to obtain permits licences andor undertake special training Prior to
conducting any testing the nearby area should always first be cleared of all other persons and measures taken to ensure that people do not
accidentally enter into an area that may expose them to a large dose of radiation
The safety equipment usually includes four basic items a radiation survey meter (such as a GeigerMueller counter) an alarming dosimeter or
rate meter a gas-charged dosimeter and a film badge or thermoluminescent dosimeter (TLD) The easiest way to remember what each of these
items does is to compare them to gauges on an automobile
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The survey meter could be compared to the speedometer as it measures the speed or rate at which radiation is being picked up When
properly calibrated used and maintained it allows the radiographer to see the current exposure to radiation at the meter It can usually be set
for different intensities and is used to prevent the radiographer from being overexposed to the radioactive source as well as for verifying the
boundary that radiographers are required to maintain around the exposed source during radiographic operations
The alarming dosimeter could be most closely compared with the tachometer as it alarms when the radiographer redlines or is exposed to
too much radiation When properly calibrated activated and worn on the radiographers person it will emit an alarm when the meter measures
a radiation level in excess of a preset threshold This device is intended to prevent the radiographer from inadvertently walking up on an
exposed source
The gas-charged dosimeter is like a trip meter in that it measures the total radiation received but can be reset It is designed to help the
radiographer measure hisher total periodic dose of radiation When properly calibrated recharged and worn on the radiographers person it
can tell the radiographer at a glance how much radiation to which the device has been exposed since it was last recharged Radiographers in
many states are required to log their radiation exposures and generate an exposure report In many countries personal dosimeters are not
required to be used by radiographers as the dose rates they show are not always correctly recorded
The film badge or TLD is more like a cars odometer It is actually a specialized piece of radiographic film in a rugged container It is meant to
measure the radiographers total exposure over time (usually a month) and is used by regulating authorities to monitor the total exposure of
certified radiographers in a certain jurisdiction At the end of the month the film badge is turned in and is processed A report of the
radiographers total dose is generated and is kept on file
When these safety devices are properly calibrated maintained and used it is virtually impossible for a radiographer to be injured by a
radioactive overexposure Sadly the elimination of just one of these devices can jeopardize the safety of the radiographer and all those who are
nearby Without the survey meter the radiation received may be just below the threshold of the rate alarm and it may be several hours before
the radiographer checks the dosimeter and up to a month or more before the film badge is developed to detect a low intensity overexposure
Without the rate alarm one radiographer may inadvertently walk up on the source exposed by the other radiographer Without the dosimeter
the radiographer may be unaware of an overexposure or even a radiation burn which may take weeks to result in noticeable injury And
without the film badge the radiographer is deprived of an important tool designed to protect him or her from the effects of a long-term
overexposure to occupationally-obtained radiation and thus may suffer long-term health problems as a result
There are three ways a radiographer will ensure they are not exposed to higher than required levels of radiation time distance shielding The
less time that a person is exposed to radiation the lower their dose will be The further a person is from a radioactive source the lower the level
of radiation they receive this is largely due to the inverse square law Lastly the more a radioactive source is shielded by either better or
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greater amounts of shielding the lower the levels of radiation that will escape from the testing area The most commanly used shielding
materials in use are sand lead (sheets or shot) steel spent (non-radioactive uranium) tungsten and in suitable situations water
Industrial radiography appears to have one of the worst safety profiles of the radiation professions possibly because there are many operators
using strong gamma sources (gt 2 Ci) in remote sites with little supervision when compared with workers within the nuclear industry or within
hospitals[3] Due to the levels of radiation present whilst they are working many radiographers are also required to work late at night when
there are few other people present as most industrial radiography is carried out in the open rather than in purpose built exposure booths or
rooms Fatigue carelessness and lack of proper training are the three most comman factors attributed to industrial radiography accidents Many
of the lost source accidents commented on by the International Atomic Energy Agency involve radiography equipment Lost source
accidents have the potential to cause a considerable loss of human life One scenario is that a passerby finds the radiography source and not
knowing what it is takes it home[4] The person shortly afterwards becomes ill and dies as a result of the radiation dose The source remains in
their home where it continues to irradiate other members of the household[5] Such an event occurred in March 1984 in Casablanca
(Mohammedia) which is part of Morocco This is related to the more famous Goiacircnia accident where a related chain of events caused
members of the public to be exposed to radiation sources Also see List of civilian radiation accidents
[edit] See also
Radiographic testing
Collimator
Industrial CT scanning
[edit] References
1 ^ httpwwwlboroacuklibrarynewSpotlighton-Archivehtml Loughborough University Library - Spotlight Archive [Accessed 22 October 2008]
[edit] External links
NISTs XAAMDI X-Ray Attenuation and Absorption for Materials of Dosimetric Interest Database
NISTs XCOM Photon Cross Sections Database
NISTs FAST Attenuation and Scattering Tables
A lost industrial radiography source event
UN information on the security of industrial sources
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Radiation Production for Industrial Radiography
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Production of Radiation for Industrial Radiography
Industrial radiography uses two sources of radiation X-radiation and Gamma radiation X-rays and Gamma rays differ only in their source of origin X-rays are produced by an X-ray generator and Gamma radiation is the product of radioactive atoms An in depth discussion on radiation production can be found in other areas of this site but will be reviewed briefly in the following sections
Production of X-Rays There are two different atomic processes that can produce X-ray photons One process produces Bremsstrahlung radiation and the other produces K-shell or characteristic emission Both processes involve a change in the energy state of electrons X-rays are generated when an electron is accelerated and then made to rapidly decelerate usually due to interaction with other atomic particles
In an X-ray system a large amount of electric current is passed through a tungsten filament which heats the filament to several thousand degrees centigrade to create a source of free electrons A large electrical potential is established between the filament (the cathode) and a target (the anode) The cathode and anode are enclosed in a vacuum tube to prevent the filament from burning up and to prevent arcing between the cathode and anode The electrical potential between the cathode and the anode pulls electrons from the cathode and accelerates them as they are attracted towards the anode or target which is usually made of tungsten X-rays are generated when free electrons give up some of their energy when they interact with the electrons or nucleus of an atom The interaction of the electrons in the target results in the emission of a continuous Bremsstrahlung spectrum and also characteristic X-rays from the target material
Production of Gamma Rays Gamma radiation is the product of radioactive atoms Depending upon the ratio of neutrons to protons within its nucleus an isotope of a particular element may be stable or unstable Over time the nuclei of unstable isotopes spontaneously disintegrate or transform in a process known as radioactive decay Various types of radiation may be emitted from the nucleus andor its surrounding electrons when an atom experiences radioactive decay Nuclides which undergo radioactive decay are called radionuclides Any material which contains measurable amounts of one or more radionuclides is a radioactive material
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Radiation Production for Industrial Radiography
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There are many naturally occurring radioactive materials but manmade radioactive isotopes or radioisotopes are used for industrial radiography Man-made sources are produced by introducing an extra neutron to atoms of the source material For example Cobalt-60 is produced by bombarding a sample of Cobalt-59 with an excess of neutrons in a nuclear reactor The Cobalt-59 atoms absorb some of the neutrons and increase their atomic weight by one to produce the radioisotope Cobalt-60 This process is known as activation As a material rids itself of atomic particles to return to a balance state energy is released in the form of Gamma rays and sometimes alpha or beta particles
Physical size of isotope materials will very slightly between manufacturer but generally an isotope is a pellet that measures 15 mm x 15 mm Depending on the activity (curies) desired a pellet or pellets are loaded into a stainless steel capsule and sealed Unlike X-ray tubes radioactive sources provide a continual source of radiation that cannot be turned off Once radioactive decay starts it continues until all of the atoms have reached a stable state The radioisotope can only be shielded to prevent exposure to the radiation In industrial radiography the instruments that are used to shield the radioisotope so that they can be safely handled and used are commonly called cameras or exposure devices Exposure devices will be discussed later in more detail
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Radiation Production for Industrial Radiography
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Radiation Decay and Half-life
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radioactive Decay and Half-Life
As mentioned previously radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation As a radioisotope atom decays to a more stable atom it emits radiation only once To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step However once the atom reaches a stable configuration no more radiation is given off For this reason radioactive sources become weaker with time As more and more unstable atoms become stable atoms less radiation is produced and eventually the material will become non-radioactive
The decay of radioactive elements occurs at a fixed rate The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material For example a source will have an intensity of 100 when new At one half-life its intensity will be cut to 50 of the original intensity At two half-lives it will have an intensity of 25 of a new source After ten half-lives less than one-thousandth of the original activity will remain Although the half-life pattern is the same for every radioisotope the length of a half-life is different For example Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days
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Radiation Decay and Half-life
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Radiation Activity and Intensity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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Rad Units
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Stochastic Effects
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Nonstochastic Effects
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Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
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Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Industrial radiography - Wikipedia the free encyclopedia
upon client requirements the radiographer would then place film cassettes on the outside of the surface to be examined This exposure
arrangement is ideal - when properly arranged and exposed all portions of all exposed film will be of the same approximate density It also has
the advantage of taking less time than other arrangements since the source must only penetrate the total wall thickness (WT) once and must
only travel the radius of the inspection item not its full diameter The major disadvantage of the panoramic is that it may be impractical to
reach the center of the item (enclosed pipe) or the source may be too weak to perform in this arrangement (large vessels or tanks)
The second SWESWV arrangement is an interior placement of the source in an enclosed inspection item without having the source centered
up The source does not come in direct contact with the item but is placed a distance away depending on client requirements The third is an
exterior placement with similar characteristics The fourth is reserved for flat objects such as plate metal and is also radiographed without the
source coming in direct contact with the item In each case the radiographic film is located on the opposite side of the inspection item from the
source In all four cases only one wall is exposed and only one wall is viewed on the radiograph
Of the other exposure arrangements only the contact shot has the source located on the inspection item This type of radiograph exposes both
walls but only resolves the image on the wall nearest the film This exposure arrangement takes more time than a panoramic as the source
must penetrate the WT twice and travel the entire outside diameter of the pipe or vessel to reach the film on the opposite side This is a double
wall exposuresingle wall view DWESWV arrangement Another is the superimposure (wherein the source is placed on one side of the item
not in direct contact with it with the film on the opposite side) This arrangement is usually reserved for very small diameter piping or parts
The last DWESWV exposure arrangement is the elliptical in which the source is offset from the plane of the inspection item (usually a weld
in pipe) and the elliptical image of the weld furthest from the source is cast onto the film
The beam of radiation must be directed to the middle of the section under examination and must be normal to the material surface at that point
except in special techniques where known defects are best revealed by a different alignment of the beam The length of weld under
examination for each exposure shall be such that the thickness of the material at the diagnostic extremities measured in the direction of the
incident beam does not exceed the actual thickness at that point by more than 6 The specimen to be inspected is placed between the source
of radiation and the detecting device usually the film in a light tight holder or cassette and the radiation is allowed to penetrate the part for the
required length of time to be adequately recorded Lead is often placed behind the film to reduce the back scattered radiation which can lead
to the film becoming over exposed
The result is a two-dimensional projection of the part onto the film producing a latent image of varying densities according to the amount of
radiation reaching each area It is known as a radiograph as distinct from a photograph produced by light Because film is cumulative in its
response (the exposure increasing as it absorbs more radiation) relatively weak radiation can be detected by prolonging the exposure until the
film can record an image that will be visible after development The radiograph is examined as a negative without printing as a positive as in
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photography This is because in printing some of the detail is always lost and no useful purpose is served
Before commencing a radiographic examination it is always advisable to examine the component with ones own eyes to eliminate any
possible external defects If the surface of a weld is too irregular it may be desirable to grind it to obtain a smooth finish but this is likely to be
limited to those cases in which the surface irregularities (which will be visible on the radiograph) may make detecting internal defects difficult
After this visual examination the operator will have a clear idea of the possibilities of access to the two faces of the weld which is important
both for the setting up of the equipment and for the choice of the most appropriate technique
[edit] Airport security
Both hold luggage and carry-on hand luggage are normally examined by X-ray machines using X-ray radiography See airport security for
more details
[edit] Non-intrusive Cargo Scanning (aka Non-Intrusive Inspection - NII)
Gamma-ray Image of intermodal cargo container
with stowaways
Gamma Radiography and High-Energy X-ray radiography are currently used to scan intermodal freight cargo containers in US and other
countries Also research is being done on adapting other types of radiography like Dual-Energy X-ray Radiography or Muon Radiography for
scanning intermodal cargo containers
[edit] Sources
[edit] X-ray sources
A high energy X-ray machine can be used It is often important to use a high accelerating voltage to provide the electrons with a very high
energy This is because in a braking radiation source the maximum photon energy is determined by the energy of the charged particles
[edit] Radioisotope sources
These have the advantage that they do not need a supply of electrical power to function but they do have the disadvantage that they can not be
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turned off Also it is difficult using radioactivity to create a small and compact source that offers the photon flux possible with a normal sealed
X-ray tube One of the leading makers of radiographic equipment is the Source Production amp Equipment Co Inc [1]
It might be possible to use Cs-137 as a photon source for radiography but this isotope is always diluted with inactive caesium isotopes This
makes it difficult to get a physically small source and a large volume of the source makes it impossible to capture fine details in a radiographic
examination
Both cobalt-60 and caesium-137 have only a few gamma energies which makes them close to monochromatic The photon energy of cobalt-
60 is higher than that of caesium-137 which allows cobalt sources to be used to examine thicker sections of metals than those that could be
examined with Cs-137 Iridium-192 has a lower photon energy than cobalt-60 and its gamma spectrum is complex (many lines of very
different energies) but this can be an advantage as this can give better contrast for the final photographs
It has been known for many years that an inactive iridium or cobalt metal object can be machined to size In the case of cobalt it is common to
alloy it with nickel to improve the mechanical properties In the case of iridium a thin wire or rod could be used These precursor materials can
then be placed in stainless steel containers that have been leak tested before being converted into radioactive sources These objects can be
processed by neutron activation to form gamma emitting radioisotopes The stainless steel has only a small ability to be activated and the small
activity due to 55Fe and 63Ni are unlikely to pose a problem in the final application because these isotopes are beta emitters which have very
weak gamma emission The 59Fe which might form has a short half life so by allowing a cobalt source to stand for a year much of this isotope
will decay away
The source is often a very small object which must be transported to the work site in a shielded container It is normal to place the film in
industrial radiography clear the area where the work is to be done add shielding (collimators) to reduce the size of the controlled area before
exposing the radioactive source A series of different designs have been developed for radiographic cameras Rather than the camera being
a device that accepts photons to record a picture the camera in industrial radiography is the radioactive photon source
[edit] Torch design of radiographic cameras
One design is best thought of as being like a torch The radioactive source is placed inside a shielded box a hinge allows part of the shielding
to be opened exposing the source allowing photons to exit the radiography camera
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This torch-type camera uses a hinge The radioactive source is
in red the shielding is bluegreen and the gamma rays are
yellow
Another design for a torch is where the source is placed in a metal wheel which can turn inside the camera to move between the expose and
storage positions
This torch-type camera uses a wheel design The radioactive
source is in red and the gamma rays are yellow
[edit] Cable based design of radiographic cameras
One group of designs use a radioactive source which connects to a drive cable contained shielded exposure device In one design of equipment
the source is stored in a block of lead or depleted uranium shielding that has a S shaped tube-like hole through the block In the safe position
the source is in the centre of the block and is attached to a metal wire that extends in both directions to use the source a guide tube is attached
to one side of the device while a drive cable is attached to the other end of the short cable Using a hand operated winch the source is then
pushed out of the shield and along the source guide tube to the tip of the tube to expose the film then cranked back into its fully-shielded
position
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A diagram of the S shaped hole through a metal
block the source is stored at point A and is driven
out on a cable through a hole to point B It often goes
a long way along a guide tube to where it is needed
[edit] Contrast agents
Defects such as delaminations and planar cracks are difficult to detect using radiography which is why penetrants are often used to enhance
the contrast in the detection of such defects Penetrants used include silver nitrate zinc iodide chloroform and diiodomethane Choice of the
penetrant is determined by the ease with which it can penetrate the cracks and also with which it can be removed Diiodomethane has the
advantages of high opacity ease of penetration and ease of removal because it evaporates relatively quickly However it can cause skin burns
[edit] Neutrons
In some rare cases radiography is done with neutrons This type of radiography is called neutron radiography (NR Nray N-Ray) or neutron
imaging Neutron radiography can see very different things than X-rays because neutrons can pass with ease through lead and steel but are
stopped by plastics water and oils Neutron sources include radioactive (241AmBe and Cf) sources electrically driven D-T reactions in
vacuum tubes and conventional critical nuclear reactors It might be possible to use a neutron amplifier to increase the neutron flux[2]
Since the amount of radiation emerging from the opposite side of the material can be detected and measured variations in this amount (or
intensity) of radiation are used to determine thickness or composition of material Penetrating radiations are those restricted to that part of the
electromagnetic spectrum of wavelength less than about 10 nanometres
[edit] Safety
Industrial radiographers are in many locations required by governing authorities to use certain types of safety equipment and to work in pairs
Depending on location industrial radiographers may have been required to obtain permits licences andor undertake special training Prior to
conducting any testing the nearby area should always first be cleared of all other persons and measures taken to ensure that people do not
accidentally enter into an area that may expose them to a large dose of radiation
The safety equipment usually includes four basic items a radiation survey meter (such as a GeigerMueller counter) an alarming dosimeter or
rate meter a gas-charged dosimeter and a film badge or thermoluminescent dosimeter (TLD) The easiest way to remember what each of these
items does is to compare them to gauges on an automobile
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The survey meter could be compared to the speedometer as it measures the speed or rate at which radiation is being picked up When
properly calibrated used and maintained it allows the radiographer to see the current exposure to radiation at the meter It can usually be set
for different intensities and is used to prevent the radiographer from being overexposed to the radioactive source as well as for verifying the
boundary that radiographers are required to maintain around the exposed source during radiographic operations
The alarming dosimeter could be most closely compared with the tachometer as it alarms when the radiographer redlines or is exposed to
too much radiation When properly calibrated activated and worn on the radiographers person it will emit an alarm when the meter measures
a radiation level in excess of a preset threshold This device is intended to prevent the radiographer from inadvertently walking up on an
exposed source
The gas-charged dosimeter is like a trip meter in that it measures the total radiation received but can be reset It is designed to help the
radiographer measure hisher total periodic dose of radiation When properly calibrated recharged and worn on the radiographers person it
can tell the radiographer at a glance how much radiation to which the device has been exposed since it was last recharged Radiographers in
many states are required to log their radiation exposures and generate an exposure report In many countries personal dosimeters are not
required to be used by radiographers as the dose rates they show are not always correctly recorded
The film badge or TLD is more like a cars odometer It is actually a specialized piece of radiographic film in a rugged container It is meant to
measure the radiographers total exposure over time (usually a month) and is used by regulating authorities to monitor the total exposure of
certified radiographers in a certain jurisdiction At the end of the month the film badge is turned in and is processed A report of the
radiographers total dose is generated and is kept on file
When these safety devices are properly calibrated maintained and used it is virtually impossible for a radiographer to be injured by a
radioactive overexposure Sadly the elimination of just one of these devices can jeopardize the safety of the radiographer and all those who are
nearby Without the survey meter the radiation received may be just below the threshold of the rate alarm and it may be several hours before
the radiographer checks the dosimeter and up to a month or more before the film badge is developed to detect a low intensity overexposure
Without the rate alarm one radiographer may inadvertently walk up on the source exposed by the other radiographer Without the dosimeter
the radiographer may be unaware of an overexposure or even a radiation burn which may take weeks to result in noticeable injury And
without the film badge the radiographer is deprived of an important tool designed to protect him or her from the effects of a long-term
overexposure to occupationally-obtained radiation and thus may suffer long-term health problems as a result
There are three ways a radiographer will ensure they are not exposed to higher than required levels of radiation time distance shielding The
less time that a person is exposed to radiation the lower their dose will be The further a person is from a radioactive source the lower the level
of radiation they receive this is largely due to the inverse square law Lastly the more a radioactive source is shielded by either better or
httpenwikipediaorgwikiIndustrial_radiography (8 of 11)21-09-2011 103049
Industrial radiography - Wikipedia the free encyclopedia
greater amounts of shielding the lower the levels of radiation that will escape from the testing area The most commanly used shielding
materials in use are sand lead (sheets or shot) steel spent (non-radioactive uranium) tungsten and in suitable situations water
Industrial radiography appears to have one of the worst safety profiles of the radiation professions possibly because there are many operators
using strong gamma sources (gt 2 Ci) in remote sites with little supervision when compared with workers within the nuclear industry or within
hospitals[3] Due to the levels of radiation present whilst they are working many radiographers are also required to work late at night when
there are few other people present as most industrial radiography is carried out in the open rather than in purpose built exposure booths or
rooms Fatigue carelessness and lack of proper training are the three most comman factors attributed to industrial radiography accidents Many
of the lost source accidents commented on by the International Atomic Energy Agency involve radiography equipment Lost source
accidents have the potential to cause a considerable loss of human life One scenario is that a passerby finds the radiography source and not
knowing what it is takes it home[4] The person shortly afterwards becomes ill and dies as a result of the radiation dose The source remains in
their home where it continues to irradiate other members of the household[5] Such an event occurred in March 1984 in Casablanca
(Mohammedia) which is part of Morocco This is related to the more famous Goiacircnia accident where a related chain of events caused
members of the public to be exposed to radiation sources Also see List of civilian radiation accidents
[edit] See also
Radiographic testing
Collimator
Industrial CT scanning
[edit] References
1 ^ httpwwwlboroacuklibrarynewSpotlighton-Archivehtml Loughborough University Library - Spotlight Archive [Accessed 22 October 2008]
[edit] External links
NISTs XAAMDI X-Ray Attenuation and Absorption for Materials of Dosimetric Interest Database
NISTs XCOM Photon Cross Sections Database
NISTs FAST Attenuation and Scattering Tables
A lost industrial radiography source event
UN information on the security of industrial sources
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Radiation Production for Industrial Radiography
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Production of Radiation for Industrial Radiography
Industrial radiography uses two sources of radiation X-radiation and Gamma radiation X-rays and Gamma rays differ only in their source of origin X-rays are produced by an X-ray generator and Gamma radiation is the product of radioactive atoms An in depth discussion on radiation production can be found in other areas of this site but will be reviewed briefly in the following sections
Production of X-Rays There are two different atomic processes that can produce X-ray photons One process produces Bremsstrahlung radiation and the other produces K-shell or characteristic emission Both processes involve a change in the energy state of electrons X-rays are generated when an electron is accelerated and then made to rapidly decelerate usually due to interaction with other atomic particles
In an X-ray system a large amount of electric current is passed through a tungsten filament which heats the filament to several thousand degrees centigrade to create a source of free electrons A large electrical potential is established between the filament (the cathode) and a target (the anode) The cathode and anode are enclosed in a vacuum tube to prevent the filament from burning up and to prevent arcing between the cathode and anode The electrical potential between the cathode and the anode pulls electrons from the cathode and accelerates them as they are attracted towards the anode or target which is usually made of tungsten X-rays are generated when free electrons give up some of their energy when they interact with the electrons or nucleus of an atom The interaction of the electrons in the target results in the emission of a continuous Bremsstrahlung spectrum and also characteristic X-rays from the target material
Production of Gamma Rays Gamma radiation is the product of radioactive atoms Depending upon the ratio of neutrons to protons within its nucleus an isotope of a particular element may be stable or unstable Over time the nuclei of unstable isotopes spontaneously disintegrate or transform in a process known as radioactive decay Various types of radiation may be emitted from the nucleus andor its surrounding electrons when an atom experiences radioactive decay Nuclides which undergo radioactive decay are called radionuclides Any material which contains measurable amounts of one or more radionuclides is a radioactive material
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Radiation Production for Industrial Radiography
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There are many naturally occurring radioactive materials but manmade radioactive isotopes or radioisotopes are used for industrial radiography Man-made sources are produced by introducing an extra neutron to atoms of the source material For example Cobalt-60 is produced by bombarding a sample of Cobalt-59 with an excess of neutrons in a nuclear reactor The Cobalt-59 atoms absorb some of the neutrons and increase their atomic weight by one to produce the radioisotope Cobalt-60 This process is known as activation As a material rids itself of atomic particles to return to a balance state energy is released in the form of Gamma rays and sometimes alpha or beta particles
Physical size of isotope materials will very slightly between manufacturer but generally an isotope is a pellet that measures 15 mm x 15 mm Depending on the activity (curies) desired a pellet or pellets are loaded into a stainless steel capsule and sealed Unlike X-ray tubes radioactive sources provide a continual source of radiation that cannot be turned off Once radioactive decay starts it continues until all of the atoms have reached a stable state The radioisotope can only be shielded to prevent exposure to the radiation In industrial radiography the instruments that are used to shield the radioisotope so that they can be safely handled and used are commonly called cameras or exposure devices Exposure devices will be discussed later in more detail
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Radiation Production for Industrial Radiography
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Radiation Decay and Half-life
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radioactive Decay and Half-Life
As mentioned previously radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation As a radioisotope atom decays to a more stable atom it emits radiation only once To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step However once the atom reaches a stable configuration no more radiation is given off For this reason radioactive sources become weaker with time As more and more unstable atoms become stable atoms less radiation is produced and eventually the material will become non-radioactive
The decay of radioactive elements occurs at a fixed rate The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material For example a source will have an intensity of 100 when new At one half-life its intensity will be cut to 50 of the original intensity At two half-lives it will have an intensity of 25 of a new source After ten half-lives less than one-thousandth of the original activity will remain Although the half-life pattern is the same for every radioisotope the length of a half-life is different For example Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days
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Radiation Decay and Half-life
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Radiation Activity and Intensity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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Rad Units
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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Exposure Limits
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Controlling Exposure
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Industrial radiography - Wikipedia the free encyclopedia
photography This is because in printing some of the detail is always lost and no useful purpose is served
Before commencing a radiographic examination it is always advisable to examine the component with ones own eyes to eliminate any
possible external defects If the surface of a weld is too irregular it may be desirable to grind it to obtain a smooth finish but this is likely to be
limited to those cases in which the surface irregularities (which will be visible on the radiograph) may make detecting internal defects difficult
After this visual examination the operator will have a clear idea of the possibilities of access to the two faces of the weld which is important
both for the setting up of the equipment and for the choice of the most appropriate technique
[edit] Airport security
Both hold luggage and carry-on hand luggage are normally examined by X-ray machines using X-ray radiography See airport security for
more details
[edit] Non-intrusive Cargo Scanning (aka Non-Intrusive Inspection - NII)
Gamma-ray Image of intermodal cargo container
with stowaways
Gamma Radiography and High-Energy X-ray radiography are currently used to scan intermodal freight cargo containers in US and other
countries Also research is being done on adapting other types of radiography like Dual-Energy X-ray Radiography or Muon Radiography for
scanning intermodal cargo containers
[edit] Sources
[edit] X-ray sources
A high energy X-ray machine can be used It is often important to use a high accelerating voltage to provide the electrons with a very high
energy This is because in a braking radiation source the maximum photon energy is determined by the energy of the charged particles
[edit] Radioisotope sources
These have the advantage that they do not need a supply of electrical power to function but they do have the disadvantage that they can not be
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turned off Also it is difficult using radioactivity to create a small and compact source that offers the photon flux possible with a normal sealed
X-ray tube One of the leading makers of radiographic equipment is the Source Production amp Equipment Co Inc [1]
It might be possible to use Cs-137 as a photon source for radiography but this isotope is always diluted with inactive caesium isotopes This
makes it difficult to get a physically small source and a large volume of the source makes it impossible to capture fine details in a radiographic
examination
Both cobalt-60 and caesium-137 have only a few gamma energies which makes them close to monochromatic The photon energy of cobalt-
60 is higher than that of caesium-137 which allows cobalt sources to be used to examine thicker sections of metals than those that could be
examined with Cs-137 Iridium-192 has a lower photon energy than cobalt-60 and its gamma spectrum is complex (many lines of very
different energies) but this can be an advantage as this can give better contrast for the final photographs
It has been known for many years that an inactive iridium or cobalt metal object can be machined to size In the case of cobalt it is common to
alloy it with nickel to improve the mechanical properties In the case of iridium a thin wire or rod could be used These precursor materials can
then be placed in stainless steel containers that have been leak tested before being converted into radioactive sources These objects can be
processed by neutron activation to form gamma emitting radioisotopes The stainless steel has only a small ability to be activated and the small
activity due to 55Fe and 63Ni are unlikely to pose a problem in the final application because these isotopes are beta emitters which have very
weak gamma emission The 59Fe which might form has a short half life so by allowing a cobalt source to stand for a year much of this isotope
will decay away
The source is often a very small object which must be transported to the work site in a shielded container It is normal to place the film in
industrial radiography clear the area where the work is to be done add shielding (collimators) to reduce the size of the controlled area before
exposing the radioactive source A series of different designs have been developed for radiographic cameras Rather than the camera being
a device that accepts photons to record a picture the camera in industrial radiography is the radioactive photon source
[edit] Torch design of radiographic cameras
One design is best thought of as being like a torch The radioactive source is placed inside a shielded box a hinge allows part of the shielding
to be opened exposing the source allowing photons to exit the radiography camera
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Industrial radiography - Wikipedia the free encyclopedia
This torch-type camera uses a hinge The radioactive source is
in red the shielding is bluegreen and the gamma rays are
yellow
Another design for a torch is where the source is placed in a metal wheel which can turn inside the camera to move between the expose and
storage positions
This torch-type camera uses a wheel design The radioactive
source is in red and the gamma rays are yellow
[edit] Cable based design of radiographic cameras
One group of designs use a radioactive source which connects to a drive cable contained shielded exposure device In one design of equipment
the source is stored in a block of lead or depleted uranium shielding that has a S shaped tube-like hole through the block In the safe position
the source is in the centre of the block and is attached to a metal wire that extends in both directions to use the source a guide tube is attached
to one side of the device while a drive cable is attached to the other end of the short cable Using a hand operated winch the source is then
pushed out of the shield and along the source guide tube to the tip of the tube to expose the film then cranked back into its fully-shielded
position
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Industrial radiography - Wikipedia the free encyclopedia
A diagram of the S shaped hole through a metal
block the source is stored at point A and is driven
out on a cable through a hole to point B It often goes
a long way along a guide tube to where it is needed
[edit] Contrast agents
Defects such as delaminations and planar cracks are difficult to detect using radiography which is why penetrants are often used to enhance
the contrast in the detection of such defects Penetrants used include silver nitrate zinc iodide chloroform and diiodomethane Choice of the
penetrant is determined by the ease with which it can penetrate the cracks and also with which it can be removed Diiodomethane has the
advantages of high opacity ease of penetration and ease of removal because it evaporates relatively quickly However it can cause skin burns
[edit] Neutrons
In some rare cases radiography is done with neutrons This type of radiography is called neutron radiography (NR Nray N-Ray) or neutron
imaging Neutron radiography can see very different things than X-rays because neutrons can pass with ease through lead and steel but are
stopped by plastics water and oils Neutron sources include radioactive (241AmBe and Cf) sources electrically driven D-T reactions in
vacuum tubes and conventional critical nuclear reactors It might be possible to use a neutron amplifier to increase the neutron flux[2]
Since the amount of radiation emerging from the opposite side of the material can be detected and measured variations in this amount (or
intensity) of radiation are used to determine thickness or composition of material Penetrating radiations are those restricted to that part of the
electromagnetic spectrum of wavelength less than about 10 nanometres
[edit] Safety
Industrial radiographers are in many locations required by governing authorities to use certain types of safety equipment and to work in pairs
Depending on location industrial radiographers may have been required to obtain permits licences andor undertake special training Prior to
conducting any testing the nearby area should always first be cleared of all other persons and measures taken to ensure that people do not
accidentally enter into an area that may expose them to a large dose of radiation
The safety equipment usually includes four basic items a radiation survey meter (such as a GeigerMueller counter) an alarming dosimeter or
rate meter a gas-charged dosimeter and a film badge or thermoluminescent dosimeter (TLD) The easiest way to remember what each of these
items does is to compare them to gauges on an automobile
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Industrial radiography - Wikipedia the free encyclopedia
The survey meter could be compared to the speedometer as it measures the speed or rate at which radiation is being picked up When
properly calibrated used and maintained it allows the radiographer to see the current exposure to radiation at the meter It can usually be set
for different intensities and is used to prevent the radiographer from being overexposed to the radioactive source as well as for verifying the
boundary that radiographers are required to maintain around the exposed source during radiographic operations
The alarming dosimeter could be most closely compared with the tachometer as it alarms when the radiographer redlines or is exposed to
too much radiation When properly calibrated activated and worn on the radiographers person it will emit an alarm when the meter measures
a radiation level in excess of a preset threshold This device is intended to prevent the radiographer from inadvertently walking up on an
exposed source
The gas-charged dosimeter is like a trip meter in that it measures the total radiation received but can be reset It is designed to help the
radiographer measure hisher total periodic dose of radiation When properly calibrated recharged and worn on the radiographers person it
can tell the radiographer at a glance how much radiation to which the device has been exposed since it was last recharged Radiographers in
many states are required to log their radiation exposures and generate an exposure report In many countries personal dosimeters are not
required to be used by radiographers as the dose rates they show are not always correctly recorded
The film badge or TLD is more like a cars odometer It is actually a specialized piece of radiographic film in a rugged container It is meant to
measure the radiographers total exposure over time (usually a month) and is used by regulating authorities to monitor the total exposure of
certified radiographers in a certain jurisdiction At the end of the month the film badge is turned in and is processed A report of the
radiographers total dose is generated and is kept on file
When these safety devices are properly calibrated maintained and used it is virtually impossible for a radiographer to be injured by a
radioactive overexposure Sadly the elimination of just one of these devices can jeopardize the safety of the radiographer and all those who are
nearby Without the survey meter the radiation received may be just below the threshold of the rate alarm and it may be several hours before
the radiographer checks the dosimeter and up to a month or more before the film badge is developed to detect a low intensity overexposure
Without the rate alarm one radiographer may inadvertently walk up on the source exposed by the other radiographer Without the dosimeter
the radiographer may be unaware of an overexposure or even a radiation burn which may take weeks to result in noticeable injury And
without the film badge the radiographer is deprived of an important tool designed to protect him or her from the effects of a long-term
overexposure to occupationally-obtained radiation and thus may suffer long-term health problems as a result
There are three ways a radiographer will ensure they are not exposed to higher than required levels of radiation time distance shielding The
less time that a person is exposed to radiation the lower their dose will be The further a person is from a radioactive source the lower the level
of radiation they receive this is largely due to the inverse square law Lastly the more a radioactive source is shielded by either better or
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Industrial radiography - Wikipedia the free encyclopedia
greater amounts of shielding the lower the levels of radiation that will escape from the testing area The most commanly used shielding
materials in use are sand lead (sheets or shot) steel spent (non-radioactive uranium) tungsten and in suitable situations water
Industrial radiography appears to have one of the worst safety profiles of the radiation professions possibly because there are many operators
using strong gamma sources (gt 2 Ci) in remote sites with little supervision when compared with workers within the nuclear industry or within
hospitals[3] Due to the levels of radiation present whilst they are working many radiographers are also required to work late at night when
there are few other people present as most industrial radiography is carried out in the open rather than in purpose built exposure booths or
rooms Fatigue carelessness and lack of proper training are the three most comman factors attributed to industrial radiography accidents Many
of the lost source accidents commented on by the International Atomic Energy Agency involve radiography equipment Lost source
accidents have the potential to cause a considerable loss of human life One scenario is that a passerby finds the radiography source and not
knowing what it is takes it home[4] The person shortly afterwards becomes ill and dies as a result of the radiation dose The source remains in
their home where it continues to irradiate other members of the household[5] Such an event occurred in March 1984 in Casablanca
(Mohammedia) which is part of Morocco This is related to the more famous Goiacircnia accident where a related chain of events caused
members of the public to be exposed to radiation sources Also see List of civilian radiation accidents
[edit] See also
Radiographic testing
Collimator
Industrial CT scanning
[edit] References
1 ^ httpwwwlboroacuklibrarynewSpotlighton-Archivehtml Loughborough University Library - Spotlight Archive [Accessed 22 October 2008]
[edit] External links
NISTs XAAMDI X-Ray Attenuation and Absorption for Materials of Dosimetric Interest Database
NISTs XCOM Photon Cross Sections Database
NISTs FAST Attenuation and Scattering Tables
A lost industrial radiography source event
UN information on the security of industrial sources
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Radiation Production for Industrial Radiography
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Production of Radiation for Industrial Radiography
Industrial radiography uses two sources of radiation X-radiation and Gamma radiation X-rays and Gamma rays differ only in their source of origin X-rays are produced by an X-ray generator and Gamma radiation is the product of radioactive atoms An in depth discussion on radiation production can be found in other areas of this site but will be reviewed briefly in the following sections
Production of X-Rays There are two different atomic processes that can produce X-ray photons One process produces Bremsstrahlung radiation and the other produces K-shell or characteristic emission Both processes involve a change in the energy state of electrons X-rays are generated when an electron is accelerated and then made to rapidly decelerate usually due to interaction with other atomic particles
In an X-ray system a large amount of electric current is passed through a tungsten filament which heats the filament to several thousand degrees centigrade to create a source of free electrons A large electrical potential is established between the filament (the cathode) and a target (the anode) The cathode and anode are enclosed in a vacuum tube to prevent the filament from burning up and to prevent arcing between the cathode and anode The electrical potential between the cathode and the anode pulls electrons from the cathode and accelerates them as they are attracted towards the anode or target which is usually made of tungsten X-rays are generated when free electrons give up some of their energy when they interact with the electrons or nucleus of an atom The interaction of the electrons in the target results in the emission of a continuous Bremsstrahlung spectrum and also characteristic X-rays from the target material
Production of Gamma Rays Gamma radiation is the product of radioactive atoms Depending upon the ratio of neutrons to protons within its nucleus an isotope of a particular element may be stable or unstable Over time the nuclei of unstable isotopes spontaneously disintegrate or transform in a process known as radioactive decay Various types of radiation may be emitted from the nucleus andor its surrounding electrons when an atom experiences radioactive decay Nuclides which undergo radioactive decay are called radionuclides Any material which contains measurable amounts of one or more radionuclides is a radioactive material
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Radiation Production for Industrial Radiography
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There are many naturally occurring radioactive materials but manmade radioactive isotopes or radioisotopes are used for industrial radiography Man-made sources are produced by introducing an extra neutron to atoms of the source material For example Cobalt-60 is produced by bombarding a sample of Cobalt-59 with an excess of neutrons in a nuclear reactor The Cobalt-59 atoms absorb some of the neutrons and increase their atomic weight by one to produce the radioisotope Cobalt-60 This process is known as activation As a material rids itself of atomic particles to return to a balance state energy is released in the form of Gamma rays and sometimes alpha or beta particles
Physical size of isotope materials will very slightly between manufacturer but generally an isotope is a pellet that measures 15 mm x 15 mm Depending on the activity (curies) desired a pellet or pellets are loaded into a stainless steel capsule and sealed Unlike X-ray tubes radioactive sources provide a continual source of radiation that cannot be turned off Once radioactive decay starts it continues until all of the atoms have reached a stable state The radioisotope can only be shielded to prevent exposure to the radiation In industrial radiography the instruments that are used to shield the radioisotope so that they can be safely handled and used are commonly called cameras or exposure devices Exposure devices will be discussed later in more detail
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Radiation Production for Industrial Radiography
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Radiation Decay and Half-life
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radioactive Decay and Half-Life
As mentioned previously radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation As a radioisotope atom decays to a more stable atom it emits radiation only once To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step However once the atom reaches a stable configuration no more radiation is given off For this reason radioactive sources become weaker with time As more and more unstable atoms become stable atoms less radiation is produced and eventually the material will become non-radioactive
The decay of radioactive elements occurs at a fixed rate The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material For example a source will have an intensity of 100 when new At one half-life its intensity will be cut to 50 of the original intensity At two half-lives it will have an intensity of 25 of a new source After ten half-lives less than one-thousandth of the original activity will remain Although the half-life pattern is the same for every radioisotope the length of a half-life is different For example Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days
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Radiation Activity and Intensity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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Exposure Limits
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Industrial radiography - Wikipedia the free encyclopedia
turned off Also it is difficult using radioactivity to create a small and compact source that offers the photon flux possible with a normal sealed
X-ray tube One of the leading makers of radiographic equipment is the Source Production amp Equipment Co Inc [1]
It might be possible to use Cs-137 as a photon source for radiography but this isotope is always diluted with inactive caesium isotopes This
makes it difficult to get a physically small source and a large volume of the source makes it impossible to capture fine details in a radiographic
examination
Both cobalt-60 and caesium-137 have only a few gamma energies which makes them close to monochromatic The photon energy of cobalt-
60 is higher than that of caesium-137 which allows cobalt sources to be used to examine thicker sections of metals than those that could be
examined with Cs-137 Iridium-192 has a lower photon energy than cobalt-60 and its gamma spectrum is complex (many lines of very
different energies) but this can be an advantage as this can give better contrast for the final photographs
It has been known for many years that an inactive iridium or cobalt metal object can be machined to size In the case of cobalt it is common to
alloy it with nickel to improve the mechanical properties In the case of iridium a thin wire or rod could be used These precursor materials can
then be placed in stainless steel containers that have been leak tested before being converted into radioactive sources These objects can be
processed by neutron activation to form gamma emitting radioisotopes The stainless steel has only a small ability to be activated and the small
activity due to 55Fe and 63Ni are unlikely to pose a problem in the final application because these isotopes are beta emitters which have very
weak gamma emission The 59Fe which might form has a short half life so by allowing a cobalt source to stand for a year much of this isotope
will decay away
The source is often a very small object which must be transported to the work site in a shielded container It is normal to place the film in
industrial radiography clear the area where the work is to be done add shielding (collimators) to reduce the size of the controlled area before
exposing the radioactive source A series of different designs have been developed for radiographic cameras Rather than the camera being
a device that accepts photons to record a picture the camera in industrial radiography is the radioactive photon source
[edit] Torch design of radiographic cameras
One design is best thought of as being like a torch The radioactive source is placed inside a shielded box a hinge allows part of the shielding
to be opened exposing the source allowing photons to exit the radiography camera
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This torch-type camera uses a hinge The radioactive source is
in red the shielding is bluegreen and the gamma rays are
yellow
Another design for a torch is where the source is placed in a metal wheel which can turn inside the camera to move between the expose and
storage positions
This torch-type camera uses a wheel design The radioactive
source is in red and the gamma rays are yellow
[edit] Cable based design of radiographic cameras
One group of designs use a radioactive source which connects to a drive cable contained shielded exposure device In one design of equipment
the source is stored in a block of lead or depleted uranium shielding that has a S shaped tube-like hole through the block In the safe position
the source is in the centre of the block and is attached to a metal wire that extends in both directions to use the source a guide tube is attached
to one side of the device while a drive cable is attached to the other end of the short cable Using a hand operated winch the source is then
pushed out of the shield and along the source guide tube to the tip of the tube to expose the film then cranked back into its fully-shielded
position
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Industrial radiography - Wikipedia the free encyclopedia
A diagram of the S shaped hole through a metal
block the source is stored at point A and is driven
out on a cable through a hole to point B It often goes
a long way along a guide tube to where it is needed
[edit] Contrast agents
Defects such as delaminations and planar cracks are difficult to detect using radiography which is why penetrants are often used to enhance
the contrast in the detection of such defects Penetrants used include silver nitrate zinc iodide chloroform and diiodomethane Choice of the
penetrant is determined by the ease with which it can penetrate the cracks and also with which it can be removed Diiodomethane has the
advantages of high opacity ease of penetration and ease of removal because it evaporates relatively quickly However it can cause skin burns
[edit] Neutrons
In some rare cases radiography is done with neutrons This type of radiography is called neutron radiography (NR Nray N-Ray) or neutron
imaging Neutron radiography can see very different things than X-rays because neutrons can pass with ease through lead and steel but are
stopped by plastics water and oils Neutron sources include radioactive (241AmBe and Cf) sources electrically driven D-T reactions in
vacuum tubes and conventional critical nuclear reactors It might be possible to use a neutron amplifier to increase the neutron flux[2]
Since the amount of radiation emerging from the opposite side of the material can be detected and measured variations in this amount (or
intensity) of radiation are used to determine thickness or composition of material Penetrating radiations are those restricted to that part of the
electromagnetic spectrum of wavelength less than about 10 nanometres
[edit] Safety
Industrial radiographers are in many locations required by governing authorities to use certain types of safety equipment and to work in pairs
Depending on location industrial radiographers may have been required to obtain permits licences andor undertake special training Prior to
conducting any testing the nearby area should always first be cleared of all other persons and measures taken to ensure that people do not
accidentally enter into an area that may expose them to a large dose of radiation
The safety equipment usually includes four basic items a radiation survey meter (such as a GeigerMueller counter) an alarming dosimeter or
rate meter a gas-charged dosimeter and a film badge or thermoluminescent dosimeter (TLD) The easiest way to remember what each of these
items does is to compare them to gauges on an automobile
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Industrial radiography - Wikipedia the free encyclopedia
The survey meter could be compared to the speedometer as it measures the speed or rate at which radiation is being picked up When
properly calibrated used and maintained it allows the radiographer to see the current exposure to radiation at the meter It can usually be set
for different intensities and is used to prevent the radiographer from being overexposed to the radioactive source as well as for verifying the
boundary that radiographers are required to maintain around the exposed source during radiographic operations
The alarming dosimeter could be most closely compared with the tachometer as it alarms when the radiographer redlines or is exposed to
too much radiation When properly calibrated activated and worn on the radiographers person it will emit an alarm when the meter measures
a radiation level in excess of a preset threshold This device is intended to prevent the radiographer from inadvertently walking up on an
exposed source
The gas-charged dosimeter is like a trip meter in that it measures the total radiation received but can be reset It is designed to help the
radiographer measure hisher total periodic dose of radiation When properly calibrated recharged and worn on the radiographers person it
can tell the radiographer at a glance how much radiation to which the device has been exposed since it was last recharged Radiographers in
many states are required to log their radiation exposures and generate an exposure report In many countries personal dosimeters are not
required to be used by radiographers as the dose rates they show are not always correctly recorded
The film badge or TLD is more like a cars odometer It is actually a specialized piece of radiographic film in a rugged container It is meant to
measure the radiographers total exposure over time (usually a month) and is used by regulating authorities to monitor the total exposure of
certified radiographers in a certain jurisdiction At the end of the month the film badge is turned in and is processed A report of the
radiographers total dose is generated and is kept on file
When these safety devices are properly calibrated maintained and used it is virtually impossible for a radiographer to be injured by a
radioactive overexposure Sadly the elimination of just one of these devices can jeopardize the safety of the radiographer and all those who are
nearby Without the survey meter the radiation received may be just below the threshold of the rate alarm and it may be several hours before
the radiographer checks the dosimeter and up to a month or more before the film badge is developed to detect a low intensity overexposure
Without the rate alarm one radiographer may inadvertently walk up on the source exposed by the other radiographer Without the dosimeter
the radiographer may be unaware of an overexposure or even a radiation burn which may take weeks to result in noticeable injury And
without the film badge the radiographer is deprived of an important tool designed to protect him or her from the effects of a long-term
overexposure to occupationally-obtained radiation and thus may suffer long-term health problems as a result
There are three ways a radiographer will ensure they are not exposed to higher than required levels of radiation time distance shielding The
less time that a person is exposed to radiation the lower their dose will be The further a person is from a radioactive source the lower the level
of radiation they receive this is largely due to the inverse square law Lastly the more a radioactive source is shielded by either better or
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Industrial radiography - Wikipedia the free encyclopedia
greater amounts of shielding the lower the levels of radiation that will escape from the testing area The most commanly used shielding
materials in use are sand lead (sheets or shot) steel spent (non-radioactive uranium) tungsten and in suitable situations water
Industrial radiography appears to have one of the worst safety profiles of the radiation professions possibly because there are many operators
using strong gamma sources (gt 2 Ci) in remote sites with little supervision when compared with workers within the nuclear industry or within
hospitals[3] Due to the levels of radiation present whilst they are working many radiographers are also required to work late at night when
there are few other people present as most industrial radiography is carried out in the open rather than in purpose built exposure booths or
rooms Fatigue carelessness and lack of proper training are the three most comman factors attributed to industrial radiography accidents Many
of the lost source accidents commented on by the International Atomic Energy Agency involve radiography equipment Lost source
accidents have the potential to cause a considerable loss of human life One scenario is that a passerby finds the radiography source and not
knowing what it is takes it home[4] The person shortly afterwards becomes ill and dies as a result of the radiation dose The source remains in
their home where it continues to irradiate other members of the household[5] Such an event occurred in March 1984 in Casablanca
(Mohammedia) which is part of Morocco This is related to the more famous Goiacircnia accident where a related chain of events caused
members of the public to be exposed to radiation sources Also see List of civilian radiation accidents
[edit] See also
Radiographic testing
Collimator
Industrial CT scanning
[edit] References
1 ^ httpwwwlboroacuklibrarynewSpotlighton-Archivehtml Loughborough University Library - Spotlight Archive [Accessed 22 October 2008]
[edit] External links
NISTs XAAMDI X-Ray Attenuation and Absorption for Materials of Dosimetric Interest Database
NISTs XCOM Photon Cross Sections Database
NISTs FAST Attenuation and Scattering Tables
A lost industrial radiography source event
UN information on the security of industrial sources
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Radiation Production for Industrial Radiography
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Production of Radiation for Industrial Radiography
Industrial radiography uses two sources of radiation X-radiation and Gamma radiation X-rays and Gamma rays differ only in their source of origin X-rays are produced by an X-ray generator and Gamma radiation is the product of radioactive atoms An in depth discussion on radiation production can be found in other areas of this site but will be reviewed briefly in the following sections
Production of X-Rays There are two different atomic processes that can produce X-ray photons One process produces Bremsstrahlung radiation and the other produces K-shell or characteristic emission Both processes involve a change in the energy state of electrons X-rays are generated when an electron is accelerated and then made to rapidly decelerate usually due to interaction with other atomic particles
In an X-ray system a large amount of electric current is passed through a tungsten filament which heats the filament to several thousand degrees centigrade to create a source of free electrons A large electrical potential is established between the filament (the cathode) and a target (the anode) The cathode and anode are enclosed in a vacuum tube to prevent the filament from burning up and to prevent arcing between the cathode and anode The electrical potential between the cathode and the anode pulls electrons from the cathode and accelerates them as they are attracted towards the anode or target which is usually made of tungsten X-rays are generated when free electrons give up some of their energy when they interact with the electrons or nucleus of an atom The interaction of the electrons in the target results in the emission of a continuous Bremsstrahlung spectrum and also characteristic X-rays from the target material
Production of Gamma Rays Gamma radiation is the product of radioactive atoms Depending upon the ratio of neutrons to protons within its nucleus an isotope of a particular element may be stable or unstable Over time the nuclei of unstable isotopes spontaneously disintegrate or transform in a process known as radioactive decay Various types of radiation may be emitted from the nucleus andor its surrounding electrons when an atom experiences radioactive decay Nuclides which undergo radioactive decay are called radionuclides Any material which contains measurable amounts of one or more radionuclides is a radioactive material
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Radiation Production for Industrial Radiography
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There are many naturally occurring radioactive materials but manmade radioactive isotopes or radioisotopes are used for industrial radiography Man-made sources are produced by introducing an extra neutron to atoms of the source material For example Cobalt-60 is produced by bombarding a sample of Cobalt-59 with an excess of neutrons in a nuclear reactor The Cobalt-59 atoms absorb some of the neutrons and increase their atomic weight by one to produce the radioisotope Cobalt-60 This process is known as activation As a material rids itself of atomic particles to return to a balance state energy is released in the form of Gamma rays and sometimes alpha or beta particles
Physical size of isotope materials will very slightly between manufacturer but generally an isotope is a pellet that measures 15 mm x 15 mm Depending on the activity (curies) desired a pellet or pellets are loaded into a stainless steel capsule and sealed Unlike X-ray tubes radioactive sources provide a continual source of radiation that cannot be turned off Once radioactive decay starts it continues until all of the atoms have reached a stable state The radioisotope can only be shielded to prevent exposure to the radiation In industrial radiography the instruments that are used to shield the radioisotope so that they can be safely handled and used are commonly called cameras or exposure devices Exposure devices will be discussed later in more detail
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Radiation Production for Industrial Radiography
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Radiation Decay and Half-life
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radioactive Decay and Half-Life
As mentioned previously radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation As a radioisotope atom decays to a more stable atom it emits radiation only once To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step However once the atom reaches a stable configuration no more radiation is given off For this reason radioactive sources become weaker with time As more and more unstable atoms become stable atoms less radiation is produced and eventually the material will become non-radioactive
The decay of radioactive elements occurs at a fixed rate The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material For example a source will have an intensity of 100 when new At one half-life its intensity will be cut to 50 of the original intensity At two half-lives it will have an intensity of 25 of a new source After ten half-lives less than one-thousandth of the original activity will remain Although the half-life pattern is the same for every radioisotope the length of a half-life is different For example Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days
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Radiation Activity and Intensity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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Exposure Limits
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Controlling Exposure
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Thermoluminescent Dosimeter
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Industrial radiography - Wikipedia the free encyclopedia
This torch-type camera uses a hinge The radioactive source is
in red the shielding is bluegreen and the gamma rays are
yellow
Another design for a torch is where the source is placed in a metal wheel which can turn inside the camera to move between the expose and
storage positions
This torch-type camera uses a wheel design The radioactive
source is in red and the gamma rays are yellow
[edit] Cable based design of radiographic cameras
One group of designs use a radioactive source which connects to a drive cable contained shielded exposure device In one design of equipment
the source is stored in a block of lead or depleted uranium shielding that has a S shaped tube-like hole through the block In the safe position
the source is in the centre of the block and is attached to a metal wire that extends in both directions to use the source a guide tube is attached
to one side of the device while a drive cable is attached to the other end of the short cable Using a hand operated winch the source is then
pushed out of the shield and along the source guide tube to the tip of the tube to expose the film then cranked back into its fully-shielded
position
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A diagram of the S shaped hole through a metal
block the source is stored at point A and is driven
out on a cable through a hole to point B It often goes
a long way along a guide tube to where it is needed
[edit] Contrast agents
Defects such as delaminations and planar cracks are difficult to detect using radiography which is why penetrants are often used to enhance
the contrast in the detection of such defects Penetrants used include silver nitrate zinc iodide chloroform and diiodomethane Choice of the
penetrant is determined by the ease with which it can penetrate the cracks and also with which it can be removed Diiodomethane has the
advantages of high opacity ease of penetration and ease of removal because it evaporates relatively quickly However it can cause skin burns
[edit] Neutrons
In some rare cases radiography is done with neutrons This type of radiography is called neutron radiography (NR Nray N-Ray) or neutron
imaging Neutron radiography can see very different things than X-rays because neutrons can pass with ease through lead and steel but are
stopped by plastics water and oils Neutron sources include radioactive (241AmBe and Cf) sources electrically driven D-T reactions in
vacuum tubes and conventional critical nuclear reactors It might be possible to use a neutron amplifier to increase the neutron flux[2]
Since the amount of radiation emerging from the opposite side of the material can be detected and measured variations in this amount (or
intensity) of radiation are used to determine thickness or composition of material Penetrating radiations are those restricted to that part of the
electromagnetic spectrum of wavelength less than about 10 nanometres
[edit] Safety
Industrial radiographers are in many locations required by governing authorities to use certain types of safety equipment and to work in pairs
Depending on location industrial radiographers may have been required to obtain permits licences andor undertake special training Prior to
conducting any testing the nearby area should always first be cleared of all other persons and measures taken to ensure that people do not
accidentally enter into an area that may expose them to a large dose of radiation
The safety equipment usually includes four basic items a radiation survey meter (such as a GeigerMueller counter) an alarming dosimeter or
rate meter a gas-charged dosimeter and a film badge or thermoluminescent dosimeter (TLD) The easiest way to remember what each of these
items does is to compare them to gauges on an automobile
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The survey meter could be compared to the speedometer as it measures the speed or rate at which radiation is being picked up When
properly calibrated used and maintained it allows the radiographer to see the current exposure to radiation at the meter It can usually be set
for different intensities and is used to prevent the radiographer from being overexposed to the radioactive source as well as for verifying the
boundary that radiographers are required to maintain around the exposed source during radiographic operations
The alarming dosimeter could be most closely compared with the tachometer as it alarms when the radiographer redlines or is exposed to
too much radiation When properly calibrated activated and worn on the radiographers person it will emit an alarm when the meter measures
a radiation level in excess of a preset threshold This device is intended to prevent the radiographer from inadvertently walking up on an
exposed source
The gas-charged dosimeter is like a trip meter in that it measures the total radiation received but can be reset It is designed to help the
radiographer measure hisher total periodic dose of radiation When properly calibrated recharged and worn on the radiographers person it
can tell the radiographer at a glance how much radiation to which the device has been exposed since it was last recharged Radiographers in
many states are required to log their radiation exposures and generate an exposure report In many countries personal dosimeters are not
required to be used by radiographers as the dose rates they show are not always correctly recorded
The film badge or TLD is more like a cars odometer It is actually a specialized piece of radiographic film in a rugged container It is meant to
measure the radiographers total exposure over time (usually a month) and is used by regulating authorities to monitor the total exposure of
certified radiographers in a certain jurisdiction At the end of the month the film badge is turned in and is processed A report of the
radiographers total dose is generated and is kept on file
When these safety devices are properly calibrated maintained and used it is virtually impossible for a radiographer to be injured by a
radioactive overexposure Sadly the elimination of just one of these devices can jeopardize the safety of the radiographer and all those who are
nearby Without the survey meter the radiation received may be just below the threshold of the rate alarm and it may be several hours before
the radiographer checks the dosimeter and up to a month or more before the film badge is developed to detect a low intensity overexposure
Without the rate alarm one radiographer may inadvertently walk up on the source exposed by the other radiographer Without the dosimeter
the radiographer may be unaware of an overexposure or even a radiation burn which may take weeks to result in noticeable injury And
without the film badge the radiographer is deprived of an important tool designed to protect him or her from the effects of a long-term
overexposure to occupationally-obtained radiation and thus may suffer long-term health problems as a result
There are three ways a radiographer will ensure they are not exposed to higher than required levels of radiation time distance shielding The
less time that a person is exposed to radiation the lower their dose will be The further a person is from a radioactive source the lower the level
of radiation they receive this is largely due to the inverse square law Lastly the more a radioactive source is shielded by either better or
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greater amounts of shielding the lower the levels of radiation that will escape from the testing area The most commanly used shielding
materials in use are sand lead (sheets or shot) steel spent (non-radioactive uranium) tungsten and in suitable situations water
Industrial radiography appears to have one of the worst safety profiles of the radiation professions possibly because there are many operators
using strong gamma sources (gt 2 Ci) in remote sites with little supervision when compared with workers within the nuclear industry or within
hospitals[3] Due to the levels of radiation present whilst they are working many radiographers are also required to work late at night when
there are few other people present as most industrial radiography is carried out in the open rather than in purpose built exposure booths or
rooms Fatigue carelessness and lack of proper training are the three most comman factors attributed to industrial radiography accidents Many
of the lost source accidents commented on by the International Atomic Energy Agency involve radiography equipment Lost source
accidents have the potential to cause a considerable loss of human life One scenario is that a passerby finds the radiography source and not
knowing what it is takes it home[4] The person shortly afterwards becomes ill and dies as a result of the radiation dose The source remains in
their home where it continues to irradiate other members of the household[5] Such an event occurred in March 1984 in Casablanca
(Mohammedia) which is part of Morocco This is related to the more famous Goiacircnia accident where a related chain of events caused
members of the public to be exposed to radiation sources Also see List of civilian radiation accidents
[edit] See also
Radiographic testing
Collimator
Industrial CT scanning
[edit] References
1 ^ httpwwwlboroacuklibrarynewSpotlighton-Archivehtml Loughborough University Library - Spotlight Archive [Accessed 22 October 2008]
[edit] External links
NISTs XAAMDI X-Ray Attenuation and Absorption for Materials of Dosimetric Interest Database
NISTs XCOM Photon Cross Sections Database
NISTs FAST Attenuation and Scattering Tables
A lost industrial radiography source event
UN information on the security of industrial sources
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Radiation Production for Industrial Radiography
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Production of Radiation for Industrial Radiography
Industrial radiography uses two sources of radiation X-radiation and Gamma radiation X-rays and Gamma rays differ only in their source of origin X-rays are produced by an X-ray generator and Gamma radiation is the product of radioactive atoms An in depth discussion on radiation production can be found in other areas of this site but will be reviewed briefly in the following sections
Production of X-Rays There are two different atomic processes that can produce X-ray photons One process produces Bremsstrahlung radiation and the other produces K-shell or characteristic emission Both processes involve a change in the energy state of electrons X-rays are generated when an electron is accelerated and then made to rapidly decelerate usually due to interaction with other atomic particles
In an X-ray system a large amount of electric current is passed through a tungsten filament which heats the filament to several thousand degrees centigrade to create a source of free electrons A large electrical potential is established between the filament (the cathode) and a target (the anode) The cathode and anode are enclosed in a vacuum tube to prevent the filament from burning up and to prevent arcing between the cathode and anode The electrical potential between the cathode and the anode pulls electrons from the cathode and accelerates them as they are attracted towards the anode or target which is usually made of tungsten X-rays are generated when free electrons give up some of their energy when they interact with the electrons or nucleus of an atom The interaction of the electrons in the target results in the emission of a continuous Bremsstrahlung spectrum and also characteristic X-rays from the target material
Production of Gamma Rays Gamma radiation is the product of radioactive atoms Depending upon the ratio of neutrons to protons within its nucleus an isotope of a particular element may be stable or unstable Over time the nuclei of unstable isotopes spontaneously disintegrate or transform in a process known as radioactive decay Various types of radiation may be emitted from the nucleus andor its surrounding electrons when an atom experiences radioactive decay Nuclides which undergo radioactive decay are called radionuclides Any material which contains measurable amounts of one or more radionuclides is a radioactive material
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Radiation Production for Industrial Radiography
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There are many naturally occurring radioactive materials but manmade radioactive isotopes or radioisotopes are used for industrial radiography Man-made sources are produced by introducing an extra neutron to atoms of the source material For example Cobalt-60 is produced by bombarding a sample of Cobalt-59 with an excess of neutrons in a nuclear reactor The Cobalt-59 atoms absorb some of the neutrons and increase their atomic weight by one to produce the radioisotope Cobalt-60 This process is known as activation As a material rids itself of atomic particles to return to a balance state energy is released in the form of Gamma rays and sometimes alpha or beta particles
Physical size of isotope materials will very slightly between manufacturer but generally an isotope is a pellet that measures 15 mm x 15 mm Depending on the activity (curies) desired a pellet or pellets are loaded into a stainless steel capsule and sealed Unlike X-ray tubes radioactive sources provide a continual source of radiation that cannot be turned off Once radioactive decay starts it continues until all of the atoms have reached a stable state The radioisotope can only be shielded to prevent exposure to the radiation In industrial radiography the instruments that are used to shield the radioisotope so that they can be safely handled and used are commonly called cameras or exposure devices Exposure devices will be discussed later in more detail
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Radiation Production for Industrial Radiography
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Radiation Decay and Half-life
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radioactive Decay and Half-Life
As mentioned previously radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation As a radioisotope atom decays to a more stable atom it emits radiation only once To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step However once the atom reaches a stable configuration no more radiation is given off For this reason radioactive sources become weaker with time As more and more unstable atoms become stable atoms less radiation is produced and eventually the material will become non-radioactive
The decay of radioactive elements occurs at a fixed rate The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material For example a source will have an intensity of 100 when new At one half-life its intensity will be cut to 50 of the original intensity At two half-lives it will have an intensity of 25 of a new source After ten half-lives less than one-thousandth of the original activity will remain Although the half-life pattern is the same for every radioisotope the length of a half-life is different For example Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days
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Radiation Activity and Intensity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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Exposure Limits
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Controlling Exposure
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Thermoluminescent Dosimeter
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Industrial radiography - Wikipedia the free encyclopedia
A diagram of the S shaped hole through a metal
block the source is stored at point A and is driven
out on a cable through a hole to point B It often goes
a long way along a guide tube to where it is needed
[edit] Contrast agents
Defects such as delaminations and planar cracks are difficult to detect using radiography which is why penetrants are often used to enhance
the contrast in the detection of such defects Penetrants used include silver nitrate zinc iodide chloroform and diiodomethane Choice of the
penetrant is determined by the ease with which it can penetrate the cracks and also with which it can be removed Diiodomethane has the
advantages of high opacity ease of penetration and ease of removal because it evaporates relatively quickly However it can cause skin burns
[edit] Neutrons
In some rare cases radiography is done with neutrons This type of radiography is called neutron radiography (NR Nray N-Ray) or neutron
imaging Neutron radiography can see very different things than X-rays because neutrons can pass with ease through lead and steel but are
stopped by plastics water and oils Neutron sources include radioactive (241AmBe and Cf) sources electrically driven D-T reactions in
vacuum tubes and conventional critical nuclear reactors It might be possible to use a neutron amplifier to increase the neutron flux[2]
Since the amount of radiation emerging from the opposite side of the material can be detected and measured variations in this amount (or
intensity) of radiation are used to determine thickness or composition of material Penetrating radiations are those restricted to that part of the
electromagnetic spectrum of wavelength less than about 10 nanometres
[edit] Safety
Industrial radiographers are in many locations required by governing authorities to use certain types of safety equipment and to work in pairs
Depending on location industrial radiographers may have been required to obtain permits licences andor undertake special training Prior to
conducting any testing the nearby area should always first be cleared of all other persons and measures taken to ensure that people do not
accidentally enter into an area that may expose them to a large dose of radiation
The safety equipment usually includes four basic items a radiation survey meter (such as a GeigerMueller counter) an alarming dosimeter or
rate meter a gas-charged dosimeter and a film badge or thermoluminescent dosimeter (TLD) The easiest way to remember what each of these
items does is to compare them to gauges on an automobile
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Industrial radiography - Wikipedia the free encyclopedia
The survey meter could be compared to the speedometer as it measures the speed or rate at which radiation is being picked up When
properly calibrated used and maintained it allows the radiographer to see the current exposure to radiation at the meter It can usually be set
for different intensities and is used to prevent the radiographer from being overexposed to the radioactive source as well as for verifying the
boundary that radiographers are required to maintain around the exposed source during radiographic operations
The alarming dosimeter could be most closely compared with the tachometer as it alarms when the radiographer redlines or is exposed to
too much radiation When properly calibrated activated and worn on the radiographers person it will emit an alarm when the meter measures
a radiation level in excess of a preset threshold This device is intended to prevent the radiographer from inadvertently walking up on an
exposed source
The gas-charged dosimeter is like a trip meter in that it measures the total radiation received but can be reset It is designed to help the
radiographer measure hisher total periodic dose of radiation When properly calibrated recharged and worn on the radiographers person it
can tell the radiographer at a glance how much radiation to which the device has been exposed since it was last recharged Radiographers in
many states are required to log their radiation exposures and generate an exposure report In many countries personal dosimeters are not
required to be used by radiographers as the dose rates they show are not always correctly recorded
The film badge or TLD is more like a cars odometer It is actually a specialized piece of radiographic film in a rugged container It is meant to
measure the radiographers total exposure over time (usually a month) and is used by regulating authorities to monitor the total exposure of
certified radiographers in a certain jurisdiction At the end of the month the film badge is turned in and is processed A report of the
radiographers total dose is generated and is kept on file
When these safety devices are properly calibrated maintained and used it is virtually impossible for a radiographer to be injured by a
radioactive overexposure Sadly the elimination of just one of these devices can jeopardize the safety of the radiographer and all those who are
nearby Without the survey meter the radiation received may be just below the threshold of the rate alarm and it may be several hours before
the radiographer checks the dosimeter and up to a month or more before the film badge is developed to detect a low intensity overexposure
Without the rate alarm one radiographer may inadvertently walk up on the source exposed by the other radiographer Without the dosimeter
the radiographer may be unaware of an overexposure or even a radiation burn which may take weeks to result in noticeable injury And
without the film badge the radiographer is deprived of an important tool designed to protect him or her from the effects of a long-term
overexposure to occupationally-obtained radiation and thus may suffer long-term health problems as a result
There are three ways a radiographer will ensure they are not exposed to higher than required levels of radiation time distance shielding The
less time that a person is exposed to radiation the lower their dose will be The further a person is from a radioactive source the lower the level
of radiation they receive this is largely due to the inverse square law Lastly the more a radioactive source is shielded by either better or
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Industrial radiography - Wikipedia the free encyclopedia
greater amounts of shielding the lower the levels of radiation that will escape from the testing area The most commanly used shielding
materials in use are sand lead (sheets or shot) steel spent (non-radioactive uranium) tungsten and in suitable situations water
Industrial radiography appears to have one of the worst safety profiles of the radiation professions possibly because there are many operators
using strong gamma sources (gt 2 Ci) in remote sites with little supervision when compared with workers within the nuclear industry or within
hospitals[3] Due to the levels of radiation present whilst they are working many radiographers are also required to work late at night when
there are few other people present as most industrial radiography is carried out in the open rather than in purpose built exposure booths or
rooms Fatigue carelessness and lack of proper training are the three most comman factors attributed to industrial radiography accidents Many
of the lost source accidents commented on by the International Atomic Energy Agency involve radiography equipment Lost source
accidents have the potential to cause a considerable loss of human life One scenario is that a passerby finds the radiography source and not
knowing what it is takes it home[4] The person shortly afterwards becomes ill and dies as a result of the radiation dose The source remains in
their home where it continues to irradiate other members of the household[5] Such an event occurred in March 1984 in Casablanca
(Mohammedia) which is part of Morocco This is related to the more famous Goiacircnia accident where a related chain of events caused
members of the public to be exposed to radiation sources Also see List of civilian radiation accidents
[edit] See also
Radiographic testing
Collimator
Industrial CT scanning
[edit] References
1 ^ httpwwwlboroacuklibrarynewSpotlighton-Archivehtml Loughborough University Library - Spotlight Archive [Accessed 22 October 2008]
[edit] External links
NISTs XAAMDI X-Ray Attenuation and Absorption for Materials of Dosimetric Interest Database
NISTs XCOM Photon Cross Sections Database
NISTs FAST Attenuation and Scattering Tables
A lost industrial radiography source event
UN information on the security of industrial sources
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Radiation Production for Industrial Radiography
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Production of Radiation for Industrial Radiography
Industrial radiography uses two sources of radiation X-radiation and Gamma radiation X-rays and Gamma rays differ only in their source of origin X-rays are produced by an X-ray generator and Gamma radiation is the product of radioactive atoms An in depth discussion on radiation production can be found in other areas of this site but will be reviewed briefly in the following sections
Production of X-Rays There are two different atomic processes that can produce X-ray photons One process produces Bremsstrahlung radiation and the other produces K-shell or characteristic emission Both processes involve a change in the energy state of electrons X-rays are generated when an electron is accelerated and then made to rapidly decelerate usually due to interaction with other atomic particles
In an X-ray system a large amount of electric current is passed through a tungsten filament which heats the filament to several thousand degrees centigrade to create a source of free electrons A large electrical potential is established between the filament (the cathode) and a target (the anode) The cathode and anode are enclosed in a vacuum tube to prevent the filament from burning up and to prevent arcing between the cathode and anode The electrical potential between the cathode and the anode pulls electrons from the cathode and accelerates them as they are attracted towards the anode or target which is usually made of tungsten X-rays are generated when free electrons give up some of their energy when they interact with the electrons or nucleus of an atom The interaction of the electrons in the target results in the emission of a continuous Bremsstrahlung spectrum and also characteristic X-rays from the target material
Production of Gamma Rays Gamma radiation is the product of radioactive atoms Depending upon the ratio of neutrons to protons within its nucleus an isotope of a particular element may be stable or unstable Over time the nuclei of unstable isotopes spontaneously disintegrate or transform in a process known as radioactive decay Various types of radiation may be emitted from the nucleus andor its surrounding electrons when an atom experiences radioactive decay Nuclides which undergo radioactive decay are called radionuclides Any material which contains measurable amounts of one or more radionuclides is a radioactive material
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Radiation Production for Industrial Radiography
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There are many naturally occurring radioactive materials but manmade radioactive isotopes or radioisotopes are used for industrial radiography Man-made sources are produced by introducing an extra neutron to atoms of the source material For example Cobalt-60 is produced by bombarding a sample of Cobalt-59 with an excess of neutrons in a nuclear reactor The Cobalt-59 atoms absorb some of the neutrons and increase their atomic weight by one to produce the radioisotope Cobalt-60 This process is known as activation As a material rids itself of atomic particles to return to a balance state energy is released in the form of Gamma rays and sometimes alpha or beta particles
Physical size of isotope materials will very slightly between manufacturer but generally an isotope is a pellet that measures 15 mm x 15 mm Depending on the activity (curies) desired a pellet or pellets are loaded into a stainless steel capsule and sealed Unlike X-ray tubes radioactive sources provide a continual source of radiation that cannot be turned off Once radioactive decay starts it continues until all of the atoms have reached a stable state The radioisotope can only be shielded to prevent exposure to the radiation In industrial radiography the instruments that are used to shield the radioisotope so that they can be safely handled and used are commonly called cameras or exposure devices Exposure devices will be discussed later in more detail
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Radiation Production for Industrial Radiography
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Radiation Decay and Half-life
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radioactive Decay and Half-Life
As mentioned previously radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation As a radioisotope atom decays to a more stable atom it emits radiation only once To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step However once the atom reaches a stable configuration no more radiation is given off For this reason radioactive sources become weaker with time As more and more unstable atoms become stable atoms less radiation is produced and eventually the material will become non-radioactive
The decay of radioactive elements occurs at a fixed rate The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material For example a source will have an intensity of 100 when new At one half-life its intensity will be cut to 50 of the original intensity At two half-lives it will have an intensity of 25 of a new source After ten half-lives less than one-thousandth of the original activity will remain Although the half-life pattern is the same for every radioisotope the length of a half-life is different For example Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days
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Radiation Activity and Intensity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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Exposure Limits
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Controlling Exposure
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Thermoluminescent Dosimeter
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Industrial radiography - Wikipedia the free encyclopedia
The survey meter could be compared to the speedometer as it measures the speed or rate at which radiation is being picked up When
properly calibrated used and maintained it allows the radiographer to see the current exposure to radiation at the meter It can usually be set
for different intensities and is used to prevent the radiographer from being overexposed to the radioactive source as well as for verifying the
boundary that radiographers are required to maintain around the exposed source during radiographic operations
The alarming dosimeter could be most closely compared with the tachometer as it alarms when the radiographer redlines or is exposed to
too much radiation When properly calibrated activated and worn on the radiographers person it will emit an alarm when the meter measures
a radiation level in excess of a preset threshold This device is intended to prevent the radiographer from inadvertently walking up on an
exposed source
The gas-charged dosimeter is like a trip meter in that it measures the total radiation received but can be reset It is designed to help the
radiographer measure hisher total periodic dose of radiation When properly calibrated recharged and worn on the radiographers person it
can tell the radiographer at a glance how much radiation to which the device has been exposed since it was last recharged Radiographers in
many states are required to log their radiation exposures and generate an exposure report In many countries personal dosimeters are not
required to be used by radiographers as the dose rates they show are not always correctly recorded
The film badge or TLD is more like a cars odometer It is actually a specialized piece of radiographic film in a rugged container It is meant to
measure the radiographers total exposure over time (usually a month) and is used by regulating authorities to monitor the total exposure of
certified radiographers in a certain jurisdiction At the end of the month the film badge is turned in and is processed A report of the
radiographers total dose is generated and is kept on file
When these safety devices are properly calibrated maintained and used it is virtually impossible for a radiographer to be injured by a
radioactive overexposure Sadly the elimination of just one of these devices can jeopardize the safety of the radiographer and all those who are
nearby Without the survey meter the radiation received may be just below the threshold of the rate alarm and it may be several hours before
the radiographer checks the dosimeter and up to a month or more before the film badge is developed to detect a low intensity overexposure
Without the rate alarm one radiographer may inadvertently walk up on the source exposed by the other radiographer Without the dosimeter
the radiographer may be unaware of an overexposure or even a radiation burn which may take weeks to result in noticeable injury And
without the film badge the radiographer is deprived of an important tool designed to protect him or her from the effects of a long-term
overexposure to occupationally-obtained radiation and thus may suffer long-term health problems as a result
There are three ways a radiographer will ensure they are not exposed to higher than required levels of radiation time distance shielding The
less time that a person is exposed to radiation the lower their dose will be The further a person is from a radioactive source the lower the level
of radiation they receive this is largely due to the inverse square law Lastly the more a radioactive source is shielded by either better or
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Industrial radiography - Wikipedia the free encyclopedia
greater amounts of shielding the lower the levels of radiation that will escape from the testing area The most commanly used shielding
materials in use are sand lead (sheets or shot) steel spent (non-radioactive uranium) tungsten and in suitable situations water
Industrial radiography appears to have one of the worst safety profiles of the radiation professions possibly because there are many operators
using strong gamma sources (gt 2 Ci) in remote sites with little supervision when compared with workers within the nuclear industry or within
hospitals[3] Due to the levels of radiation present whilst they are working many radiographers are also required to work late at night when
there are few other people present as most industrial radiography is carried out in the open rather than in purpose built exposure booths or
rooms Fatigue carelessness and lack of proper training are the three most comman factors attributed to industrial radiography accidents Many
of the lost source accidents commented on by the International Atomic Energy Agency involve radiography equipment Lost source
accidents have the potential to cause a considerable loss of human life One scenario is that a passerby finds the radiography source and not
knowing what it is takes it home[4] The person shortly afterwards becomes ill and dies as a result of the radiation dose The source remains in
their home where it continues to irradiate other members of the household[5] Such an event occurred in March 1984 in Casablanca
(Mohammedia) which is part of Morocco This is related to the more famous Goiacircnia accident where a related chain of events caused
members of the public to be exposed to radiation sources Also see List of civilian radiation accidents
[edit] See also
Radiographic testing
Collimator
Industrial CT scanning
[edit] References
1 ^ httpwwwlboroacuklibrarynewSpotlighton-Archivehtml Loughborough University Library - Spotlight Archive [Accessed 22 October 2008]
[edit] External links
NISTs XAAMDI X-Ray Attenuation and Absorption for Materials of Dosimetric Interest Database
NISTs XCOM Photon Cross Sections Database
NISTs FAST Attenuation and Scattering Tables
A lost industrial radiography source event
UN information on the security of industrial sources
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Radiation Production for Industrial Radiography
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Production of Radiation for Industrial Radiography
Industrial radiography uses two sources of radiation X-radiation and Gamma radiation X-rays and Gamma rays differ only in their source of origin X-rays are produced by an X-ray generator and Gamma radiation is the product of radioactive atoms An in depth discussion on radiation production can be found in other areas of this site but will be reviewed briefly in the following sections
Production of X-Rays There are two different atomic processes that can produce X-ray photons One process produces Bremsstrahlung radiation and the other produces K-shell or characteristic emission Both processes involve a change in the energy state of electrons X-rays are generated when an electron is accelerated and then made to rapidly decelerate usually due to interaction with other atomic particles
In an X-ray system a large amount of electric current is passed through a tungsten filament which heats the filament to several thousand degrees centigrade to create a source of free electrons A large electrical potential is established between the filament (the cathode) and a target (the anode) The cathode and anode are enclosed in a vacuum tube to prevent the filament from burning up and to prevent arcing between the cathode and anode The electrical potential between the cathode and the anode pulls electrons from the cathode and accelerates them as they are attracted towards the anode or target which is usually made of tungsten X-rays are generated when free electrons give up some of their energy when they interact with the electrons or nucleus of an atom The interaction of the electrons in the target results in the emission of a continuous Bremsstrahlung spectrum and also characteristic X-rays from the target material
Production of Gamma Rays Gamma radiation is the product of radioactive atoms Depending upon the ratio of neutrons to protons within its nucleus an isotope of a particular element may be stable or unstable Over time the nuclei of unstable isotopes spontaneously disintegrate or transform in a process known as radioactive decay Various types of radiation may be emitted from the nucleus andor its surrounding electrons when an atom experiences radioactive decay Nuclides which undergo radioactive decay are called radionuclides Any material which contains measurable amounts of one or more radionuclides is a radioactive material
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Radiation Production for Industrial Radiography
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There are many naturally occurring radioactive materials but manmade radioactive isotopes or radioisotopes are used for industrial radiography Man-made sources are produced by introducing an extra neutron to atoms of the source material For example Cobalt-60 is produced by bombarding a sample of Cobalt-59 with an excess of neutrons in a nuclear reactor The Cobalt-59 atoms absorb some of the neutrons and increase their atomic weight by one to produce the radioisotope Cobalt-60 This process is known as activation As a material rids itself of atomic particles to return to a balance state energy is released in the form of Gamma rays and sometimes alpha or beta particles
Physical size of isotope materials will very slightly between manufacturer but generally an isotope is a pellet that measures 15 mm x 15 mm Depending on the activity (curies) desired a pellet or pellets are loaded into a stainless steel capsule and sealed Unlike X-ray tubes radioactive sources provide a continual source of radiation that cannot be turned off Once radioactive decay starts it continues until all of the atoms have reached a stable state The radioisotope can only be shielded to prevent exposure to the radiation In industrial radiography the instruments that are used to shield the radioisotope so that they can be safely handled and used are commonly called cameras or exposure devices Exposure devices will be discussed later in more detail
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Radiation Production for Industrial Radiography
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Radiation Decay and Half-life
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radioactive Decay and Half-Life
As mentioned previously radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation As a radioisotope atom decays to a more stable atom it emits radiation only once To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step However once the atom reaches a stable configuration no more radiation is given off For this reason radioactive sources become weaker with time As more and more unstable atoms become stable atoms less radiation is produced and eventually the material will become non-radioactive
The decay of radioactive elements occurs at a fixed rate The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material For example a source will have an intensity of 100 when new At one half-life its intensity will be cut to 50 of the original intensity At two half-lives it will have an intensity of 25 of a new source After ten half-lives less than one-thousandth of the original activity will remain Although the half-life pattern is the same for every radioisotope the length of a half-life is different For example Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days
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Radiation Activity and Intensity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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Exposure Limits
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Controlling Exposure
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Industrial radiography - Wikipedia the free encyclopedia
greater amounts of shielding the lower the levels of radiation that will escape from the testing area The most commanly used shielding
materials in use are sand lead (sheets or shot) steel spent (non-radioactive uranium) tungsten and in suitable situations water
Industrial radiography appears to have one of the worst safety profiles of the radiation professions possibly because there are many operators
using strong gamma sources (gt 2 Ci) in remote sites with little supervision when compared with workers within the nuclear industry or within
hospitals[3] Due to the levels of radiation present whilst they are working many radiographers are also required to work late at night when
there are few other people present as most industrial radiography is carried out in the open rather than in purpose built exposure booths or
rooms Fatigue carelessness and lack of proper training are the three most comman factors attributed to industrial radiography accidents Many
of the lost source accidents commented on by the International Atomic Energy Agency involve radiography equipment Lost source
accidents have the potential to cause a considerable loss of human life One scenario is that a passerby finds the radiography source and not
knowing what it is takes it home[4] The person shortly afterwards becomes ill and dies as a result of the radiation dose The source remains in
their home where it continues to irradiate other members of the household[5] Such an event occurred in March 1984 in Casablanca
(Mohammedia) which is part of Morocco This is related to the more famous Goiacircnia accident where a related chain of events caused
members of the public to be exposed to radiation sources Also see List of civilian radiation accidents
[edit] See also
Radiographic testing
Collimator
Industrial CT scanning
[edit] References
1 ^ httpwwwlboroacuklibrarynewSpotlighton-Archivehtml Loughborough University Library - Spotlight Archive [Accessed 22 October 2008]
[edit] External links
NISTs XAAMDI X-Ray Attenuation and Absorption for Materials of Dosimetric Interest Database
NISTs XCOM Photon Cross Sections Database
NISTs FAST Attenuation and Scattering Tables
A lost industrial radiography source event
UN information on the security of industrial sources
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Industrial radiography - Wikipedia the free encyclopedia
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Categories Nondestructive testing | Radiography | Loughborough University | Casting (manufacturing)
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httpenwikipediaorgwikiIndustrial_radiography (11 of 11)21-09-2011 103049
Radiation Production for Industrial Radiography
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Production of Radiation for Industrial Radiography
Industrial radiography uses two sources of radiation X-radiation and Gamma radiation X-rays and Gamma rays differ only in their source of origin X-rays are produced by an X-ray generator and Gamma radiation is the product of radioactive atoms An in depth discussion on radiation production can be found in other areas of this site but will be reviewed briefly in the following sections
Production of X-Rays There are two different atomic processes that can produce X-ray photons One process produces Bremsstrahlung radiation and the other produces K-shell or characteristic emission Both processes involve a change in the energy state of electrons X-rays are generated when an electron is accelerated and then made to rapidly decelerate usually due to interaction with other atomic particles
In an X-ray system a large amount of electric current is passed through a tungsten filament which heats the filament to several thousand degrees centigrade to create a source of free electrons A large electrical potential is established between the filament (the cathode) and a target (the anode) The cathode and anode are enclosed in a vacuum tube to prevent the filament from burning up and to prevent arcing between the cathode and anode The electrical potential between the cathode and the anode pulls electrons from the cathode and accelerates them as they are attracted towards the anode or target which is usually made of tungsten X-rays are generated when free electrons give up some of their energy when they interact with the electrons or nucleus of an atom The interaction of the electrons in the target results in the emission of a continuous Bremsstrahlung spectrum and also characteristic X-rays from the target material
Production of Gamma Rays Gamma radiation is the product of radioactive atoms Depending upon the ratio of neutrons to protons within its nucleus an isotope of a particular element may be stable or unstable Over time the nuclei of unstable isotopes spontaneously disintegrate or transform in a process known as radioactive decay Various types of radiation may be emitted from the nucleus andor its surrounding electrons when an atom experiences radioactive decay Nuclides which undergo radioactive decay are called radionuclides Any material which contains measurable amounts of one or more radionuclides is a radioactive material
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Radiation Production for Industrial Radiography
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There are many naturally occurring radioactive materials but manmade radioactive isotopes or radioisotopes are used for industrial radiography Man-made sources are produced by introducing an extra neutron to atoms of the source material For example Cobalt-60 is produced by bombarding a sample of Cobalt-59 with an excess of neutrons in a nuclear reactor The Cobalt-59 atoms absorb some of the neutrons and increase their atomic weight by one to produce the radioisotope Cobalt-60 This process is known as activation As a material rids itself of atomic particles to return to a balance state energy is released in the form of Gamma rays and sometimes alpha or beta particles
Physical size of isotope materials will very slightly between manufacturer but generally an isotope is a pellet that measures 15 mm x 15 mm Depending on the activity (curies) desired a pellet or pellets are loaded into a stainless steel capsule and sealed Unlike X-ray tubes radioactive sources provide a continual source of radiation that cannot be turned off Once radioactive decay starts it continues until all of the atoms have reached a stable state The radioisotope can only be shielded to prevent exposure to the radiation In industrial radiography the instruments that are used to shield the radioisotope so that they can be safely handled and used are commonly called cameras or exposure devices Exposure devices will be discussed later in more detail
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Radiation Production for Industrial Radiography
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Radiation Decay and Half-life
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radioactive Decay and Half-Life
As mentioned previously radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation As a radioisotope atom decays to a more stable atom it emits radiation only once To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step However once the atom reaches a stable configuration no more radiation is given off For this reason radioactive sources become weaker with time As more and more unstable atoms become stable atoms less radiation is produced and eventually the material will become non-radioactive
The decay of radioactive elements occurs at a fixed rate The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material For example a source will have an intensity of 100 when new At one half-life its intensity will be cut to 50 of the original intensity At two half-lives it will have an intensity of 25 of a new source After ten half-lives less than one-thousandth of the original activity will remain Although the half-life pattern is the same for every radioisotope the length of a half-life is different For example Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days
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Radiation Decay and Half-life
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Radiation Activity and Intensity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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Rad Units
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
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Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
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Nonstochastic Effects
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Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
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Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Radiation Production for Industrial Radiography
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Production of Radiation for Industrial Radiography
Industrial radiography uses two sources of radiation X-radiation and Gamma radiation X-rays and Gamma rays differ only in their source of origin X-rays are produced by an X-ray generator and Gamma radiation is the product of radioactive atoms An in depth discussion on radiation production can be found in other areas of this site but will be reviewed briefly in the following sections
Production of X-Rays There are two different atomic processes that can produce X-ray photons One process produces Bremsstrahlung radiation and the other produces K-shell or characteristic emission Both processes involve a change in the energy state of electrons X-rays are generated when an electron is accelerated and then made to rapidly decelerate usually due to interaction with other atomic particles
In an X-ray system a large amount of electric current is passed through a tungsten filament which heats the filament to several thousand degrees centigrade to create a source of free electrons A large electrical potential is established between the filament (the cathode) and a target (the anode) The cathode and anode are enclosed in a vacuum tube to prevent the filament from burning up and to prevent arcing between the cathode and anode The electrical potential between the cathode and the anode pulls electrons from the cathode and accelerates them as they are attracted towards the anode or target which is usually made of tungsten X-rays are generated when free electrons give up some of their energy when they interact with the electrons or nucleus of an atom The interaction of the electrons in the target results in the emission of a continuous Bremsstrahlung spectrum and also characteristic X-rays from the target material
Production of Gamma Rays Gamma radiation is the product of radioactive atoms Depending upon the ratio of neutrons to protons within its nucleus an isotope of a particular element may be stable or unstable Over time the nuclei of unstable isotopes spontaneously disintegrate or transform in a process known as radioactive decay Various types of radiation may be emitted from the nucleus andor its surrounding electrons when an atom experiences radioactive decay Nuclides which undergo radioactive decay are called radionuclides Any material which contains measurable amounts of one or more radionuclides is a radioactive material
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There are many naturally occurring radioactive materials but manmade radioactive isotopes or radioisotopes are used for industrial radiography Man-made sources are produced by introducing an extra neutron to atoms of the source material For example Cobalt-60 is produced by bombarding a sample of Cobalt-59 with an excess of neutrons in a nuclear reactor The Cobalt-59 atoms absorb some of the neutrons and increase their atomic weight by one to produce the radioisotope Cobalt-60 This process is known as activation As a material rids itself of atomic particles to return to a balance state energy is released in the form of Gamma rays and sometimes alpha or beta particles
Physical size of isotope materials will very slightly between manufacturer but generally an isotope is a pellet that measures 15 mm x 15 mm Depending on the activity (curies) desired a pellet or pellets are loaded into a stainless steel capsule and sealed Unlike X-ray tubes radioactive sources provide a continual source of radiation that cannot be turned off Once radioactive decay starts it continues until all of the atoms have reached a stable state The radioisotope can only be shielded to prevent exposure to the radiation In industrial radiography the instruments that are used to shield the radioisotope so that they can be safely handled and used are commonly called cameras or exposure devices Exposure devices will be discussed later in more detail
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Radiation Production for Industrial Radiography
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Radiation Decay and Half-life
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Radioactive Decay and Half-Life
As mentioned previously radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation As a radioisotope atom decays to a more stable atom it emits radiation only once To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step However once the atom reaches a stable configuration no more radiation is given off For this reason radioactive sources become weaker with time As more and more unstable atoms become stable atoms less radiation is produced and eventually the material will become non-radioactive
The decay of radioactive elements occurs at a fixed rate The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material For example a source will have an intensity of 100 when new At one half-life its intensity will be cut to 50 of the original intensity At two half-lives it will have an intensity of 25 of a new source After ten half-lives less than one-thousandth of the original activity will remain Although the half-life pattern is the same for every radioisotope the length of a half-life is different For example Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days
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Radiation Activity and Intensity
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Radiosensitivity
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Rad Units
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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Rad Units
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Stochastic Effects
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Nonstochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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Nonstochastic Effects
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
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Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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Pocket Dosimeter
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Audible Alarm Rate Meters
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film Badges
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Industrial radiography - Wikipedia the free encyclopedia
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Radiation Production for Industrial Radiography
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Production of Radiation for Industrial Radiography
Industrial radiography uses two sources of radiation X-radiation and Gamma radiation X-rays and Gamma rays differ only in their source of origin X-rays are produced by an X-ray generator and Gamma radiation is the product of radioactive atoms An in depth discussion on radiation production can be found in other areas of this site but will be reviewed briefly in the following sections
Production of X-Rays There are two different atomic processes that can produce X-ray photons One process produces Bremsstrahlung radiation and the other produces K-shell or characteristic emission Both processes involve a change in the energy state of electrons X-rays are generated when an electron is accelerated and then made to rapidly decelerate usually due to interaction with other atomic particles
In an X-ray system a large amount of electric current is passed through a tungsten filament which heats the filament to several thousand degrees centigrade to create a source of free electrons A large electrical potential is established between the filament (the cathode) and a target (the anode) The cathode and anode are enclosed in a vacuum tube to prevent the filament from burning up and to prevent arcing between the cathode and anode The electrical potential between the cathode and the anode pulls electrons from the cathode and accelerates them as they are attracted towards the anode or target which is usually made of tungsten X-rays are generated when free electrons give up some of their energy when they interact with the electrons or nucleus of an atom The interaction of the electrons in the target results in the emission of a continuous Bremsstrahlung spectrum and also characteristic X-rays from the target material
Production of Gamma Rays Gamma radiation is the product of radioactive atoms Depending upon the ratio of neutrons to protons within its nucleus an isotope of a particular element may be stable or unstable Over time the nuclei of unstable isotopes spontaneously disintegrate or transform in a process known as radioactive decay Various types of radiation may be emitted from the nucleus andor its surrounding electrons when an atom experiences radioactive decay Nuclides which undergo radioactive decay are called radionuclides Any material which contains measurable amounts of one or more radionuclides is a radioactive material
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Radiation Production for Industrial Radiography
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There are many naturally occurring radioactive materials but manmade radioactive isotopes or radioisotopes are used for industrial radiography Man-made sources are produced by introducing an extra neutron to atoms of the source material For example Cobalt-60 is produced by bombarding a sample of Cobalt-59 with an excess of neutrons in a nuclear reactor The Cobalt-59 atoms absorb some of the neutrons and increase their atomic weight by one to produce the radioisotope Cobalt-60 This process is known as activation As a material rids itself of atomic particles to return to a balance state energy is released in the form of Gamma rays and sometimes alpha or beta particles
Physical size of isotope materials will very slightly between manufacturer but generally an isotope is a pellet that measures 15 mm x 15 mm Depending on the activity (curies) desired a pellet or pellets are loaded into a stainless steel capsule and sealed Unlike X-ray tubes radioactive sources provide a continual source of radiation that cannot be turned off Once radioactive decay starts it continues until all of the atoms have reached a stable state The radioisotope can only be shielded to prevent exposure to the radiation In industrial radiography the instruments that are used to shield the radioisotope so that they can be safely handled and used are commonly called cameras or exposure devices Exposure devices will be discussed later in more detail
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Radiation Production for Industrial Radiography
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Radiation Decay and Half-life
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radioactive Decay and Half-Life
As mentioned previously radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation As a radioisotope atom decays to a more stable atom it emits radiation only once To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step However once the atom reaches a stable configuration no more radiation is given off For this reason radioactive sources become weaker with time As more and more unstable atoms become stable atoms less radiation is produced and eventually the material will become non-radioactive
The decay of radioactive elements occurs at a fixed rate The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material For example a source will have an intensity of 100 when new At one half-life its intensity will be cut to 50 of the original intensity At two half-lives it will have an intensity of 25 of a new source After ten half-lives less than one-thousandth of the original activity will remain Although the half-life pattern is the same for every radioisotope the length of a half-life is different For example Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days
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Radiation Activity and Intensity
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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Exposure Limits
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Controlling Exposure
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film Badges
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Radiation Production for Industrial Radiography
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Production of Radiation for Industrial Radiography
Industrial radiography uses two sources of radiation X-radiation and Gamma radiation X-rays and Gamma rays differ only in their source of origin X-rays are produced by an X-ray generator and Gamma radiation is the product of radioactive atoms An in depth discussion on radiation production can be found in other areas of this site but will be reviewed briefly in the following sections
Production of X-Rays There are two different atomic processes that can produce X-ray photons One process produces Bremsstrahlung radiation and the other produces K-shell or characteristic emission Both processes involve a change in the energy state of electrons X-rays are generated when an electron is accelerated and then made to rapidly decelerate usually due to interaction with other atomic particles
In an X-ray system a large amount of electric current is passed through a tungsten filament which heats the filament to several thousand degrees centigrade to create a source of free electrons A large electrical potential is established between the filament (the cathode) and a target (the anode) The cathode and anode are enclosed in a vacuum tube to prevent the filament from burning up and to prevent arcing between the cathode and anode The electrical potential between the cathode and the anode pulls electrons from the cathode and accelerates them as they are attracted towards the anode or target which is usually made of tungsten X-rays are generated when free electrons give up some of their energy when they interact with the electrons or nucleus of an atom The interaction of the electrons in the target results in the emission of a continuous Bremsstrahlung spectrum and also characteristic X-rays from the target material
Production of Gamma Rays Gamma radiation is the product of radioactive atoms Depending upon the ratio of neutrons to protons within its nucleus an isotope of a particular element may be stable or unstable Over time the nuclei of unstable isotopes spontaneously disintegrate or transform in a process known as radioactive decay Various types of radiation may be emitted from the nucleus andor its surrounding electrons when an atom experiences radioactive decay Nuclides which undergo radioactive decay are called radionuclides Any material which contains measurable amounts of one or more radionuclides is a radioactive material
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Radiation Production for Industrial Radiography
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There are many naturally occurring radioactive materials but manmade radioactive isotopes or radioisotopes are used for industrial radiography Man-made sources are produced by introducing an extra neutron to atoms of the source material For example Cobalt-60 is produced by bombarding a sample of Cobalt-59 with an excess of neutrons in a nuclear reactor The Cobalt-59 atoms absorb some of the neutrons and increase their atomic weight by one to produce the radioisotope Cobalt-60 This process is known as activation As a material rids itself of atomic particles to return to a balance state energy is released in the form of Gamma rays and sometimes alpha or beta particles
Physical size of isotope materials will very slightly between manufacturer but generally an isotope is a pellet that measures 15 mm x 15 mm Depending on the activity (curies) desired a pellet or pellets are loaded into a stainless steel capsule and sealed Unlike X-ray tubes radioactive sources provide a continual source of radiation that cannot be turned off Once radioactive decay starts it continues until all of the atoms have reached a stable state The radioisotope can only be shielded to prevent exposure to the radiation In industrial radiography the instruments that are used to shield the radioisotope so that they can be safely handled and used are commonly called cameras or exposure devices Exposure devices will be discussed later in more detail
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Radiation Production for Industrial Radiography
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Radiation Decay and Half-life
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radioactive Decay and Half-Life
As mentioned previously radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation As a radioisotope atom decays to a more stable atom it emits radiation only once To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step However once the atom reaches a stable configuration no more radiation is given off For this reason radioactive sources become weaker with time As more and more unstable atoms become stable atoms less radiation is produced and eventually the material will become non-radioactive
The decay of radioactive elements occurs at a fixed rate The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material For example a source will have an intensity of 100 when new At one half-life its intensity will be cut to 50 of the original intensity At two half-lives it will have an intensity of 25 of a new source After ten half-lives less than one-thousandth of the original activity will remain Although the half-life pattern is the same for every radioisotope the length of a half-life is different For example Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days
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Radiation Activity and Intensity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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Rad Units
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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Exposure Limits
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Controlling Exposure
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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There are many naturally occurring radioactive materials but manmade radioactive isotopes or radioisotopes are used for industrial radiography Man-made sources are produced by introducing an extra neutron to atoms of the source material For example Cobalt-60 is produced by bombarding a sample of Cobalt-59 with an excess of neutrons in a nuclear reactor The Cobalt-59 atoms absorb some of the neutrons and increase their atomic weight by one to produce the radioisotope Cobalt-60 This process is known as activation As a material rids itself of atomic particles to return to a balance state energy is released in the form of Gamma rays and sometimes alpha or beta particles
Physical size of isotope materials will very slightly between manufacturer but generally an isotope is a pellet that measures 15 mm x 15 mm Depending on the activity (curies) desired a pellet or pellets are loaded into a stainless steel capsule and sealed Unlike X-ray tubes radioactive sources provide a continual source of radiation that cannot be turned off Once radioactive decay starts it continues until all of the atoms have reached a stable state The radioisotope can only be shielded to prevent exposure to the radiation In industrial radiography the instruments that are used to shield the radioisotope so that they can be safely handled and used are commonly called cameras or exposure devices Exposure devices will be discussed later in more detail
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Radiation Decay and Half-life
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Radioactive Decay and Half-Life
As mentioned previously radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation As a radioisotope atom decays to a more stable atom it emits radiation only once To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step However once the atom reaches a stable configuration no more radiation is given off For this reason radioactive sources become weaker with time As more and more unstable atoms become stable atoms less radiation is produced and eventually the material will become non-radioactive
The decay of radioactive elements occurs at a fixed rate The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material For example a source will have an intensity of 100 when new At one half-life its intensity will be cut to 50 of the original intensity At two half-lives it will have an intensity of 25 of a new source After ten half-lives less than one-thousandth of the original activity will remain Although the half-life pattern is the same for every radioisotope the length of a half-life is different For example Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days
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Radiation Activity and Intensity
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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Rad Units
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Stochastic Effects
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Nonstochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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Nonstochastic Effects
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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Pocket Dosimeter
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film Badges
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Radiation Production for Industrial Radiography
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Radiation Decay and Half-life
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radioactive Decay and Half-Life
As mentioned previously radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation As a radioisotope atom decays to a more stable atom it emits radiation only once To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step However once the atom reaches a stable configuration no more radiation is given off For this reason radioactive sources become weaker with time As more and more unstable atoms become stable atoms less radiation is produced and eventually the material will become non-radioactive
The decay of radioactive elements occurs at a fixed rate The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material For example a source will have an intensity of 100 when new At one half-life its intensity will be cut to 50 of the original intensity At two half-lives it will have an intensity of 25 of a new source After ten half-lives less than one-thousandth of the original activity will remain Although the half-life pattern is the same for every radioisotope the length of a half-life is different For example Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days
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Radiation Decay and Half-life
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Radiation Activity and Intensity
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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Exposure Limits
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Controlling Exposure
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Thermoluminescent Dosimeter
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Radiation Decay and Half-life
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radioactive Decay and Half-Life
As mentioned previously radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation As a radioisotope atom decays to a more stable atom it emits radiation only once To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step However once the atom reaches a stable configuration no more radiation is given off For this reason radioactive sources become weaker with time As more and more unstable atoms become stable atoms less radiation is produced and eventually the material will become non-radioactive
The decay of radioactive elements occurs at a fixed rate The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material For example a source will have an intensity of 100 when new At one half-life its intensity will be cut to 50 of the original intensity At two half-lives it will have an intensity of 25 of a new source After ten half-lives less than one-thousandth of the original activity will remain Although the half-life pattern is the same for every radioisotope the length of a half-life is different For example Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days
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Radiation Decay and Half-life
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Radiation Activity and Intensity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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Rad Units
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Biological Effects
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Stochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Stochastic Effects
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Nonstochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Radiation Decay and Half-life
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Radiation Activity and Intensity
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Controlling Exposure
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film Badges
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Thermoluminescent Dosimeter
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Radiation Activity and Intensity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Energy Activity Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates It is important to understand the terms used to describe the energy and intensity of the radiation The four terms used most for this purpose are energy activity intensity and exposure
Radiation Energy As mentioned previously the energy of the radiation is responsible for its ability to penetrate matter Higher energy radiation can penetrate more and higher density matter than low energy radiation The energy of ionizing radiation is measured in electronvolts (eV) One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV) An electronvolt is a measure of energy which is different from a volt which is a measure of the electrical potential between two positions Specifically an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt X-ray generators have a control to adjust the keV or the kV
The energy of a radioisotope is a characteristic of the atomic structure of the material Consider for example Iridium-192 and Cobalt-60 which are two of the more common industrial Gamma ray sources These isotopes emit radiation in two or three discreet wavelengths Cobalt-60 will emit 133 and 117 MeV Gamma rays and Iridium-192 will emit 031 047 and 060 MeV Gamma rays It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192 From a radiation safety point of view this difference in energy is important because the Co-60 has more material penetrating power and therefore is more dangerous and requires more shielding
Activity The strength of a radioactive source is called its activity which is defined as the rate at which the isotope decays Specifically it is the number of atoms that decay and emit radiation in one second Radioactivity may be thought of as the volume of radiation produced in a given amount of time It is similar to the current control on a X-ray generator The International System (SI) unit for activity is the becquerel (Bq) which is that quantity of radioactive material in which one atom transforms per second The becquerel is a small unit In practical situations radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq) The curie (Ci) is also commonly used as the unit for activity of a particular source material The curie is a quantity of radioactive
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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Rad Units
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Biological Effects
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Stochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Stochastic Effects
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Nonstochastic Effects
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Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Radiation Detectors
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Survey Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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Pocket Dosimeter
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Radiation Activity and Intensity
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material in which 37 x 1010 atoms disintegrate per second This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226 One curie equals approximately 37037 MBq New sources of cobalt will have an activity of 20 to over 100 curies and new sources of iridium will have an activity of similar amounts
Once a radioactive nucleus decays it is no longer possible for it to emit the same radiation again Therefore the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present Additionally two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source The concentration of radioactivity or the relationship between the mass of radioactive material and the activity is called the specific activity Specific activity is expressed as the number of curies or becquerels per unit mass or volume The higher the specific activity of a material the smaller the physical size of the source is likely to be
Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time The intensity of an X-ray or gamma-ray source can easily be measured with the right detector Since it is difficult to measure the strength of a radioactive source based on its activity which is the number of atoms that decay and emit radiation in one second the strength of a source is often referred to in terms of its intensity Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period which is directly related to the number of disintegrations in the same time period (the activity)
Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air The amount of ionization in air produced by the radiation is called the exposure Exposure is expressed in terms of a scientific unit called a roentgen (R or r) The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0degC and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgenshour which is known as the dose rate
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Interaction of Electromagnetic Radiation and Matter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Interaction of Electromagnetic Radiation and Matter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms But subatomically matter is made up of mostly empty space For example consider the hydrogen atom with its one proton one neutron and one electron The diameter of a single proton has been measured to be about 10-15 meters The diameter of a single hydrogen atom has been determined to be 10-10 meters therefore the ratio of the size of a hydrogen atom to the size of the proton is 1000001 Consider this in terms of something more easily pictured in your mind If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm) its electron would be approximately 10 kilometers away Therefore when electromagnetic waves pass through a material they are primarily moving through free space but may have a chance encounter with the nucleus or an electron of an atom
Because the encounters of photons with atom particles are by chance a given photon has a finite probability of passing completely through the medium it is traversing The probability that a photon will pass completely through a medium depends on numerous factors including the photonrsquos energy and the mediumrsquos composition and thickness The more densely packed a mediumrsquos atoms the more likely the photon will encounter an atomic particle In other words the more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur Similarly the more material a photon must cross through the more likely the chance of an encounter
When a photon does encounter an atomic particle it transfers energy to the particle The energy may be reemitted back the way it came (reflected) scattered in a different direction or transmitted forward into the material Let us first consider the interaction of visible light Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate If the material is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object If the material is opaque then the vibrations of the electrons are not passed from atom to atom through the bulk of the material but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave The light may be reemitted from the surface of the material at a different wavelength thus changing its color
X-Rays and Gamma Rays X-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei However X-rays and gamma rays have enough energy to do more than just make the electrons vibrate When these high energy rays encounter an atom the result is an ejection of energetic electrons from the atom or the excitation of electrons The term excitation is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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Nonstochastic Effects
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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Pocket Dosimeter
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Audible Alarm Rate Meters
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film Badges
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Interaction of Electromagnetic Radiation and Matter
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Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles With each interaction the energy may be directed in a different direction The higher the energy of a photon the more likely the energy will continue traveling in the same direction As the radiation moves from point to point in matter it loses its energy through various interactions with the atoms it encounters If the radiation has enough energy it may eventually make it through the material
Photon Interaction with Matter is Key From the previous paragraph it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planckrsquos constant (E = hƒ) The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =cλ ) However it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating The key is its interaction with matter or more specifically whether the photons energy is right to excite some transition of a charged particle For instance microwaves penetrate glass very easily but they are strongly absorbed by water Move up to slightly higher frequency and infrared is strongly absorbed by both glass and water but both substances transmit visible light Ultraviolet is stopped by glass but not so readily by water
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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Ionization
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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Rad Units
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Stochastic Effects
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Nonstochastic Effects
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Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Ionization
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Ionization and Cell Damage
As previously discussed photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material This creates electrons which carry a negative charge and atoms without electrons which carry a positive charge Ionization in industrial materials is usually not a big concern In most cases once the radiation ceases the electrons rejoin the atoms and no damage is done However ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed Ionization may cause unwanted changes in some materials such as semiconductors so that they are no longer effective for their intended use
Ionization in Living Tissue (Cell Damage) In living tissue similar interactions occur and ionization can be very detrimental to cells Ionization of living tissue causes molecules in the cells to be broken apart This interaction can kill the cell or cause them to reproduce abnormally
Damage to a cell can come from direct action or indirect action of the radiation Cell damage due to direct action occurs when the radiation interacts directly with a cells essential molecules (DNA) The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA) DNA is found in every cell and consists of molecules that determine the function that each cell performs When radiation interacts with a cell wall or DNA the cell either dies or becomes a different kind of cell possibly even a cancerous one
Cell damage due to indirect action occurs when radiation interacts with the water molecules which are roughly 80 of a cells composition The energy absorbed by the water molecule can result in the formation of free radicals Free radicals are molecules that are highly reactive due to the presence of unpaired electrons which result when water molecules are split Free radicals may form compounds such as hydrogen peroxide which may initiate harmful chemical reactions within the cells As a result of these chemical changes cells may undergo a variety of structural changes which lead to altered function or cell death
Various possibilities exist for the fate of cells damaged by radiation Damaged cells can
completely and perfectly repair themselves with the bodys inherent repair mechanisms die during their attempt to reproduce Thus tissues and organs in which there is substantial cell
loss may become functionally impaired There is a threshold dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome Exceeding the threshold dose increases the level of harm Such outcomes are called
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
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Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
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Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
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Cancer
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Nonstochastic Effects
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Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
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Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
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Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
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Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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deterministic effects and occur at high doses repair themselves imperfectly and replicate this imperfect structure These cells with the
progression of time may be transformed by external agents (eg chemicals diet radiation exposure lifestyle habits etc) After a latency period of years they may develop into leukemia or a solid tumor (cancer) Such latent effects are called stochastic (or random)
Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here Non-ionizing radiation behaves exactly like ionizing radiation but differs in that it has a much greater wavelength and therefore less energy Although this non-ionizing radiation does not have the energy to create ion pairs some of these waves can cause personal injury Anyone who has received a sunburn knows that ultraviolet light can damage skin cells Non-ionizing radiation sources include lasers high-intensity sources of ultraviolet light microwave transmitters and other devices that produce high intensity radio-frequency radiation
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Radiosensitivity
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Rad Units
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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Exposure Limits
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Controlling Exposure
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Radiosensitivity
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells tissues organs organisms or other substances to the injurious action of radiation In general it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation In short this means that actively dividing cells or those not fully mature are most at risk from radiation The most radio-sensitive cells are those which
have a high division rate have a high metabolic rate are of a non-specialized type are well nourished
Examples of various tissues and their relative radiosensitivities are listed below
High Radiosensitivity
Lymphoid organs bone marrow blood testes ovaries intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea oral cavity esophagus rectum bladder vagina uterine cervix ureters)
Moderate Radiosensitivity
Optic lens stomach growing cartilage fine vasculature growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones salivary glands respiratory organs kidneys liver pancreas thyroid adrenal and pituitary glands
Low Radiosensitivity
Muscle brain spinal cord
Reference Rubin P and Casarett G W Clinical Radiation Pathology (Philadelphia W B Saunders 1968)
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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Exposure Limits
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Controlling Exposure
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Rad Units
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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Exposure Limits
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Controlling Exposure
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Rad Units
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Measures Relative to the Biological Effect of Radiation Exposure
There are five measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays These measures are Exposure Dose Dose Equivalent and Dose Rate A short summary of these measures and their units will be followed by more in depth information below
Exposure Exposure is a measure of the strength of a radiation field at some point in air This is the measure made by a survey meter The most commonly used unit of exposure is the roentgen (R)
Dose or Absorbed Dose Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter In other words the dose is the amount of radiation absorbed by and object The SI unit for absorbed dose is the gray (Gy) but the ldquoradrdquo (Radiation Absorbed Dose) is commonly used 1 rad is equivalent to 001 Gy Different materials that receive the same exposure may not absorb the same amount of radiation In human tissue one Roentgen of gamma radiation exposure results in about one rad of absorbed dose
Dose Equivalent The dose equivalent relates the absorbed dose to the biological effect of that dose The absorbed dose of specific types of radiation is multiplied by a quality factor to arrive at the dose equivalent The SI unit is the sievert (SV) but the rem is commonly used Rem is an acronym for roentgen equivalent in man One rem is equivalent to 001 SV When exposed to X- or Gamma radiation the quality factor is 1
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Dose rate is usually presented in terms of Rhour mRhour remhour mremhour etc
For the types of radiation used in industrial radiography one roentgen equals one rad and since the quality factor for x- and gamma rays is one radiographers can consider the Roentgen rad and rem to be equal in value
More Information on Exposure Dose Dose Equivalent and Dose Rate
Exposure Exposure is a measure of the strength of a radiation field at some point It is a measure of the ionization of the molecules in a mass of air It is usually defined as the amount of charge (ie the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass The most commonly used unit of exposure is the Roentgen (R) Specifically a Roentgen is the amount of photon energy required to produce 1610 x 1012 ion pairs in one gram of dry air at 0degC A radiation field of one Roentgen
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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Exposure Limits
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Controlling Exposure
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Responsibilities
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Survey Technique
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Radiation Detectors
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Survey Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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Pocket Dosimeter
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Rad Units
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will deposit 258 x 10-4 coulombs of charge in one kilogram of dry air The main advantage of this unit is that it is easy to directly measure with a survey meter The main limitation is that it is only valid for deposition in air
Dose or Absorbed Dose Whereas exposure is defined for air the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field The most commonly used unit for absorbed dose is the ldquoradrdquo (Radiation Absorbed Dose) A rad is defined as a dose of 100 ergs of energy per gram of the given material The SI unit for absorbed dose is the gray (Gy) which is defined as a dose of one joule per kilogram Since one joule equals 107 ergs and since one kilogram equals 1000 grams 1 Gray equals 100 rads
The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source the distance from the source to the irradiated material and the time over which the material is irradiated The activity of the source will determine the dose rate which can be expressed in radhr mrhr mGysec etc
Dose Equivalent When considering radiation interacting with living tissue it is important to also consider the type of radiation Although the biological effects of radiation are dependent upon the absorbed dose some types of radiation produce greater effects than others for the same amount of energy imparted For example for equal absorbed doses alpha particles may be 20 times as damaging as beta particles In order to account for these variations when describing human health risks from radiation exposure the quantity called ldquodose equivalentrdquo is used This is the absorbed dose multiplied by certain ldquoqualityrdquo or ldquoadjustmentrdquo factors indicative of the relative biological-damage potential of the particular type of radiation
The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness Radiation with higher Q factors will cause greater damage to tissue The rem is a term used to describe a special unit of dose equivalent Rem is an abbreviation for roentgen equivalent in man The SI unit is the sievert (SV) one rem is equivalent to 001 SV Doses of radiation received by workers are recorded in rems however sieverts are being required as the industry transitions to the SI unit system
The table below presents the Q factors for several types of radiation
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
More on Specific Stochastic Effects
Cancer
Leukemia
Genetic Effects
Cataracts
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Nonstochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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Nonstochastic Effects
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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Exposure Limits
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Controlling Exposure
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Procedures
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Rad Units
Type of Radiation Rad Q Factor RemX-Ray 1 1 1Gamma Ray 1 1 1Beta Particles 1 1 1Thermal Neutrons 1 5 5Fast Neutrons 1 10 10Alpha Particles 1 20 20
Dose Rate The dose rate is a measure of how fast a radiation dose is being received Knowing the dose rate allows the dose to be calculated for a period of time Fore example if the dose rate is found to be 08remhour then a person working in this field for two hours would receive a 16rem dose
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Biological Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
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Nonstochastic Effects
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Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
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Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
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Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
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Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Biological Effects
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Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including
Type of radiation involved All kinds of ionizing radiation can produce health effects The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues
Size of dose received The higher the dose of radiation received the higher the likelihood of health effects
Rate the dose is received Tissue can receive larger dosages over a period of time If the dosage occurs over a number of days or weeks the results are often not as serious if a similar dose was received in a matter of minutes
Part of the body exposed Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso See radiosensitivity page for more information
The age of the individual As a person ages cell division slows and the body is less sensitive to the effects of ionizing radiation Once cell division has slowed the effects of radiation are somewhat less damaging than when cells were rapidly dividing
Biological differences Some individuals are more sensitive to the effects of radiation than others Studies have not been able to conclusively determine the differences
The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic These two terms are discussed more in the next few pages
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Stochastic Effects
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Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
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Nonstochastic Effects
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Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Stochastic Effects
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Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
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Nonstochastic Effects
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Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
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Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Stochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects Stochastic effects often show up years after exposure As the dose to an individual increases the probability that cancer or a genetic effect will occur also increases However at no time even for high doses is it certain that cancer or genetic damage will result Similarly for stochastic effects there is no threshold dose below which it is relatively certain that an adverse effect cannot occur In addition because stochastic effects can occur in individuals that have not been exposed to radiation above background levels it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure
While it cannot be determined conclusively it often possible to estimate the probability that radiation exposure will cause a stochastic effect As mentioned previously it is estimated that the probability of having a cancer in the US rises from 20 for non radiation workers to 21 for persons who work regularly with radiation The probability for genetic defects is even less likely to increase for workers exposed to radiation Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur
Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation hereditary effects could occur in the progeny of the individual Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and during certain periods in early pregnancy may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high
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Nonstochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
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Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Responsibilities
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Stochastic Effects
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Nonstochastic Effects
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
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Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Nonstochastic Effects
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Nonstochastic (Acute) Effects
Unlike stochastic effects nonstochastic effects are characterized by a threshold dose below which they do not occur In other words nonstochastic effects have a clear relationship between the exposure and the effect In addition the magnitude of the effect is directly proportional to the size of the dose Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time These effects will often be evident within hours or days Examples of nonstochastic effects include erythema (skin reddening) skin and tissue burns cataract formation sterility radiation sickness and death Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (ie acute vs chronic exposure)
There are a number of cases of radiation burns occurring to the hands or fingers These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1768 Rs Contact with the source for two seconds would expose the hand of an individual to 3536 rems and this does not consider any additional whole body dosage received when approaching the source
More on Specific Nonstochastic Effects
Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy) This disease is characterized by depression or ablation of the bone marrow and the physiological consequences of this damage The onset of the disease is rather sudden and is heralded by nausea and vomiting within several hours after the overexposure occurred Malaise and fatigue are felt by the victim but the degree of malaise does not seem to be correlated with the size of the dose Loss of hair (epilation) which is almost always seen appears between the second and third week after the exposure Death may occur within one to two months after exposure The chief effects to be noted of course are in the bone marrow and in the blood Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs In this case however spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow
Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater and is a consequence of the desquamation of the intestinal epithelium All the signs and symptoms of hemopoietic syndrome are seen with the addition of severe nausea vomiting and diarrhea which begin very soon after exposure Death within one to two weeks after exposure is the most likely outcome
Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
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Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
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Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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as well as all the other organ systems in the body Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days The rapidity of the onset of unconsciousness is directly related to the dose received In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy) the victim was ataxic and disoriented within 30 seconds In 10 minutes he was unconscious and in shock Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident
Other Acute Effects Several other immediate effects of acute overexposure should be noted Because of its physical location the skin is subject to more radiation exposure especially in the case of low energy x-rays and beta rays than most other tissues An exposure of about 300 R (77 mCkg) of low energy (in the diagnostic range) x-rays results in erythema Higher doses may cause changes in pigmentation loss of hair blistering cell death and ulceration Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century
The reproductive organs are particularly radiosensitive A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men For women a 300 rad (3 Gy) dose to the ovaries produces temporary sterility Higher doses increase the period of temporary sterility In women temporary sterility is evidenced by a cessation of menstruation for a period of one month or more depending on the dose Irregularities in the menstrual cycle which suggest functional changes in the reproductive organs may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization
The eyes too are relatively radiosensitive A local dose of several hundred rads can result in acute conjunctivitis
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Exposure Symptoms
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Exposure Symptoms
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Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period
Dosages are in Roentgen Equivalent Man (Rem)
bull 0-25 No injury evident First detectable blood change at 5 rem bull 25-50 Definite blood change at 25 rem No serious injury bull 50-100 Some injury possible bull 100-200 Injury and possible disability bull 200-400 Injury and disability likely death possible bull 400-500 Median Lethal Dose (MLD) 50 of exposures are fatal bull 500-1000 Up to 100 of exposures are fatal bull 1000-over 100 likely fatal
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure
Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period
100 - 200 Rem First Day No definite symptomsFirst Week No definite symptomsSecond Week No definite symptomsThird Week Loss of appetite malaise sore throat and diarrhea
Fourth WeekRecovery is likely in a few months unless complications develop because of poor health
400 - 500 Rem First Day Nausea vomiting and diarrhea usually in the first few hours First Week Symptoms may continueSecond Week Epilation loss off appetite
Third WeekHemorrhage nosebleeds inflammation of mouth and throat diarrhea emaciation
Fourth Week Rapid emaciation and mortality rate around 50
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Exposure Limits
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Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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Controlling Exposure
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Exposure Limits
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Exposure Limits
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Exposure Limits
As discussed in the introduction concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895 Over the years numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups In general the guidelines established for radiation exposure have had two principle objectives 1) to prevent acute exposure and 2) to limit chronic exposure to acceptable levels
Current guidelines are based on the conservative assumption that there is no safe level of exposure In other words even the smallest exposure has some probability of causing a stochastic effect such as cancer This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure as low as reasonable achievable (ALARA) ALARA is a basic requirement of current radiation safety practices It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible
Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world In the United States annual radiation exposure limits are found in Title 10 part 20 of the Code of Federal Regulations and in equivalent state regulations For industrial radiographers who generally are not concerned with an intake of radioactive material the Code sets the annual limit of exposure at the following
1) the more limiting of
A total effective dose equivalent of 5 rems (005 Sv) or
The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (05 Sv)
2) The annual limits to the lens of the eye to the
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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skin and to the extremities which are
A lens dose equivalent of 15 rems (015 Sv)
A shallow-dose equivalent of 50 rems (050 Sv) to the skin or to any extremity
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 0007 cm averaged over and area of 10 cm2 The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 03 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation This value is the dose equivalent at a tissue depth of 1 cm The total effective dose equivalent is the dose equivalent to the whole-body
Declared Pregnant Workers and Minors Because of the increased health risks to the rapidly developing embryo and fetus pregnant women can receive no more than 05 rem during the entire gestation period This is 10 of the dose limit that normally applies to radiation workers Persons under the age of 18 years are also limited to 05remyear
Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit Therefore a non-radiation worker can receive a whole body dose of no more that 01 remyear from industrial ionizing radiation This exposure would be in addition to the 03 remyear from natural background radiation and the 005 remyear from man-made sources such as medical x-rays
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Controlling Exposure
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Controlling Exposure
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Controlling Radiation Exposure
When working with radiation there is a concern for two types of exposure acute and chronic An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time An acute exposure has the potential for producing both nonstochastic and stochastic effects Chronic exposure which is also sometimes called continuous exposure is long-term low level overexposure Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures
The three basic ways of controlling exposure to harmful radiation are 1) limiting the time spent near a source of radiation 2) increasing the distance away from the source 3) and using shielding to stop or reduce the level of radiation
Time The radiation dose is directly proportional to the time spent in the radiation Therefore a person should not stay near a source of radiation any longer than necessary If a survey meter reads 4 mRh at a particular location a total dose of 4mr will be received if a person remains at that location for one hour In a two hour span of time a dose of 8 mR would be received The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area
Dose = Dose Rate x Time (click here for more information on using this formula)
When using a gamma camera it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators This is illustrated in the images at the bottom of this page
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Distance Increasing distance from the source of radiation will reduce the amount of radiation received As radiation travels from the source it spreads out becoming less intense This is analogous to standing near a fire The closer a person stands to the fire the more intense the heat feels from the fire This phenomenon can be expressed by an equation known as the inverse square law which states that as the radiation travels out from the source the dosage decreases inversely with the square of the distance
Inverse Square Law I1 I2 = D22 D1
2
(click here for more information on using this formula)
Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation In general the more dense the material the more shielding it will provide The most effective shielding is provided by depleted uranium metal It is used primarily in gamma ray cameras like the one shown below The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera Depleted uranium and other heavy metals like tungsten are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials Concrete is commonly used in the construction of radiation vaults Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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HVL-Shielding
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Half-Value Layer (Shielding)
As was discussed in the radiation theory section the depth of penetration for a given photon energy is dependent upon the material density (atomic structure) The more subatomic particles in a material (higher Z number) the greater the likelihood that interactions will occur and the radiation will lose its energy Therefore the more dense a material is the smaller the depth of radiation penetration will be Materials such as depleted uranium tungsten and lead have high Z numbers and are therefore very effective in shielding radiation Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults
Since different materials attenuate radiation to different degrees a convenient method of comparing the shielding performance of materials was needed The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level At some point in the material there is a level at which the radiation intensity becomes one half that at the surface of the material This depth is known as the half-value layer for that material Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value
Sometimes shielding is specified as some number of HVL For example if a Gamma source is producing 369 Rh at one foot and a four HVL shield is placed around it the intensity would be reduced to 230 Rh
Each material has its own specific HVL thickness Not only is the HVL material dependent but it is also radiation energy dependent This means that for a given material if the radiation energy changes the point at which the intensity decreases to half its original value will also change Below are some HVL values for various materials commonly used in industrial radiography As can be seen from reviewing the values as the energy of the radiation increases the HVL value also increases
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 445 (175) 127 (05) 48 (019) 33 (013) 28 (011)
Cobalt-60 605 (238) 216 (085) 125 (049) 79 (031) 69 (027)
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Half-Value Layer mm (inch)
Peak Voltage (kVp) Lead Concrete
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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50 006 (0002) 432 (0170)
100 027 (0010) 1510 (0595)
150 030 (0012) 2232 (0879)
200 052 (0021) 250 (0984)
250 088 (0035) 280 (1102)
300 147 (0055) 3121 (1229)
400 25 (0098) 330 (1299)
1000 79 (0311) 4445 (175)
Note The values presented on this page are intended for educational purposes Other sources of information should be consulted when designing shielding for radiation sources
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Safe controls
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Safe controls
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Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent a variety of safety controls are used to limit exposure The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls Engineered controls include shielding interlocks alarms warning signals and material containment Administrative controls include postings procedures dosimetry and training
Engineered Controls Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a radiation vault Fixed shielding materials are commonly high density concrete andor lead Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced Warning lights are used to alert workers and the public that radiation is being used Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present Safety controls should never be tampered with or bypassed
When portable radiography is performed it is most often not practical to place alarms or warning lights in the exposure area Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation Occasionally radiographers will use battery operated flashing lights to alert the public to the presence of radiation Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection Sheets of steel steel beams or other equipment may be used for temporary shielding It is the responsibility of the radiographer to know and understand the absorption value of various materials More information on absorption values and material properties can be found in the radiography section of this site
Administrative Controls As mentioned above administrative controls supplement the engineered controls These controls include postings procedures dosimetry and training It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation Normal operating procedures and emergency
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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procedures must also be prepared and followed In the US federal law requires that any individual who is likely to receive more than 10 of any annual occupational dose limit be monitored for radiation exposure This monitoring is accomplished with the use of dosimeters which are discussed in the radiation safety equipment section of this material Proper training with accompanying documentation is also a very important administrative control
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Responsibilities
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Responsibilities
Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials Depending on the size of the organization specific responsibilities may be assigned to various individuals andor committees
Radiation Safety Officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements Some of the common responsible for the RSO include
Ensuring that all individuals using radiation equipment are appropriately trained and supervised
Ensuring that all individuals using the equipment have been formally authorized to use the equipment
Ensuring that all rules regulations and procedures for the safe use of radioactive sources and X-ray systems are observed
Ensuring that proper operating emergency and ALARA procedures have been developed and are available to all system users
Ensuring that accurate records of the use of the sources and equipment are maintained Ensuring that required radiation surveys and leak tests are performed and documented Ensuring that systems and equipment are protected from unauthorized access or removal
The minimum qualifications training and experience for RSOs for industrial radiography are as follows (1) Completion of the training and testing requirements of Sec 3443(a) of Part 10 of the Federal Code of Regulations (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations and (3) Formal training in the establishment and maintenance of a radiation protection program
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Responsibilities
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Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO The RSC often provides oversight of the policies procedures and responsibilities of an organizations radiation safety program
System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that
All rules regulations and procedures for the safe use of the X-ray system are followed An accurate record of the use of the system is maintained All safety problems with the system are reported to the RSO and corrected before further use The system is protected from unauthorized access or removal
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Procedures
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment These procedures must be specific to the equipment and its use in a particular application Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer
Standard Operating Procedures As a minimum operating procedures must include instructions for the following
Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits
Methods and occasions for conducting radiation surveys Methods for controlling access to radiographic areas Methods and occasions for locking and securing radiographic exposure devices transport and
storage containers and sealed sources Personnel monitoring and the use of personnel monitoring equipment Transporting sealed sources to field locations including packing of radiographic exposure
devices and storage containers in the vehicles placarding of vehicles when needed and control of the sealed sources during transportation
The inspection maintenance and operability checks of radiographic exposure devices survey instruments transport containers and storage containers
The procedure(s) for identifying and reporting defects and noncompliance Maintenance of records
Emergency Procedures Procedures must also be developed that guide workers in the event of an emergency A few of the items that could be covered include
Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly
Steps for minimizing exposure of persons in the event of an accident The procedure for notifying proper persons in the event of an accident Radioactive source recovery procedure if licensee will perform the recovery
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Survey Technique
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Technique
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers A survey must be performed occasionally to verify that vaults are not leaking radiation and that the safety devices are performing properly However when conducting radiography with gamma emitters in the field the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field
Approaching the Exposure Device A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential Before removing the exposure device (camera) from storage the calibration of the survey meter must be verified and the battery level must be checked When approaching the exposure device to remove it from the storage location the survey meter should be in hand and operational The survey meter should be placed next to the exposure device to verify that the source is contained inside the projector and to verify that the survey meter is working properly Survey meter readings should be compared to previous readings and recorded
Transporting the Exposure Device When transporting the exposure device it must be stowed securely in the vehicle A lockable metal box is often bolted in the rear of the vehicle A survey of the over pack the outside of the vehicle and the drivers compartment is then conducted and documented
Preparing for an Exposure Once on the job site the exposure area will be assessed distance calculations made for restricted area boundaries and ropes and signs placed appropriately Once this is complete the radiographer is ready to remove the exposure device from its storage compartment in the vehicle The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment Daily safety checks should then be made Once these checks are completed the radiographer and assistant may then move the exposure device to the exposure location As the cranks and guide tubes are attached in preparation for the first exposure the survey meter should be monitored Before the source is exposed the assistant should check the area for persons who may have crossed into the restricted area and then move outside the rope boundary
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
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Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Making an Exposure The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place As the source moves out of the exposure device the survey meter will increase to a very high level and then reduce once the source is inside the collimator During the exposure the assistant will survey the established boundary to determine the levels of radiation present If the survey meter indicates levels are higher than calculated the boundary must be extended
Retracting the Source On retraction of the source the radiographers will see a rise in readings as the source moves from the collimator and is retracted into the projector When the source is inside the exposure device the radiographer should approach it while monitoring the survey meter If the source is properly retracted no increase in the survey meter reading should be seen when approaching the exposure device The exposure device should be surveyed on all sides paying special attention to the front of the device The entire length of the guide tube must then be surveyed
This process is repeated for each exposure The survey results must be documented when the exposure device is returned to the vehicle for transportation and when it is placed back into its storage location
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Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Radiation Detectors
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Radiation Detectors
Instruments used for radiation measurement fall into two broad categories - rate measuring instruments and - personal dose measuring instruments
Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity) Survey meters audible alarms and area monitors fall into this category These instruments present a radiation intensity reading relative to time such as Rhr or mRhr An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time
Dose measuring instruments are those that measure the total amount of exposure received during a measuring period The dose measuring instruments or dosimeters that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units
The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Survey Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Audible Alarm Rate Meters
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Survey Meters
The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter
There are many different models of survey meters available to measure radiation in the field They all basically consist of a detector and a readout display Analog and digital displays are available Most of the survey meters used for industrial radiography use a gas filled detector
Gas filled detectors consists of a gas filled cylinder with two electrodes Sometimes the cylinder itself acts as one electrode and a needle or thin taut wire along the axis of the cylinder acts as the other electrode A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge) The gas becomes ionized whenever the counter is brought near radioactive substances The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode This results in an electrical signal that is amplified correlated to exposure and displayed as a value
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Audible Alarm Rate Meters
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Film Badges
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film Badges
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Thermoluminescent Dosimeter
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Depending on the voltage applied between the anode and the cathode the detector may be considered an ion chamber a proportional counter or a Geiger-Muumlller (GM) detector Each of these types of detectors have their advantages and disadvantages A brief summary of each of these detectors follows
Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode which results in a collection of only the charges produced in the initial ionization event This type of detector produces a weak output signal that corresponds to the number of ionization events Higher energies and intensities of radiation will produce more ionization which will result in a stronger output voltage
Collection of only primary ions provides information on true radiation exposure (energy and intensity) However the meters require sensitive electronics to amplify the signal which makes them fairly expensive and delicate The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used
Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode Due to the strong electrical field the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas The electrons produced in these secondary ion pairs along with the primary electrons continue to gain energy as they move towards the anode and as they do they produce more and more ionizations The result is that each electron from a primary ion pair produces a cascade of ion pairs This effect is known as gas multiplication or amplification In this voltage regime the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Hence these gas ionization detectors are called proportional counters
Like ion chamber detectors proportional detectors discriminate between types of radiation However they require very stable electronics which are expensive and fragile Proportional detectors are usually only used in a laboratory setting
Geiger-Muumlller (GM) Counter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Audible Alarm Rate Meters
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film Badges
Dosimeter
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
httpwwwndt-edorgEducationResourcesCommunityCollegeadiationSafetyradiation_safety_equipmentfilm_badgeshtm (2 of 2)21-09-2011 113209
Thermoluminescent Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Thermoluminescent Dosimeter
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Survey Meters
Geiger-Muumlller counters operate under even higher voltages between the anode and the cathode usually in the 800 to 1200 volt range Like the proportional counter the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas However this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions This all happens in a fraction of a second and results in an electrical current pulse of constant voltage The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse In other words the size of the current pulse is independent of the size of the ionization event that produced it
The GM counter was named for Hans Geiger who invented the device in 1908 and Walther Muumlller who collaborated with Geiger in developing it further in 1928
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute If the instrument has a speaker the pulses can also produce an audible click When the volume of gas in the chamber is completely ionized ion collection stops until the electrical pulse discharges Again this only takes a fraction of a second but this process slightly limits the rate at which individual events can be detected
Because they can display individual ionizing events GM counters are generally more sensitive to low levels of radiation than ion chamber instruments By means of calibration the count rate can be displayed as the exposure rate over a specified energy range When used for gamma radiography GM meters are typically calibrated for the energy of the gamma radiation being used Most often gamma radiation from Cs-137 at 0662 MeV provides the calibration Only small errors occur when the radiographer uses Ir-192 (average energy about 034 MeV) or Co-60 (average energy about 125 MeV)
Since the Geiger-Muumlller counter produces many more electrons than a ion chamber counter or a proportional counter it does not require the same level of electronic sophistication as other survey meters This results in a meter that is relatively low cost and rugged The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge)
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage In the ion chamber region the voltage between the anode and cathode is relatively low and only primary ions are collected In the proportional region the voltage is higher and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected In the GM region a maximum number of secondary ions are collected when the gas around the anode is completely ionized Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and
proportional regions Radiation at different energy levels forms different numbers of primary ions in the detector However in the GM region the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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Pocket Dosimeter
Dosimeter
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Audible Alarm Rate Meters
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film Badges
Dosimeter
Video Clips
References
Quizzes
Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
httpwwwndt-edorgEducationResourcesCommunityCollegeadiationSafetyradiation_safety_equipmentfilm_badgeshtm (2 of 2)21-09-2011 113209
Thermoluminescent Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Thermoluminescent Dosimeter
Dosimeter
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Survey Meters
the event The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters
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Pocket Dosimeter
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Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
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Pocket Dosimeter
Dosimeter
Video Clips
References
Quizzes
By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
httpwwwndt-edorgEducationResourcesCommunityCollegonSafetyradiation_safety_equipmentpocket_dosimeterhtm (2 of 3)21-09-2011 112201
Pocket Dosimeter
httpwwwndt-edorgEducationResourcesCommunityCollegonSafetyradiation_safety_equipmentpocket_dosimeterhtm (3 of 3)21-09-2011 112201
Audible Alarm Rate Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
httpwwwndt-edorgEducationResourcesCommunityCollegeiationSafetyradiation_safety_equipmentaudible_alarmhtm (1 of 2)21-09-2011 113122
Audible Alarm Rate Meters
Dosimeter
Video Clips
References
Quizzes
httpwwwndt-edorgEducationResourcesCommunityCollegeiationSafetyradiation_safety_equipmentaudible_alarmhtm (2 of 2)21-09-2011 113122
Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
httpwwwndt-edorgEducationResourcesCommunityCollegeadiationSafetyradiation_safety_equipmentfilm_badgeshtm (1 of 2)21-09-2011 113209
Film Badges
Dosimeter
Video Clips
References
Quizzes
Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
httpwwwndt-edorgEducationResourcesCommunityCollegeadiationSafetyradiation_safety_equipmentfilm_badgeshtm (2 of 2)21-09-2011 113209
Thermoluminescent Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
httpwwwndt-edorgEducationResourcesCommunityCollegnSafetyradiation_safety_equipmentthermoluminescenthtm (1 of 2)21-09-2011 113333
Thermoluminescent Dosimeter
Dosimeter
Video Clips
References
Quizzes
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Pocket Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays As the name implies they are commonly worn in the pocket The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter
Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen The dosimeter contains a small ionization chamber with a volume of approximately two milliliters Inside the ionization chamber is a central wire anode and attached to this wire anode is a metal coated quartz fiber When the anode is charged to a positive potential the charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects the quartz fiber and the greater the charge the greater the deflection of the quartz fiber Radiation incident on the chamber produces ionization inside the active volume of the chamber The electrons produced by ionization are attracted to and collected by the positively charged central anode This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position The amount of movement is directly proportional to the amount of ionization which occurs
httpwwwndt-edorgEducationResourcesCommunityCollegonSafetyradiation_safety_equipmentpocket_dosimeterhtm (1 of 3)21-09-2011 112201
Pocket Dosimeter
Dosimeter
Video Clips
References
Quizzes
By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
httpwwwndt-edorgEducationResourcesCommunityCollegonSafetyradiation_safety_equipmentpocket_dosimeterhtm (2 of 3)21-09-2011 112201
Pocket Dosimeter
httpwwwndt-edorgEducationResourcesCommunityCollegonSafetyradiation_safety_equipmentpocket_dosimeterhtm (3 of 3)21-09-2011 112201
Audible Alarm Rate Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Audible Alarm Rate Meters
Dosimeter
Video Clips
References
Quizzes
httpwwwndt-edorgEducationResourcesCommunityCollegeiationSafetyradiation_safety_equipmentaudible_alarmhtm (2 of 2)21-09-2011 113122
Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
httpwwwndt-edorgEducationResourcesCommunityCollegeadiationSafetyradiation_safety_equipmentfilm_badgeshtm (1 of 2)21-09-2011 113209
Film Badges
Dosimeter
Video Clips
References
Quizzes
Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
httpwwwndt-edorgEducationResourcesCommunityCollegeadiationSafetyradiation_safety_equipmentfilm_badgeshtm (2 of 2)21-09-2011 113209
Thermoluminescent Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
httpwwwndt-edorgEducationResourcesCommunityCollegnSafetyradiation_safety_equipmentthermoluminescenthtm (1 of 2)21-09-2011 113333
Thermoluminescent Dosimeter
Dosimeter
Video Clips
References
Quizzes
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Pocket Dosimeter
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By pointing the instrument at a light source the position of the fiber may be observed through a system of built-in lenses The fiber is viewed on a translucent scale which is graduated in units of exposure Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts During the shift the dosimeter reading should be checked frequently The measured exposure should be recorded at the end of each shift
The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure It also has the advantage of being reusable The limited range inability to provide a permanent record and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter The dosimeters must be recharged and recorded at the start of each working shift Charge leakage or drift can also affect the reading of a dosimeter Leakage should be no greater than 2 percent of full scale in a 24 hour period
Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter These dosimeters record dose information and dose rate These dosimeters most often use Geiger-Muumlller counters The output of the radiation detector is collected and when a predetermined exposure has been reached the collected charge is discharged to trigger an electronic counter The counter then displays the accumulated exposure and dose rate in digital form
Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure Some models can also be set to provide a continuous audible signal when a preset exposure has been reached This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary
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Pocket Dosimeter
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Audible Alarm Rate Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Audible Alarm Rate Meters
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film Badges
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
httpwwwndt-edorgEducationResourcesCommunityCollegeadiationSafetyradiation_safety_equipmentfilm_badgeshtm (2 of 2)21-09-2011 113209
Thermoluminescent Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Thermoluminescent Dosimeter
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Pocket Dosimeter
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Audible Alarm Rate Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Audible Alarm Rate Meters
Dosimeter
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
httpwwwndt-edorgEducationResourcesCommunityCollegeadiationSafetyradiation_safety_equipmentfilm_badgeshtm (1 of 2)21-09-2011 113209
Film Badges
Dosimeter
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References
Quizzes
Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
httpwwwndt-edorgEducationResourcesCommunityCollegeadiationSafetyradiation_safety_equipmentfilm_badgeshtm (2 of 2)21-09-2011 113209
Thermoluminescent Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Thermoluminescent Dosimeter
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Audible Alarm Rate Meters
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Audible Alarm Rate Meters and Digital Electronic Dosimeters
Audible alarms are devices that emit a short beep or chirp when a predetermined exposure has been received It is required that these electronic devices be worn by an individual working with gamma emitters These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount Typical alarm rate meters will begin sounding in areas of 450-500 mRh It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters
Most audible alarms use a Geiger-Muumlller detector The output of the detector is collected and when a predetermined exposure has been reached this collected charge is discharged through a speaker Hence an audible chirp is emitted Consequently the frequency or chirp rate of the alarm is proportional to the radiation intensity The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen
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Audible Alarm Rate Meters
Dosimeter
Video Clips
References
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httpwwwndt-edorgEducationResourcesCommunityCollegeiationSafetyradiation_safety_equipmentaudible_alarmhtm (2 of 2)21-09-2011 113122
Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
httpwwwndt-edorgEducationResourcesCommunityCollegeadiationSafetyradiation_safety_equipmentfilm_badgeshtm (1 of 2)21-09-2011 113209
Film Badges
Dosimeter
Video Clips
References
Quizzes
Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
httpwwwndt-edorgEducationResourcesCommunityCollegeadiationSafetyradiation_safety_equipmentfilm_badgeshtm (2 of 2)21-09-2011 113209
Thermoluminescent Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
httpwwwndt-edorgEducationResourcesCommunityCollegnSafetyradiation_safety_equipmentthermoluminescenthtm (1 of 2)21-09-2011 113333
Thermoluminescent Dosimeter
Dosimeter
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Audible Alarm Rate Meters
Dosimeter
Video Clips
References
Quizzes
httpwwwndt-edorgEducationResourcesCommunityCollegeiationSafetyradiation_safety_equipmentaudible_alarmhtm (2 of 2)21-09-2011 113122
Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
httpwwwndt-edorgEducationResourcesCommunityCollegeadiationSafetyradiation_safety_equipmentfilm_badgeshtm (1 of 2)21-09-2011 113209
Film Badges
Dosimeter
Video Clips
References
Quizzes
Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
httpwwwndt-edorgEducationResourcesCommunityCollegeadiationSafetyradiation_safety_equipmentfilm_badgeshtm (2 of 2)21-09-2011 113209
Thermoluminescent Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
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Thermoluminescent Dosimeter
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Film Badges
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Film Badges
Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays X-rays and beta particles The detector is as the name implies a piece of radiation sensitive film The film is packaged in a light proof vapor proof envelope preventing light moisture or chemical vapors from affecting the film
A special film is used which is coated with two different emulsions One side is coated with a large grain fast emulsion that is sensitive to low levels of exposure The other side of the film is coated with a fine grain slow emulsion that is less sensitive to exposure If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted the fast emulsion is removed and the dose is computed using the slow emulsion
The film is contained inside a film holder or badge The badge incorporates a series of filters to determine the quality of the radiation Radiation of a given energy is attenuated to a different extent by various types of absorbers Therefore the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter By comparing these results the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy The badge holder also contains an open window to determine radiation exposure due to beta particles Beta particles are effectively shielded by a thin amount of material
The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record it is able to distinguish between different energies of photons and it can measure doses due to different types of radiation It is quite accurate for exposures greater than 100 millirem The major disadvantages are that it must be developed and read by a processor (which is time consuming) prolonged heat exposure can affect the film and exposures of less than 20 millirem of gamma radiation cannot be accurately measured
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
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Thermoluminescent Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
httpwwwndt-edorgEducationResourcesCommunityCollegnSafetyradiation_safety_equipmentthermoluminescenthtm (1 of 2)21-09-2011 113333
Thermoluminescent Dosimeter
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Film Badges
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Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives Whole body badges are worn on the body between the neck and the waist often on the belt or a shirt pocket The clip-on badge is worn most often when performing X-ray or gamma radiography The film badge may also be worn when working around a low curie source Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body
httpwwwndt-edorgEducationResourcesCommunityCollegeadiationSafetyradiation_safety_equipmentfilm_badgeshtm (2 of 2)21-09-2011 113209
Thermoluminescent Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
httpwwwndt-edorgEducationResourcesCommunityCollegnSafetyradiation_safety_equipmentthermoluminescenthtm (1 of 2)21-09-2011 113333
Thermoluminescent Dosimeter
Dosimeter
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Thermoluminescent Dosimeter
Home - Education Resources - NDT Course Material - Radiation -
Radiation Safety Introduction Background Information X-Radiation Gamma Radiation Health Concerns Radiation Theory Nature of Radiation Sources of High Energy Rad Rad for Ind Radiography Decay and Half-life Energy Activity Intensity and Exposure Interaction with Matter Ionization Radiosensitivity Measures Related to Biological Effects Biological Effects Biological Factors Stochastic (Delayed) Effects -Cancer -Leukemia -Genetic Effects -Cataracts Nonstochastic (Acute) Effects Symptoms Safe Use of Radiation NRC amp Code of Federal Regs Exposure Limits Controlling Exposure -Time-Dose Calculation -Distance-Intensity Calc HVL Shielding Safety Controls Responsibilities Procedures Survey Techniques Radiation Safety Equipment Radiation Detectors Survey Meters Pocket Dosimeter Audible Alarm Rate Meters Film Badges Thermoluminescent
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used instead of the film badge Like a film badge it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received if any Thermoluminescent dosimeters can measure doses as low as 1 millirem but under routine conditions their low-dose capability is approximately the same as for film badges TLDs have a precision of approximately 15 for low doses This precision improves to approximately 3 for high doses The advantages of a TLD over other personnel monitors is its linearity of response to dose its relative energy independence and its sensitivity to low doses It is also reusable which is an advantage over film badges However no permanent record or re-readability is provided and an immediate on the job readout is not possible
How it works A TLD is a phosphor such as lithium fluoride (LiF) or calcium fluoride (CaF) in a solid crystal structure When a TLD is exposed to ionizing radiation at ambient temperatures the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons called holes Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place
Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process Released electrons return to the original ground state releasing the captured energy from ionization as light hence the name thermoluminescent Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor
Instead of reading the optical density (blackness) of a film as is done with film badges the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured The glow curve produced by this process is then related to the radiation exposure The process can be repeated many times
httpwwwndt-edorgEducationResourcesCommunityCollegnSafetyradiation_safety_equipmentthermoluminescenthtm (1 of 2)21-09-2011 113333
Thermoluminescent Dosimeter
Dosimeter
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Thermoluminescent Dosimeter
Dosimeter
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