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RADIOGRAPHIC
TESTING
&
RADIOGRAPHIC
INTERPRETATION
( MAIN LECTURE NOTES
ANC-RAD- TD-OOl RUANE & T P O'NElllISSUE9 31/03/09
ACKNOWLEDGEMENT
The literature within is supplied by Argyll Ruane Ltd by way of contract agreement
whereby terms and conditions apply.
This document remains the copyright of Argyll Ruane Ltd and should not be copied
without prior consent from Argyll Ruane Ltd directly.
This document is reviewed on a regular basis and amended accordingly to meet
industry standards that apply.
We would like to thanks Argyll Ruane Ltd for their continued support.
30th April 2009
ANC-RAD-TD-001 RUANE & T P O'NEILLISSUE9 31/03/09
TABLE OF CONTENTS
RADIOGRAPHIC OVERVIEW RIPrinciples of film radiography R 1-1Radiographic quality Rl-lCapabilities and limitations of radiography R 1-1Duties of a radiographic interpreter R]-I
X AND GAMMA RADIA TION R2Comparison of x and gamma rays for industrial radiography R2-1
BASIC PHYSICS R3Elements R3-2Atoms R3-2Isotopes R3-3Ions R3-3Radionuclides (radio-isotopes) R3-3Gamma ray generation R3-3Types of radiation R3-5Activity R3-8Specific activity R3-8Decay ~ R3-8Half life R3-8Ionisation R3-8
ABSORPTION AND SCATTERING R4Scatter R4-1
RADIOGRAPIDC EQUIPMENT R5Gamma sources RS-lX-ray generation RS-3Electrical circuits in x-ray tubes RS-4
HALF VALUE THICKNESS ~ ~..R6RADIOGRAPHIC FILM R7
The make-up of a radiographic film R7-]Film types R7-2Film speed R7-2
CHARACTERISTIC CURVES OF FILMS R8INTENSIFYING SCREENS R9
General R9-1Lead screens R9-1Fluorescent (salt) screens R9-1Fluorometallic screens R9-2Comparison of intensifying screens R9-2
IMAGE FORMATION RIOFILM PROCESSING •....•......................................•.............•..••...................•..........................•....•......•.•...Rll
Darkrooms Rl ]-1Processing Rl ]-3
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Developer R 11-4Stopbath RII-5Fixer RI1-5Final wash RII-5Wetting agent R11-6Drying the film Rl1-6
RADIOGRAPmC QUALITy ..•...•..•...•..............•.•...........••.•..•.•.....•......•...••.•...•.•.....•...•...•.........•....•.......R12Density RI2-1Radiographic contrast R12-2Definition R12-3Processing and handling faults RI2-6Artifacts R 12-7Sensitivity R 12-8Assessing sensitivity R12-10
RADIOGRAPIC TECIINIQUES ••..•...•...........•...........•.....................•................••...............••...........•...... R13SWSI : source outside, film inside R13-1SWSI: (panoramic) source inside, film outside R13-2DWSI. RI3-2DWDI RI3-3Sandwich technique RI3-3Location of defects RI3-3Image shifts RI3-5
DETERMINATION OF EXPOSURE ••.••....••...............•.•...............•••••...•.•...................•..................•..••.•R14Considerations for exposures R 14-1Exposure charts R14-3Exposure calculations for gamma rays R14-4Exposure calculations using gamma slide rule RI4-4Equivalence charts R14-9
FILTERS R15GLOSSARY OF TERMS R16
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Xsradiography typically uses/50·300 k V on steel weldmentsup to approximately 30 mm totalthickness.
Cobalt 60 (C060) has a veryhigh penetrating power - veryshort wavelength - and can beused on materials up /0 200 mmthick. Iridium 192 (JrI92) is 40commonly used on steelweldments up to 60 mm thick.
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. .. . .'UNIT Rl . RADIOGRAPHIC OVERVIE\V .
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X-radiography vs gamma radiographyX-radiography requires bulky and expensive machinery in comparison with gammaradiography, but x-radiography generally produces better quality radiographs and issafer. X-ray machines can be switched on and off, unlike gamma sources.
PRINCIPLES OF FILM RADIOGRAPHY
20
Film radiography is carried out using x-ray machines or artificial gamma sources(radio-isotopes).
X-rays or gamma rays pass through the object to be radiographed and record an imageon a radiographic film placed on the opposite side. The quality and amount of radiationreaching the film will be largely determined by the objects thickness and density, e.g. acrack in a weld will increase the amount of radiation falling on the film in that area dueto a reduction in thickness.
It is the wavelength of the radiation which governs its penetrating power. This isgoverned by the kilovoltage (kV) setting when using x-rays and isotope type withgamma rays. The intensity of the radiation is governed by the milli-amperage (mA)setting when using x-rays and by the activity of the isotope type with gamma rays.Activity is measured in curies or gigabecquerels.
When the film is processed a negative is produced. The thin areas of an object will bedarker than the thicker areas, therefore most weld defects will show up dark in relationto the surrounding areas, exceptions are excess weld metal, spatter, copper inclusionsand tungsten inclusions.
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RADIOGRAPHIC QUALITYAn overall assessment of radiographic quality is made by the use of image qualityindicators (IQI's), the commonly used type consists of seven thin wires decreasing inthickness. At least one IQI is pre-placed transversely across the weld being examined.After exposure, some of the wires will be visible on the resultant radiograph - the morewires visible the better the sensitivity.
The density of an image on a radiograph, Le. its degree of blackness, is also measuredto ensure it lies within a specified range for optimum quality.
60CAPABILITIES AND LIMITATIONS OF RADIOGRAPHY
70
A major advantage of radiographic testing is that a permanent record is produced, i.e.the radiograph.
A major limitation of radiography is that it will only detect defects which havesignificant depth in relation to the axis of the x-ray beam. As a rough guide, theminimum through thickness depth of a defect capable of being detected is about 2% ofthe wall thickness in the same axis as the x-ray beam, e.g. radiography will not usuallydetect plate laminations, lack of inter-run fusion or cracks perpendicular to the x-raybeam.
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liNIT RI • RADIOGRAPHIC OVERVIE\\'
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] . Mask off any unwanted light on the viewer.
2. View radiographs under subdued background light.
3. Ensure, as far as is reasonably practicable, each radiograph is correctly identified tothe weld it represents.
4. Ensure that the weld locations are identified, e.g. has the correct number tape beenused.
5. Assess the quality of the radiograph:a. Measure radiographic density.b. Calculate IQI sensitivity - also ensure the IQl's are of the correct type and
correctly positioned.c. Assess radiographic contrast; e.g. has gamma been used when only x-
radiography is permitted?d. Assess definition/graininess; e.g. have salt intensifying screens been used
when only lead intensifying screens are permitted? Has a fast film been usedinstead of a slow film?
e. Do artifacts interfere with interpretation?6. Check the radiograph to determine if any obstruction between the source of
radiation and the film interferes with interpretation, e.g. lead numbers.
7. Identify the type of weld if possible - normally already known.
8. Check the parent material on the radiograph for arc strikes, hard stamping, gouges,minimum seam offset etc., when applicable.
9. Check the weld on the radiograph for defects, stating type and region.
10. State action to be taken, e.g. accept the radiograph and weld, reshoot, repair,remove the entire weld, visual check, grind and investigate, MP] check, ultrasoniccheck.
DUTIES OF A RADIOGRAPHIC INTERPRETER10 It is the duty of a radiographic interpreter to ensure that all radiographic interpretation
and any associated actions are carried out in accordance with the relevantspecification(s) for the work being carried out.
A radiographic interpreter must have access to the relevant specification(s) and mustknow where to find and interpret relevant information.
Specific duties when interpreting radiographs of welds are typically as follows:
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UNIT R2 • X AND GAMMA RAJ)IATION
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VersatilityThe intensity and wavelengths of x-rays can be adjusted from the x-ray control panel.The intensity and wavelengths of gamma radiation cannot be adjusted, although theintensity (activity) reduces with time - see half-lives.
Certain gamma sources have a very high penetrating power which enables them to beused on very thick material, e.g. 150 mm steel. Most conventional x-ray machines willnot penetrate more than 50 mm of steel although there are huge x-ray machines, e.g. thelinear accelerator and the betatron which can produce radiation of a wavelength whichcan penetrate as much as, and usually more than, gamma radiation.
COMPARISON OF X AND GAMMA RAYS FOR INDUSTRIALRADIOGRAPHY
20
SafetyUsing x-ray machines is normally safer than using gamma sources because x-raymachines may be switched off like a light bulb, whereas there is a constant emission ofradiation with a gamma source. Gamma sources must always be returned to theirshielding containers when not in use.
30
Quality of radiographic imagesAssuming variables such as test material thickness, film type etc. remains constant, x-rays produced by conventional x-ray equipment, say up to 300 kV, produce betterquality radiographic images than Ir192 or C060 isotopes, because these x-rays havelonger wavelengths than the gamma sources.
Ytterbium 169 (Yb169) may produce radiographs comparable to those produced byusing x-rays. If the wavelength from the gamma source is the same as the wavelengthfrom the x-ray set, the quality will be the same.
40 HandlingGamma sources are easier to handle in comparison with bulky and fragile x-rayequipment. The size also allows for gamma sources to be used in difficult andinaccessible areas for x-ray machines, e.g. on pipe racks.
50 CostGamma sources and containers are much cheaper than x-ray equipment, however,gamma sources deplete in output and must be replaced regularly. This makes gammamore expensive in the long run.
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UNIT R3 • BASIC PHYSICS
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Element SymbolNumber of Electrons
K L M N 0 PI
Hydrogen IH 1
Helium4 22He
Lithium 7L· 2 13 1
Beryllium 9 2 24Be
Carbon 12C 2 46
Aluminium 27 A 2 8 313
Cobalt 59 Co 2 8 15 227
Nickel 59N· 2 8 16 228 1
Barium 137Ba 2 8 18 18 8 256
Tungsten (Wolfram) 134W 2 8 18 32 12 274
Iridium 192I 2 8 18 32 15 277 r
N shell.-.--~-~M shell
, .... ----.-.Lshell • • Proton (+ charge)
0 Neutron (no charge)
• '- Electron (- charge)•• ~ •
~
• !.'
• ---_ .._,...K shell
•:' •
,./
20 •••
30
-,
40A [MASS NUMBER] Neutrons and protons
E Element
Z [ATOMIC NUMBER] Number of protons in the nucleus50
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Atomic numberThe atomic number or Z number is the total number of protons in the nucleus and thisdefmes the element, e.g. H = I; He = 2; C = 6; 0 = 8.
ELEMENTSAn element is a substance that cannot be separated into any other constituents. Thisstatement is with reference to the chemical nature only.
There are over one hundred elements known to man and these have been placed withina table referred to as the periodic table; this places elements into groups and periodswith reference to their chemical characteristics.
Hydrogen (H) is the lightest element and is taken as the reference element. Helium(He), neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe) are grouped together becausethese are inert gases or gases that cannot react chemically with other elements.
The halogen group includes fluorine (F), chlorine (Cl), bromine (Br) and iodine (I);these are very active elements which readily combine with most of the other elements inthe table.
Elements range from hydrogen (H), with an atomic number of I, to uranium (U) with30 an atomic number 92; between these are all the elements that make up everything on
earth.
20
ATOMS40 An atom is the smallest part of an element that can have the element's properties. All
atoms of the same element are similar in construction, however, atoms of differentelements have different constructions.
50
An atom is a very small particle which is made up from a number of sub-atomicparticles grouped together. The size of the sub-atomic particles are small, with most ofeach atom consisting of free space.
The sub-atomic particles in the centre (core or nucleus) of each atom contain theheavier particles consisting of protons which carry a positive charge, and neutronswhich carry no charge. Protons and neutrons have an unusual attraction for each otherand tend to pair together.
The lighter particles, electrons, are said to be held in stable orbits around the nucleus bythe attraction of the protons in the nucleus. These orbits are referred to as shells, e.g. K.L. Mshells.
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There are other sub-atomic particles, e.g. the positron, which is of similar size and massto the electron but with a positive charge.
70 Sub-atomic particles• Protons are along with neutrons, the heavy particles in an atom and are found in
the nucleus. They are positively charged and have a rest mass of 1.673 x 10.27 kg.
• Neutrons are similar in mass to a proton having a rest mass of 1.675 x 10-27 kg.They have no charge, are neutral and are found in the nucleus.
• Electrons are small, very light weight particles and have a rest mass of9. I09 x 10-31 Kg. They have a negative charge and orbit the nucleus in restrictedshells according to the rules of quantum mechanics.
Atoms will have the same number of protons and electrons when the atom is inequilibrium, i.e. when it is not an ion.
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'Activity' is a term which 70relates to the number ofdlsintegrations per unittime. Activity is measuredin becquerels (Bq) or Curiestco.
Radium produces radongas.
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UNIT R3 • BASIC PHYSICS
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Ganuna rays used in industrial radiography are emitted from artificial radioactiveisotopes, also known as radionuclides. A radioactive isotope is an unstable state of achemical element which has a different number of neutrons to the normal state of thesame element.
As with all isotopes, the different number of neutrons will result in a change in mass,therefore, the mass number or A number will be different to the mass number of theother isotopes possible for the specific element. The atomic number or Z numberhowever will be the same for all the isotopes of the specific element, because thisnumber refers to the number of protons in the nucleus which have not changed.
If a material is radioactive, it spontaneously emits corpuscular and electromagneticenergy, the ganuna radiation is a by-product produced from the disintegration of theradioactive isotope.
Mass numberThe mass number or A number essentially refers to the weight of an atom and is thenumber of protons and neutrons in the nucleus. Mass (Aj number for He = 4, C = 12
10 and 0 = 16. Note that the mass number is not always twice the atomic number.
ISOTOPESElements that have the same number of protons but different numbers of neutrons arevarieties of the same element and are called isotopes. Among the 100 or so knownelements there are some 300 different isotopes, e.g. HII, H/ and HI3 are three isotopesof hydrogen HI2 = deuterium, HI
3 = tritium.
Carbon also has three isotopes: C612
, C6\3 and C614 conunonly referred to as carbon 12,
carbon 13 and carbon 14 respectively.
30IONS
40
An ion is an electrically charged particle which may be positive (+ve) or negative(-ve).
When particles or photons of energy (quanta) pass through matter, all the energy isabsorbed in exciting the atoms or molecules so that electrons are ejected producingelectrical imbalance. The ejected electrons (having negative charges) are negative ions,whilst the atoms losing electrons are positive ions due to their unpaired proton(s) ineach nucleus.
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Ions are created when x-rays, gamma rays, alpha particles, beta particles or neutronspass through matter.
The process of producing ions is known a ionisation.
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RADIONUCLIDES (RADIO-ISOTOPES)Radionuclides are radioactive isotopes, Le. the disintegrate by releasing sub-atomicparticles, and also give off excess energy known as gamma radiation.
All elements with atomic numbers higher than bismuth (atomic number 83) areradioactive and are elements which result from the decay of either uranium 235,uranium 238 or thorium 232.
Every radionuclide has a half life, this is the time it takes for the activity to drop to onehalf of its initial strength; this varies from a fraction of a second for some isotopes andto thousands of years for others.
GAMMA RAY GENERATION
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pSv/hlGBq, divide by 37 90then multiply by 10,000.
UNIT R3 • BASIC PHYSICS
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Cobalt 60 (Co60) is produced by bombarding C059 with neutrons in a reactor.
20
The activity or strength of a radioactive isotope is expressed in curies (Ci) orbecquerels (Bq). The higher the activity value, the greater the intensity of gamma raysproduced.
I becquerel = I disintegration per second;
3.7 x 1010 becquerels = 1 curie;
therefore, 3.7 x 1010 disintegrations per second = I curie.
For industrial radiography, it is usually more practical to talk in terms ofgigabecquerels (GBq).
Giga= 109•
I gigabecquerel = 109 becquerels.
37 gigabecquerels = 1 curie.
The activity of a radioactive isotope does not relate to the penetrating power of thegamma rays produced; penetrating power depends on the wavelength of the gammarays produced and this depends on the specific radioactive element involved. Forexample, Cobalt 60 (C060) has a very high penetrating power and may be used on steelcomponents up to 200 mm thick, because the gamma radiation emitted has a very shortwavelength.
There are four main radioactive isotopes used for industrial radiography;Iridium 192 (IrI92), Cobalt 60 (Co60), Ytterbium 169 (Yb169) and Selenium (Se75).
Radioactive isotopes are used taking into consideration their half-lives; the half-life of aradioactive isotope is the time it takes for the activity to drop to one-half of its initialstrength.
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Natural occurring radionucIidesThere are two main radionuclides which occur naturally: Radon and Radium. Radonhas a half life of3.825 days and Radium has a halflife of 1,590 years.
Radium 226 is no longer used for radiography because of the hazards presented by its60 alpha decay and its gaseous radioactive daughter Radon. Bones are especially
susceptible to damage from radiation emitted from radium 226.
Artificial radionucIidesArtificially produced radionuclides have replaced natural radionuclides for use inindustrial radiography. There are three methods of producing artificial radionuclides:
70 1. Neutron activation (neutron bombardment in a reactor).2. Fission produce separation.3. Charged particle bombardment (via high energy x-ray machine).
The most widely used radioisotopes are shown in the following table:
80 Characteristics of Gamma Ray Sources
Source Half life Output* Gamma ray Approx. x-ray Range in steelenerales MeV equivalent kV -mm
Cobalt 60 5.26 years 1.32 1.17- 1.33 1200 50 - 200 mmSelenium 75 118.5 days 0.203 0.066 - 0.401 400 4 -28 mmCaesium 137 30 years 0.33 0.66 700 45 -75 mmIridium 192 74 days 0.48 0.29 - 0.61 600 12-70 mmYtterbium 169 31 days 0.125 0.063 - 0.308 300 2-17mmThulliuml70 127days 0.0025 0.052 - 0.084 80 1-\3mm* - Exposure rate factor: Emission in roentgens per curie per hour at I metre (RlCi/hr at I metre).
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Source Sizes and Maximum ActivitySource Dimensions Activity in Curies
Dia (mm) Length (mm) Ir 192 Co601.0 1.0 7.0 1.52.0 1.0 30 8.02.0 2.0 50 153.0 2.0 95 323.0 3.0 140 453.0 4.0 180 904.0 4.0 210 120
Corpuscular (particulate) radiationCorpuscular radiation is the flow of sub-atomic particles. These particles mayor maynot have an electrical charge.
This type of radiation is different to x and gamma radiation by having mass and nottravelling at the speed of light. There are three main types of corpuscular radiation:alpha, beta and neutron radiation.
20Alpha radiationAn alpha particle is a large sub-atomic particle consisting of two protons and twoneutrons (the nucleus ofa helium atom) and therefore has a positive charge.
Alpha radiation travels comparatively slowly leaving the source at about 16,000 km.s"(10,000 miles/sec) but the particles soon slow down and only travel a total distance ofafew centimetres through the air.
Alpha particles ionise atoms by removing electrons as they pass through matter but theydo not penetrate deeply and can be stopped by a sheet of paper and human skin. Themain hazard is that they may enter the body through a cut in the skin or they may beingested.
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40Beta radiationA beta particle is a very light high speed electron and will possess a negative charge.
Beta particles travel faster than alpha particles. They are small and lightweight andtherefore do not have a high ionising potential compared with alpha radiation. Theycan travel through 3 meters of air or 1 mm of lead and are more penetrating than alphaparticles but they can be stopped by a few millimetres of most solid or liquid materials.
If beta particles are emitted from a radioactive source, they are normally preventedfrom entering the surrounding air space by absorption by the mass of the radioactivepellet or its surrounding capsule.
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Neutron radiationNeutron radiation simply consists of flowing neutrons which have no electrical charge.
Neutrons are produced from nuclear reactors, accelerators and certain radioactiveisotopes, e.g. califomium 252, all of which produce fast neutrons. These neutronsnormally have to be slowed down by using a moderator before they are used inradiography; these slower, lower energy, neutrons are called thermal neutrons.
70 Neutron radiation can penetrate many materials made from heavy elements with easebut it is absorbed by many lighter materials, particularly those containing hydrogen.Hydrogen has an affinity for neutrons.
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UNIT R3 . BASIC PHYSICS
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IONISATION
SPECIFIC ACTIVITYSpecific activity relates the curie output to the physical size of the source and ismeasured in curies per gram (Ci/gm). From the table above, it can be seen that a2 mm x 2 mm Irl92 source can have an activity of up to 50 Ci but a 2 mm x 2 mmC060 source can only have an activity of 15 Ci. In order to increase Ci output, thesource size must be increased. Irl92 has a higher specific activity than C060.
20 DECAY
30
Decay is the process of spontaneous transformation of a radionuclide. A loss ofactivity will be the result of decay and most radionucJide will decay throughdisintegration.
Radioactive materials decay by at least one of five primary modes:I. Emission of alpha particles (helium nucleus).
2. Emission of beta particles.
3. Electron capture or positron emission.
4. Emission of gamma rays (photons).
5. Spontaneous fission.40
HALF LIFE
50
Half life is the time taken for a radioactive isotope to reduce its output by half. AfterI half life has occurred, an exposure needs to be doubled to achieve the same density.
Radioactive Decay
70
60Vb 169 half life 31 days
Ir 192 half life 74 days
Co 60 half life 5.3 years
Typical replacemente.g. alter 3half lives.
11
Vb 169 31
If" 192 74
co 60 5.3
2 3 ~ 5 662 93 124 155 186 days
148 222 296 370 444 days
10.6 15.9 21.2 26.5 38.8 years
HafflivesX&:G __ •. Rl·!
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90 Ionisation is simply the formation of ions which are positively or negatively chargedparticles.
ionising radiation means gamma rays, x-rays or corpuscular radiations which arecapable of producing ions either directly or indirectly.
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Photoelectric effectThe photoelectric effect is an interaction between a photon and an orbiting electronwhich causes an electron to be ejected. The photon is consumed and the excess energyimparts kinetic energy to the electron.
SCATTER
20
When radiographic exposures are being made, some of the radiation scatters in alldirections by the atoms which form the object. This scatter results in an overallfoggingof the film and reduces the contrast and sharpness of the radiographic image. Thethicker the object being radiographed, the greater the amount of scatter.
Furthermore, the ground, a wall, or another object close to the object beingradiographed which is struck by the radiation, will partially re-emit the rays in the formof back seal/er; this is also liable to fog the film.
Scatter radiation is less penetrating than primary radiation from which it is derived,i.e. they have a longer wavelength. Because scatter rays are less penetrating, they canbe intercepted by a sheet of lead; this is one reason for using lead screens on either sideof the film in a film cassette during exposure, although heavier filters may also beneeded if the scatter is heavy.
The intensity of ionising radiation is reduced by at least one of the following types ofinteraction:30a. Rayleigh scattering.b. Photoelectric effect.c. Compton effect.d. Pair production
40 The extent of absorption and scattering is governed by the energy of the primaryradiation and the atomic number of the elements making up the medium through whichthe radiation is traveIling.
Scattered radiation may seriously effect the quality of a radiographic image and mayalso increase the radiation dose levels in the working viscinity.
50Rayleigh scattering
In the process, photons are deflected by outer electrons but do not change in energy orrelease any electrons. The photon scattering is in the forward direction.
This process accounts for less than 20% of the total attenuation of a radiation beam.60 Rayleigh scattering is most relevant when dealing with low energies of radiation
passing through materials consisting of elements with a high atomic number.
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Photons·.....Vi I-#:,...~< ! . -c-,~•..•••_.!" ....."""
" •.....•................~~-.: ..7'-.,/ -----0.-. .....
0/ .-----""0.----
/
eo o
cl 0
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UI'iIT R4 • ABSORPTIO~ ANI> sexTTERING
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Ejected electron (-)
This process applies to ionising radiation of relatively low energy, e.g. less than100 keY in steel, and also to higher energy radiation up to about 2 MeV when passingthrough materials containing elements of high atomic number.
Photons
20
, ,• : r I' •••
,~#i..~oc·---~~
Ejected electron (-)"e---
.....--.,.o
I
~
----,.30
40
Compton scatteringThis is also called the Compton effect. In this process, a photon interacts with a free orweakly bonded outer electron, part of the photon's energy is transferred to the electronwhich is ejected. The photon emerges from the collision as scattered radiation ofreduced energy.
PhotonEjected electron (-)_0
~......... -----~---------~-",o
/ /.--o-·-'....J...;.··!.,:.'i .~.J._/"-0, -- Scattered radiation
50
ri
!60
.c.:>"0. ._c..,'
o.~--~/70
80
Pair productionThis effect occurs at very high radiation energies (above 1.02 MeV). When a highenergy photon collides with the nucleus of the atom, the energy of the photon isabsorbed and produces an electron and a positron. Very soon after, the electron and thepositron collide and both are destroyed but release two photons each with energies of0.5 MeY.
CoIlison andannih~alion
Photons> 1.02 MeV
o o Ejected positron (+)....,. (> ••....- . ,'•....8
.,..- 0.5 MeV.' ~
.•..._-"'-0
0.5 MeV
ee
-._ ..(:.. " Photons
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BS 5650: /978:Specification for opparatusfor gamma radiography.
An exposure head will be aform of collimator.
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UNIT R5 • RADIOGRAPHIC EQUIPMENT
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Classification and types of exposure containerTo comply with BS 5650 (ISO 3999), apparatus for gamma radiography is classifiedaccording to the mobility of the exposure container.
• Class P - A portable exposure container designed to be carried by one man alone.
• Class M - A mobile but not portable exposure container designed to be movedeasily by a suitable means provided for the purpose.
• Class F - A fixed installed exposure container or one with mobility restricted tothe confines of a particular working area.
An exposure container must be provided either with an integral lock or with haspsthrough which separate padlocks can be fitted. The locks must be either lockablewithout the key or an integral lock from which the key cannot be removed when thecontainer is in the working position. On all exposure containers the radiation can onlybe exposed after an unlocking operation.There are a number of different designs for containers, the most common types are:
• Shutter type (Category I).
• Rotating type (Category I).
• Projection type (Category 11).BS 5650 Category J containers are containers from which the sealed source is notremoved for exposure. Category IJ containers are those from which the sealed sourceis projected from the container via a projection sheath (guide tube) to an exposurehead, they may operate electrically, mechanically or pneumatically.Another type of container is the larch type. This type of container should no longer beused because of relatively high radiation doses received by the user and the high risk ofoverexposure.
GAMMA SOURCES
Sealed sourcesThe source of gamma radiation, i.e. the radioisotope, which is typically in disc orcylindrical form, is enclosed in a capsule sometimes referred to as a pill.
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The sources available range in size and configuration from 0.5 mm diameter disc to a4 mm x 4 mm cylinder. Example configurations are:
• Thin discs: typically up to 3.0 mm diameter x 1.0 mm thick. These can be stackedtogether.
• Cylindrical: typically up to 4 mm in length.
• Spherical: 0.6 - 3.0 mm diameter.The capsule is made from either 3 16 S 12 grade stainless steel or titanium.
Titanium is used for Yb169 capsules and is an alternative to stainless steel for Ir192andCo60.
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BS 5650 does not coverapparatus operated by 10removing the sealed sourcefrom the exposurecontainer by using amanual handling devicebecause its use isprohibited in certainnational regulations.
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Rotating type
Shutter and rotating types can now only be used with remote control operation. Theyare mostly used for casting and forgings and give a directional coned beam only.
Torch typeThe container houses the source within a torch assembly and also a short handle. Thehandle is fitted to the torch assembly, this is secured in the main container by a bayonetfixing. As the torch assembly is withdrawn from the container, a spring load plungerpushes part of the assembly down producing a shielding effect so as to produce anarrow beam of radiation. Direct handling of torch assembly types is no longerpermitted. This type of container is now obsolete.
.mnhandle
source holder
30: .
sealed source ~-........•.•.shielding material
Torch type40
Shutter type (Category I type to BS 5650)shutter
50.\
j, :; I--'----'-....shielding /
material II
60 ~~, .•...-
.\ sealed source
70 Shutter type
Rotating type {Category I type to BS 5650)
shieldingmaterial - .---P'" rotates
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X-rays used in industrial radiography are produced from electrical machines usuallyreferred to as x-ray sets; the x-rays themselves being produced from within an x-raytube.
An x-ray tube consists of an evacuated glass bulb, encompassing an anode (the positiveelectrode), and a cathode (the negative electrode). The cathode contains a filamentwithin a curved reflector or focusing cup.
When the filament is heated to a white hot state by a current flow of a few amperes,electrons are emitted and are attracted towards the anode in a concentrated beamformed by the focusing cup. The beam strikes a target set into the anode which resultsin the release of energy; this energy consists of approximately 97-99% heat and 1-3 %x-rays for conventional x-ray tubes up to 300 kV.
X-radiation is also a form of electromagnetic radiation and differs from y rays only inits mechanism of production. While y rays are a product of spontaneous radioactivedecay, x-rays are generally created artificially by an x-ray set. X-rays are producedwhen high speed electrons, produced for example in an x-ray tube, strike a solid target.There are two interactions responsible for the production of x-rays. These are:
Projection type (Category 11type BS 5650)
This type is also known as a remote control or wind out type. The source is attached toa special connector called a pigtail; the pigtail and source are moved along a guide tube
10 by means of a cable until the source reaches the exposure head (which is fixed in theworking position). The cable is driven along by means of a hand-cranked wind outmechanism, or it can be pneumatically or electrically controlled. The cable is retractedto return the source to its container at the end of the exposure.
The projection type can be further classified as an S-type or straight-through type.
handle
iII Ire .
i ~ lock assembly30
40.....'.
source assemblyconnector
S-lube
50 shielding materialsealed source
Projection type
CoUimators60 Collimators are usually used with gamma sources during exposures for safety reasons
and sometimes to improve radiographic quality by reducing scatter from walls orobjects close to the beam.
X-RAY GENERATION70
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Because of the high amount of heat energy produced, the anode is made from copper toconduct away the heat. But, copper has a low melting point, so to prevent the coppermelting, a slip of metal with a high melting point is recessed into the anode at the pointwhich is struck by the electron beam.
a. The incoming electrons have sufficient energy to eject an inner orbitalelectron from the target atoms. An electron from a higher orbit falls into thevacant space that remains in the inner orbit and in doing so emits a pulse ofelectromagnetic radiation, the energy of which is equal to the energydifference between the two orbits. The x-radiation produced by this processis referred to as 'characteristic' x-radiation,
Incoming electrons will also be slowed down by the field of force around thenucleus, and this process again results in the emission of x-radiation. Theradiation produced by this interaction is referred to as 'bremsstrahlung'radiation (bremsstrahlung is German for braking radiation'). Bremsstrahlungradiation is emitted in a wide spectrum of energies.
b.
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Production of x-rays
30
characteristicx-rays
40
path of incoming ebremsstrahtungx-rays
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Thus a typical x-ray energy spectrum will be of a continuous nature and will showcharacteristic spikes at discrete energies that are dependent on the target material andthe difference in the energies of its electron orbits (see figure 9). Except for specialapplications, it is the bremsstrahlung radiation that constitutes most of the x---rnyoutput.
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IGloss envelope
~----------~------~------~~
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.. . . . ." .UNIT U5 . RADIOGRAPHIC EQUIPMENT
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By reversing the half cycle by rectification, this produces full wave rectified d.c ..When used in x-ray sets, FWRC is known as a Graetz circuit.
20
The target serves another purpose, because, the higher the atomic number of theelement struck by electrons, the greater will be the intensity and energy of the x-raysproduced. The target is usually made of tungsten because of its high melting point of3370°C, and its high atomic number of74.
The area on the target which is struck by the electrons is called the focal spot; this areashould be large enough to avoid local overheating, although from the radiographicimage quality point of view, the focal spot should be as small as possible to providegood definition (sharpness) on the radiograph.
Additional cooling is required to cool the anode; gas, oil or water normally beingemployed for this purpose.
The cooling system and the insert are contained together in an earthed, lead linedcontainer, the complete unit commonly being referred to as the x-ray tubehead. Thetubehead is controlled from the control panel.
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ELECTRICAL CIRCUITS IN X-RAY TUBES
A.C. circuit - (self rectified)
/,,,-,I '.I \
40 •.•..'\.
\\
....-...., +, \I \I I
/I
50
The effect of a.c, on the direction of current flow. In an x-ray tube, x-rays can only beproduced when the current is travelling from the cathode (-ve) to the anode (+ve).
60 Graetz circuit
+
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. . .UNIT RS . RADIOGRAPHIC EQl1IPME!,;T
Villard circuit
,-"' ...,, ,I ,
j \I I
j \
I I
.....•..•..••. ,,\,
\I
+-- - ." "
I 'I \
I \I I
,~IIII
\-,,
Another means of obtaining d.c, from a.c, is to use a circuit incorporating diodes andcapacitors in series with the high voltage transformer. This circuit doubles the peak
30 voltage from the transformer and produces a waveform as shown above. Although thewaveform is oscillating, it is all in the -ve half of the cycle and is therefore directcurrent. When used in x-ray sets which use this kind of double waveform, it is knownas a Villard circuit.
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Greinacher circuit - (constant potential)
+
Further improvements can be made to the FWRC waveform by introducing capacitorswhich flatten or smooth the rippling to produce the waveform shown above. When
60 used in x-ray sets, this smooth constant potential (CP) waveform is known as aGreinacher circuit.
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. .U~IT R5 . RADIOGRAPHIC EQUIPMENT
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The x-ray equipment control panelThe three controls that govern a radiographic exposure using x-rays are the timer, themA control and the kV control.
20 ~~o
rox.raJIuk(20 PI ",lnimum cable length)
o
TDp(1Wer2
ra warning l]Slcm
30 4Timer •••--
TimerThe timer is usually calibrated in minutes. The exposure time for an exposure is pre-
50 set; when the equipment is activated, the timer counts down from the pre-set value. Theexposure time will partially govern how much radiation is going to reach the film.
Milliamps (mA)The mA controls the intensity or quantity of x-rays. When the mA is increased, thecurrent flow through the filament is increased, which causes the filament to get hotter
60 resulting in an increase in the intensity of electrons released. The greater the intensityof electrons striking the target, the greater the intensity of the x-rays produced.
The mA control on conventional x-ray equipment may only allow for a maximum of 6to 12 mA to be used, the value being measured across the tube, i.e. between the cathodeand the anode. The value required for a specific exposure is usually pre-set on the
70 panel, this value is usually at, or close to, the maximum mA possible with theequipment for the purpose of minimising exposure time.
Kilovoltage (kV)
The kV governs the wavelength or quality of'x-rays produced which practically governspenetrating power. When the kV is increased, the speed of the electron flow from the
80 cathode to the anode is increased. Therefore, when the electrons strike the target, thekinetic energy is increased, which results in a reduction of wavelength.
An increase in kV, i.e. a shortening of wavelength, has an adverse affect on the contrastand definition of a radiographic image. Certain standard specifications,e.g. BS EN 1435 Radiography of welds, states the maximum kV values for this reason.
90 The kV meters on the control panels for conventional x-ray equipment are peak kVvalues measured across the tube, i.e. between the cathode and the anode. Themaximum kV which can be used is primarily governed by the tubehead; typicalmaximum values are 200 kV, 250 kV and 300 kV. The value required for a specificexposure is usually pre-set on the panel.
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X-Ray Tube Voltage required to penetrate steel of various thicknesses
S Fine-qrain filmT Medium-speed film
T400r-------.--------r--,----.r-----~
20
300
~30
a)ClCI:S.•..'0> 200G>.D.::I->-CI:S
40 ~•><100
50
o 50 75 10025
Penetrated thickness, mm
Note: The curves for voltage are not extended beyond 400 kV as there is nocommercial x-ray equipment in use in this country operating between 400 kV and1000 kV.
Pipeline crawler equipmentMachines have been developed specifically for the radiographic examination ofpipeline welds using either x-ray units or gamma sources. These machines may have apower source attached to the radiation source, i.e. battery pack or generator, or theymay be operated remotely via a cable with the power source outside the pipeline.
Because pipeline crawlers are used inside the pipeline, they are not visible from theoutside of the pipeline, therefore, it is essential that suitable warning signals are givenand are capable of alerting persons in the vicinity of the crawler.
Signals that operate automatically should be linked by some method to the crawler, thisis normally achieved by using sensors linked to warning lights which operate as soon asthey detect ionising radiation. Crawlers available usually have an integrated audiblepre-exposure alarm and an exposure alarm. A separate warning signal is sometimesintegrated when the crawler is in motion.
The useful beam from crawlers should be restricted so that the beam width does notexceed 120 mm at the circumference of the pipe.
Any control isotope used should not exceed 100 J.lSv.h-1 at the accessible surface of thepipe when exposed.
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Betatrons can be manufactured up to 300 MeV and an 11 MeV can penetrate steel up to300 mm thick, but is not transportable.
Portable x-ray betatrons are available with energy outputs up to 6 MeV.
High energy unitsRadiography using x-ray energies of one million electron volts (1 MeV) or greater isconsidered to be in the high energy range.
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Electrostatic generators
The Van de GrafJe electrostatic generator consists of a rapidly moving insulated beltonto which is sprayed an electric charge which is carried to a hemispherical highvoltage terminal. This produces a high voltage difference with respect to the lower end.
Electrically charged particles are made available for acceleration from a heated cathodeand injected into a very high vacuum tube and collimated to bombard special targetsand produce x-rays, The target size is about 2.5 mm.
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Electron linear acceleratorsThese are commonly referred to as linacs or simply linear accelerators. Linacsaccelerate electrons down a guide by means of radio frequency (rf) voltages. Thevoltages are applied so that the electrons reach an acceleration point in the field at aprecise time. The guide consists of a series of cavities which produce gaps when the rfpower is applied. With phased power, the electrons are accelerated along the guide to atarget, the rays energy at the other side.
The energy in electron volts increases with the length of the tube.
The focal spots can be as small as 0.1 mm.
As an example, the 100 mm thick steel shell of a nuclear reactor at a power station inWales was radiographed at a distance of9 m using ultrafine grain film with a 20 minuteexposure. Each exposure covered 3 m of weld. The 4 MeV linac was mountedcentralIy on a rotating stand in the centre of the shell.
This 4 MeV was transportable and could readily be moved with lifting equipment.
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The BetatronThis machine is based on the same principle as the linac but the electron guide is aspiral. This means that the path of the electrons can be increased over a smaller overallarea.
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Half value layer (HVL) isalternative terminology \0used.
The tenth value thicknesses 90(TVT) 0/ a material willreduce the radiationintensity by one tenth.
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UNIT R6 . HALF YALUE THICK!\'ESS
20
The lower the kV (longer the wavelength), the higher the subject contrast and thereforethe higher the radiographic contrast.
Note: The radiographic density produced in Figure J will be lower than Figure 2 if theexposures are identical, so it is assumed that the exposure time for Figure I is higher tocompensate in order to give the same density on either side A or side B.
The half value thickness (HVT) of a material may be used as a guide for determiningthe thickness of a material to be used for shielding from radiation, e.g. for theconstruction of a radiation work bay in a factory.
The HVT of a specific material is the thickness which cuts down the radiation intensityby one half.
If the initial intensity of radiation increases, e.g. by increasing the mA when using x-rayequipment, the HVT will remain the same. However, if the wavelength (penetratingpower) of the radiation is changed, e.g. by changing kV or isotope type, the HVT of aspecific material will alter.
The following table shows examples of the HVT for lead, concrete and steel.
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Energy Lead Steel ConcreteHVT(mm) HVT(mm) HVT(mm)
l50kV 0.3 4 22200kV 0.5 6 26250kV 1.0 12 28300 kV 1.5 15 31lrl92 6 13 40Co60 12 20 65
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SO
The HVT of a material can also be used to explain subject contrast in relation towavelength (kV):
Figure J shows that side A of the specimen has four times the intensity of radiationemerging from it in comparison with side B.
Figure 2 shows that side A of the specimen has two times the intensity of radiationemerging from it in comparison with side B.
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Figure 1 - 200 kV - steel
!! 1 16R !!! !!! 16R !! !
12mmn-u ~~~~~~~~~~~TVTI':i,..----Luu-uuIH~l~!! !!
4R IR 8R 4R
Figure 2 - 250 kV - steel
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Therefore, the resultant radiograph from the specimen in Figure I will display higher80 radiographic contrast (because of an increase in subject contrast) compared to the
radiograph produced in Figure 2.
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The base is normally tintedblue and will thereforepossess some density, i.e. thebase of a film is not totallytransparent.
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Vl\IT R7 • RAI>IOGRAPHIC FILM
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Film emulsion is produced by mixing solutions of silver nitrate and salts, such aspotassium bromide, with a solution of gelatine. The rate and temperature of mixinggoverns the grain size; rapid mixing at low temperature produces the finest grainstructure, whereas slow mixing at high temperature produces emulsions with largergrains. When large grain structures are required, to produce a fast emulsion, somesilver iodide is usually included in the formula.
The sizes of these crystals and the distribution, effect the final radiographicquality/appearance; the larger the crystal size the greater the sensitivity to radiation.Various shapes of crystals exist, but these shapes have virtually no effect on the finalimage.
The reason for two layers of emulsion is to give a faster film speed, i.e. the radiographscan be produced quicker, and higher radiographic contrast.
THE MAKE-UP OF A RADIOGRAPHIC FILM10 Radiographic film is usually made up of seven layers: a central base layer and three
coatings on either side consisting of a subbing layer, emulsion and supercoat.
30 BaseThe physical characteristics of emulsion do not allow it to be used by itself withoutsupport, therefore it is applied to a substrate known as the base. The base must betransparent, chemically inert and must not be susceptible to expansion and contraction.Glass is an ideal substrate to meet these requirements, but for applications where theobjects to be radiographed are curved, e.g. on pipes, it is necessary for a flexible baseto be used. Polyester and cellulose triacetate, although not quite as stable as glass, arewidely employed for such applications.
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Subbing layer (substratum)The subbing layers adhere the emulsion to the base; the material employed for this isgelatine plus a base solvent.
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EmulsionThe layers of primary importance are the two emulsion layers. These layers consist ofmillions of silver halide crystals (usually silver bromide); the sizes of the crystals areusually between 0.1 and 1.0 micrometers (urn) and are suspended in a gelatine bindingmedium.
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Supercoat (anti-abrasion layer)90 Radiographic emulsion is susceptible to mechanical and chemical damage, so to
prevent, or at least reduce this, the emulsion is coated with a layer of hardened gelatine.
Although the supercoat otTers some protection against chemical attack., e.g. oil fromthe skin during handling, it must allow for chemical reactions to take place in theprocessing tanks.
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U~IT R7 • RADIOGRAlll-llC FILM
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Manufacturer Name Speed Grain Film Factor
Agfa Gevaert RCF Fast CoarseDupont NDT91 Fast Coarse
Dupont NDT75 Medium Fine 20Kodak CX Medium Fine 25Kodak AX Medium Fine 30Agfa Gevaert 07 Medium Fine 35
Oupont NOT 55 Slow Very fine 80Agfa Gevaert D4 Slow Very fme 95Kodak MX Slow Very fme 120
Agfa Gevaert 02 Very slow Ultra fine 200
FILMTVPESRadiographic film may be graded in terms of grain size or speed:
• Ultra fine grain - exceptional radiographic quality but very slow speed.
• Fine grain - slow speed.• Medium grain - medium speed.• Coarse grain - poor radiographic quality but fast speed.
Radiographic films are also divided into two types: direct-type or salt screen type.
Direct-type films are intended for direct exposure to gamma or x-rays or for exposureusing lead intensifying screens. Some of these films may be suitable for use withfluorometallic or salt (fluorescent) intensifying screens.
Salt screen type films are designed to be used exclusively with salt (fluorescent)intensifying screens. They are able to produce radiographs with minimum exposureand are widely used in medical radiography.
FILM SPEED
40 A film factor is a number which relates to the speed of a particular film and is obtainedfrom a films characteristic curve.
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The SCRATA scale is a scale often used for film factors; the smaller the film factor thefaster the film. Film manufacturers may have their own scale which may work in thesame or opposite way to the SCRATA scale.
Example to the SCRA TA scale:
A film with a factor of 10 will be twice as fast compared to a film with a factor of20.This means to say of the film with a factor of20 took four minutes to expose, then thefilm with a factor of 10 will require two minutes to give the same density.
Types of film with their corresponding SCRATA film factors:60
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RCF & Iluorometallic screensWhen characteristic curves of various films are superimposed on one graph, it will beseen that the faster films lie closer to the left vertical axis, because faster films attaindensity at lower exposures. Therefore, it should be appreciated that it is possible toobtain the relative film factors from the characteristic curves of films.
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A characteristic curve is a curve on a graph produced for a particular film which showsthe relationship between different exposures applied and the resulting densities.
Information which can be gained from a characteristic curve is as follows:
a. The position of the curve on the exposure axis gives information on film speed.b. The gradient on the curve gives information about film contrast - a high contrast
film will display a steep gradient.c. The position of the straight line portion of the curve against the density axis will
show the density range within which the film contrast will be at its highest (usuallyoptimum).
d. A new exposure time can be determined for a change of film type. For example, itwould be possible to determine the new exposure for film type x in order to achievea density of3.0, if the exposure for film type y was 5 mA-mins to achieve a densityof2.0.
30A characteristic curve will also show that the density does not vary in the sameproportion as the applied exposure.
A curve is produced by applying increasing exposures to adjacent areas of a strip offilm. After development, the densities are measured with a densitometer and thenplotted on a graph against the corresponding exposures. Both the vertical axis (density)and horizontal axis (exposure) are calibrated in a logarithmic scale (logloE); thismethod is the most practical method for the size and interpretation of a curve. Whenthe points obtained are joined together a curve will be produced.
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Sensitometric curve of STRUCTURIXAutomatic processing: 8 minutes cycle using developer G 121/G 135 at 29-300
50 RC1 pi D4 021
3.5
I I 3.0
I I I 2.5
/ / j I 20
/ / / / 1,5
1/ V / I1 /1,0
/ / / 1/ ~liizwCl 0,5
~
V / / V--- - LOG. EL. EXP.
1,0 2,0 3,0
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Other metallic screens existfor less common 20applications.
Scatter radiation has a 50longer wavelength than theprimary beam/ram which itis derived and is thereforeless penetrating.
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UI\'IT R9 • INTEI\'SIFYING SCREENS
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I. High definition (fine grain) screens.
2. High speed or rapid screens.
GENERALA radiographic film is normally sandwiched between two intensifying screens whenexposed to x-rays or gamma rays. Intensifying screens have an extra photographiceffect on the emulsion thereby reducing the exposure needed to attain the requireddensity.
There are three main types of intensifying screens:
1. Lead screens.
2. Fluorescent (salt) screens.
3. FluorometaJlic screens.
Close contact between screens and film is essential in order to obtain sharp images.Screens must be kept free from dust and scratches, if this is not done they may be seenas light indications on the radiographic image - especially if using fluorometallic or
30 fluorescent screens.
LEAD SCREENSLead screens consist of a thin lead foil of uniform thickness, usually stuck onto a thinbase card in the case of reusable screens, or stuck onto a thin sheet of paper when used
40 with pre-packed film.
Lead screens intensify the image by emitting beta radiation (electrons) when struck byx-rays or gamma rays of sufficient energy. The intensification action is only achievedwith x-rays above approximately 120 kV and gamma rays above similar energy levels.
Lead screens will also improve the radiographic image by partially filtering out scatterradiation.
60
Two lead screens are used to sandwich the film; the thickness of the front screen mustbe matched to the wavelength of radiation being used, so that it will pass the primaryradiation while stopping as much of the secondary radiation as possible. The rear screencuts down the effect of back scattered radiation.
If it is technically feasible, it is better to use screens of the same thickness, thusavoiding the problem of accidentally loading a film cassette with the rear screen at thefront. Screen thicknesses are usually between 0.02 mm and 0.15 mm.
Lead screens are pliable and should be handled with care if buckling is to be avoided.If the lead screens are to be used more than once, e.g. in cassettes as opposed to rollfilm or pre-packed film, they become dusty and should be frequently dusted with a finebrush. If screens become too dirty or splashed with liquid, they may be cleaned withcotton wool damped with a weak detergent solution. When the screens become tooscratched or dirty causing the radiographic quality to be impaired, they should bereplaced by new screens.
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80 FLUORESCENT (SALT)SCREENSFluorescent screens are made up from micro crystals of a suitable metallic salt, usuallycalcium tungstate, applied to a supporting thin base card.
These screens, when subjected to x-rays or gamma rays, emit light radiation to whichthe film is sensitive. This light radiation results in a large increase of effectiveradiation.
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UNIT R9 • INTENSIFYING SCREENS
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Screen type Order of Order of Intensification How intensification isimage quality speed factor achieved
Lead 1 3 2-4 Beta particlesFluorescent 4 1 8-15 Light radiation and UV
Fluorometallic 3 2 5-10 Light radiation, UVand beta particles
None 2 4 N/A N/A
A radiograph obtained using fluorescent screens will have a grainy appearance due tothe screens salt grains resulting in low definition compared to a radiograph taken usinglead screens or no screens at all.
10 Because of the resulting loss of image quality, fluorescent screens are only used toavoid excessively long exposure times, e.g. on very thick specimens.
FLUOROMETALLIC SCREENS20 Fluorometallic screens are a combination of a salt screen and a lead screen; they are
made up of from a base card, a lead layer, a salt layer (calcium tungstate) and a thinprotective layer.
There is more than one type of fluorometaIlic screen:
• Type I - for x-rays up to 300 kV.
• Type 2 - for x-rays 300-1000 kV, Ir 192.• Type 3 - for C060.
40
Providing the correct type of fluorometallic screen and film are used with the range of ~radiation being used, substantial reductions in exposure time or kV can be achieved.Because the lead layer will partially filter out scatter radiation, the image produced onthe radiograph will be better than one obtained using fluorescent screens, but the imagewill still retain a grainy appearance due to the salt crystals.
These screens are not commonly used due to high cost. Their application is similar tothose applications where fluorescent screens may be used, i.e. on thick specimens.
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COMP ARISON OF INTENSIFYING SCREENS
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Actinic radiation. in thiscontext. is that which willaffect the film emulsion.i.e. form a latent image.
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UNIT RIO· IMAGE FORMATION
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When radiation passes through an object it is differentially absorbed depending uponthe thickness and any differing material densities. The radiation finally emerging at thefilm side of the object will largely determine the final characteristics of the radiograph.
The portions of radiographic film which receive sufficient quantities of actinic radiationundergo minute changes. These changes are so small they are invisible to the nakedeye and also invisible when using conventional microscopes; this hidden image isknown as the latent image. The latent image can be defined as the hidden image on aradiographic film after exposure to actinic radiation but before development.
Therefore, radiation alone does not convert a radiographic film into a visible readableimage. The sequence of processes to attain a radiographic image are as follows:
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1. The silver halide crystals which have absorbed a sufficient quantity of radiation arepartially converted into metallic silver - this is the latent image.
2. The affected crystals are then essentially amplified by the developer; the developer30 completely converts the affected crystals into metallic silver.
3. The radiograph attains its final appearance by fixation; the fixer removes theunexposed and therefore undeveloped crystals.
4. Washing removes the chemicals (fixer).
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Viewer.
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Processing of radiographs may be carried out manually or by using automatic processors.
Manual processing takes place in a darkroom under the illumination of safelights whichusually consist of ordinary light bulbs behind orange filters. Other colours for filters exist,but the colour chosen must emit light of a wavelength which does not detrimentally affectthe emulsion.
The darkroom should preferably be divided into two sides, a dry side for loading andunloading of cassettes and a wet side for processing; this is so the films are not splashedprior to development. The wet side of the darkroom will usually have five tanks arrangedin the following sequence:
I. Developer tank.
2. Stopbath or rinse tank.
3. Fixer tank.
4. Final wash tank.]05. Wetting agent tank.
When the exposed film has been unloaded from its cassette, it is placed into aframe (orspiral if its a long film) and placed into the developer.
40 DARKROOMS
General rulesDarkrooms must be light-tight, must be kept clean and everything must be kept in its place.
50LayoutThe loading bench (the dry side) must be on the opposite side to the processing tanks (thewet side). The distance between should be wide enough for two people to pass. Theloading bench should have storage space (drawers and cupboards) underneath for films,chemicals etc ..
There must be at least one central white light and two safelights, one over the loadingbench, one over the processing tanks.
There must be electric sockets conveniently placed for extra electrical equipment.
There must be ventilation baffled against light and an exhaust fan, also baffled.
The entrance door should be spring loaded for self-closing and baffled all round againstlight.
The entrance door should be lockable from the outside but not from the inside.
The darkroom walls should be painted washable white or cream, except for the walls by theentrance which should be matt black.
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80 ServicesAn electric supply is essential (mains or generator).
A running water supply is desirable but in some cases on isolated sites, water may have tobe carried.
90 EquipmentProcessing tanks - There should be a minimum of four processing tanks; one for developer,one for rinse, one for fixer and one twice as large for the wash. An extra tank is desirablefor wetting agent.
Drying cabinet - Desirable but not essential for a low output of radiographs.
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Immersion heater - plunger type.
Timer.
Film hangers.
Film clips.
Cassettes.
Screens.
Films.
Chemicals - Developer, replenisher and fixer.
Miscellaneous items - Plastic bucket, mop, swabs, brush, paper towels, large waste paperbasket or box and a chair.
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30 Layout of a typical industrial darkroom
DRY SIDE WET SIDE
DEV40
STRIPLIGHTS
S
Beloware DRY BENCHcupboards (For loading &for unloading filmstoring cassettes)cassettes,films &chemicals
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60 S
70DRYER
LIGHT TRAP I'~RED WARNING LIGHT
[!] WALL MOUNTED SAFELlGHTS
~ SAFElIGHTS SUSPENDED FROM CEILING FOR GENERAL ILLUMINATION
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Washing film in hot weatherIn the summer, excessive washing should be avoided. Prolonged immersion in warm watermay cause the emulsion to frill. To determine the correct rate of water flow, measure thetime required to refill the tank after removing a given quantity of water and adjust the flowso that water in the tank changes at least 10 times each hour.
PROCESSINGRadiation causes a latent image to form on the film. A latent image cannot be discerned
10 with the naked eye.
Developing changes the latent image into a visual image by blackening the irradiated silverhalides.
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Stop bath or rinse stops the action of the developer by neutralisation and removes thesurplus chemicals.
Fixer removes unaffected silver halides and hardens the gelatine.
Final wash removes all chemicals from the film, preventing chemical fogging.
Developer> film to be developed for 4 minutes at 68°F (20°C) regularly agitated. It shouldbe topped up with replenisher and changed after twice its own volume of replenisher hasbeen added. Concentrated developer is mixed to a dilution of I part plus 4 parts water butwhen used as a replenisher, the ratio is I part plus 3 parts of water, i.e. I gallon ofconcentrate makes 5 gallons of developer.
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Hot weather processing. Through the summer months, darkrooms and chemical solutions frequently get warmer than
normal. For best results, the developer, fixer and wash water should be kept at the sametemperature. Ice should not be placed in the solution because excessive dilution will resultas the ice melts.Although processing films in hot solution is not recommended, satisfactory radiographs canbe produced in solution up to 35°C. Water temperatures can shoot up to dangerous heights,even in air conditioned darkrooms.Prolonged washing at high temperatures may damage film, therefore, if the water is toowarm, washing must be kept to a minimum.Automatic water mixes will require watching too, they cannot keep water any cooler thanthe temperature of the cold water supply.
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60RestrainerWith temperatures up to 24°C, no extra precautions are needed. However, whentemperatures range between 27°C and 35°C, restrainer can be added to the developer.
A restrainer for developing solutions is made up of 18 g of sodium bicarbonate per litre ofdiluted developer, or 4.5 g of concentrated solution.
The total amount of proper restrainer needed for a full tank of developer should be weighedout and then dissolved in approximately 200 ml of warm water. The resulting solutionshould be added to the developer and the mixture stirred thoroughly.
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RinseThe gelatin in the emulsion swells more in warm solutions and absorbs more developer.Therefore longer rinsing times are required at higher temperatures. Poorly rinsed filmscarry more alkali into the fixer and thereby reduce the speed and hardening action of thefixer.
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Fixing at high temperaturesA fixing bath that contains an acid hardener minimises the tendency of the emulsion to frillduring the final washing. Even when rinsing is done carefully, the fixer acidity declineswith use. The addition of fixer replenisher will maintain pH 4.5 and the fixer's hardeningability.
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Developer
Constituents Action Chemicals incommon use
DevelopingPreferentially reduces the exposed Metol.silver halide crystals (+ve ions) to Hydroquinone.
agent(s) black metallic silver. Phenidone
A chemical which gives an alkaline Borax.Accelerator Sodium carbonate.reaction which speeds up development Sodium hydroxide.Preservative Prevents oxidation of the developer. Sodium sulphate.
RestrainerControls the level of development Potassium bromide.fogging.
Sequestering Prevents the formation of scale. Sodium.a~ent Hesarnetaphosphate.
Drying film in hot weather
The high relative humidity generally prevailing in hot weather increases the time requiredto dry an Xsray film. Three of the factors that affect drying time are:
I. the degree to which the film has been hardened in the fixer;
2. the length oftime it was washed;
3. the water absorbing property of the gelatin used to make the emulsion.
Methods of controlling the first two factors have been described previously. Faster20 processible film is recommended, especially because it absorbs a minimum of water.
Overnight coolingIn laboratories where 10 - 20 litre solution tanks are used, the following recommendationsmay prove useful.
30 Before closing the laboratory for the day, remove 4 litres of developer and 4 litres of fixerand place them in separate labelled glass containers. Store them in a refrigerator overnightand in the morning, add chilled solutions to the warm solution to bring the workingtemperature closer to normal. Make certain the bottles are dedicated and correctly labelled.~--
40 DEVELOPER
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Developer is an alkali and is usually supplied as a liquid concentrate and is diluted withwater at a ratio governed by the manufacturers instructions, e.g. 1 part developer to 4parts water.
Developer temperature and development time should be in accordance with themanufacturers recommendations or specification, but for manual processing is typically 20°± I °C for 4 to 5 minutes. The time should be taken from when the film hits the developerwith a suitable darkroom timer.
Once the film is in the developer it is agitated for approximately 20 seconds and then forapproximately 10 seconds every minute. Agitation allows for fresh developer to flow overthe film and prevents the possibility of bromide streaking; agitation also cuts downdevelopment time. The developer supplies a source of electrons (-ve ions) which cause thechemical changes in the emulsion. The frames or spirals should be tapped against the tanksto prevents any air bubbles settling on the film which can cause light spots on the fmishedradiograph.
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If/he crystals are unexposedthey will not have beendeveloped. 70
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UI\IT RII . FILM PROCESSI~G
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Fixer is an acid which is supplied as a liquid concentrate and is to be diluted with water,typically at a ratio of I part fixer to 3 parts water (follow manufacturers instructions); ahardening agent is also added.
Fixation is the process which removes the undeveloped silver halide crystals and fixes theremaining developed crystals, thereby producing radiographs of a diagnostic (readable)quality.
The fixer contains chemicals, e.g. ammonium or sodium thiosulphate, which convert theunwanted unexposed halides into water soluble compounds; they are then readily dissolvedor removed at the fmal wash stage.
The films must be agitated in the fixer, failing to do so may result in light spots on the film.The fixing time is twice the time it takes for the image to clear, e.g. if the milky imagedisappears in 3 minutes, after looking under the illumination of the safe lights, the films arereturned to the fixing tank for another 3 minutes, i.e. total fixing time 6 minutes.
When the fixer becomes exhausted, e.g. as a guideline - when the fixing time is over 10minutes, the fixer should be replaced. Fixers are not usually replenished. The exhaustedfixer is retained because silver may be reclaimed via electrolysis methods.
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ReplenishmentThe activity of the developer gradually decreases with use and age. Replenishment ensuresthat the activity of the developer and the developing time required remains constantthroughout the useful life of the developer. When approximately I m2 of film has beendeveloped, about 400 ml (2 cups) of replenish er needs to be added.
After continuous replenishment the quality of the image will be affected and the developerwill have to be changed. A common guide for the remixing time is when the replenisheradded exceeds twice the volume of the original developer.
STOPBATHThe stopbath may be:
30 • An acid stopbath.
• A water spray rinse.• A fresh water tank.
40The most efficient type of stopbath is an acid stopbatb which is typically made up of 2%glacial acetic acid in water. This stops the reaction of the developer, due to the developerbeing an alkali and the stopbath an acid.
Films should be placed and agitated in the stopbathlrinse tank for at least 10 seconds; if thisis not done properly, the fixer will soon become neutralised.
50 FIXER
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FINAL WASH90 Films should be washed preferably in a tank with constant running water, for at least 20
minutes. This removes any soluble silver compounds left behind in the emulsion afterfixing and removes the fixer which is an acid. Yellow fog appears on films which have notbeen sufficiently washed.
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Wetting agent reduces the surface tension of the water and results in even drying of thefilm; this prevents black spots or streaks. Wetting agents are supplied as a liquidconcentrate and is to be diluted with water at a ratio of approximately I part wetting agentto 4000 parts of water.
Films are only dipped in and out of the wetting agent.
WETTING AGENT
20 DRYING THE FILMInitially excess water is removed from the films with a squeegee and then placed in either adrying cabinet, other specially designed drying apparatus or a dust free drying room. Caremust be taken not to allow drops of water to fall onto the drying films, otherwise blackmarks will remain on the radiograph.
30 The drying time will depend on the temperature, air circulation and the relative humidity ofthe warm air. Typical drying times are 15 minutes in a drying cabinet, 45 minutes in adrying room.
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The viewer must be capable 70of white light intensitiessuitablefor viewingradiographs up to themaximum permissibledensities.
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UNIT nI2 . nADIOGRAPHIC QUALITY
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Density % light transmitted throughthe radiograph
l.0 10%2.0 1%3.0 0.1%4.0 0.01%5.0 0.001%
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Radiographic quality can be discussed using four main terms:
I. Density - The density of a radiograph relates its degree of blackness.
2. Contrast - Radiographic contrast is the degree of difference between density fields ona radiograph.
3. Definition - Radiographic definition is the degree of sharpness at the boundaries ofdensity fields.
4. Sensitivity - Sensitivity is a term used to give an indication of overall radiographicquality.
There are two qualities of a radiograph usually measured: density and sensitivity. Densityis measured using a densitometer and sensitivity is measured using an image qualityindicator (IQI).
Sensitivity measurements give an overall guide as to the radiographic technique's ability todetect fine defects. Sensitivity is affected directly by the contrast and definition, i.e. ifeither of these qualities are lacking then the sensitivity is lacking.30
DENSITY
40The density of a radiograph relates its degree of blackness.
A high density or dark area absorbs more light than a low density or light area. The greaterthe amount of black metallic silver grains present in an area on a radiograph, the more lightis absorbed and the denser the area appears.
More radiation passes through the thinner sections of a specimen, e.g. areas where cracksor lack of fusion are present, therefore these areas will eventually show up on theradiograph as dark (dense) areas.50
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Measuring densityDensity on a processed radiograph is measured using an instrument called a densitometer,this compares the incident light (I.) with the transmitted fight (IJ and expresses the result asa logarithmic ratio. Incident light is light from the viewer; transmitted light is lighttransmitted through a film when the film is on the viewer.
~Density = LOglOIt
Example:
If the incident light was 100 times greater than the transmitted light:
Density = LoglO100I
80Density = 2.0
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BS EN /435 states that theminimum optical densityshall be greater or equal to2.0 or 2.3. depending on theclass. 30
Latitude: The range ofthicknesses which can beviewed on a radiograph.e.g. C060 gives goodlatitude. Low k V x-ray givespoor latitude
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UNIT IU2 • RADIOGRAPHIC QUALITY
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Note: We are assuming that there are thickness changes or material density changes presentin order to display density changes.
The following chart shows the criteria which affect radiographic contrast:
The ratio of transmitted light for densities of 1.0 and 2.0 is a factor of 10, i.e. 10 times mor,light passes through the radiograph for a density of 1.0 than for a density of 2.0.
The ratio of transmitted light for densities of 1.0 and 3.0 is a factor of 100, i.e. 100 times10 more light passes through the radiograph for a density of 1.0 than for a density of3.
Before use, densitometers should be calibrated using a calibrated density strip - a strip offilm containing known densities on the same viewer which is to be used for interpreting theradiograph.
The minimum density in the area of interest, i.e. the weld, required by specifications istypically between 1.5 and 2.5. However, this is not always practical to determine when thearea of interest has many thickness changes and therefore density changes - as is the casewith certain types of m.m.a. welds. In this situation the specification may specify that thedensity is to be measured inunediately adjacent to the weld reinforcement.
The maximum density stated in a specification will typically be 3.0 or 3.5.
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Lack of density - causes• Under exposure to radiation.• Insufficient development time.• Developer temperature too low.• Exhausted developer.
• Incorrect developer.
• Solution of developer too weak.
50Excessive density - causes
• Over exposure to radiation.
• Excessive development time.
• Developer temperature too high.
• Incorrect developer.
• Solution of developer too strong.60
RADIOGRAPHIC CONTRAST
70Radiographic contrast is the degree of difference between density fields on a radiograph. .........••When a radiograph contains only blacks and whites and no intermediate tones the contrast.high; when only tones of a similar density exist the contrast is low; the optimum contrastmay lie between these two extremes, it depends on the aim of the radiographic technique.
If an application specification is not permitting any detected defects in the weld whatsoever,then the contrast should ideally be as high as possible, i.e. high contrast is ideal fordetecting defects.
If, however, an application specification permitted certain defects, depending on the defectsthrough thickness dimensions, as well as length and/or width, then it would be necessary tohave a range of tones on the radiographs so that the through thickness depth of the defectsand the height of weld reinforcements can be assessed.
Therefore, to gain more information about the through thickness dimensions of any defectsand the weld itself, we need to have intermediate tones, i.e. greys in addition to black andwhites.
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Measuring radiographic definitionRadiographic defmition is not usually measured exclusively; it is normally assessedsubjectively, but can be measured by the use of a duplex type IQI.
Radiographic contrast
ISubject contrast
IAffected by:a. Thickness differences in specimenb. Radiation qualityc. Scattered radiation
IFilm contrast
Affected by:a. Type of filmb. Development time, temperature
and agitationc. Activity of the developer
20 Subject contrast is the ratio of x-ray or gamma ray intensities transmitted by two selectedportions of a specimen. Subject contrast depends on the nature of the specimen, thewavelength of the radiation used and the intensity and distribution of the scattered radiationbut is independent of time, milliamperage of source strength, distance and thecharacteristics or treatment of the film.
Film contrast refers to the slope (steepness) of the characteristic curve of the film. Itdepends on the type of film, the processing it receives and the density. It also depends onwhether the film's exposure is direct, with lead screens or with fluorescent screens. Filmcontrast is independent, for most practical purposes, of the wavelengths and distribution ofthe radiation reaching the film and hence is independent of subject contrast.
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40 Measuring radiographic contrastRadiographic contrast is not usually measured exclusively; it is normally assessedsubjectively, but could be measured by the use of a step wedge type J.Q.I..
A wire type IQI used to assess sensitivity primarily gives information about theradiographic contrast, but the degree of definition also affects the result.
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Insufficient contrast - causes• Radiation wavelength too short, Le. kVIpenetrating power too high.• Over exposure to radiation, compensated for by shortened development time.• Prolonged development in too cold a developer or exhausted developer.
60 • Unsuitable or wrongly mixed developer.• Insufficient fixation.
• Fog.
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Excessive contrast - causes• Radiation wavelength too long, i.e. kV/penetrating power too low.• Incorrect developer.• Wrongly mixed developer.• Under exposure, compensated for by a prolonged developer.
80 DEFINITIONRadiographic definition is the degree of sharpness at the boundaries of density fields.
There are many factors that govern the final definition on a radiograph, including thegeometry of the set-up during exposure and the film type used. Perfect defmition can neverbe obtained due to the existence of penumbra and the films inherent unsharpness.
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1. Place a lead sheet, approximately 4 mm thick containing a small hole about 0.25 mmdiameter, exactly halfway between the focal spot and a radiographic film.
2. Expose - the exposure should not be excessive otherwise the image will be blurred.The image on the film will be the size of the focal spot plus twice the diameter of thehole.
3. Calculate the focal spot size by measuring the total diameter of the image and thendeduct 2 x hole diameter.
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-r-,A duplex type IQJ (BS EN 462 : Part 5) - Image quality indicators (Duplex) consists o.pairs of parallel platinum or tungsten wires of decreasing thickness, the thickness of thepairs usually being the same as the gap between them. If a pair of parallel wires blend intoone on the radiographic image it will be due to the poor definition. The largest pair ofwires, the image of which has just merged from that of two separate wires into the singleform, is taken as the criterion of discemability.
Unsharpness is given in BS EN 462 : Part 5 as U = 2d, where d is the width of the wire andthe wire spacing distance.
Note: Duplex IQIs are placed on the source side of the object being examined and alignedas closely as possible to the axis of the radiation beam.
Radiographic definitionI
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IGeometrical factors
IAffected by:a. Focal spot sizeb. Focus film distancec. Specimen film distanced. Abrupt thickness changes in specimene. Screen film contact
Affected by:a. Type of filmb. Type of screenc. Radiation qualityd. Development
IGraininess factors
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Inherent (film) unsharpnessInherent unsharpness is the unsharpness on a radiograph caused by stray electronstransmitted from exposed crystals which have affected adjacent crystals. Inherentunsharpness always exists, its magnitude depending on grain size, grain distribution andradiation energy used; it increases with a reduction in wavelength.
Geometric unsharpness (Ug)Geometric unsharpness or penumbra is the unsharpness on a radiograph caused by thegeometry of the radiation beam in relation to the object being radiographed and the film.Penumbra always exists and borders all density fields.
The dimensions of the focal spot or gamma source, object to film distance (o.f.d.) and focalspot to film distance (f.f.d.) all affect penumbra.
To minimise penumbra we must adhere to the following conditions:
70 • The source or focal spot should be as small as possible.• D.f.d. should be as small as possible.
• F.f.d.ls.f.d. should be as long as practicable.
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Determination of focal spot sizeThe focal spot size of x-ray tubes can change over a period of time. To determine the sizeof the focal spot, e.g. for penumbra calculations, the following procedure may be adopted.
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Using the nomogramBS EN 1435 uses a nomogram which is based on calculations for minimum Ug. The Ug isnot stated but using the nomogram gives minimum source to object (sod) distances whichwill give acceptable Ug.
Calculation of geometric unsha rpness (Ug)
sxofdUg=
sfd - ofd (SOD)
Where: s = the maximum dimension of the gamma source or focal spot.
This is calculated using the Pythagorus theorem, e.g.:
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2 mmdiameter
AI2mmL.-J length
s= =2.82 mm
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ofd = object to film distancesfd = source to film distancesod = source to object distance
Note: sod + ofd = sfd
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s------------SOURCEI\7\------------1-
1/ X \
f I \ \I I \ \II \\ sed
srd / I \ \ 1f/ \~;'1 \\OBIE;I \;----T
If \\ ord
If \\ I60
FILM
70The maximum penumbra allowed on radiographs is specified in certain standards. Incontractual situations where the standards do not quote maximum penumbra values, theymay be agreed with the client; a typical maximum penumbra of 0.25 mm is often used.
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10
b = OFD = sample thickness
10.\1"
20
8
7
6
s
1
30
" 2
40
0.550
1) Class B
--......
50001101400
)00
2005000
"'101 2000
3000100
2000 100010
601000 soo
SO
)00 ~o500 30
200
)0020 1
200 100 "'c <Ici i'- "-
lOO SO 10
)0W 6
20 S
)0 4
20 10 3
10 S
2) Class A
60Figure 21. Nomogram for the determination of minimum source-to-object distancefmin in relation to the object-to-liIm distance and the source size.
d = source size
70PROCESSING AND HANDLING FAULTS
FogFog is unwanted density on a radiograph and appears on radiographs as darkened areas oroverall darkening which has not been caused by thickness variations in the subject.
80Grey fog
• Accidental exposure to actinic radiation light, x-rays, gamma rays. When fog iscaused by light leaks, e.g. because ofa faulty cassette, it is often termed lighlfog.
• Scatter.
• Unsuitable darkroom lighting, e.g. wrong safelights, white light enteringdarkroom.
• Film exposed to heat.• Bad film storage.
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Light patches - possible causes• Film was not agitated/tapped during development or fixation.• Film insufficiently rinsed after development.
• Drops of fixer fell onto film prior to development.• Mechanical damage causing pressure marks to emulsion before exposure.• Impurities between screen and film.
• Marked or cracked fluorescent screens.
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Yellow fog
• Insufficient final wash.• Exhausted fixer.• Prolonged development in badly oxidized developer.
Dichroic rog
• Greenish colour by reflected light, pink via transmitted light.
• Prolonged development in exhausted developing bath.
• Film stuck to another film in fixer.• Developing tank contaminated with fixer.
Mottled fog
• Film badly stored, e.g. in damp surroundings.
• Film out of date.30
ARTIFACTS
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An artifact is a spurious indication on the radiographic image, e.g. a fault in or on the filmusually caused by mishandling or incorrect developing. An artifact may be mistaken for adefect in the weld or parent material; an artifact may also mask a fault in the weld,therefore, it is essential that artifacts should be avoided.
Static dischargeStatic discharge marks may occur when the film is pulled quickly from between the
50 intensifying screens in a dry atmosphere. The appearance on the radiograph is usuallylightning like, but it may also be mottled.
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ReticulationReticulation is a net like structure appearing in the emulsion due to rupture caused byexcessive temperature differences between the processing tanks. It is a rare artifactnowadays to the flexible/plastic nature of modem day emulsions.
Diffraction mottleDiffraction mottle may occur in a weld area on a radiographic image because of the grainstructure and grain orientation of certain materials matching the wavelength of the radiation
70 in a certain way. Austenitic stainless steels and aluminium welds are particularlysusceptible.
Diffraction mottle has the-appearance of fme porosity throughout the weld area. It may bereduced or eliminated by changing the wavelength of radiation, i.e. increasing kV, or bychanging the radiation angle by approximately 5°.
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The wire gauges and range of wires used in IQl's for BS EN 462-1 are the same as DIN 62,i.e. a DIN 62 10-16 has become an EN 462 WIO. The WI3 was added to increase the rangeto cope with thinner materials. This covers the wire gauges 13 to 19.
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Dark patches, lines or streaks - possible causes• Drops of developer fallen onto film prior to development.• Drops of water fallen onto film prior to development.• Mechanical damage causing pressure marks to emulsion after exposure.• Buckled or scratched lead screens.• Slow and uneven drying of film, i.e. when there are still droplets of water on the
film.• Uneven drying.• Bending of film after exposure (usually between two fingers causes dark crescent
shaped marks).
Whitish deposit - possible causes• Water used to make up processing solutions too hard.
Solarisation• Solarisation is lightening of the image, or reversal, due to exposure to white light
whilst the film is in the developer.30
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SENSITIVITYThe term sensitivity, when used in a general sense, is the ability to detect small changeT"The term sensitivity when applied to radiography is an overall assessment of qualitywhich relates to the radiographic technique's ability to detect fine defects on aradiograph.
The sensitivity associated with a radiograph is directly affected by the radiographic contrastand defmition, therefore all those factors which affect contrast and definition will also affectthe sensitivity.
50 Calculating sensitivity using IQl'sSensitivity is measured by the use of image quality indicators (IQIs), also known as apenetrameters. There are various types of IQI; the type commonly used consists of seventhin wires within a plastic packaging. The wires are placed transversely across the weldarea being examined during exposure. The sensitivity on the resultant radiograph is thengiven a numerical value by dividing the thickness of the smallest wire visible on theradiograph by the thickness of the specimen in the area being examined; this is thenmultiplied by 100 in order to express the result as a percentage of the specimen thickness.Alternatively, some specifications simply specify the minimum number of wires which haveto be visible on the radiograph.
60
70thickness of thinnest wire visible x 100
Sensitivity % = . .thickness of specimen
80
The lower the figure obtained, the better, i.e. the higher the sensitivity.
It must be noted however, that the obtained IQI sensitivity value does not directly relate tothe minimum defect size detectable by the radiographic technique used because of defectorientation.
BS EN 462 Image Quality Indicators is the standard which supersedes BS 3971 and DIN62. It is in 5 parts and covers the following:
BS EN 462-1BS EN 462-2BS EN 462-3BS EN 462-4BS EN 462-5
Wire typeStep/wedge typeClasses for ferrous metalsImage quality values and image quality tablesDuplex wire type
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Table 2. Types of IQI and wire materials used for selected { rOUDSof materialsImage quality indicator Wire number Wire material Suitable for test - the
followlna materialsW 1 CU W I to W 7 Copper Copper, zinc, tin andW 6CU W 6 to W 12 their alloysW IOCU W 10to W 16WI3CU W 13 to W 19W 1 FE W 1 to W 7 Steel (low Ferrous materialsW 6FE W 6 to W 12 alloyed)W IOFE W 10 to W 16W J3FE WJ3toW19W ITI W I to W 7 Titanium Titanium and their alloysW6T1 W 6to W 12W 10Tl W 10to W 16W 13 TI W13toWI9W I AL W I to W 7 Aluminium Aluminium and theirW 6AL W 6 to W 12 alloysW 10AL WIOtoWI6W 13AL Wl3toWI9
Table 1 gives the wire number and nominal wire
20
Table l. Wire numbers. diameters and limit deviations Dimensions in millimetres
Image quality indicator Wire Wire centrelineincluding spacing, aWl W6 WIO W13 Wire number Nominal wire Tolerances
diameterX WI 3,20 9,(;X W2 2,50 ±0,03 7,$X W3 2,00 6 -:X W4 1,60X W5 1,25X X W6 1,00 ±0,02X X W7 0,80
X W8 0,63X W9 0,50X X W 10 0,40X X WlI 0,32 .r
X X W 12 0,25±0,01 '5
X X W 13 0,20X X W 14 0,16X X W 15 0,125X X W 16 0,100
X W 17 0,080 ±0,005X W 18 0,063X W 19 0,050
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50 Table 2 gives types ofIQI and wire materials used for selected groups of materials
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90 B S EN 462-1 5.2 states the IQ [ should be placed on the side of the section under test facingthe source of radiation and remote from the film. If this is not possible, the IQI may beplaced adjacent to the side under test nearest the film and a letter F near to the IQ!.
The IQ! shall be placed on the object in an area where the thickness is as uniform aspossible.
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Thickness sensitivity
The smallest change in thickness which can be detected by radiography, usually expressedas a percentage of the specimen thickness.
20
ASSESSING SENSITIVITYIn accordance with BS EN 1435 Radiographic examination of welded joints, IQI wiresshall be directed perpendicular to the weld and ensure that at least IQ mm of the wire lengthwill show in a section of uniform optical density, which is normally in the parent metaladjacent to the weld.
For double wall double image and perpendicular shots, the wire can be placed across thepipe axis and should not project into the weld.
Step wedge/hole type IQl's are placed adjacent to the weld in the centre of the film. Thesensitivity is assessed in the same way as for wire types except you use the hole diameterinstead of a wire thickness.
With the exception of duplex wires, IQl's are made of the same material as the specimenbeing examined and are available in a variety of thickness ranges.
Although it is desirable for the IQI and the specimen to be of the same material, it is notalways possible or practicable to accomplish due to lack of availability. For test specimensmade from alloyed elements, the IQI material chosen should have similar radiationabsorption/transmission properties to the test specimen.
BS EN 1435 requires minimum image quality values to be assessed from tables BI to Bl;~The tables are compiled from calculations of minimum acceptable sensitivity.
30
40
Specific sensitivity termsThere are many specific terms relating to sensrtivrty which may be encountered; thefollowing terms are in accordance with BS EN 1330 - Terms used in non-destructivetesting : Part 3 <Industrial Radiographic Testing:
50 Contrast sensitivityThe smallest thickness change in a specimen which produces a discernible change in opticaldensity on a radiographic image, usually expressed as a percentage of the total specimenthickness.
60 Flaw sensitivity
The minimum flaw size detectable under specified test conditions usually expressed as apercentage of the specimen thickness.
70
Image quality indicator sensitivityThe dimension in the direction of the radiation of the thinnest step-with-hole or wire thatcan be clearly identified, expressed as a percentage of the thickness of the material unde •.-examination.
Note: The duplex-wire image quality indicator is based on a different principle and gives ameasure ofunsharpness only.
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Source
Radiographic techniques for welds on steel are listed in BS EN 1435 : RadiographicExamination of Welded Joints.
The radiographic examination of a plate weld would result in a single wall, singleimage technique being used; however, there are essentially four ways to radiograph agirth/pipe weld:
I. Single wall, single image (SWSI) - film inside, source outside.
2. Single wall, single image (SWSI) - film outside, source inside (internal exposure,usually full panoramic).
3. Double wall, single image (DWSI) - film outside, source outside (externalexposure).
4. Double wall, double image (DWDJ) - film outside, source outside (ellipticalexposure).
30 The panoramic technique is usually the preferred technique if the equipment isavailable, access permits and the minimum f.f.d.ls.f.d. requirements are met. This isdue to the fact that the entire weld can be examined in one exposure and goodsensitivity can be achieved because of a lower level of scatter and kV in comparisonwith a double walled exposure.
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SWSI: SOURCE OUTSIDE, FILM INSIDEFor standard exposures, the radiation beam is positioned at normal incidence to theweld face and film passing through the centre of the weld.
This technique is primarily intended for 100 mm diameter pipe welds and above,where access to the internal weld area permits. The main disadvantages of thistechnique are the number of exposures required due to a large amount of fade off, andthe practical aspects of positioning the radiation source at sufficient f.f.d. when dealingwith fabrications in situ. It is a technique more suited to large diameter pipes, vesselsand tanks where the curvature is closer to a flat plate and therefore has a reduced effecton the amount offade off.
The required minimum number of exposures to cover the full circumference of the welddepends on the wall thickness, pipe diameter and f.f.d.ls.f.d.; see Figure Al and A3 inBS EN 1435.
60
Note: AI is for 10% fade off and A3, 20% fade off.
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80 ---- ----- ---- ----- --- --- ------------
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~---Offset
SWSI: (PANORAMIC) SOURCE INSIDE, FILM OUTSIDEFor standard exposures, the radiation beam is positioned at normal incidence to theweld face and film passing through the centre of the weld, with equal f.f.dJs.f.d. around
ID the circumference. This technique cannot be used if the minimum f.f.d.ls.f.d.requirements cannot be met. See BS EN 1435.
30
40 DWSI
50
This technique is commonly applied to all welds where the use of a panoramictechnique is not possible or practicable, e.g. on small diameter pipe welds.
For standard exposures on any diameter of pipe weld, the radiation beam is positionedat approximately 850 to the weld face and film. With this technique the radiation beamcannot be positioned at normal incidence to the weld portion being examined becausethe weld on the radiation source side will superimpose over the film side weld resultingin an unreadable radiograph. This problem mainly applies when using x-ray tubes; thex-ray tube must be moved approximately 60 mm to the side of the weld, so the centralline of the x-ray beam shoots past the tube side weld resulting in a diagnostic image ofthe film side weld. Care must a)so be taken to ensure that the number tape on thesource side does not interfere with the image, i.e. shoot through from the opposite sideof the weld to that which the number tape is positioned.
The required minimum number of exposures to cover the full circumference of the welddepends on the wall thickness, pipe diameter and f.f.d.; see Figure A2 and A4 in BSEN 1435.
60
70Note: A2 is for 10% fade off and A4, 20% fade off.
Film Film80
90X __ I
Section on X - X
.1.Source
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The parallax technique issometimes reJerred 10 as thetube shift method when an 100x-ray tube is used.
UNIT RJ3 . RAHIOGRAPHIC TECHI\"IQUES
10
Parallax techniqueThe parallax radiographic technique may be used to determine the depth of defectsbelow the surface of a component; this may be useful to know for repair purposes. It isa technique more applicable to thick specimens, e.g. over 50 mm, but is rarely used
DWDI
20
This technique is only applied to welds on pipe or fittings 100 mm diameter or below.
A minimum of two exposures are usually required at 90° to each other; this results in atotal of four interpretable areas on the radiograph which should cover the fullcircumference ofthe weld.
The cassette is placed flat on one side of the pipe. The source is positioned at theminimum sfd (calculated using the Ug formula or nomogram) and is offset from theweld centre line to give an elliptical image. In most cases an offset of about one fifthsfd will separate the top image from the bottom. This, however, becomes more difficultas the wall thickness increases and the pipe diameter reduces.
On small bore heavy wall pipework, it is often permitted by specification or client forthe radiation to pass through the centre of the weld at normal incidence to the pipe; thiswill produce a radiograph with the tube side weld superimposed over the film sideweld.30
·-offset·'Source ------~fr.------------------------------------~t~.
:.: :.;:
40
50 I ---,-.- XI
Section on X - X
_._-----,-
60 ----Film
Film
SANDWICH TECHNIQUE70 The sandwich technique is a radiographic technique sometimes used in order to save
time. It may be used on components where there are substantial thickness differencescausing the density on a single radiograph to be out of specification on either thethicker side, the thinner side or both. Rather than carry out two separate shots atdifferent exposures for each weld or position, cassettes may be loaded with two films.Two radiographs will therefore be produced - one for the thick side and the other forthe thinner side but they will have been produced in a single exposure.
The films are usually of different speeds, e.g. a fine grained film loaded with a veryfine grained film, however, the same effect will be produced by placing a lead screen,thicker than usual, between two films of the same speed.
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LOCATION OF DEFECTS
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D = T(b-c)(a-c)
because ultrasonic testing can usually give the same information quicker and at a lowercost.
20
The technique is used after a defect has already been detected by conventional methods.The procedure involves the placement of a lead marker on the source side of aspecimen's surface close to the plan view location of the defect. Two exposures aremade, each at half the normal exposure, and offset to each other in order to produce adouble image ofthe lead marker and defect.
The following criteria are used to calculate the distance of the defect from the filmeither by means of a formula or graph:
a. Distance between defect images.b. Distance between lead marker images.c. F.f.d.ls.f.d.d. Specimen thickness.e. Dimension of shift between source of radiation.
30Right-angled methodTwo shots of specimen taken at right angles. The position of defect may be found bymeasurement. This is the most straight forward method for cube shaped or similarspecimens. This method cannot be used for welds or plates.
40 Mathematical (tube or source shift) methodFind the defect by normal radiography. Position the tube over the defect - move thetube a distance of exactly one fifth of the FFD sideways and give half the originalexposure. Move the tube some distance in the opposite direction, i.e. one fifth of theFFD from centre, and give another half exposure on the same sheet of film:
50lh
d = I +s
60Where d
Ih
distance of defect from filmdistance of image movementFFDtwo fifths of the FFDs
70
Lead marker (tube or source shift) methodFind the defect by normal radiography. Position the tube over the defect. Place onelead wire on top of the specimen to one side of the defect. Place a second lead wireunderneath the specimen on the other side of the defect.
Move the tube approximately one fifth of the FFD to one side and give half the originalexposure, then move the tube approximately one fifth of the FFD to the other side andgive halfthe original exposure, again on the same sheet offilm.
Draw graph as shown80
T total specimen thicknessimage shift of top lead wireimage shift of defectimage shift of bottom lead wiredistance of defect from bottom of specimen
abcd
Or, instead of drawing graph, use:
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IMAGESHIFfS
SPECIMEN 2
--:--- Pb marker(source side)
Source shiftdirection
20 -<'>."'= Defect
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Pb marker(film side)
4
RADIOGRAPH 2
_________ T.
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SOURCE POSITIONS
ill ®
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{ ,T t..;
Top Pb wire '--j'
t¥ ---------- -.!,,'
, '.I J
'f/
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'T' - thicknessof specimen
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HeighlofDefect'd'
Above bottomsurface
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Remember that densityaffects contrast and contrastaffects sensitivity.
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l1NIT R14 • DETERl\lIl'"ATION OF EXPOSl1RE
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Intensity of radiation and exposure timeThe intensity of the radiation reaching the film and exposure time will affect the density ofthe image.
Radiation intensity and exposure time are related. Exposure time is proportional to theintensity of radiation; this relationship is known as the reciprocity law:
Exposure = time x intensity
X-ray equipment - If you had an exposure of say 4 minutes and 3 mA, then 4 x 3 = 12,therefore you would be using 12 mA-mins. You could also use 3 minutes and 4 mA togive you the same amount of exposure because 3 x 4 = 12, or I minute at 12 mA,l x12 = 12, or 2 minutes 6 mA, 2 x 6 = 12 etc.; all these give you the same amount ofexposure.
The higher the mA setting on the control panel, the greater the intensity of radiationproduced, and therefore the darker the image will be, unless the time is reduced tocompensate.
Gamma isotopes - If you had an exposure of say 5 minutes using an isotope with anactivity of 4 curies, then 5 x 4 = 20, therefore you would be using 20 Ci-rnins.
20
Many factors govern the fmal quality of a radiographic image; all these factors must beconsidered and controlled in order to meet with a specifications requirements.
The time to use for an exposure is only one factor to consider for an exposure, but it is thisfactor which changes most often. Gamma exposure times are usually calculated fromspecial slide rules, usually referred to as gamma exposure calculators, these take intoconsideration the following:
a. Film density to be achieved.b. Source type.c. Activity of source.d. Film speed.e. Source to film distance.f. Material type.g. Material thickness.
When using x-ray equipment, the determination of exposure is less straightforward. This isbecause the wavelength and intensity of radiation may be adjusted, and different machinesproduce different quantities and qualities of x-radiation even though they may be operatedon the same panel settings. The following methods are used to determine correct exposureswhen using x-ray equipment:
a. By reference to previous exposure records.b. By trial and error test shots.c. A combination of the above.d. By using exposure charts.
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40
CONSIDERATIONS FOR EXPOSURES
50 Wavelength of radiationThe wavelength of radiation used will affect the density, contrast and defmition of aradiographic image.
60
X-ray equipment - The lower the kV used to penetrate the specimen, the higher will be thecontrast, but enough kV must be used to ensure penetration and keep the exposure timereasonable.Gamma isotopes - Different radioactive isotopes produce different wavelengths of gammaradiation, e.g. C060 produces shorter wavelength radiation than Ir 192 and is therefore morepenetrating, but a radiograph produced on the same specimen using Co60 will have muchlower contrast and definition.
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E2 = 2.8 mAmins
The higher the activity of the isotope used, the greater the intensity of radiation produced,and therefore the darker the image will be, unless the time is reduced to compensate.
X-ray equipmentThe intensity of radiation (governed by mA) and quality of radiation (governed by kV) canbe affected by the electric circuit of the equipment being used. The kV and mA may be onthe same panel setting, but the radiation intensity and wavelengths can vary from one set toanother.
20 Filter types and thicknesses also differ between x-ray tubes. Filters are used to reduce longwavelength primary radiation to provide a more homogeneous x-ray beam with lowerresultant scatter levels. Filters affect the exposure time, e.g. an x-ray tube with a thick filterwill require more exposure than an x-ray tube with a thinner filter.
30Type of filmThe higher the speed of the film, the denser the image compared to that of a slow film at thesame exposure. However, the radiograph's definition for a slow film at the correct exposurewill be better than that for a fast film at the correct exposure.
40
Intensifying screensUsing intensifying screens reduces the exposure required to attain the required density, butfluorescent and fluorometallic screens have an adverse affect on the definition of theradiographic image.
Exposures made with direct x-ray and lead screens obey the law of reciprocity (E = mAT).Fluorescent screens emit light of various wavelengths, including UV. Where intensificationis due to light exposure, the law of reciprocity cannot be strictly applied. Exposures withfluorescent screens are, therefore, less predictable and more likely to be attained by trial anderror.
50
Ffd/sfdThe greater the ffd/sfd the smaller the penumbra, therefore the better the radiographic
60 defmition. But, x-rays and gamma rays obey the inverse square law. Therefore, with regardto exposure, the greater the ffd/sfd, the greater the exposure should be to attain a givendensity.
The following formula, based on the inverse square law, can be used to determine newexposures when the ffdlsfd changes:
70 El D/ --E) = D/
Where: E, = original exposure;E2 = new exposure;
01 = original distanceO2= new distance
80 Example:
El = 5 mAmins; 01 = 1000 mm
E2 = ? mAmins; O2 = 750 mm
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10
EXPOSURE CHARTS
Object being radiographcdThe radiation absorption and transmission characteristics of a material depends upon itsthickness, density and atomic mass. This will primarily govern penetrating power required.
Processing the filmThe density, contrast and definition of a radiograph are affected by the type, temperature,agitation and time in the developer. The development process should not be adjustedoutside a specifications requirements in order to compensate for incorrect exposures, i.e. toadjust the density of a radiograph, the exposure should be changed; not the developing time.20
Exposure charts provide the exposure conditions for a given thickness of material using x-ray equipment. An exposure chart will show the exposure to use in mA-min for a chosenspecimen thickness and kV in order to attain the density that the chart is based on.
Exposure charts are drawn up from preliminary charts made up from exposures usingdifferent kilovoltages on step wedges.
The vertical scale on an exposure chart is logarithmic and the horizontal scale is arithmetic.
Each chart must show the variables to which the chart is applicable to:
a. Type of x-ray set.b. Film density.c. Film type.d. Intensifying screense. Focus to film distance.f. Development conditions.g. Material tested.
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The gamma slide rule enables very accurate calculations to be made providing the followinginformation is available:
EXPOSURE CALCULATIONS FOR GAMMA RA VS
20
The following information is required to obtain the exposure from an exposure chart.
a. Weld thickness.b. Source strength from decay chart.c. Type of film
From the gamma ray exposure chart for Ir192, select the weld thickness, follow this lineuntil it strikes the density required line, say 02.0 and then follow the line down onto the Cihour line and read off this value.
Example
Weld thickness = 20 mm density required = 2.0.
The Ir 192 source is at 15 CL30
From the chart, the exposure is 5.1 Ci hours
5.1 Ci hours x 60 = 306 Ci mins
40E=CiT E 306 .
:. T= - = - = 20AmmutesCi 15
The exposure time at 900 mm with 15 Ci = 20.4 minutes.
50 If the sfd was changed to 600 mm, then:
D 2 6002E2 = _2_ x El = -- x 20.4 = 9 minutes
012 9002
The new exposure time at 600 mm with 15 Ci = 9 minutes.60
EXPOSURE CALCULATIONS USING GAMMA SLIDE RULE
70 a. Film speed.b. Density required.c. Source strength in Ci.d. SFD.e. Material thickness.
80 The results give the exposure time.
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(mm) SS;)U:Ij:l!II.L 1~31S
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The information required to produce a radiograph is as follows:
20
From the test piece
• Plate thickness• Weld thickness• Pipe diameter (for pipes)• Length of weld
From this information the data required to produce the radiograph can be produced.
30
1. Minimum Source to Film distance (sfd) or Focus to Film Distance (ffd)To calculate the minimum FFD/SFD so that the unsharpness of the image is better than theresolution ofthe eye - 0.25 mm
Minimum tTd= (source size x ofd) + ofd0.25
ofd can be taken as the sample thickness.
40
r rsod sfd
orffd
10
1< dfl >1
50
60 ffd = Focus to Film Distance
sfd = Source to Film Distance
sod = Source to Object Distance
ofd = Object to Film Distance70
T = Sample Thickness
1.1T = 10% fade off (the edge of the diagnostic length)
dfl = Diagnostic Film Length
80 2. The DFL is derived from the following (for flat plates only)
90
Source X
x= I1.1sod sod
r:T
:? < ~ dfl dfl >
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UNIT RI ..•• nETERl\1INATION OF EXPOSURE
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6. Mark up the test piece according to the technique requirements
The diagram shows two similar triangles, the small triangle comprising of:
T == perpendicular; 1.1T == hypotenuse; ? = base
10 The larger triangle consists of:
sod = perpendicular; 1.1 sod = hypotenuse; Y:zdfl = base
The pythagoras theory states that the sum of the squares of the base and the perpendicularequals the square on the hypotenuse, therefore:
sod2 + (Y:z dfl)2 = (1.1 sod)2
Require to find the value of the dfl:
Therefore (~dfl r = (1.1 sod)2 - sod2
~(~ dflr = ~(l.1 sod)2 - sod '
! dfl = ~1.l sod2 - sod2
2
30
Therefore
Therefore40
Therefore dfl = 2 x ~(].I sod)2 - sod2
The SOD is taken from the calculation of the minimum ffd/sfd - for plate only
50 The DFL for pipes is calculated from EN 1435 - Radiographic Examination of WeldedJoints.
60
3. To work out the Image Quality Indicator - IQI (see Unit 7)
The IQI sensitivity should be better than 2% with respect to the sample thickness.
To calculate the IQI wire diameter the following is used:
. Sample thickness 2IQl dia = 100 x
To find the wire number, consult Table 2 ofBS EN 462-1, which is the current standard, orBS 3971 which has been superseded.
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4. Working out the exposureAn exposure chart is required for x-ray sets and may also be used for gamma ray, however,the use of a gamma slide rule is often used and is generally more accurate and quicker.
From the x-ray exposure chart on R9-3, it is possible to obtain kV values and exposurevalues for any given thickness working on the exposure chart within the 15 to 60 mAminutes range.
90
5. Correcting the exposureThe exposure obtained will be for a fixed distance and fixed density, film, material andprocessing conditions. To change the exposure the following is used:
New distance/ [ 0 2)New exposure = Old exposure x 2 E2 = El X D2\2
Old distance
Where the new distance is your selected ffd and the old distance is the chart distance.
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Steps for radiographing a butt weld in a plate1. Measure:
Plate thickness, weld thickness and weld length.
2. Calculate:The minimum Film to Focus Distance (ffd)and the Source to Focus Distance (sfd).
20Th .. ffd (source size x Sample thiCkness) I h' ke mmunum - = + samp e t IC ess
sfd 0.25
This is the minimum, therefore select an FFD/SFD greater than this for plate butt welds.The figure should be in the order of 1.5 times the length of the weld to be covered in oneshot.
30 3. Work out the diagnostic film length (dfl):Using the ffd/sfd selected:
40 sod = Source to Object Distance is equal to the ffd - ofd (Object to Film Distance).
The ofd can be taken as the weld thickness.
If the ffd/sfd will not cover the required length, then the ffd/sfd must be increased if theweld is to be covered in one shot.
so 4. To calculate the Image Quality Indicator (lQI):
IQI . thi Subject thickness 2wire rcness = xlOO
Look up the wire thickness on Table 1 in BS EN 462-1 for the wire number and wire group.
60 5. To calculate the exposure:Using the exposure chart supplied for the x-ray set to be used, the weld thickness is thenused to find the kV and corresponding exposure in mA minutes within the 15-60 mA minsbox.
This will give one or two kV and corresponding mA minute exposure for fixed conditionsof distance, film type, density and development.
The kV will be fixed but the exposure in mA minutes will require to be adjusted for theffd/sfd to be used.
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Correcting tire exposureNew distance/
New exposure = Old exposure x 2Old distance
The new distance is the selected ffd
The old distance is the chart ffd
90 6. Marking up the plate
A1 2ID Date
Weld
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UNIT RI ..•. DETERMINATION OF EXPOSURE
Butt Welds in PipesButt welds in pipes are worked out in a similar manner with the exception of the diagnosticfilm length which is calculated from a series of charts in EN 1435 - Radiographic
10 Examination of Welded Joints, depending on the radiographic method used.
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EQUIVALENCE CHARTSGenerally exposure charts are made for either aluminium or steel. This can cause problemswhen it is required to radiograph other materials.
The following chart shows a radiographic equivalence chart which relates other materials toaluminium and steel.
The figures given in the chart are multiplication factors and are used to convert a particularthickness of the selected material to the equivalent thickness of the standard material.
X-Rays kV Gamma Rays
50 100 150 220 400 1000 2000 Ir192 CE-137 Co60
Magnesium 0.6 0.6 0.05 0.08 0.22 0.22 0.22
Aluminium 1.0 1.0 0.12 0.18 0.34 0.34 0.34
Titanium 8.0 0.63 0.71 0.71 0.9 0.9 0.9 0.9 0.9
Steel 12.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
Copper 18.0 1.6 lA lA 1.1 1.1 1.1 1.1 1.1
Zinc 1.4 1.3 J.3 1.1 1.0 \.1 1.0 1.0
Brass 1.4 1.3 J.3 1.2 1.2 1.1 1.I 1.0
Lead 14.0 12.0 5.0 2.5 4.0 3.2 2.3
For the x-ray range 50-100 kV, aluminium is taken as the standard and uses a factor of 1.0.
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UNIT R15 . FILTERS
10
In practice higher kV's are used with filters of lead, copper or tin which have highatomic numbers.
DefinitionA relative thin layer ofa heavy metal (e.g. lead or copper), interposed in the path of theradiation before it reaches the film.
There are two types of filters:
1. A tube head filter, e.g. the beryllium window, or thin layer of heavy metal.
2. A cassette filter.
20 Tube head filterPositioned inside the tube head window, the action of the tube filter depends on the factthat an x-ray beam is heterogeneous (a mixture of wavelengths) and the longerwavelengths are more easily scattered. The filter removes much of the soft radiationgiving a marked reduction in scatter. Thus the beam becomes more nearlymonochromatic in wavelength and also effectively of shorter wavelength.
NB. A filter of a higher atomic number will be equivalent to a thicker filter with a loweratomic number.
30
In general, the filter thickness should be less than 10% of the specimen thickness. Theeffectively shorter wavelength reduces the contrast obtained.
40
X-\SC__ T----.:specimen I X-rays Masking
___\LI ~ J2d~A '\;t
Film50 8
In (A), soft radiation is scattered by the edge of the object giving undercutting. Thiseffect is reduced if a tube filter is used.
60 For (B), a tube filter will merely decrease contrast.
e.g. 50 kV with tungsten target
Unfiltered beamUse of1 mmUse of5 mm
70
80
0.6 0.80.1 0.2 0.4
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lJ~IT R15 . FILTERS
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The filter removes a greater proportion of the scatter than of the primary beam,however, it adds to the total thickness thus decreasing the contrast.
NB. The specimen itself acts as a filter for the main beam and for any scatter whichpasses through it.
The cassette filter will produce its own characteristic radiation and may be a source ofscatter, therefore, on thin sections will give no advantage.
If metal intensifying screens are used inside the cassette they will have the same effectas a cassette filter, also metal cassettes will act similarly. Thus, cassette filters are notnormally used with metal intensifying screens.
Cassette filterScatter Image forming beam
20
30
40GeneralFor similar exposure conditions, use of a tube head filter will give reduced contrastcompared to no filter used and a cassette filter will give even more reduction in contrastcompared to the use of a tube filter.
50 Gamma rays have a much shorter wavelength than x-rays and cause much less scatter sofilters are seldom used.
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UJ\IT R16 . GLOSSARY OF TERMS
lOO
10
Clearing time
BS EN 1330-3 : 1997 - Terms used in industrial radiographic testing
Absorption
Activity
Ageing fog
20Anode
Artefact (false indication)
30
Attenuation
Attenuation coefficient J140
Average gradient
50 Back scatterlback scatteredradiation
Beam angle
60 Betatron
Blocking medium
70 Build-up factor
Cassette
80Cathode
Calibrated density step wedge
90 Characteristic curve (of a film)
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The process whereby the incident photons arereduced in number as they pass through matter.
The number of nuclear disintegrations per unit timetaking place in a radioactive source.
The increase in optical density on an unexposed film,measured after processing, due to long-term storage.
The electrons passing from the cathode to the anodein an x-ray tube.
A spurious indication on a radiograph caused e.g. byfaults in the manufacturing, handling, exposing orprocessing of a film.
The reduction in intensity of a beam of x or gammaradiation during its passage through matter caused byabsorption and scattering.
The relationship between the intensity (/0) of aradiation incident on one side of an absorber and thetransmitted intensity (I) for an absorber thickness (r)as expressed by 1= 10 - exp (- ut).
The slope of a line drawn between two specifiedpoints on the sensitometric curve.
That part of the scattered x or gamma radiationwhich is emitted at an angle of more than 90° inrelation to the direction of the incident beam.
The angle between the central axis of the radiationbeam and the lane of the film.
A machine in which electrons are accelerated in acircular orbit before being deflected onto a target toproduce high energy x-rays.
A material used to reduce the effect of scatteredradiation on the film or on the image detector.
The radio of the intensity of the total radiationreaching a point, to the intensity of the primaryradiation reachingthe same point.
A rigid or flexible light-tight container for holdingradiographic film or paper with or withoutintensifying screens, during exposure.
The negative electrode of an x-ray tube.
A piece of film having a series of different opticaldensities which have been calibrated to be used asreference densities.
A curve showing the relationship between thecommon logarithm of exposure, log K. and the opticaldensity, D.
The time required for the first stage of fixing of afilm, during which the cloudiness disappears.
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10
Dose rate meter
Collimation
Collimator
Compton scatter
20
30Computerized tomography (CT)
40
Constant potential circuit
50
Continuous spectrum
Contrast
60Contrast medium
70
Contrast sensitivity(thickness sensitivity)
Decay curve
80 Densitometer
Development (ora film or paper)
90Diffraction mottle
Dosemeter (dosimeter)
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The limiting of a beam of radiation to a form o.required dimensions, by the use of diaphragms madeof absorbing material.
A device made from radiation absorbent materialsuch as lead or tungsten, designed to limit and definethe direction and area of the radiation beam.
A form of scattering caused by a photon of x orgamma radiation interacting with an electron andsuffering a reduction of energy, the scatteredradiation being emitted at an angle to the incidentdirection.
Note: For radiation in the energy range 100 keY to10 MeV, it is the main factor contributing to radiationattenuation.
A procedure by which an image of the detail in achosen plane, perpendicular to the axis of thespecimen, is computed from a large number of x-rayabsorption measurements made from many directior ..--...perpendicular to the axis.
Note: This is computerized axial tomography anddoes not apply to other means of performingtomography,
An electronic configuration which is designed toapply and maintain a substantially constant potentialwithin an x-ray tube.
The range of wavelengths or quantum energiesgenerated by an x-ray set.
See image contrast, radiation contrast, objectcontrast and visual contrast.
Any suitable substance, solid or liquid, applied to amaterial being radiographed, to enhance its radiationcontrast in total or in part.
The smallest thickness change in a specimenwhich produces a discernible change in opticaldensity on a radiographic (or radioscopic) image:">usually expressed as a percentage of the tota,specimen thickness.
The activity of a radioisotope plotted against time,usually as a log/linear relationship.
A device for the measurement of the optical densityof a radiographic film or reflective density of aphotographic print.
The chemical or physical process which converts alatent image into a visible image.
A superimposed pattern on a radiographic image dueto diffraction of the incident radiation by the materialstructure.
An instrument for measuring the accumulated dose ofx or gamma radiation.
An instrument for the measurement of x or gammaradiation dose rate.
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Edge-blocking material
Dual focus tube An x-ray tube with two different size of focus.
Duplex wire image quality indicator An image quality indicator specifically designed toassess the overall unsharpness of a radiographic
10 image and composed of a series of pairs of wireelements made of high density metal.
Material applied around a specimen or in cavities toobtain a more uniform absorption, to reduceextraneous scattered radiation. and to prevent localover-exposure. e.g. fine lead shot (see also blockingmedium).
Equalizing filter (beam flattener) A device used to equalize the intensity across theprimary x-ray beam in megavoltage radiography andso extend the useful field size.
30 Equivalent x-ray voltage
Exposure
40 Exposure calculator
Exposure chart
50 Exposure latitude
Exposure times
Film base60
Film gradient (G)
Film illuminator (viewing screen)
70Film processing
-----
80 Film system speed
Filter
90
Fixing
Flaw sensitivity
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The voltage of an x-ray tube which produces aradiograph most nearly equivalent to a gammaradiograph taken with a particular gamma ray source.
The process whereby radiation is recorded on animaging system.
A device (for example a slide rule) which may beused to determine the exposure time required.
A chart indicating the time for radiographic exposuresfor different thicknesses of a specified material andfor a given quality of a beam radiation.
The range of exposures corresponding to the usefuloptical density range of the emulsion.
Duration of the process of exposing a recordingmedium to radiation.
The support material on which the photosensitiveemulsion is coated.
The slope of the characteristic curve of a film at aspecified optical density D.
Equipment containing a source of light and atranslucent screen used for viewing radiographs.
The operations necessary to transform the latentimage on the film into a permanent visible image,consisting normally of developing. fixing, washingand drying a film.
A quantitative measure of the response of a filmsystem to radiation energy, for specific exposureconditions.
Uniform layer of material, usually of higher atomicnumber than the specimen, placed between theradiation source and the film for the purpose ofpreferentially absorbing the softer radiations.
The chemical removal of silver halides from a filmemulsion after development.
The minimum flaw size detectable under specifiedtest conditions.
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Image enhancement
...-......
A screen consisting of a coating of phosphors whic.,fluoresce when exposed to x or gamma radiation.
Fluorometallic intensifying screen A screen consisting of a metallic foil (usually lead)10 coated with a material that fluoresces when exposed
to x or gamma radiation.
100
Fluorescent intensifying screen
Fluoroscopy
Focal spot
Focal spot size
30Focus-to-film distance (ffd)
Fog density
40Gamma radiation
Gamma rays
50 Gamma-ray source
Gamma-ray source container
60 Geometric unsharpness
Graininess70
Granularity
Half life
80 Halfvalue thickness (HVT)
Illuminator
Image contrast90
Image definition
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The production of a visible image on a fluorescentscreen by x-rays and for direct viewing of the screen.
The x-ray emitting area on the anode of the x-raytube, as seen from the measuring device.
The dimension across the focal spot of an x-ray tube,measured parallel to the plane of the film or thefluorescent screen.
The shortest distance from the focus of an x-ray tubeto a film set up for a radiographic exposure.
A general term used to denote the optical density of aprocessed film caused by anything other than thedirect action of image - forming radiation. It can i:' ~ageing fog, chemical fog, dichroic fog, exposure f05or inherent fog.
Radiography using a gamma ray source.
Electromagnetic ionizing radiation, emitted byspecific radioactive materials.
Radioactive material sealed into a metal capsule.
A container made of dense material and having a wallthickness sufficient to produce a very great reductionin the intensity of the radiation emitted by the source,so as to make it safe to handle.
Unsharpness of a radiographic image arising from thefinite size of the source of radiation. Its magnitudealso depends on the distances of source-to-object andobject-to-film. Also called geometric blurring orpenumbra.
The visual appearance of granularity.~
The stochastic density fluctuations in the radiograpisuperimposed on the object image.
The time in which the activity of a radioactive sourcedecays to half its value.
The thickness of specified material which, whenintroduced into the beam of x or gamma radiation,reduces its intensity by a half.
Equipment for viewing radiographs.
The relative change of optical density between twoadjacent areas in a radiographic image.
The sharpness of delineation of image detail in aradiograph.
Any process which increases the quality of an imageby improving contrast and/or definition, or reducingnoise. Often done by computer programmes, when itis known as digital image processing.
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Microfocus radiography
20
An electronic device designed to provide a brighterimage than produced by the unaided action of the x-ray beam on a fluorescent screen.
That characteristic of a radiographic image whichdetermines the degree of detail which it shows.
A device comprising a series of elements of gradedthickness which enables a measure of the imagequality to be obtained. The elements of an IQI arecommonly wires or steps with holes.
Image quality value, IQI sensitivity Measure of the image quality required or achieved.
Image intensifier
Image quality
Image quality indicator (IQI)
Incident beam axis The axis of the beam cone defmed by the focal spotand the tube window.
50
The science and application of x-rays, gamma rays,neutrons and other penetrating radiation in non-destructive testing.
The filtration of a radiation beam by the parts of thetube, set up or source incapsulation, through whichthe primary beam will pass.
The blurring of a radiographic image caused byphotons of radiation dislodging electrons in thephotographic emulsion and these electrons renderingsilver halide grains developable.
The ratio of the exposure time without intensifyingscreens, to that when screens are used, otherconditions being the same, to obtain the same opticaldensity.
A material that converts a part of the radiographicenergy into light or electrons and that, when incontact with a recording medium during exposure,improves the quality of the radiograph, or reduces theexposure time required to produce a radiograph orboth. See metal screen, fluorometallic intensifyingscreen or fluorescent intensifying screen.
An invisible image produced in a film by radiationand capable of being converted into a visible imageby film processing.
Linear electron accelerator (LINAC) A machine for producing high energy electrons byaccelerating them along a waveguide. The electronsstrike a target to produce x-rays.
Masking The application of material which limits the area ofirradiation of an object to the region undergoingradiographic examination.
30Industrial radiology
Inherent filtration
40 Inherent unsharpness
Intensifying factor
Intensifying screen
60
Latent image70
80
Metal screen A screen consisting of dense metal (usually lead) thatfilters radiation and emits electrons when exposed tox or gamma rays.
Radiography using an x-ray tube having a very smalleffective focus size of less than 100 urn in size.Commonly used for direct geometric enlargement ofthe image by projection.
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UNIT RIG· GLOSSARY OF TERMS
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Rod anode tube
Modulation transfer function (MTF) The spatial frequency response of an imaging system.
Movement unsharpness A blurring of the radiographic or radioscopic imagedue to relative movement of the radiation source,object or radiation detector.
Relative difference of radiation transmission betweentwo considered zones of the irradiated object.
The distance between the radiation side of the testobject and the film surface measured along the centralaxis of the radiation beam.
Object contrast
Object-to-film distance
20
Panoramic exposure A radiographic set-up utilizing the multi-directionalproperties of a gamma ray source or a panoramic x-ray set, e.g. by radiographic several specimenssimultaneously, or the full circumference of acylindrical specimen.
See image quality indicator.
A variation in density of a radiograph, which may belight or dark in appearance, according •~circumstances, caused by local pressure to the film.
Radiation which travels directly along a straight linefrom the source to the detector without deviation.
30Penetrameter
Pressure mark
40 Primary radiation
Projective magnification The amount of image size enlargement.
50
Projective magnification technique A method of radiography or radioscopy involvingprimary enlargement of the image by the use of adistance between the specimen and imaging system(see microfocus radiography).
The penetrating power of the radiation, oftenmeasured as a half-value thickness.
Quality (ofa beam of radiation)
Radiation contrast differences in radiation intensity arising fromvariation in radiation opacity within an irradiatedobject.
An equipment (e.g. x-ray tube or gamma ray source)capable of emitting ionising radiation.
A visible image after processing produced by a bearr-r-,of penetrating ionising radiation on a radiographi,film or paper. The term is also used for imagesproduced by neutrons, electrons, protons, etc ..
A film consisting of a transparent base, usually coatedon both sides with a radiation sensitive emulsion.
60
Radiation source
70 Radiograph
Radiographic film
80Radiography The production of radiographs on a permanent
imaging support.
An isotope of an element with the property ofspontaneously emitting particles or gamma radiationor of emitting x-radiation.
The production of a visual image by ionisingradiation on a radiation detector such as fluorescentscreen and displayed on a television monitor screen.
A type of x-ray tube in which the target is situated atthe extremity of a tubular anode; such tubes canproduce a panoramic beam of radiation.
Radioisotopes
90Radioscopy
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Vacuum cassette
Scattered radiation Radiation which has suffered a change in direction,with or without a change in energy, during its passagethrough matter.
Radiographic film designed for use with fluorescentintensifying screens.
A holding, carrying, or attachment device, by meansof which the gamma ray source (sealed source) can befixed in the exposure container, or at the head of aremote control device.
Screen type film
Source holder
20Source size The size of the source of radiation.
The distance between the source of radiation and thefilm measured in the direction of the beam.
The distance between details which can just beseparated in an image.
The activity per unit mass of a radioisotope.
Object in the form of a series of steps of a samematerial.
Source-to-film distance (sfd)
30Spatial resolution
Specific activity
Step wedge
40 Stereo radiography The production of a pair of radiographs suitable forstereoscopic viewing.
The area on the surface of the anode of an x-ray tubeon which the electron beam impinges and from whichthe primary beam of x-rays is emitted.
A device, normaIly fixed to a tube shield or head, tolimit the extent of the emergent x-ray beam.
That part of an x-ray installation that contains thetube in its shield.
The housing of an x-ray tube which reduces theleakage radiation to defined values.
A device attached to a tube shield, generally of leadand usually remotely operated, used to control theemergence of the x-ray beam.
The area of an x-ray tube through which the radiationis emitted.
The high voltage applied between the anode and thecathode of an x-ray tube.
Any radioactive source which is not sealed into acapsule.
Due to image blurring a loss of image definition. It iscombination of geometric unsharpness, inherentunsharpness and movement unsharpness.
The range of optical density on a radiograph that isused for image interpretation. The upper limit isdetermined by the film illuminator and the lower limitby the loss in flaw sensitivity.
A light-tight container that where operated under avacuum, holds film and screen in intimate contactduring radiographic exposure.
Target
50 Tube diaphragm
Tube head
Tube shield60
Tube shutter
Tube window70
Tube voltage
Unsealed source
80Unsharpness
Useful density range
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X-ray film
X-ray tube
Viewing mask
Visual contrast
An attachment to an illuminator to exclude glare.
The visual density difference between two adjacentareas on an illuminated radiograph.
Penetrating electromagnetic radiation, within theapproximate wavelength range of 1 nm to 0,0001nanometres, produced when high velocity electronsimpinge on a metal target.
See radiographic film.
A vacuum tube, usually containing a filament toproduce electrons which are accelerated to strike aanode, on the surface of which x-rays are produced.
X-rays
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RADIOG RAPH IC
INTERPRETATION
ADDITIONAL COURSE NOTES
Radiograph Interpretation - Welds
In addition to producing high quality radiographs, the radiographer must also be skilled in radiographicinterpretation. Interpretation of radiographs takes place in three basic steps: (1) detection, (2) interpretation,and (3) evaluation. All of these steps make use of the radiographer's visual acuity. Visual acuity is the ability toresolve a spatial pattern in an image. The ability of an individual to detect discontinuities in radiography is alsoaffected by the lighting condition in the place of viewing, and the experience level for recognizing variousfeatures in the image. The following material was developed to help students develop an understanding of thetypes of defects found in weldments and how they appear in a radiograph.
Discontinuities
Discontinuities are interruptions in the typical structure of a material. These interruptions may occur in the basemetal, weld material or "heat affected" zones. Discontinuities, which do not meet the requirements of thecodes or specifications used to invoke and control an inspection, are referred to as defects.
General Welding Discontinuities
The following discontinuities are typical of all types of welding.
Cold lap is a condition where the weld filler metal does not properly fuse with the base metal or the previousweld pass material (interpass cold lap). The arc does not melt the base metal sufficiently and causes the slightlymolten puddle to flow into the base material without bonding.
~
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Porosity is the result of gas entrapment in the solidifying metal. Porosity can take many shapes on a radiographbut often appears as dark round or irregular spots or specks appearing singularly, in clusters, or in rows.Sometimes, porosity is elongated and may appear to have a tail. This is the result of gas attempting to escapewhile the metal is still in a liquid state and is called wormhole porosity. All porosity is a void in the material andit will have a higher radiographic density than the surrounding area.
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Cluster porosity is caused when flux coated electrodes are contaminated with moisture. The moisture turnsinto a gas when heated and becomes trapped in the weld during the welding process. Cluster porosity appearjust like regular porosity in the radiograph but the indications will be grouped close together. ....
Slag inclusions are nonmetallic solid material entrapped in weld metal or between weld and base metal. In aradiograph, dark, jagged asymmetrical shapes within the weld or along the weld joint areas are indicative ofslag inclusions. r
Incomplete penetration (IP) or lack of penetration (LOP) occurs when the weld metal fails to penetrate thejoint. It is one of the most objectionable weld discontinuities. Lack of penetration allows a natural stress risrfrom which a crack may propagate. The appearance on a radiograph is a dark area with well-defined, straightedges that follows the land or root face down the center of the weldment.
~
!Inadequate or Lack of Penetration
Incomplete fusion is a condition where the weld filler metal does not properly fuse with the base metal.Appearance on radiograph: usually appears as a dark line or lines oriented in the direction of the weld seamalong the weld preparation or joining area.
~
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Internal concavity or suck back is a condition where the weld metal has contracted as it cools and has beendrawn up into the root of the weld. On a radiograph it looks similar to a lack of penetration but the line hasirregular edges and it is often quite wide in the centre of the weld image.
Internal or root undercut is an erosion of the base metal next to the root of the weld. In the radiographic imageit appears as a dark irregular line offset from the centreline of the weldment. Undercutting is not as straightedged as LOPbecause it does not follow a ground edge.
~
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. ,\c I~,,~!~~.~~~~~~Internal Undercut
External or crown undercut is an erosion of the base metal next to the crown of the weld. In the radlograpr e,
appears as a dark irregular line along the outside edge of the weld area.
Offset or mismatch are terms associated with a condition where two pieces being welded together are notproperly aligned. The radiographic image shows a noticeable difference in density between the two pieces. Thedifference in density is caused by the difference in material thickness. The dark, straight line is caused by thefailure of the weld metal to fuse with the land area.
Inadequate weld reinforcement is an area of a weld where the thickness of weld metal deposited is less thanthe thickness of the base material. It is very easy to determine by radiograph if the weld has inadequatereinforcement, because the image density in the area of suspected inadequacy will be higher (darker) than theimage density of the surrounding base material.
Excess weld reinforcement is an area of a weld that has weld metal added in excess of that specified byengineering drawings and codes. The appearance on a radiograph is a localized, lighter area in the weld. A visualinspection will easily determine if the weld reinforcement is in excess of that specified by the engineeringrequirements.
Cracks can be detected in a radiograph only when they are propagating in a direction that produces a change inthickness that is parallel to the x-ray beam. Cracks will appear as jagged and often very faint irregular lines.Cracks can sometimes appear as "tails" on inclusions or porosity.
Discontinuities in TIG weldsThe following discontinuities are unique to the TIG welding process. These discontinuities occur in most metalswelded by the process, including aluminium and stainless steels. The TIG method of welding produces a cleanhomogeneous weld which when radiographed is easily interpreted.
Tungsten inclusions. Tungsten is a brittle and inherently dense material used in the electrode in tungsten inertgas welding. If improper welding procedures are used, tungsten may be entrapped in the weld.Radiographically, tungsten is more dense than aluminium or steel, therefore it shows up as a lighter area with adistinct outline on the radiograph.
~
Oxide inclusions are usually visible on the surface of material being welded (especially aluminium). Oxideinclusions are less dense than the surrounding material and, therefore, appear as dark irregularly shapeddiscontinuities in the radiograph.
Discontinuities in Gas Metal Arc Welds (GMAW)
The following discontinuities are most commonly found in GMAW welds.
Whiskers are short lengths of weld electrode wire, visible on the top or bottom surface of the weld orcontained within the weld. On a radiograph they appear as light, "wire like" indications.
Burn-Through results when too much heat causes excessive weld metal to penetrate the weld zone. Oftenlumps of metal sag through the weld, creating a thick globular condition on the back of the weld. These globs ofmetal are referred to as icicles. On a radiograph, burn-through appears as dark spots, which are oftensurrounded by light globular areas (icicles).
