Manual onPANORAMICGAMMA IRRADIATORS(CATEGORIES II AND IV)

Incorporating:Applications GuideProcedures GuideBasics Guide

IKlERiiAl'iONAL

PRACTICAL RADIATION SAFETYMANUAL

Manual onPANORAMIC GAMMA IRRADIATORS

(CATEGORIES II AND IV)

Incorporating:Applications GuideProcedures Guide

Basics Guide

INTERNATIONAL ATOMIC ENERGY AGENCY

MANUAL ON PANORAMIC GAMMA IRRADIATORS(CATEGORIES II AND IV)

IAEA, VIENNA, 1996IAEA-PRSM-8 (Rev.1)

© IAEA, 1996

Permission to reproduce or translate the informationin this publication may be obtained by writing to the

International Atomic Energy Agency,Wagramerstrasse 5, P.O. Box 100, A-1400 Vienna, Austria.

Printed by the IAEA in AustriaMarch 1996

These drafts were again revised by R. Wheelton from theNational Radiation Protection Board in the UK and B. Thomadsenfrom Wisconsin University in the USA. In a second Advisory Groupmeeting held in Vienna in September 1990, the first six reviseddrafts were reviewed by P. Beaver (UK), S. Coornaert (France),P. Ferruz (Chile), J. Glenn (USA), B. Holliday (Chairman; UK),J. Karlberg (Sweden), A. Mendonca (Brazil), M.A. Mohamad-Yusuf(Malaysia), J.C. Rosenwald (France), R. Wheelton (UK),A. Schmitt-Hannig (Germany), and P. Ortiz and P. Zuniga-Bello(IAEA). The drafts of the practical radiation safety manuals Nos 7and 8 were reviewed by R.G McKinnon (Canada), R.A. Gauthier(Canada), J. Gilat (Israel), G.J. Koeteles (Hungary), E. Sgrilli (Italy),R. Wheelton (UK) and P. Zuniga-Bello (IAEA) in a consultantsmeeting in December 1992. Finalization of the first six manuals wascarried out by A. Schmitt-Hannig, Federal Office of RadiationProtection (Germany) and P. Zuniga-Bello (IAEA). Finalization ofmanuals Nos 7 and 8 was carried out by P. Zuniga-Bello (IAEA).

FOREWORD

The use of radiation sources of various types and activities iswidespread in industry, medicine, research and teaching in virtu-ally all Member States of the IAEA and is increasing. Although anumber of accidents have caught the attention of the public in re-cent years, the widespread use of radiation sources has generallybeen accompanied by a good safety record. However, the controlof radiation sources is not always adequate. Loss of control ofradiation sources has given rise to unplanned exposures to workers,patients and members of the public, sometimes with fatal results.

In 1990 the IAEA published a Safety Series book (SafetySeries No. 102) providing guidance on the safe use and regulationof radiation sources in industry, medicine, research and teaching.However, it was felt necessary to have practical radiation safetymanuals (PRSM) for different fields of application aimed primarilyat persons handling radiation sources on a daily routine basis,which could at the same time be used by the competent authorities,supporting their efforts in the radiation protection training of work-ers or medical assistance personnel or helping on-site manage-ment to set up local radiation protection rules.

A new publication series has therefore been established. Eachdocument is complete in itself and includes three parts:— Applications Guide — which is specific to each application of

radiation sources and describes the purpose of the practice, thetype of equipment used to carry out the practice and the precau-tions to be taken.

— Procedures Guide — which includes step by step instructionson how to carry out the practice. In this part, each step isillustrated with drawings to stimulate interest and facilitateunderstanding.

— Basics Guide — which explains the fundamentals of radiation,the system of units, the interaction of radiation with matter, radi-ation detection, etc., and is common to all documents.

The initial drafts were prepared with the assistance of S. Orr(UK), T. Gaines (USA) and R. Wheelton (UK) acting as consultants,and the help of the participants of an Advisory Group meetingwhich took place in Vienna in May 1989: J.C.E. Button (Australia),A. Mendonga (Brazil), A. Olombel (France), F. Kossel (Germany),Fatimah, M. Amin (Malaysia), R. Siwicki (Poland), J. Karlberg(Sweden), A. Jennings (Chairman; UK), R. Wheelton (UK), J. Glenn(USA) and A. Schmitt-Hannig and P. Zuniga-Bello (IAEA).

CONTENTS

Applications Guide 7

Gamma irradiation 7Panoramic gamma irradiators 9Source containment and storage 12Safety systems 14Operating and maintaining irradiators 21Dealing with incidents 25

Procedures Guide 27

Basics Guide for Users of Ionizing Radiation 43

Production of radiation 44

Radiation energy units 46Radiation travelling through matter 46Containment of radioactive substances 50The activity of sources 51Measurement of radiation 53Radiation and distance 56Examples of calculations 58Radiation and time 59Radiation effects 59

APPLICATIONS GUIDE:PANORAMIC GAMMA IRRADIATORS

(CATEGORIES II AND IV)

Gamma Irradiation

Gamma irradiators are facilities in which matter may bedeliberately and safely irradiated, i.e. exposed to largedoses of gamma radiation. By ionizing the atoms andmolecules that compose the matter the radiation can frac-ture molecular links. The ions are then potentially free tomake new connections and form different molecules. Theprocess has many applications.

Irradiation can be used as a catalyst in chemical reactions.Ionized molecules may initiate processes which are neces-sary to form new substances. This method has been uti-lized for the industrial production of ethyl bromide, whichis a volatile liquid used in the synthesis of organicmaterials.

Matter may also be irradiated to modify its behaviour or toinduce beneficial properties. This may be achieved when,following their ionization, the molecules simply reform,realign or recombine in an altered format. For example,polyethylene molecules 'cross-link', bestowing the materialwith the ability to stretch without breaking. Irradiated poly-ethylene is used as transparent packaging for food andother products.

The precisely structured molecules essential to life do notbenefit from irradiation. The changes introduced may notonly disrupt the life form, they may also cause its death.This provides irradiators with some of their most beneficialand commercially important applications: the preservationof foodstuffs and the sterilization of medical supplies. Thebacteria and other life forms which infect and infest theseproducts may be destroyed by irradiation. Substancessuch as human and animal wastes have also been steri-lized by irradiation.

Radiation induced effects are dose dependent and thosementioned above all require very large doses. For exam-ple, the different bacteria which infect food and medicalsupplies vary in their sensitivity to radiation. Some arekilled by a dose of 4 kGy while others survive up to 40 kGy.

7

Product

Plastics

Meat, poultry, fish,shellfish, somevegetables, bakedgoods, preparedfoods

Surgical dressings,sutures, catheters,syringes, somePharmaceuticals,implants

Spices and otherseasonings

Meat, poultry andfish

Strawberries andsome other fruits

Grain, fruit andvegetables

Bananas, avocados,mangoes, papayas,guavas and certainother non-citrusfruits

Pork

Potatoes, onionsand garlic

Typical doses(kGy)

1-2500.2-30

20-70

15-25

1-30

1-7

1-4

0.01-1

0.25-0.35

0.08-0.15

0.05-0.15

Intended effect

Cross-linking.Grafting or bondingpolymers

Sterilization. Productcan be stored atroom temperature

Sterilization. Productscan be pre-packed,ready for single use

Kills a variety of micro-organisms andinsects

Delays spoilage. Killscertain bacteria whichspoil food (e.g.salmonella)

Extends shelf life bydelaying mould growthand retarding decay

Kills insects orprevents them fromreproducing. Maypartially replaceinsecticides

Delays ripening

Kills embryo parasiticworms (trichinae) inmuscle tissue

Inhibits sprouting

Materials which may damage the irradiator or source, such asexplosives, pyrophoric substances and corrosive or heat sen-sitive materials should not be irradiated.

8

Irradiation may also introduce undesirable effects. Somematerials degrade and become brittle or discoloured atdoses of between 5 and 15 kGy. A dose of 2 kGy signifi-cantly retards the decay of strawberries without adverselyaffecting the quality but at similar doses cherries and rasp-berries tend to soften. The texture of nectarines isadversely affected, as is the taste of some oranges.

Industrial irradiators must accurately and uniformly deliverthe appropriate doses under conditions of maximumproduction rate. Typical doses required to achieveintended desired effects on various products are listed inthe table.

Panoramic Gamma Irradiators

The doses described are delivered using radiation sourcesthat produce very high dose rates. These very high doserates, together with panoramic exposures which allowsimultaneous irradiations, maximize product throughput.

Radionuclides with high gamma factors (see BasicsGuide), particularly 60Co and sometimes 137Cs, are uti-lized. Very large activities of about 10-200 PBq (1 PBq =1015 Bq) 60Co are generally used. The radioactive materialis encapsulated in special form sealed source capsules.

MODULE

SEALEDSOURCE

HOIST CABLE

SOURCERACKGUIDECABLES

PLAQUESOURCERACK

PENCILCYLINDRICALSOURCERACK

CAGE MODULE

Irradiator source configurations.

The capsules are given secondary encapsulation withinmetallic, usually stainless steel, tubes called pencils. Typi-cally, forty-two pencils are mounted within a flat or cylindri-cal form called a module and arranged to provide uniformirradiation of the product. A module may also contain non-radioactive spacers or dummy pencils to provide therequired spatial array of active material. The modules (upto twenty-four) are assembled onto a source rack whichensures that the pencils cannot be removed. Special toolsare required to remove the pencils. Each pencil has anengraved serial number which enables its position on therack to be recorded.

The product, production rates and the overall size of eachfacility will dictate the most suitable manner in which toirradiate products. The magnitude and uniformity of thedose are assured by product movements around thesource. These take into account: the source to product dis-tance, the product box size, the product densities and theexposure times. Among the industrial irradiators inwidespread use there are two popular design systems:

(1) Overlapping product: A system in which the totalheight of the product being irradiated is muchgreater than the height of the source. This requiresthe product to make two or more complete passesthrough the irradiator at different elevations to

IRRADIATION PATHWITH FOUR PASSES

CARRIER

SOURCE RACK

An overlappingproduct arrangement.

10

achieve a uniform dose. This system can be furthersubdivided into two designs:

(a) Conveyor bed systems in which the products areplaced in cardboard or aluminium containerscalled tote boxes and loaded onto a conveyorbed. A typical box size is 0.25 m3. Pneumaticpistons called cylinders move the tote boxesaround the source. Multiple passes increase themagnitude and distribution of the dosedelivered.

(b) Overhead monorail systems which convey alu-minium carriers past the source. A typical carriersize is 1.8 m3. Elevators and cylinders automati-cally transfer products between shelves in thecarriers and with multiple passes the productsaccumulate a uniform dose distribution.

(2) Overlapping source: A system in which the sourceis taller than the product being irradiated. Theproduct normally requires only one complete passthrough the irradiator to achieve a uniform dose. Themost common design employs overhead monorailsystems which convey large carriers (up to 3 m high)

IRRADIATION PATHWITH FOUR PASSES

CARRIER

SOURCE RACK

An overlappingsource arrangement.

11

past even larger (4 m high) source racks which over-lap the products. Multiple passes, and on somefacilities rail networks which recycle selected car-riers for incremental irradiations, improve the dosedistribution.

The timing and sequential operation of cylinders are auto-matically controlled and monitored by a programmableelectronic system. This may also be referred to as aprogrammable logic control. Older units employ relay logicpanels. The conveyor or monorail may also automaticallyconvey the products to and from the start of the irradiationpath in a continuous mode of operation. At facilities whichoperate only in batch mode, groups of totes or carriersmight be manually moved to and from the irradiation pathbut only while the source is shielded. Some batch facilitiesalso use automatic movement of carriers to and from theirradiation room in which irradiations take place.

Source Containment and Storage

The control console for the process is outside the irradia-tion room. A massive concrete biological shield surroundsthe irradiation room to protect the operator and othersworking outside while panoramic irradiations are inprogress. Roof plugs, access ports and other potentialweaknesses in the shield which are necessary to allow the

HOIST

PLANTROOM

CONTROL/DISPLAY PANEL

PERSONNEL DOOR INTO MAZE

PRODUCT FEED/DISCHARGE THROUGH MAZE

SOURCE RACKGUIDE CABLES

Construction and layout of atypical Category IV irradiator.

12

facility to be serviced are designed to prevent radiationleakage. Further information on the design of shielding isgiven in the IAEA Practical Radiation Safety Manual onShielded Enclosures. Product feed and discharge pointsand personnel doors into the irradiation room are con-trolled by a range of devices described below. Humanaccess is only permitted when the radiation source is fullyshielded when it is not in use. The radiation level in theirradiation room returns to normal background when thesource has returned to this shielded position.

Industrial panoramic irradiators are categorized1 accord-ing to the type of source storage arrangement:

A Category II irradiation room contains a dry storage pit ora container constructed of solid materials into which thesource is moved by remote control to be shielded when itis not in use.

A Category IV irradiation room contains a deep water filledpool into which the radiation source is lowered by remotecontrol when it is not in use. The water acts as a shield andinteracts with the gamma emissions producing a visible,harmless, bright blue underwater glow called Cerenkovradiation in the vicinity of the source. The water is recircu-lated through a treatment plant where it is deionized,filtered and chilled to minimize potential corrosion of thepool liner, source pencils and source rack and to keep thepool clean.

During irradiations, the atmosphere inside the irradiationroom is ionized, forming noxious gases such as ozone(O3) and nitrogen oxides (NXOX). After the radiation sourcehas returned to its stored position, entry to the cell can bedelayed whilst the ventilation continues to extract thegases and replace them with fresh air. The smell of ozoneis usually noticeable even at relatively low concentrations.At higher concentrations it may cause breathing difficul-ties.

1 Irradiators in categories I and III have small irradiationchamber volumes which are inaccessible to humans. They aremainly used in research applications and are the subject of theIAEA Practical Radiation Safety Manual on Self-Contained Irradia-tors (Categories I and III).

13

The water treatment units, ventilation fans, air compres-sors, source hoist and other equipment which are essentialto the operation of the facility are housed in rooms adjacentto and above but outside the cell. These may be referredto as plant rooms, equipment rooms and penthouses.

All equipment (irradiators, sources, auxiliary equipment,etc.) must be designed and produced by experienced andreputable manufacturers in accordance with acceptedinternational standards and approved by national compe-tent authorities.

Safety Systems

Industrial irradiators are equipped with safety systemsdesigned to prevent injury and radiation exposure of theoperators and other persons, to prevent or limit damage tothe facility, and to control the doses to products withinprescribed limits. Some are part of the normal operation ofthe facility; others provide warnings or caution indicatorsthat will draw attention to abnormal but not hazardous con-ditions; and the remainder are fault indicators which willalert the operator to serious problems whilst automaticallyimplementing appropriate actions such as returning thesource to the shielded position and preventing entry to theirradiation room.

Safety systems are usually based on the use of timers,radiation detectors and electromechanical or physicaldevices. These systems and devices are expected to meetcertain criteria:

(a) Systems and devices should combine to provide pro-tection in depth so that in the event of one systemfailing, a second or third system can be relied uponto provide the intended protection.

(b) There should be redundancy and diversity built intoeach system to reduce the risk of failure. Principalcomponents in each system might be duplicated(redundancy) by supporting devices of different type(diversity) to reduce the risk of multiple failures.

(c) Systems and devices should be independent so thata fault in the irradiator does not impair the safety sys-tem which is intended to mitigate the fault. A fault inone system should not cause the collapse of others.

14

(d) Systems should always fail to safety, that is theyshould be designed and installed to be tamper-proofand difficult to override without the use of specialtools. Failure of a system should result in safeconditions.

Examples of safety systems which are typical of those fit-ted to common irradiator facilities are given below.

(1) Procedural control systems

The most acceptable means of preventing access to theirradiation room during irradiations is to ensure that onlyone master key is available to operate the control consoleand the personnel door lock. In other words, the controlconsole and personnel door must be interlocked. When thesource is fully shielded the master key can be removedfrom the control console to prevent its use. The key maythen be used to unlock the door. The same key must betaken into the irradiation room and used to operate a delaytimer key switch to begin the startup sequence. Thestartup sequence may also be described as a search andlock-up procedure because, by requiring the operator toenter the irradiation room, it confirms that the room is clearof other personnel prior to exposure of the source. Promi-nent beacons and audible signals both inside and outsidethe irradiation room automatically provide warnings thatthe startup sequence has been initiated and that a radia-tion hazard exists inside the room.

(2) Feedback control systems

Switches are installed at the limits of travel of the sourcerack, individual cylinders and product line doors toproduce signals which indicate their positions to theprogrammable logic control (PLC). When actions areinitiated, the PLC monitors the time taken to complete themovement. Individual or co-ordinated actions that takelonger than preset values result in appropriate fault indica-tions on a display panel. These may suggest that mechani-cal breakdown, product blockages or other problems haveoccurred.

15

(3) Timed control systems

The PLC logs the time periods between product move-ments and provides a fault indication when no movementoccurs within an allotted time. Two separate timers controland monitor each irradiation cycle to ensure the accuracyof the product dose. Disparities between the timers resultin a fault indication. These fault indications may suggestproduct overdose or the improper arrangement of carriers.

(4) Radiation monitoring systems

Fixed monitors and portable radiation survey meters areutilized.

A special radiation monitor must be mounted on the walladjacent to the personnel entrance, with its radiationdetector mounted on the wall of the irradiation room. It isinterlocked with the maze access door such that personnelcannot enter unless the source is safely shielded andnormal (background) radiation dose rates exist in the irradi-ation room.

Monitors shall be mounted at the discharge conveyor adja-cent to the product output port barrier and interlocked withthe machine operation. If the radiation level in that arearises above a preset level, the source pass mechanism andconveyor system will automatically stop and the source, ifup, will return to its fully shielded position.

In Category IV irradiators monitors should be located adja-cent to the water deionizer beds. Their purpose is to pro-vide early detection in the unlikely event of a source pencilleak. The radioactive substance 137Cs is not recom-mended for wet storage. These monitors should be inter-locked with the machine operation so that if radiation isdetected in the water deionizer beds, the source passmechanism, conveyor system and water circulation systemwill automatically stop and the source will return to its fullyshielded position.

If any of the monitors provides an alarm, the personnelaccess door, which is interlocked with these monitors,cannot be opened.

The portable dose rate meter must always be carried bythe operator when entering the irradiation room. This is

16

achieved by chaining it to the single master key which hasto be removed for the control panel to unlock the personneldoor. A second portable dose rate meter should beprovided as a back-up during calibrations and repairs.These instruments must be capable of measuring occupa-tional exposure rates of, for example, 1 ̂ Sv per hour up toseveral millisieverts per hour, without saturation.

Sensor monitoring systems

A number of transducers are installed to monitor condi-tions and to provide fault indications including the auto-matic shutdown of the irradiator in the event of problemsoccurring. The source rack is protected against excessiveheat by heat sensors mounted on the irradiation room ceil-ing and a smoke detector which samples irradiation roomair will stop ventilation and return the source to safestorage. The irradiation chamber should be equipped witha fire extinguishing system capable of extinguishing fireswithout personnel entering the irradiation room. A seismicdetector on the biological shield responds to vibration toadvise of potential damage due to an earthquake. A flowswitch in the ventilation duct indicates failures which couldresult in the accumulation and leakage of noxious gasesfrom the irradiation room. A pressure switch indicateswhen there is low air pressure to the source hoist, possiblyas a result of a compressor fault. Sensors located alongboth edges of the source rack guide detect any collision ofthe carrier or product with the source rack that couldobstruct the source. On tote facilities a metal shroudaround the source rack provides protection against suchcollisions. Switches are mounted on pallet turntables toindicate when rotation ceases prematurely and productsare being irradiated non-uniformly.

Typical control room and safety systems

The personnel door at the entrance to the maze is electri-cally interlocked with the source and monitor so that thesource cannot be raised if the door is open. Conversely,the door cannot be opened if there are higher than normal(background) radiation fields in the irradiation room even ifthe controls indicate that the source is in the fully shieldedposition. The door can be opened from the inside at anytime. The source interlock chain, or other systems located

17

PERSONNEL DOORINTERLOCKS

SOURCE UNSHIELDEDWARNING LIGHTS \

IRRADIATION CELL MONITORALARMS AND TEST SOURCE \

SEISMIC DETECTOR

INDICATORS ONDISPLAY PANEL

RADIATIONMONITORS

TESTSOURCE

0 U C TDISCHARGE

BARRIER DCX)RHOIST SAFETY VALVE CHAIN CONTROL/DOOR KEY

INTERLOCK

Control room and safety systems.

across the maze entrance, must be secured to allow opera-tion of the irradiator. The source cannot be raised if thechain is not secured.

An independent access control system, usually in the formof pressure mats or photoelectric cells, is normallyinstalled at each entry or exit port. This second systemdetects inadvertent entry of personnel when the source isexposed and acts as a backup if the primary door inter-locks fail. The source will automatically return to the fullyshielded position and visible and audible alarms will beprovided.

(7) Process stop systems

An emergency stop button on the control console andemergency stop cables inside the irradiation room, if used,immediately cause the automatic shutdown of the irradia-tor. Failure of the control system, any safety system trip ormalfunction in a safety system will also cause the auto-matic return of the source to the fully shielded position. Atthe end of a batch irradiation an indication is provided onthe display panel to provide a controlled stop upon thecompletion of a batch load, a batch run or a batch unload.

(a) Visual indicators

Lights on the control console indicate the source position,and a radiation warning light is illuminated above the per-

18

sonnel door when the source is not in its fully shielded('DOWN') position. While the source rack is travelling, orif it is not in its fully exposed ('UP') or fully shielded('DOWN') position, an audible signal sounds and a lightlocated above the personnel door will flash.

(b) Pool water level alarm

If the pool water drops below a preset level, a visible andaudible alarm will be activated and the personnel door,which is interlocked with this alarm, cannot be openedfrom the outside without special procedures.

(c) Emergency stop cable and button

An emergency stop cable, located around the walls of theirradiation room and in the access maze, will abort themachine startup procedure or return the source to thestorage position. In either event the conveyors will stop. Anemergency stop button located on the console performsthe same function.

(d) Fire

In the event of smoke, fire or an excessive rise in tempera-ture in the irradiation room, sensors interlocked with thecontrols automatically stop plant operation, lower thesource to the fully shielded position, shut down the ventila-tion system and even trigger fire fighting apparatus withoutpersonnel entering the irradiation room.

(e) Ventilation system

The ventilation system is interlocked with the control con-sole so that the plant cannot be operated unless the venti-lation system is on. In the event of fire the ventilationsystem will automatically shut down.

(f) Power supply

If a power failure occurs when the source is exposed, thesource will automatically be lowered to the fully shieldedposition. The personnel door cannot be opened during apower failure. An emergency power supply should beavailable.

19

ROOF PLUG SWITCH(S) ^HEAT AND SMOKE SENSORS

1163

, , ALARM (MAZE)RADIATION

1-SAFETY DELAY

TIMER KEY SWITCH

DETECTORS

Q -

fvu* M f t * AM*

MNWW»»M.Tfty

SOURCE UPSWITCH(S)

NORMAL LEVELFLOAT SWITCH

LOW LEVEL FLOAT SWITCH

Irradiation cell and safety systems (Category IV)(for clarity, product transport system has been

omitted).

(8) Source storage alerts

In Category IV irradiators normal level, high level and lowlevel float switches are installed in the pool to maintain thevolume of water within their limits and to control the fillingoperation. The low level switch prevents entry to the irradi-ation room and provides audible and visible alarms. InCategory II, extremity switches are installed to shieldingblocks to control their movement and to provide confirma-tion of their closure. These devices also provide signals forcaution indicators on the display panel.

(9) Entry alerts

Caution indications are displayed when the personnel doorhas been opened and someone in the interim may bepreparing to start up the irradiator. A separate indication isprovided following irradiations when there is an entry delayto allow the extraction of noxious gases from the irradiationroom.

(10) Status alerts

Indications are displayed when: the programmable logiccontrol (PLC) is on and functioning correctly; the PLC is not

20

functioning correctly; the source is exposed; the source issafe; and the source is travelling.

Operating and Maintaining Irradiators

The decision to install and operate Category II and IVirradiators (as any other radiation facility) must be madeknown to the national competent authority, which mayspecify certain limitations or conditions under the authori-zation to operate a facility.

Preparations should include the appointment of a Radia-tion Protection Officer to be responsible for the equipmentand to ensure that it is maintained and operated in a safemanner.

Work with Category II and IV irradiators extends over fourdifferent operational modes: routine operations; normalmaintenance; special maintenance and routines; andmajor modifications and repairs.

(1) Routine operations

Although industrial irradiators contain a great number ofautomatic systems, their safe operation is also very depen-dent on the skill of the operators. It is important to developa strong managerial structure that has clearly definedresponsibilities, organized procedures and appropriate for-mal training at all operational levels. The names andresponsibilities of individuals should be contained in writ-ten operating procedures.

The operators, who satisfy the standards required and areauthorized to carry out the routine irradiation of products,are unlikely to accumulate a fraction of any relevant doselimit in a year. Nevertheless, it is strongly recommendedthat each operator wear a suitable personal dosimeter (seeBasics Guide) and that dose records be maintained. Careshould be taken not to confuse personal dosimeters andproduct dosimeters. The latter type comprise a range ofaqueous solutions, plastics, acrylics, perspex and dyefilms which, in general, change colour/shade in responseto very high doses. Product dosimeters are used to confirmthe delivery of satisfactory irradiation doses.

21

A record called the Irradiator Log should contain relevantdetails of every irradiation and maintenance carried out. Aregister of visitors should also be kept including, if visitorsenter the irradiation room, any dose recorded by a directreading dosimeter.

(2) Normal maintenance

Electrical, mechanical and building services make animportant contribution to the radiation safety of irradiatorfacilities. Even simple faults can develop into seriousproblems affecting the source rack. The operator mustcontinually be alert to even minor malfunctions and equip-ment wear and tear. All safety systems should be tested toverify correct operation on a monthly basis. Abnormal con-ditions should be reported to a supervisor and if a problemoccurs with any of the safety systems, irradiations must notcontinue until the problem and its cause have been cor-rected. Safety systems must not be bypassed as this couldlead to serious injury or even death from overexposure toradiation. Routine maintenance and repair may preventproblems arising and should not involve exposure toradiation.

Regular inspections of the facility should be carried out bysuitably trained technicians and parts which are defectiveor worn should be repaired or replaced. Replacement partsshould be obtained from or approved by the supplier of thefacility, especially parts that are likely to be exposed toradiation. Apart from other considerations, componentswhich are not 'hardened to radiation' may degrade, withserious consequences. Records should be maintaineddescribing all work carried out on the irradiator.

Normal maintenance must include regular, recorded radia-tion measurements by the operator. These should showthe dose rates at all readily accessible surfaces outside thebiological shield while the source is exposed and similarmeasurements inside the irradiation room while the sourceis safely shielded.

(3) Special maintenance and routines

In order to maintain the full function and safe operation ofirradiators, periodic special maintenance and routines willneed to be performed. This work may require the special-

22

ized equipment and skills of the manufacturer's agentsand other experts. Because of the potential for radiationexposure, workers classified as radiation workers whoreceive medical surveillance should be employed to carryout this work.2

As the activity of the source decays, progressively longerirradiations become necessary. The activity on the rackneeds to be periodically replenished and possibly redis-tributed to maintain dose uniformity. New source pencilsare delivered to the irradiator in a container which meetsthe requirements of IAEA Safety Series No. 6 (Regulationsfor the Safe Transport of Radioactive Material). The Type Bcontainers used provide sufficient shielding to transport upto about 7.5 PBq. When a container arrives, controlledareas may need to be set up around it by means of a bar-rier and suitable notices or other means.

Source loading.

A container loaded with pencils for a Category II irradiatortypically keys into a wall duct and is bolted to the outsideof the biological shield. When a channel is established,

2 This requirement may vary from country to country depend-ing on national regulations.

23

long tools are used to release an old source; it is pushedinto an empty tube in the container and a new source ispulled into its place. Some Category II irradiators have thesource lifted vertically from the shielded container. Thecontainer for a Category IV irradiator is lowered into thepool through a roof duct; a cage containing source pencilsis removed from the container and old and new pencils aremanipulated under water by means of long tools. Oldsources must be transported from the facility for controlleddisposal, usually by the supplier. This work is normally car-ried out by the manufacturer or agents.

Sources are continuously tested for gross leakage of radio-active material by the radiation monitors installed on theresin columns. Separate checks on the water and air filtersare made at regular intervals and before their disposal.

The installed and portable radiation monitors must betested by the operator as part of the irradiation room entryprocedure. The installed monitors have a built-in test andadditional checks on the radiation detector responseshould be performed using a low activity test source. Theportable dose rate meter checks include a battery test anda response test using a low activity source. All radiationmeasuring instruments should be accurately tested andcalibrated by a qualified person at periodic intervals.

(4) Major modifications and repairs

The design parameters3 of some irradiator facilitiesexceed the initial requirements in order to allow for futureupgrading. Greater activities may then later be installed tomatch production increases without the dose rates outsidethe irradiation room exceeding certain levels, for example7.5 /xSv/h or preferably 2.5 jtSv/h.

By sharing collective experiences, the operators can assistthe manufacturer to develop and improve all facilities.The manufacturer may recommend improvements whichreduce the risks of problems, incidents and accidentsoccurring. Proposals for major modifications and repairsshould be discussed with the manufacturer's agents andapproved by the competent authority. It is possible that the

3 See the IAEA Practical Radiation Safety Manual onShielded Enclosures.

24

advice obtained will help to ensure that the work is carriedout safely and enhances the safety of the facility.

Records containing precise details of all modificationsmust be permanently retained.

Dealing with Incidents

A thorough assessment of each facility and the equipmentand procedures in use will enable problems or abnormalsituations to be foreseen so that contingency plans can beprepared to deal with them.

For example, the plans might define immediate actions todeal with the following:

— An earthquake, tornado or other naturalphenomenon;

— A fire or explosion inside or outside the irradiationroom;

— Sources failing to expose or return to the safestorage position;

— Damage affecting the source rack or product holder;— Radioactive substance on the product, water/air

filters, or leak test wipe, etc.;— Damage to the pool liner, plumbing or storage pit;— Loss of water;— Degradation of electrical, mechanical or structural

components;— Failure of ventilation or water treatment equipment;— Higher than normal radiation readings from installed

monitors, portable dose rate meters or personaldosimeters;

— Failure of access control or any safety system;— Fault and caution indications or apparent problems

that the control display panel fails to indicate.

The contingency plans should contain the names and/orjob title and contact telephone number of each person whohas a responsibility to implement any action defined in theplans. The names of experts that are familiar with the facil-ity and are available to discuss possible solutions or pro-vide other assistance should be kept in the contingencyplans. This should include the name of the supplier. Theregulatory bodies that must be notified should also bespecified. Plans that involve outside agencies, such asemergency services, should be discussed and agreed to in

25

advance. Appropriate training must be given to enable allnamed persons to carry out their part in the plans. Suitabledrills should be rehearsed to ensure competence. Practis-ing may also indicate whether any special equipmentshould be readily available, for example, barrier equip-ment, signs or additional dosimeters.

An irradiator may experience periodic stoppages due toproduct jams that do not compromise the safety of thesource. Before taking action to correct such a situation, theoperator should consult with the Radiation ProtectionOfficer to confirm that the source is in the fully shieldedposition.

In other situations the security of the source may be inquestion or radiation monitors may indicate the presenceof radiation. The best course of action to take when sucha problem occurs is usually to vacate the irradiation room,close the door and immediately report the problem to asupervisor, the Radiation Protection Officer or manager.Only those actions are justifiable which stabilize theproblem and do not involve risk to human life. It is impor-tant to fully assess the severity and implications of everyabnormal situation which arises before implementingremedial measures. Display panel indications, observa-tions and radiation measurements provide useful clues.

Any abnormal situation which may have resulted in aninternal or external dose to a person or any high dosereported on a dosimeter must be investigated and a writtenreport prepared. It is important to determine whether thesuspected or reported dose was actually received and alsowhether some part of the body has received a higher dosewhich might result in localized tissue injury. If a high doseof radiation has been received by an individual, urgentmedical treatment will be required.

26

PROCEDURES GUIDE:PANORAMIC GAMMA IRRADIATORS

(CATEGORIES II AND IV)

27

to00

Follow authorized procedures when working in and around panoramic gammairradiators.

Only trained and authorized workers should assist in irradiations. Dosimetersshould be worn by workers who are permitted to enter the irradiation room.Some work in Controlled Areas may be carried out only by workers who havealso had medical examinations. No worker should receive a dose greater thana dose limit (currently 50 mSv to the whole body) in a year.

Before proceeding with the work, read and ask questions about these safetyguides, operating procedures and contingency plans. Keep them available forreference.

Discuss the contributions and tasks of all workers involved.

DANGER

o

CD

Examine the irradiator and identify its main features.

Become familiar with the irradiation process and the safety systems which areessential for the safe conduct of the work.

Ensure that legible warning notices are displayed and that the warning signalsare labelled, understood and obeyed.

Each worker has a responsibility to ensure that only those persons who aretrained and authorized to operate controls do so. The master control key shouldbe kept by the responsible person in a secure place at all times when the irradia-tor is not in use.

8Examine the irradiator room, carriers and/or tote boxes frequently and keep arecord of regular (for example three-monthly) inspections for damage and wear.Stop irradiations if safety is likely to be compromised.

Formally test the safety systems as recommended by the manufacturer. Forexample, to confirm that fault and caution indicators operate and that all otheractions prescribed actually occur when the sensors are triggered:

— depress or flex microswitches and float switches;— use a soft hammer to tap near seismic sensors;—- blow smoke towards smoke detectors;— apply a gentle flame to the heat sensors, being careful to isolate any auto-

matic sprinkler systems that may be triggered by these actions; and— test radiation detectors using a low activity source.

Check that emergency stop controls are capable of preventing the source beingexposed.

Report any faults to your supervisor, Radiation Protection Officeror manager

DANGER

CONTROLLED. i AREA. . /

CO

Periodically use a portable dose rate meter and record the measurements at allreadily accessible positions outside the irradiation room during irradiations andinside the room while the source is shielded.

Be aware that if the whole detector is not irradiated, for example during mea-surements close to narrow gaps around doors, the reading may be less than thedose rates which actually exist. The dose rate meter should slowly scan thesurface.

Compare readings with those previously obtained at the same positions andnote any variations. These could indicate that problems are developing with theirradiator's shielding or the radiation detector.

CON>

Keep a record of regular (weekly, monthly, quarterly, annual or biennial) main-tenance, for example:

— Check that the irradiation room, product loading and unloading areas,plant and equipment rooms are tidy and free from obstructions, flamm-able materials or other potential problems.

— Clean and examine tote boxes or carriers.— Check screws, nuts and bolts for tightness and look for damage to screw

threads and springs.— Use recommended lubricants to clean and maintain any accessible

moving parts.— Confirm that warning signals and indicator bulbs are functioning correctly.— At the recommended intervals, and only when trained and authorized to

do so, carry out tests for leakage of radioactive material in the appropriatemanner required by the competent authority or recommended by themanufacturer of the sealed sources in use.

— Have portable survey meters calibrated at the required intervals.

Report any fault to your supervisor, Radiation Protection Officeror manager

GOCO

Load product containers into tote boxes or carriers carefully. Ensure thatnothing is likely to snag or obstruct their movement up to or throughout theirradiation.

Place product dosimeters to measure the doses delivered.

Move the products to the start of the irradiation path for manual batch process-ing or into a take-up position for automatic irradiations.

oHave a reliable dose rate meter available and checked to ensure that it is oper-ating when entering and working inside the irradiation room.

Before initiating a source exposure, search the irradiation room and recheckanything which seems abnormal.

The operator must be the last to leave. After activating the delay timer keyswitch, walk directly to the personnel door, connect the source hoist safety valvechain barrier (if provided), close and lock the door, and remove the key. Confirmthat the warning signals have operated.

1/X

1r

CAUSESIF JkLAKM SOOHWPULL OORO ANO

LEAVE ROOM/' kIMMtDUkTEf )

-irAnyone who is inside the irradiation room when the startup warnings operatemust move immediately to press the nearest emergency stop button or to pullthe cable to cancel the process. He or she should then go to the personnel door.

CO

After locking the door, insert the key in the control panel and set the controlsto carry out the irradiation process.

Pay attention while the irradiation is in progress. Do not be distracted but watchfor problems arising around the facility as well as indications on the controldisplay panel.

Stop the irradiation if any abnormal situation begins to develop.

Write details of the irradiation in the Irradiator Log.

Report abnormal conditions to your supervisor,Radiation Protection Officer or manager

CO

At the end of the process when the source has returned to a stored position,wait for the required time for dissipation of the gases inside the irradiation room.Make sure the source has returned to the fully shielded position. Remove thekey from the control panel and take it with the portable dose rate meter(attached by chain to the key) to the personnel door.

Switch the dose rate meter on, first testing its batteries and then setting it to alow range value. Holding it close to the test source by the door, check for theexpected response.

Check that the installed monitors are functioning. This is usually done by press-ing the test button on the panel by the door, waiting for the alarm to signal andthen return to normal level (background), indicating that it is safe to enter theirradiation room. If the indicated level does not return to normal, do notunlock the door.

Report any abnormal situation to your supervisor,Radiation Protection Officer or manager

If the irradiation room has to be entered and indications are satisfactory, unlockand open the personnel door.

Unhook the source hoist safety valve chain barrier (if provided) and cautiouslyenter the irradiation room, checking the portable dose rate meter. If it shows anunusual reading, leave immediately and lock the door. If the backup accesscontrol provides an alarm, leave immediately and lock the door.

Inform your supervisor, Radiation Protection Officeror manager immediately

COCD

If a fault develops during an irradiation, the control is likely to automatically stopthe process. Determine the reasons by analysing the sequence of events andindications or data logging entries that preceded the interruption.

Do not attempt to enter the cell

Check which cylinders or mechanisms were operating in case an obstructionhas occurred at that position; examine the indicators to determine which sen-sors) triggered the shutdown; measure the dose rates outside the biologicalshield; and discuss the situation with others.

Check the installed monitor and indicators to confirm that the source is shielded.

Do not attempt to defeat safety systems

Implement only those immediate actions which the contingency plans prescribeto stabilize the situation. For example, if it appears that the installed monitorshave detected a leakage of radioactive substance, the contaminant may be con-tained by turning off the water circulation and/or irradiation room ventilation, ifthis has not automatically occurred.

Inform your supervisor, Radiation Protection Officer or manager

a

Do not act alone

Before taking any action to rectify a problem, discuss it with suppliers or withothers who may have experience of similar situations or may be able to giveadvice about the cause and the methods to correct the problem.

Plan to execute any recovery plan in stages to provide time to carefully considereach action in advance. Do not begin any action before all the necessary equip-ment is available to complete the task safely.

•3

Record details of all maintenance, repairs and replaced parts. Use onlyapproved replacement parts, established methods and suitable equipment andtools to work on the irradiator.

Barriers may need to be set up to mark Controlled Areas before certain worksuch as handling source transport containers, installing new source pencils andtesting sources for leakage of radioactive material is carried out.

BASICS GUIDE FOR USERS OFIONIZING RADIATION

43

BASICS GUIDE FOR USERS OFIONIZING RADIATION

Production of Radiation

Radioactive substances are predictable and continuousemitters of energy. The energy emitted can be in the formof alpha (a) particles, beta (/3) particles and gamma (7)rays. Interaction of these radiations with matter can, incertain circumstances, give rise to the emission of X raysand neutron particles.

Gamma and X rays consist of physical entities called pho-tons that behave like particles, suffering collisions withother particles when interacting with matter. However,large numbers of photons behave, as a whole, like radio orlight waves. The shorter their wavelength the higher theenergy of the individual photons.

The very high energy of gamma rays and their ability topenetrate matter results from their much shorter wave-lengths.

Y-rays Optical light MicrowaveX-rays Heat Radio

& aiO 3> 25

c ?a» oa> ̂ ~en >,

a>encoo

• a

Spectrum of radiations similar to gamma rays.

44

X rays are produced by an X ray machine only when it iselectrically supplied with thousands of volts. Although theyare similar to gamma rays, X rays normally have longerwavelengths and so they carry less energy and are lesspenetrating. (However, X rays produced by linear accelera-tors can surpass the energies of gamma radiation in theirability to penetrate materials.) The output of X radiationgenerated by a machine is usually hundreds or even thou-sands of times greater than the output of gamma radiationemitted by a typical industrial radioactive source. However,typical teletherapy sources are usually thousands of timesgreater in output than industrial radiography sources.

The gamma rays from iridium-192 (192lr) are of lower ener-gies than those of cobalt-60 (^Co). These are usefuldifferences which allow selection from a wide range ofman-made radionuclides of the one that emits those radia-tions best suited to a particular application.

Beta particles are electrons and can also have a range ofenergies. For example, beta particles from a radionuclidesuch as hydrogen-3 (3H) travel more slowly and so havealmost one hundredth of the energy of the beta particlesfrom a different radionuclide such as phosphorus-32 (32P).

Neutron particle radiation can be created in several ways.The most common is by mixing a radioactive substancesuch as americium-241 (241Am) with beryllium. When it isstruck by alpha particles emitted by the americium-241,beryllium reacts in a special way. It emits high energy, fastneutrons. Americium-241 also emits gamma rays and sofrom the composite americium-241 /beryllium source areproduced. Another way to create neutrons is using aradiation generator machine combining high voltages andspecial targets. Special substances in the machine com-bined with high voltages can generate great numbers ofneutrons of extremely high energy.

Alpha particles in general travel more slowly than beta par-ticles, but as they are heavier particles they are usuallyemitted with higher energy. They are used in applicationswhich require intense ionization over short distances suchas static eliminators and smoke detectors.

45

Radiation Energy Units

A unit called the electron-volt (eV) is used to describe theenergy of these different types of radiation. An electron-volt is the energy aquired by an electron acceleratedthrough a voltage of one volt. Thus, one thousand voltswould create a spectrum (range) of energies up to1000 eV. Ten thousand volts would create X rays of up to10 000 eV. A convenient way of expressing such largenumbers is to use prefixes, for example:

1000 eV can be written as 1 kiloelectron-volt (1 keV);

10 000 eV can be written as 10 kiloelectron-volts (10 keV);

1 000 000 eV can be written as 1 megaelectron-volts (1 MeV);

5 000 000 eV can be written as 5 megaelectron-volts (5 MeV).

Radiation Travelling Through Matter

As radiation travels through matter it collides and interactswith the component atoms and molecules. In a single colli-sion or interaction the radiation will generally lose only asmall part of its energy to the atom or molecule. However,the atom or molecule will be altered and becomes an ion.Ionizing radiation leaves a trail of these ionized atoms andmolecules, which may then behave in a changed way.

After successive collisions an alpha particle loses all of itsenergy and stops moving, having created a short, densetrail of ions. This will occur within a few centimetres in air,the thickness of a piece of paper, clothing or the outsidelayer of skin on a person's body. Consequently, radio-nuclides that emit alpha particles are not an external haz-ard. This means that the alpha particles cannot causeharm if the alpha emitter is outside the body. However,alpha emitters which have been ingested or inhaled are aserious internal hazard.

Depending upon their energy, beta particles can travel upto a few metres in air and up to a few centimetres in sub-stances such as tissue and plastic. Eventually, as the betaparticle loses energy, it slows down considerably and isabsorbed by the medium. Beta emitters present an internalhazard and those that emit high energy beta particles arealso an external hazard.

46

Radionuclide

Americium-241

Hydrogen-3

Phosphorus-32

lodine-131

Technetium-99m

Caesium-137

(Barium-137m)

lridium-192

Cobalt-60

Americium-241/beryllium

Strontium-90/(Yttrium-90)

Promethium-147

Thalium-204

Gold-198

lodine-125

Radium-226

Type of radiation

alphagamma

beta

beta

betagamma

gamma

beta

gamma

betagamma

betagamma

neutrongamma

betabeta

beta

beta

betagamma

Xraygamma

alphabetagamma

Range of energies (MeV)

5.5 to 5.30.03 to 0.37

0.018 maximum

1.7 maximum

0.61 maximum0.08 to 0.7; 0.36

0.14

0.51 maximum

0.66

0.67 maximum0.2 to 1.4

0.314 maximum1.17 and 1.33

4 to 50.06

2.272.26

0.23

0.77

0.960.41

0.0280.035

4.59 to 6.00.67 to 3.260.2 to 2.4

Heavier atoms such as those of lead do absorb a greaterpart of the beta's energy in each interaction but as a resultthe atoms produce X rays called bremsstrahlung. Theshield then becomes an X ray emitter requiring furthershielding. Lightweight (low density) materials are thereforethe most effective shields of beta radiation, albeit requiringlarger thicknesses of material.

47

Promethium-147Thalium-204Phosphorus-32Strontium-90/Yttrium-90

Maximum betaparticleenergy(MeV)

0.230.771.71

2.26

Air(mm)

40024007100

8500

Maximum range

Plastic(mm)

0.63.3

11.7

Softwood(mm)

0.74.0

14.0

Aluminium(mm)

0.261.5

5.2

Gamma rays and X rays are more penetrating. However,as they cause ionization they may be removed from thebeam or lose their energy. They thus become progres-sively less able to penetrate matter and are reduced innumber, that is attenuated, until they cease to be a seriousexternal hazard.

One way of expressing the quality or penetrating power ofgamma and X rays also provides a useful means ofestimating the appropriate thickness of shields. The halfvalue thickness (HVT) or the half value layer (HVL) is thatthickness of material which when placed in the path of theradiation will attenuate it to one half its original value. Atenth value thickness (TVT) similarly reduces the radiationto one tenth of its original value.

Radiationproducer

Technetium-99mlodine-131Caesium-137lridium-192Cobalt-60100 kVp X rays200 kVp X rays

HVT and TVT

Lead

HVT TVT

0.020.72 2.40.65 2.20.55 1.91.1 4.00.026 0.0870.043 0.142

values (cm) in various materials

Iron

HVT TVT

1.61.32.0

5.44.36.7

Concrete

HVT

4.74.94.36.31.652.59

TVT

15.716.314.020.3

5.428.55

Material which contains heavy atoms and molecules suchas steel and lead provide the most effective (thinnest)shields for gamma radiation and X rays.

48

PAPER PLASTIC STEEL LEAO WAX

NEUTRONS

The penetrating properties of ionizing radiations.

Neutrons behave in complex ways when travelling throughmatter. Fast neutrons will scatter (bounce) off much largeratoms and molecules without losing much energy.However, in a collision between a neutron and a smallatom or molecule, the latter will absorb a proportion of theneutron's energy. The smallest atom, the hydrogen atom,is able to cause the greatest reduction in energy.

Hydrogenous materials such as water, oil, wax and poly-thene therefore make the best neutron shields. A compli-cation is that when a neutron has lost nearly all its energyit can be 'captured', that is absorbed whole by an atom.This often results in the newly formed atom becoming aradionuclide, which in many instances would be capable ofemitting a gamma ray of extremely high energy. Specialneutron absorbing hydrogenous shields contain a smallamount of boron which helps to absorb the neutrons.

Damage to human tissue caused by ionizing radiation is afunction of the energy deposited in the tissue. This isdependent on the type and energies of the radiations beingused. Hence the precautions needed to work with differentradionuclides also depend on the type and energy of theradiation.

49

Containment of Radioactive Substances

Radioactive substances can be produced in any physicalform: a gas, a liquid or a solid. Many medical and mostindustrial applications use sources in which the radioactivesubstance has been sealed into a metal capsule orenclosed between layers of non-radioactive materials.Often these sources are in 'Special Form' which meansthat they are designed and manufactured to withstand themost severe tests, including specified impact forces,crushing forces, immersion in liquid and heat stress,without leaking radioactive substance.

1cm

A sealed source, showing the encapsulatedradioactive substance.

All sealed sources are leak tested after manufacture andthe test (also called a wipe test) must be repeated periodi-cally throughout the working life of the source. More fre-quent testing is required for sealed sources which are usedin harsh environments or in applications that are likely tocause them damage. Most sealed sources can remainleak-free and provide good, reliable service for many yearsbut eventually must be safely disposed of and replacedbecause the activities have decayed below usable levels.

Sealed sources present only an external hazard. Providedthat the source does not leak there is no risk of the radio-active substance being ingested, inhaled or otherwisebeing taken into a person's body.

Unsealed radioactive substances such as liquids, powdersand gases are likely to be contained, for example within abottle or cylinder, upon delivery, but may be released and

50

manipulated when used. Some unsealed sources remaincontained but the containment is deliberately weak toprovide a window for the radiation to emerge. Unsealedradioactive substances present both external and internalhazards.

A bottle of radioactive liquid.The rubber cap sealing the bottle may be removed

or pierced to extract liquid.

The Activity of Sources

The activity of a source is measured in becquereis (Bq) andindicates the number of radionuclide atoms disintegratingper second (dps or s~1).

1 Becquerel is equivalent to 1 atom disintegratingper second

Industrial and medical applications usually require sealedsources with activities of thousands or millions of bec-quereis. A convenient method of expressing such largenumbers is to use prefixes, for example:

1 000 becquereis is written 1 kilobecquerel (1 kBq);

1 000 000 becquereis is written 1 megabecquerel (1 MBq);

1 000 000 000 becquereis is written 1 gigabecquerel (1 GBq);

1 000 000 000 000 becquereis is written 1 terabecquerel (1 TBq).

51

The activity of a source is dependent on the half-life of theparticular radionuclide. Each radionuclide has its owncharacteristic half-life, which is the time it will take for theactivity of the source to decrease to one half of its originalvalue. Radionuclides with short half-lives are generallyselected for medical purposes involving incorporation intothe body via oral, injection or inhalation, whereas thosewith relatively longer half-lives are often of benefit for medi-cal, therapeutic (external or as temporary inserts) andindustrial applications.

Radionuclide

Technetium-99m

lodine-131

Phosphorus-32

Cobalt-60

Caesium-137

Strontium-90

lridium-192

Radium-226

lodine-125

Americium-241

Hydrogen-3

Ytterbium-169

Promethium-147

Thalium-204

Gold-198

Thulium-170

Half-life3

6.02 h

8.1 d

14.3 d

5.25 a

28 a

28 a

74 d

1620 a

60 d

458 a

12.3 a

32 d

2.7 a

3.8 a

2.7 d

127 d

Application

Medical diagnostic imaging

Medical diagnostic/ therapy(incorporated)

Medical therapy (incorporated)

Medical therapy (external)Industrial gauging/radiography

Medical therapy (temporaryinserts)Industrial gauging/radiography

Industrial gauging

Industrial radiography, ormedical therapy

Medical therapy (temporaryinserts)

Medical diagnostic/therapy

Industrial gauging

Industrial gauging

Industrial radiography

Industrial gauging

Industrial gauging

Medical therapy

Industrial radiography

a The abbreviation 'a' stands for 'year'.

When radioactive substances are dispersed throughoutother materials or dispersed over other surfaces in the

52

form of contamination, the units of measurement which aremost commonly used are:

(a) for dispersion throughout liquids BqmL"1

(b) for dispersion throughout solids Bqg~1

(c) for dispersion throughout gases(most particularly air) Bqm" 3

(d) for dispersion over surfaces Bqcm"2

An older unit of activity which is still used, the curie (Ci),was originally defined in terms of the activity of 1 gram ofradium-226. In modern terms:

1 Curie is equivalent to 37 000 000 000 dps, that is 37 GBq:

1 nCi 1 /xCi 1 mCi 1 Ci 10 Ci

37 Bq 37 kBq 37 MBq 37 GBq 37 TBq

Measurement of Radiation

Ionizing radiation cannot be seen, felt or sensed by thebody in any other way and, as has already been noted,damage to human tissue is dependent on the energyabsorbed by the tissue as a result of ionization. The termused to describe energy absorption in an appropriate partor parts of the human body is 'dose'.

The modern unit of dose is the gray (Gy). However, in prac-tical radiation protection, in order to take account of certainbiological effects, the unit most often used is the sievert(Sv). For X ray, gamma and beta radiation, one sievertcorresponds to one gray. The most important item ofequipment for the user is a radiation monitoring device.There are instruments and other devices that depend onthe response of film or solid state detectors (for example,the film badge or thermoluminescent dosimeters).

Two types of instruments are available: dose rate meters(also called survey meters) and dosimeters.

Modern dose rate meters are generally calibrated to readin microsieverts per hour (/iSvh"1). However, many ins-truments still use the older unit of millirem per hour(mremh"1). 10 /xSvh"1 is equivalent to 1 mrem-h"1.

53

•— BATTERIES

A typical dose rate meter.

Neutron radiation can only be detected using special doserate meters.

Most dose rate meters are battery powered and some havea switch position that enables the user to check the batterycondition, i.e. that it has sufficient life remaining to powerthe instrument. It is important that users are advised not toleave the switch in the battery check position for longperiods and to switch off when not in use. Otherwise thebatteries will be used unnecessarily.

A check that an instrument is working can be made byholding it close to a small shielded source but some instru-ments have a small inbuilt test source. Workers should beinstructed on the use of test sources since regular checkswill not only increase their own experience but give themconfidence and provide early indication of any faults. It isimportant that users recognize the great danger of relyingon measurements made using a faulty instrument.

A dosimeter measures the total dose accumulated by thedetector over a period of time. For example, a dosimeterwould record 20 /xSv if it was exposed to 10 /xSv-rT1 fortwo hours. Some dosimeters can give an immediate read-ing of the dose. Others, like the film badge and the thermo-luminescent dosimeter (TLD), can only provide a readingafter being processed by a laboratory.

54

(a) Electronicdosimeter

\

368845D]

(b) Thermoluminescentdosimeter

(c) Film badgedosimeter

Personnel dosimeters.

A third type of instrument will be needed by users ofunsealed sources: a surface contamination meter. This isoften simply a more sensitive detector which should beused to monitor for spillages. When the detector is placedclose to a contaminated surface the meter normally onlyprovides a reading in counts per second (cps or s~1) orsometimes in counts per minute (cpm or min"1). It needsto be calibrated for the radionuclide in use so that the read-ing can be interpreted to measure the amount of radio-active substance per unit area (BqcnrT2). There are manysurface contamination meters of widely differing sensi-tivities. The more sensitive instruments will indicate avery high count rate in the presence of, for example1000 Bq-cnrT2 of iodine-131, but different detectors mea-suring the same surface contamination will provide a lowerreading or possibly no response at all. When choosing adetector it is best to use one that has a good detection effi-ciency for the radionuclide in use and gives an audible indi-cation. The internal hazard created by small spillages canthen be identified and a safe working area maintained.

55

>A typical surface contamination meter.

Radiation and Distance

Ionizing radiation in air travels in straight lines. In suchcircumstances the radiation simply diverges from a radio-active source and the dose rate decreases as the inversesquare of the distance from the source.

For example:

If the measured dose rate at 1 m is 400 1

the expected dose rate at 2 m is 100the expected dose rate at 10 m is 4 jiSv-h"1;the expected dose rate at 20 m is 1 /tSv-h"1; etc.

Distance has a major effect in reducing the dose rate.

Solid shields in the radiation path will cause the radiationto be attenuated and also cause it to be scattered invarious directions. The actual dose rate at a point somedistance from a source will not be due only to the primaryradiation arriving from the source without interaction.Secondary radiation which has been scattered will alsocontribute to the dose rate.

However, it is simple to calculate the dose rate at a dis-tance from a source. The primary radiation energies will beconstant and known if the radionuclide is specified.

56

£0,000 nSv h'1 «0.im1,600 «0.5m

1002516

1 2 3 4DISTANCE FROM SOURCE

1 mttr«

After measuring the dose rate, estimates can bemade of the dose rates at different distances

from the source.

The dose rate is obtained using the equation:

Dose rateGamma factor x Source activity

(Distance)2

Gamma factor is the absorbed dose rate in mSvh"1 at1 m from 1 GBq of the radionuclide;

Activity of the source is in gigabecquerels;Distance is in metres from the source to the point of

interest.

Gamma emittingradionuclide

Gamma factorr

Ytterbium-169Technetium-99mThulium-170Caesium-137lridium-192Cobalt-60

0.00070.0220.0340.0810.130.351

However, the dose rate from the source is best determinedusing a reliable dose rate meter.

57

U-o-

Notation for the examples of calculations.

Examples of Calculations

(1) What will be the dose rate at 5 m from 400 GBq ofiridium-192?

T x A 0.13 x 400 ,Dose rate = — = mSv-h

= 2.08 mSv-rT1

(2) A dose rate of 1 mGy-h"1 is measured at 15 cmfrom a caesium-137 source. What is the source'sactivity?

Dose rate = 1 mSv-h-1

0.081 x activity

0.0225mSv-h - i

1 x 0.0225Activity = GBq = 0.278 GBq

0.081

(3) A dose rate of 780 jiGy-h"1 is measured from320 GBq cobalt-60. How far away is the source?

Dose rate • 0.78 mSv-h"1

0.351 x 320mSv-rr

58

Distance0.351 x 320

0.78m = 12 m

(4) A 1.3 TBq iridium-192 source is to be used. What dis-tance will reduce the dose rate to 7.5 /xGyh"1?

Dose rate « 0.0075 mGyh"1

0.13 x

/ 0.13

1.3d2

x 1.

X

3

1000

x 1000Distance = / m = 150 m

N 0.0075

(5) A dose rate of 3 mSvh"1 is measured at 4 m froma gamma emitting source. At what distance will thedose rate be reduced to 7.5 /xSvh~1?

Gamma factor x ActivityDose rate = ;

(Distance)

Gamma factor x Activity is the source output and is con-stant. Therefore, Dose rate x (Distance)2 is constant.

Hence, 0.0075 x d2 = 3 x 42

V 0.x 42

m.0075

d = 80 m

Radiation and Time

Radiation dose is proportional to the time spent in the radi-ation field. Work in a radiation area should be carried outquickly and efficiently. It is important that workers shouldnot be distracted by other tasks or by conversation.However, working too rapidly might cause mistakes tohappen. This leads to the job taking longer, thus resultingin greater exposure.

Radiation Effects

Industrial and medical uses of radiation do not presentsubstantial radiation risks to workers and should not leadto exposure of such workers to radiation in excess of anylevel which would be regarded as unacceptable.

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Possible radiation effects which have been considered bythe international bodies (e.g. the International Commissionon Radiological Protection, International Atomic EnergyAgency) are:

(a) Short term effects such as skin burns and eyecataracts;

(b) Long term effects such as an increased dispositionto leukaemia and solid cancers.

Current recommendations for dose limitations are con-tained in IAEA Safety Series No. 115. In summary, theseare:

(a) No application of radiation should be undertakenunless justified;

(b) All doses should be kept as low as achievable, eco-nomic and social factors being taken into account;and

(c) In any case, all doses should be kept below doselimits.

For reference, the principal dose limits specified in IAEASafety Series No. 115 are:

Adult workers 20 mSv per year(averaged over five years)

Members of the public 1 mSv per year.

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Printed by the IAEA in AustriaMarch 1996

IAEA-PRSM-8(Rev.1)