XA9/43492

f Jfe i

Manual onSHIELDED ENCLOSURES

Incorporating:Applications GuideProcedures GuideBasics Guide

q

PRACTICAL RADIATION SAFETYMANUAL

Manual onSHIELDED ENCLOSURES

Incorporating:

Applications Guide

Procedures Guide

Basics Guide

INTERNATIONAL ATOMIC ENERGY AGENCY

MANUAL ON SHIELDED ENCLOSURESIAEA, VIENNA, 1996IAEA-PRSM-2 (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

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 (Safety Se-ries No. 102) providing guidance on the safe use and regulation ofradiation sources in industry, medicine, research and teaching.However, it was felt necessary to have practical radiation safetymanuals for different fields of application aimed primarily at per-sons handling radiation sources on a daily routine basis, whichcould at the same time be used by the competent authorities, sup-porting their efforts in the radiation protection training of workers ormedical assistance personnel or helping on-site management toset 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) and T. Gaines (USA), acting as consultants, and the help ofthe participants of an Advisory Group meeting which took place inVienna in May 1989: J.C.E. Button (Australia), A. Mendonca(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) andA. Schmitt-Hannig and P. Zufiiga-Bello (IAEA).

These drafts were revised by R. Wheelton from the NationalRadiation Protection Board in the UK and B. Thomadsen from Wis-consin University in the USA. In a second Advisory Group meetingheld in Vienna in September 1990, the revised drafts werereviewed 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. Zufiiga-Bello (IAEA). Finalization ofall six manuals was carried out by A. Schmitt-Hannig, FederalOffice for Radiation Protection (Germany) and P. Zufiiga-Bello(IAEA).

CONTENTS

Applications Guide 7

Need for shielded enclosures 7Forms of shielded enclosure 7Primary shielding for enclosures 8Secondary shielding for enclosures 10Enclosure design considerations 13Shielding design considerations 14Control of access to enclosures 16Worker protection 18Dealing with emergencies 19

Procedures Guide 21

Basics Guide for Users of Ionizing Radiation 33

Production of radiation 34

Radiation energy units 36Radiation travelling through matter 36Containment of radioactive substances 40The activity of sources 41Measurement of radiation 43Radiation and distance 46Examples of calculations 48Radiation and time 49Radiation effects 49

APPLICATIONS GUIDE:SHIELDED ENCLOSURES

Need for Shielded Enclosures

In all applications of ionizing radiation, the radiation doseto the users, and to others in the vicinity, must be kept aslow as reasonably achievable and, in any case, belowinternationally agreed dose limits. This is possible byapplying a combination of four methods:

(1) By using a source which is the most suitable for theparticular application. As radioactive sealed sourcesand machines involve only an external hazard theyare, in general, preferable to open sources. The haz-ard is further minimized by choosing a source ofsufficient activity, and the most appropriate radiationenergies for the application.

(2) By keeping the time that the user and others areexposed to the radiation as short as possible.

(3) By maintaining the necessary distance between thesource and the user and greater distances betweenthe source and other persons.

(4) By placing suitable shielding materials of sufficientthickness around the source or the application.

Physical means exist to ensure that: exposure times arekept to a minimum; barriers remain in place to keep peopleaway from the hazardous areas; and shielding materialsare in place before a source can be exposed. Theseengineered controls are preferable to administrative con-trols which rely on individuals obeying instructions not toremain longer than necessary near sources, not to passbarriers, and to use shielding materials.

Forms of Shielded Enclosure

A shielded enclosure is an enclosed space engineered tocontain ionizing radiation and to provide adequate shield-ing for persons in the vicinity. They range in size from rela-tively small cabinets, for example to contain the X raymachines used to examine unopened mail and baggage;through walled compounds in which radiography is carried

7

out; to large chambers in which very high doses are deli-vered for irradiation purposes such as disinfestation,sterilization and the induction of changes in the matterbeing irradiated. An enclosure is essential for work with aradiation source that has a high dose rate output. To relyon distance alone to reduce the dose rate would require aprohibitively large exclusion zone. Similarly, other sourcesneed to be suitably enclosed to be used close to areas thatare either occupied, or to which persons have a right ofaccess.

The design principles are similar for all enclosuresalthough different characteristics are incorporateddepending on whether the enclosure is to be suitable forX ray, gamma or neutron radiation.

Primary Shielding for Enclosures

Sufficient shielding will be required to reduce the accessi-ble, transmitted dose equivalent rates to an acceptablelevel, for example 7.5 ^Svh ' 1 . There may be a need for aControlled Area outside an enclosure.

The amount of shielding needed, and consequently thecost, can be minimized by restricting the number and areaof internal surfaces that primary radiation is allowed tostrike. This higher energy radiation will need primary bar-riers, which are either comparatively thicker or made usingdifferent materials. The restrictions can often be achievedby collimating the radiation source, that is reducing andshaping the radiation to a useful beam which can then bedirected only towards suitable barriers.

, D, with shieldD2 without shield

T = D,

"07

rr •—---_.USEFULBEAM

PRIMARYBARRIER

The transmission factor as an indicationof the effect of the primary barrier.

To estimate the necessary thickness of a primary barrier itstransmission factor T must first be calculated. If D, isthe maximum dose rate to be allowed (for example7.5 jxSv-h'1) at a position close to the outside surface ofthe barrier and D2 is the dose rate at the same place whenthe barrier is not there, then T is equal to D, divided byD2. To calculate D2, the enclosure's designer will needdesign parameters, which are basic assumptions about:

— which radionuclides or machines are to be used;— the upper limit of activity of the radioactive source or

the electrical parameters of the machines that canbe used; and

— the minimum distances between the radiation sourceand the barrier's internal surface.

An accurate estimate of the thickness of the shield neededrequires transmission graphs which are published fordifferent radionuclides (and machine parameters) anddifferent shielding materials. However, a simplified estima-tion of the primary barrier's thickness which may tend tooverestimate the shielding is possible. If the calculatedtransmission factor, D,/D2, is one-tenth then a primarybarrier equivalent to a tenth value thickness (TVT, seeBasics Guide) is used; to obtain a reduction of one-hundredth, two TVTs are used; for a reduction of one-thousandth, three TVTS are used; and so on. The HVTs(half value thicknesses) and TVTs are dependent on theprimary radiation's energy.

Typical primary barrierthicknesses

Radiation source

Dental, veterinary X rayMedical, diagnostic X rayIndustrial X ray (250 kV)lridium-192 (1 TBq)Cobalt-60 (185 GBq)Linear accelerator (8 MV)

Lead (mm)

0.52

1070

180300

Concrete (cm)

515506080

200

Secondary Shielding for Enclosures

Barriers (often called secondary barriers) are required toprovide sufficient shielding for secondary radiation whichis scattered out of the useful beam and also leakage radia-tion which has been transmitted through the sides of thehousing which contains the radioactive source or X raymachine. Although the secondary radiation has lower ener-gies than the primary radiation, leakage radiation (whichmust not be confused with tests for leakage of radioactivesubstance which are carried out on sealed sources) some-times has energies as high as those of the primary radia-tion. This often dictates the thickness of secondaryshielding required, which is then calculated in a mannersimilar to that used to calculate the primary barrierthickness.

nn SECONDARY ANDLEAKAGE DOSERATES COMBINE.

tSECONDARY ; -^

BARRIER

" - v SECONDARYLEAKAGE ^ (SCATTEREDIRADIATION "RADIATIONS

PRIMARYBARRIER.

Secondary barriers are used toshield secondary radiations.

Various standards set limits for leakage of radiation fromX ray machine housings and gamma radiographyexposure containers. The latter are used for industrial radi-ography and are described in the Applications Guide onGamma Radiography.

The radiation leakage from an industrial X ray machinemust not exceed 10 mSv-h"1 at 1 m from where the X raysoriginate. Linear accelerators and other very high voltagemachines should not leak more than 0 .1% of the dose rate10

in the useful beam. Such machines only leak radiationwhilst they are electrically supplied. However, dose rateswill exist constantly around exposure containers. The con-tainers may be Class M — mobile, or Class F — fixed.The latter are designed to shield much higher sourceactivities than those usable outside an enclosure. If theISO 3999-**** Apparatus for Gamma Radiography —specifications concerning the construction, markings anddose rate limits are complied with, the maximum doserates around an exposure container will be as follows:

Maximum allowed dose rate (/iSv-h ')

Class

of On external 50 mm from 1 m fromexposure surface of external surface external surfacecontainer container of container of container

Class M 2000 or 1000 50Class F 2000 or 1000 100

Complex calculations are necessary to obtain precisevalues of the dose rates due to scattered radiation. Thesetake into account the energy of the radiation before it inter-acts, the size of the beam, the nature of the medium withwhich the radiation interacts and the direction of scatter.However, for large area primary beams, a very much sim-plified estimation of the dose rates due to scattered radia-tion can be made. This assumes that the dose rate at 1 mfrom the scattering point will be a small, fixed percentageof the dose rate at the scattering point. The followingvalues would tend to overestimate the scattered doserates:

_, .. .. Maximum scatter to 1 mRadiation source , . .

from the scattering point

Industrial X rays (100 to 300 kV) 3.6%lridium-192 gamma radiations 2%Cobalt-60 gamma radiations 1 %

11

SECONDARYRADIATION

6".'

For 192|rDB = DOSE RATE AT BARRIERDOSE RATE AT 1m Ds = DB a 2%DOSE RATE AT 2 m = ^S.

DOSE RATE AT 3 m = Es

Using the figures given to estimate the scattered dose rateat 1 m from the wall, floor, radiographed object or otherscattering point, the consequent dose rates at greater dis-tances can be calculated using the inverse square law.

DOSE RATE AT CRANE. C

Ds

c " (distance B—— C)2

DOSE RATE AT TOP OF WALL, T.

DT =Idistance B — - Tl2 ' 'DOSE RATE AT 1m FROM T, W.

Dw -- D, x 1%DOSE RATE AT MAN. M

Dw

DM

12

Calculation of dose ratesoutside an open-topped enclosure.

When the scattering medium is either a very large area ora large volume, direct radiation measurements suggestthat it is prudent to assume that the dose rate decreasesonly as the inverse of the distance. The calculations usedto obtain estimates of dose rate outside an enclosure, forexample at a crane driver's position and at ground level,are shown in the illustration.

Determining the effective energy of scattered radiation isdifficult but, because specific shielding materials are moreeffective attenuators of this lower energy radiation, theshield thicknesses required will normally be a fraction ofthose necessary for the primary radiation energies. Secon-dary barriers which shield combined leakage and scat-tered radiation are often about one half the thickness of theprimary barriers required for an enclosure.

Enclosure Design Considerations

Doors that are to form part of the secondary barriers toshield industrial radiography sources are very heavy andcostly to engineer. Only small doors need to be fitted if theobjects to be radiographed are small or the enclosure isopen-topped to allow cranes to be used to lift the objectsinto the enclosure. The height of the wall then becomes animportant feature of the shielding. This is a commondesign but radiation scattered over the wall may becomeexcessive if the design parameters are not followed inpractice. The design may be compromised if, for example,X radiography is introduced into an enclosure which hasbeen designed only for gamma radiography, the usefulbeam is raised above the horizontal or the collimation isnot maintained. The amount of scatter may also beaffected by changes above the enclosure such as theinstallation of new ventilation ducting. The users of theseenclosures need to be aware of crane drivers or other per-sonnel working above ground level or plans to constructnew multilevel buildings in the vicinity of the enclosure.

The dose rate at the door can be significantly reduced,thereby relieving the engineering problems, by incorporat-ing a maze entrance in the design of the enclosure. Thedose rate 1 m down the sheltered leg of a maze, the cornerof which is irradiated by gamma radiation, is about 10% of

13

the dose rate at the corner centre; it decreases approxi-mately as the inverse square of the distance from thecorner towards the door. A similar effect is achieved forother radiation although the percentage scatter may besignificantly higher, for example about 25% for neutronradiation.

An open-topped enclosure with maze entrance.

DOSE RATE AT CORNER OF MAZE C

(distance B- •CVDOSE RATE AT 1m. INTO MAZE. M.

DM = Dc x 10%

DOSE RATE AT DOOR, d.n DMu " " Idislance C d) !

Calculations of the dose rateat the maze entrance to an enclosure.

Shielding Design Considerations

Radiation which either penetrates or scatters around weak-nesses in the shielding can cause significant dose ratesoutside an enclosure. Such weaknesses often occur whereshielding is penetrated by pipework, ducting or fasteners14

(for example, screws), around doors and windows, andwhere shielding materials abut. The weaknesses are notalways foreseen during the design of the enclosure andthey sometimes occur after a period of wear (for example,door supports), shielding damage, movement of shielding,or building settlement.

Various design techniques can be used to avoid introduc-ing weaknesses. Radiation monitoring using a dose ratemeter outside the enclosure and regular maintenance willidentify faults developing before they become serious.

WALL WALL

/

WALL

V o 0 o

Methods to preventradiation scattering under a wall.

Methods to closeshielding weaknesses.

15

Methods to preventradiation scattering around doors.

Control of Access to Enclosures

An enclosure may contain a Controlled Area requiringaccess to be restricted at all times. However, when aprimary beam is about to be exposed there is a particularneed to ensure that no one accidentally remains inside. Itis also necessary to prevent anyone accidentally enteringwhilst a useful beam is exposed. Devices which areinstalled to achieve these objectives must be effective.They must operate in such a way that failure of the deviceautomatically prevents or terminates the radiation hazard.

When a primary beam is about to be exposed, a clear pre-exposure warning signal should be given both inside andoutside the enclosure. The signal should last at least tenseconds to allow anyone inside the enclosure who hears orsees the signal to take action. Notices should explain themeaning of the signal and the actions which must betaken. When the radiation source is a machine or a sourceoperated electrically, some form of control device shouldbe installed to enable any person trapped to shut off theelectricity supply. For example an emergency stop buttonor wire should be located where it can be reached withoutpassing through the primary beam. In other cases, atrapped person should be able to either leave the16

enclosure immediately or take refuge in a shielded areasuch as a maze entrance where there should be provisionto communicate with the people outside.

Whilst a primary beam is exposed, exposure warning sig-nals, which are separate and distinguishable from the pre-exposure signals, should be provided and explained bothinside and outside the enclosure. Small cabinets, forexample of less than 0.2 m3, will only need an exposurewarning and no internal signals or control devices.

Suitable interlocks should be installed to form a mechani-cal or an electrical link between the exposure control andthe door or other means of access to an enclosure. Thesewill either prevent a person entering during an exposure orimmediately make safe the radiation source as someoneattempts to enter. One such device which has wide appli-cation is a mechanical interlock called a captured-key sys-tem. In its simplest form, the system is a door lock whichsimultaneously holds two unique keys: a door key and acontrol key. One or other of the keys is always held by thedoor lock. When the door is locked closed the door key isheld but the control key can be removed and used tounlock the control and expose the useful beam. The con-trol key cannot be removed from the control panel until theradiation source is made safe. The door key cannot beturned to unlock the door until the control key is back in thedoor lock. Anyone entering the enclosure should take thedoor key to prevent the source being exposed. The samekey can be used inside the enclosure to initiate the pre-exposure signal thereby ensuring that a check is made tosee that no person is accidentally shut inside theenclosure. Such a device is called a search and lock-upsystem and is essential in enclosures which contain radia-tion sources of very high output.

Electrical switches can be mounted on doors in variousways to operate as interlocks. Simple push button switcheswhich are depressed by a closing door are not always reli-able. The primary beam could be accidentally emitted if theswitch develops a fault. A more effective device, for exam-ple, is one mounted at the trailing edge of a sliding door sothat the open door depresses the button and the primarybeam can be emitted only when the door is closed and thebutton is released. In this position the interlock will fail tosafety.

17

/

D

Fail-to-safety installation of a door interlock:(a) door open; (to) door closed.

It is important to ensure that if a radiation source is madesafe automatically by the action of an interlock, it shouldremain so until the primary beam is exposed by the opera-tor at the control. The exposure of a primary beam in anenclosure must not commence on the mere act of closinga door.

Where reasonably practicable, the control point for all radi-ation sources should be outside the enclosure. This is anessential requirement for sources with an extremely highoutput, for example more than 10 mSvmin'1 at 1 m. Inany case, the dose rate at any control point should notexceed 2 mSv-h'1.

Worker Protection

Each time an enclosure is entered, after a primary beamhas been emitted, the operator should carry a dose ratemeter which is switched on. Measurements should bemade close to where the primary beam would be, to con-firm that the source has been made safe. Additionally, adose rate alarm may be installed inside the enclosure toindicate when a fault has occurred.18

At regular intervals of perhaps four months and wheneveran unusual primary beam direction is used, a radiation sur-vey should be carried out by making dose rate measure-ments around the outside of the enclosure. If practicable,physical restrictions should be placed on the collimator toensure that the primary beam cannot be used other thanwithin the constraints allowed by the design parameters.

By using a well designed and maintained enclosure, it ispossible that the operator will rarely, if ever, be required towork in dose rates which exceed 7.5 /*Svh 1. It is unlikelythat the worker will accumulate in excess of three-tenths ofany relevant dose limit in a calendar year. Nevertheless, itmay be desirable for workers who regularly enterenclosures to wear personal dosimeters.

Dealing with Emergencies

The high output of radiation sources that are often used inenclosures can cause accidents and incidents of a seriousnature. However, the protection offered by the permanentnature of the facility should reduce these risks. Even whena problem arises, it should be easier to regain control thanis normally the case for portable equipment.

Nevertheless, a thorough assessment of the equipment,the enclosure and the procedures used is necessary toidentify any abnormal situations which might occur. Con-tingency plans are needed which can be implementedquickly and effectively to regain control in the event that aproblem arises. Practising the contingency plans will indi-cate whether any special equipment will be needed to dealwith reasonably foreseeable incidents.

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

— physical damage to a source housing that has beencrushed or involved in a fire or explosion;

— leakage of radioactive substance from a sealedsource;

— the discovery of unacceptably high dose rates as aresult of damage to the enclosure or failure to workwithin the design parameters;

19

— the exposure of a person because of one of the inci-dents described or because of the failure of an inter-lock or a warning signal;

— a radioactive source failing to return to its safe,shielded position.

If a source is suspected of leaking radioactive substance,surfaces that may come into both direct and indirect con-tact with the radioactive substance will become contami-nated. Precautions should be taken to prevent ingestion ofthe radioactive substances that can result when clothingand surfaces of the body become contaminated. Effectivedecontamination will require expert assistance.

Any incident which may have resulted in an internal orexternal dose to a person or any high dose reported on adosimeter should be investigated. It is important to deter-mine whether the suspected or reported dose was actuallyreceived and also whether some part of the body hasreceived a much higher dose which might result in local-ized tissue injury.

20

PROCEDURES GUIDE: SHIELDED ENCLOSURES

21

Use and maintain the shielded enclosure for the purpose for which it wasintended.

Only trained and authorized workers should enter the enclosure. If appropriate,such persons should have had medical examinations and wear dosimeters.

Before proceeding with the work, read and ask questions about these safetyguides. Discuss with your colleagues your contributions to this important work.

—JlARE*

^RADIATION

PUDIATW"

SIGNALS / — \

r MMNLEAVC ROOM

/

h

/ / /

RADIATIONSMMALS

K(I1//

Examine the enclosure and identify its main features such as the primary andsecondary barriers.

Read the enclosure's design parameters, if they are available, and compare theradiation source in use with that originally intended. Establish that the maximumoutput of the source in use and its collimation are within the specifications. Inves-tigate the constraints on the beam direction and recent typical workload for thebeam directions normally used.

It may be prudent to display the design parameters inside the enclosure.

ro

Regularly examine the installed pre-exposure and exposure warning signals.Check that they work and that anyone inside the enclosure would always receivea warning that an exposure sequence has started. If necessary, two pre-exposure signals should be designed so that at least one will always work. Con-firm that audible signals will be heard if there are noisy surroundings and thatvisible signals are prominent enough to be seen.

Keep the signals clearly labelled so that both authorized and unauthorized per-sons, inside and outside the enclosure, will understand their meaning and anyactions which need to be taken when the signals are given.

Report any faults to your supervisor

"•^--rfe"

r\301

Examine the installed safety features, including the means provided for any per-son trapped inside the enclosure to stop an exposure and to contact the outside.

Check that the interlocks function and will either prevent anyone accidentallyentering or will immediately cancel an exposure taking place when the door isopened.

Report any faults to your supervisor

to

In the case of sealed sources, use a dose rate meter to check that the sourceis safely shielded before carrying out routine maintenance on the installedexposure container or source housing. Keep a record to show that the regular(weekly, monthly, annual or biannual) maintenance has included, for example:

(1) Cleaning the container, removing any grit and moisture.(2) Using recommended lubricants to clean and maintain any moving parts.(3) Checking screws and nuts for tightness and looking for damage to screw

threads and springs.(4) Confirming that the source locking mechanism works.(5) Checking that the source housing displays: the trefoil symbol and a suita-

ble warning; the name of any radionuclide contained; the activity and refer-ence date of sealed sources; and identification numbers for the containerand sources.

(6) Carrying out, at the recommended intervals, and only when trained andauthorized to do so, out tests for leakage of radioactive substance in theappropriate manner required by the regulatory authority or recommendedby the manufacturer of any sealed sources in use.

Report any faults to your supervisor

CONTROLLEDA R E *

(J) W OuT

[go]

l

Check the enclosure at an appropriate frequency, for example at three-monthintervals, for damage to doors, pipe and cable shield boxes, or other potentialshielding defects.

Measure the accessible dose rates during normal source exposures. Be awarethat if the whole detector is not irradiated, for example when making measure-ments close to narrow gaps around doors, the reading may be less than the doserates which actually exist. In addition, radiation scattered over a wall may com-bine with transmitted (leakage and primary) radiation to produce a maximumreading a few metres away from outside surfaces of the enclosure.

Dose rate measurements should be made at positions above ground level evensome distance from the enclosure.

Repeat the measurements on every occasion that the useful beam is used inabnormal situations or is operated at the limits of the design parameters.

rooo

CONTROLLED Ij' AREA \l/~\\ VCCD OUT

If it is impossible to use an enclosure which is not completely closed, that is toadapt an existing enclosure to carry out gamma radiography using a portable,projection-type exposure container with the control point outside the enclosure,ensure that the dose rate at the control point is always less then 2 mSv-h"1. Bar-riers and notices should be set up to mark any Controlled Areas near the dooror elsewhere and other procedures should be used as described in the Proce-dures Guide.

ro(O

At the end of an automatically timed exposure, or after operating the control toterminate the exposure, cautiously enter the enclosure carrying a dose ratemeter which is switched on.

Make measurements close to where the primary beam would be expected andthen around the source housing to confirm that the source has been fully andsafely shielded.

The irradiated objects or tubes through which sealed sources are guided cannow be safely handled.

Keep the dose rate meter operating until you leave the enclosure.

If the radiation source, or the enclosure, develops a fault or an incident becomesapparent whilst someone is inside the enclosure, stay calm. The person shouldleave the enclosure immediately.

If it is possible, the controls should be used to return the source to its shieldedposition. If this cannot be done, make dose rate measurements in the vicinity ofthe enclosure and set up barriers to mark the extent of any Controlled Areas.

If it is possible without receiving unnecessary doses, seal the enclosure door andstay close to the area to prevent people entering. Send someone to inform yoursupervisor.

Give careful consideration before implementing any plan to rectify a fault orrecover sealed sources which are exposed but nevertheless shielded inside anenclosure. Options may be available which involve minimal risk or dose to thosepersons involved.

The advice of the equipment manufacturer or other qualified experts should besought.

The examination of similar equipment may present a solution to the problem orsuggest special tools which may be helpful.

As soon as you have no further use for a radioactive source or the equipmentwhich houses it, it should preferably be returned to the manufacturer or supplier.If any other method is used it must comply with your government's laws for thatparticular method of disposal.

Radioactive substances being sent for disposal must be appropriately packagedand transported in accordance with the IAEA Regulations for the Safe Transportof Radioactive Material.

BASICS GUIDE FOR USERS OFIONIZING RADIATION

33

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 (j3) 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

44HI Q, 0) O C

> .O CT> >N O I_

Spectrum of radiations similar to gamma rays.

34

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.

35

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.

36

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

X raygamma

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.

37

Maximum beta Maximum rangeRadionuclide particle

energy Air Plastic Softwood Aluminium(MeV) (mm) (mm) (mm) (mm)

Promethium-147 0.23 400 0.6 0.7 0.26Thalium-204 0.77 2400 3.3 4.0 1.5Phosphorus-32 1.71 7100Strontium-90/Yttnum-90

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 values (cm) in various materials

Lead

HVT

0.020.720.650.551.10.0260.043

TVT

2.42.21.94.00.0870.142

Iron

HVT

1.61.32.0

TVT

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.38

PAPER PLASTIC STEEL LEAD 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.

39

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.

tcm

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 and40

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 becquerels (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-querels. A convenient method of expressing such largenumbers is to use prefixes, for example:

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

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

1 000 000 000 becquerels is written 1 gigabecquerel (1 QBq);

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

41

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

Hall-lite"

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

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

When radioactive substances are dispersed throughout

other materials or dispersed over other surfaces in the

42

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

(a) for dispersion throughout liquids Bq-mL'1

(b) for dispersion throughout solids Bq-g~1

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

(d) for dispersion over surfaces Bq-crrT2

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 nC\ 1 mCi 1 Ci 10 Ci—I 1 1 1 1-37 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 sieved(Sv). For X ray, gamma and beta radiation, one sievedcorresponds to one gray. The most impodant 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 microsieveds per hour (/iSvh"1). However, many ins-truments still use the older unit of millirem per hour(mrem-rT1). 10 /tSv-h'1 is equivalent to 1 mremh"1

43

— 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 pSv if it was exposed to 10 pSvh~1 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.44

(a) Electronicdosimeter

a n a

(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~') 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 (Bqcm"2). 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 Bqcm'2 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.

45

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 400the expected dose rate at 2 m is 100the expected dose rate at 10 m is 4^Sv-h~1;the expected dose rate at 20 m is 1 jiSv-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.46

40,000 |iSv h'1

1,60010025

I 16

0 1 2 3 4 5 6DISTANCE FROM SOURCE

• O.ir• 0.5• 2m• 4m• 5m

Attar 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 mSv-rr1 at1 m from 1 GBq of the radionuclide;

Activity of the source is in gigabecquereis;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.

47

Notation for the examples of calculations.

Examples of Calculations

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

Dose rater x A 0.13 x 400

mSv-h"

2.08 mSvh"

(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-rT1

0.081 x activity ,mSvh" 1

0.0225

1 x 0.0225Activity = GBq = 0.278 GBq

0.081

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

Dose rate = 0.78 mSvh"1

0.351 x 320rnSv-h"

DistanceJa351 x :

N 0.78x 320

m « 12 m

48

(4) A1.3 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 mGy-h'1

0.13 x 1.3 x 1000

, 0.13 x 1.3 x 1000Distance = / m = 150 m

0.0075

(5) A dose rate of 3 mSv I r 1 is measured at 4 m froma gamma emitting source. At what distance will thedose rate be reduced to 7.5 S h 1 ?

Gamma factor x ActivityDose rate =

(Distance)2

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

Hence, 0.0075 x d 2 = 3 x 42

- I1

N 0.

x 42

m1.0075

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.

49

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.

50

Printed by the IAEA in AustriaMarch 1996

IAEA-PRSM-2 (Rev.1)