Monitoring of Radioactive Contamination on Surfaces
46
TECHNICAL REPORTS SERIES No.120 Monitoring of Radioactive Contamination on Surfaces ^INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA,1970
Transcript of Monitoring of Radioactive Contamination on Surfaces
TECHNICAL REPORTS SERIES No.120
Monitoring of Radioactive Contamination
on Surfaces
^ I N T E R N A T I O N A L A T O M I C E N E R G Y A G E N C Y , V I E N N A , 1 9 7 0
MONITORING OF RADIOACTIVE CONTAMINATION
ON SURFACES
The fo l lowing States are Members of the International A t o m i c Energy A g e n c y .
AFGHANISTAN GREECE NORWAY ALBANIA GUATEMALA PAKISTAN ALGERIA HAITI PANAMA ARGENTINA HOLY SEE PARAGUAY AUSTRALIA HUNGARY PERU AUSTRIA ICELAND PHILIPPINES BELGIUM INDIA POLAND BOLIVIA INDONESIA PORTUGAL BRAZIL IRAN ROMANIA BULGARIA IRAQ SAUDI ARABIA BURMA IRELAND SENEGAL BYELORUSSIAN SOVIET ISRAEL SIERRA LEONE
SOCIALIST REPUBLIC ITALY SINGAPORE CAMBODIA IVORY:COAST SOUTH AFRICA CAMEROON JAMAICA SPAIN. CANADA JAPAN SUDAN CEYLON JORDAN 1 ! SWEDEN"' CHILE KENYA. SWITZERLAND CHINA KOREA, REPUBLIC OF SYRIAN ARAB REPUBLIC COLOMBIA KUWAIT THAILAND CONGO, DEMOCRATIC LEBANON TUNISIA
REPUBLIC OF LIBERIA TURKEY COSTA RICA LIBYAN' ARAB REPUBLIC UGANDA
CUBA LIECHTENSTEIN UKRAINIAN SOVIET SOCIALIST CYPRUS LUXEMBOURG REPUBLIC
CZECHOSLOVAK SOCIALIST MADAGASCAR UNION OF SOVIET SOCIALIST
REPUBLIC MALAYSIA REPUBLICS
DENMARK MALI UNITED ARAB REPUBLIC
DOMINICAN REPUBLIC MEXICO UNITED KINGDOM OF GREAT. ECUADOR MONACO BRITAIN AND NORTHERN EL SALVADOR MOROCCO IRELAND ETHIOPIA NETHERLANDS UNITED STATES OF AMERICA FINLAND NEW ZEALAND URUGUAY FRANCE NICARAGUA VENEZUELA
GABON NIGER V I E T - N A M
GERMANY, FEDERAL REPUBLIC OF NIGERIA YUGOSLAVIA
GHANA ZAMBIA
The A g e n c y ' s Statute was approved on 23 October 1956 by the C o n f e r e n c e on the Statute of the IAEA held at United Nations Headquarters, New York ; it entered into f o rce on 29 July 1957 . T h e Headquarters of the A g e n c y are situated in V ienna . Its principal o b j e c t i v e is " t o a c c e l e r a t e and enlarge the contribution of a t o m i c energy to p e a c e , health and prosperity throughout the w o r l d " .
© I A E A , 1970
Permission to reproduce or translate the information contained in this publ icat ion may be obtained by writing to the International A t o m i c Energy A g e n c y , Karntner Ring 11, P . O . Box 590, A - 1 0 1 1 Vienna, Austria.
Printed by the IAEA in Austria August 1970
TECHNICAL REPORTS SERIES No. 120
MONITORING OF RADIOACTIVE CONTAMINATION
ON SURFACES
A manual prepared by
R. F. CLAYTON Health Physics and Medica l Division,
Atomic Energy Research Establishment, United Kingdom Atomic Energy Authority,
Harwell, Berks, United Kingdom
INTERNATIONAL ATOMIC ENERGY AGENCY •VIENNA-, 1970
MONITORING OF RADIOACTIVE CONTAMINATION ON SURFACES IAEA, • VIENNA, 1970
STI/DOC/10/1-20
FOREWORD
The assessment of radioactive surface contamination f o rms an i m -portant part of the application of safety standards to work involving the use of unsealed radiation sources' .
This manual is intended to help those concerne.d with the implementa-tion of these standards to select and apply suitable procedures and tech-niques. .It has been prepared on behalf o fthe Agency by Mr. _R. F . Clayton of the Health Physics and Medical Division, Atomic Energy Research Establishment, United Kingdom Atomic Energy Authority, Harwell, Berks.
The manual outlines the objectives of surface contamination m o n i t o r -ing and discusses the relative advantages and disadvantages of various available methods. Examples of derived working limits of surface conta-mination at present in use are quoted. The factors to be considered in selecting contamination monitoring instruments .are also discussed and a few instruments commonly used in the United Kingdom are illustrated by way of example.
In a manual of this type it is difficult to give a balanced exposition of pract ices adopted by different countries throughout the world. An attempt to do so would result in a text of bewildering complexity. It is acknowledged'that the present text is largely based on the approach adop-ted in the United Kingdom Atomic Energy Authority. A bibliography is provided, however, which may to some extent counteract this bias by directing attention to publications dealing with pract ices adopted in other countries.
CONTENTS
•GENERAL CONSIDERATIONS , : i " . ' . . . - . : . . ' . ' . '
1. 1. Purpose and scope ,1,. 2. '. Definitions . ' . 1. 3. The undesirability of surface contamination .1. 4. Derived working limits of surface contamination 1.5. Monitoring techniques
.1.6. Recordkeep ing
DERIVED WORKING LIMITS . . . . . ' ; . . . . . . ; ' . . .
2 .1 . Health considerations 2.2. . Limits derived from'inhalation risk 2. 3.- Limits derived f r om external'.radiation hazard 2.'4. . Limits derived f r om ingestion risk 2. 5. Examples of derived working limits currently in use 2..6. . Relaxation of derived working limits, .
SURFACE MONITORING TECHNIQUES '3. 1. • General-3. 2. Selection of method
DIRECT METHODS OF SURFACE MONITORING' 4. 1. Alpha contamination monitoring 4. 2. Beta contamination monitoring — general techniques 4 .3 . - Beta contamination monitoring — low-energy emitters
INDIRECT METHODS OF SURFACE MONITORING
5.1. : General . 5. 2. . . Dry smears 5 .3 . Wet smears 5 .4 . Large area swabs 5. 5. Adhesive tape samples 5. 6. 'Smair! sampling 5: 7. ^Sniffing1 techniques 5. 8. Examination of rinsing and washing solutions 5. 9. Tritium smears 5.10. Skin and wound monitoring 5.11. Instruments used for indirect methods
6. SUPPLEMENTARY TECHNIQUES 16
6. 1 . Autoradiography 6. 2. Thermoluminescent dosimetry 6. 3. Radiation spectrometry 6. 4. Air monitoring 6. 5. Deposition trays 6. 6. Overshoe and cleaning material monitoring 6. 7. Personnel dosimetry 6. 8. Biological monitoring 6. 9. Pinhole cameras
7. CALIBRATION AND PERFORMANCE CHECKING OF SURFACE MONITORING EQUIPMENT 20
8. INSTRUMENTATION' ." ; . . 23
8. 1. Criteria for selection of instruments 8. 2. Detailed assessment for choice of instruments
8. 2. 1. Mechanical per formance 8 . 2 . 2 . Electr ical per formance • • ' . 8. 2. 3. Radiological performance
8. 3. Examples of instruments in use .
REFERENCES • : 3 0
BIBLIOGRAPHY 32
1. GENERAL CONSIDERATIONS
1.1. Purpose and scope
The manual is intended as a practical handbook on methods of moni -toring the working environment, f o r health physicists and other persons using radioactive materials . Section 2 deals br ie f ly with the derivation of working limits of contamination.
The applications, advantages and disadvantages of direct and indirect monitoring methods are discussed, and useful supplementary techniques to assist the health physicist in his interpretation of measurements are
^ described. The manual does not discuss monitoring of surface contamination in
the environment of nuclear installations. This aspect of monitoring has already been dealt with in two IAEA manuals, one covering normal o p e r -ations and the other emergency situations [1, 2],
Although the manual is not pr imari ly concerned with the detection of contamination in or on humans, permiss ib le levels of skin contamination are quoted. Many of the methods described f o r surface contamination monitoring can be applied to measuring the amounts of radioactive con -tamination on skin, but internal personnel contamination monitoring requires special techniques not within the scope of this manual.
The surface monitoring techniques described are m o r e applicable to situations in which one is concerned with preventing or controlling a radiological hazard rather than to those in which experimental or oper - • ational inconvenience can arise . In these latter cases , permitted levels may be lower and greater prec is ion in monitoring assessments may be required than in the case of radiological protection.
1 .2 . Definitions
Monitoring. Committee 4 of the International Commiss ion of Radiological Protection has defined monitoring as 'the measurement of radiation or radioactivity f o r reasons related to the assessment or control of exposure to radiation or radioactive material ' and quotes the principal objects of a surface contamination monitoring program [3, 4],
Radioactive surface contamination [5] is unwanted radioactive material deposited in an uncontrolled manner in or on animate o r inanimate objects , i rrespect ive of their situations, in such concentrations that either oper -ational inconvenience or radiological hazard is caused.
Classif ication of radioactive contamination. Several ways are used to c lass i fy radioactive contamination: by the type of radiation emitted, by the availability f o r or ease of transfer to other objects , or by the physical f o rm of the contaminant.
(a) Classif ication by emission is pr imari ly into alpha, beta and l o w -energy beta emitters. F o r surface monitoring techniques , -X- and gamma-ray emissions are of secondary importance, although the radiation hazard due to their presence must never be ignored.
(b) Contamination is further c lassi f ied as being ' f ixed' or ' l oose ' . 'Fixed ' contamination is that which is not transferred f r om a contaminated
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surface to an uncontaminated surface when the two surfaces accidentally touch; conversely , ' l oose ' contamination is that which may be readily transferred under these c ircumstances .
(c) Contamination of surfaces by solid material may occur by spillage of powders , by contact between the surface and radioactive material , by the drying of radioactive liquids and solutions on the surface of the material , o r by deposition f r om airborne radioactive material. Surface contamination may also occur by sorption of radioactive gas such as tritium, or vapour such as tritiated water.
Act ive areas. There are areas in which persons may be exposed to rad io -active contamination, radiation, or the risk of inhaling or ingesting radio -active material f rom airborne or surface radioactive contamination at levels in excess of those recommended by the International Commiss ion on Radiological Protection or the International Atomic Energy Agency; these may include change r o o m s and o f f i ces as well as laborator ies and p r o c e s s areas.
Inactive areas, are all areas other than those c lassi f ied as active areas as defined above. j
1. 3. The undesirability of surface contamination
A potential radiological hazard arises f r o m the presence of rad io -active surface contamination. It can become airborne and subsequently may be inhaled, or it may be transferred by contact to the hands and subsequently to food and ingested, or it may penetrate the intact skin or • enter an open wound giving r i se to an internal radiological hazard. In the case of beta and gamma emitters an external radiation hazard may also exist. •
If the contaminated article is portable, it might be moved to an in-active area where, as the presence of radioactive materials is not expected, it would be handled without the precautions normally taken in active areas.
The presence of radioactive contamination at levels, which may not be a health hazard may interfere with experimental work, leading to erroneous low- leve l counting results, or cross-contamination of radiochemical experiments.
Because of the undesirability of radioactive contamination the risk of its occurring, should be minimized by careful design of laboratories and radioactive plants. The choice of proper equipment for handling rad io -active materials and the selection of correct- operating procedures together with the strict management and disposal of radioactive waste all serve to reduce'this risk.
1 ;4 . Derived working l imits,of surface contamination
Derived working limits (DWL) have been set as an indication of the level of surface contamination which can be tolerated. These levels are such.that the risk of exposure to radiation f r om contamination within, or without the body in excess of the maximum permiss ib le level is minimal.
In this manual derived working limits of surface contamination (which are discussed in greater detail in Session 2) are based on radiological
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protection considerations only. Interference with sensitive measurements , o r the prevention of cross-contamination between experiments may impose m o r e stringent l imits. It is not poss ib le in a manual such as this to attempt to give f igures f or these cases as each must be individually assessed . Methods of measurement are too specialized, involving the.use of low background counting equipment in proper ly equipped laboratories .
1.5. Monitoring'techniques
Monitoring techniques may be broadly divided into 'd irect ' and ' indirect ' methods. Direct methods are those in' which a radiation detector is presented directly to the surface 'under examination or v i ce versa. When direct methods are inapplicable because o f unfavourable geometry, o r because of interference by other radiations, or when it is necessary to determine whether or not contamination is ' f ixed' , indirect methods are used. These usually involve taking samples of the surface contamination,' e. g. on smears , . which are then examined for radioactivity. In .many c ircumstances the two methods are complementary and both must be. used to attain a complete picture of the state .of a surface under examination.
The points to be considered which govern the choice, of.technique to be-employed, and the relative advantages and limitations of the methods are described in Section 3. . . . . . .
1. 6. ' Record keeping • •
Records of monitoring provide a check that monitoring is being done correc t ly and at the required frequency; they enable the responsible organization to maintain a c lose surveil lance of hazardous operations so that the effectiveness of control measures can be-assessed. They also f o r m the basis f r om which a health physicist may make recommendations f or altering control procedures . In the event of litigation, records of conditions during operations may be related to reported or alleged exposures to radiological hazard. . ' '
The type and extent of records of surface contamination monitoring and their period' of retention must depend largely on local conditions and relevant administrative and legal requirements.
2. DERIVED WORKING LIMITS
2. 1. Health considerations
Radiological safety controls must always b e related to the maximum permiss ib le .exposure of workers to radiation whether arising f r o m radior active materials outside or inside the body.
T.o restrict the dose to the person f r om internally deposited radio -nuclides, the concept of the maximum .permissible body burden [6] ,is used. Mathematical models [6, 7] to descr ibe the behaviour of radionuclides in-the body following.an intake have been proposed and f r om these, data the maximum permiss ib le concentrations of radionuclides in air and water have
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been calculated [6, 8], F r o m these primary levels it is poss ible to derive levels of surface contamination which should result in these pr imary levels not being exceeded [9],
Calculations on the basis of the maximum permiss ib le airborne con-centrations of individual, radionuclides result in a range of derived working l imits. Additionally the amount of activity removed f r om a surface depends upon the type of surface, the operations being per formed and the physical f o rm of the contaminant. It would not be realistic to attempt to tabulate DWLs to cover every contingency. To arrive at a rational solution, the worst conditions likely to be encountered must be assumed, i. e. that the most hazardous radionuclide likely to be present is spread over a wide area in a condition in which it will become readily available f o r inhalation or ingestion.
2 .2 . Limits derived f rom inhalation risk 1
To estimate the amount of surface contamination which may b e c o m e airborne under a variety of conditions, resuspension factors , defined as the ratio of the air contamination (/uCi/m3) to the surface contamination (pCi /m 2 ) , have been determined [10-14], Values quoted vary f r om 3 X 10"3 /m to 2 X 10"6 /m; a mean value of 5 X 10"5 /m appears to be ap-propriate for general conditions. F r o m this f igure and the'occupational mpCjjj for plutonium-239 (2 X 10"12 / jC i / cm 3 ) , the most toxic alpha emitter normally encountered which is likely to be inhaled, the derived working limit f o r contamination by this radionuclide is calculated to be 4 X 10"6 f iC i / cm 2 .
Widespread uniform contamination is not usually found in pract i ce , the resuspension factor can be reduced by careful selection of suitable surfaces and footwear, and all the resuspended material is not of respirable size. Because of these mitigating factors , a relaxation to 10-5fiCi/cm2 is recommended. Under some c ircumstances , when large areas of surface contamination at this level occur , the permitted air concentration may be exceeded.
By applying similar cr iteria , the derived working limit for contami-nation by strontium-90, the most toxic beta emitter normally encountered, is calculated to be 2 X 10"3 ( jC i / cm 2 .
2. 3. Limits derived f rom external radiation hazard
All beta emitters are a source of external radiation. The dose - ra te at a planar source of beta partic les of maximum energy in the range 0. 5 to 3. 0 MeV is about 7 rad /h per/LiCi/cm2 [15]. Back-scattering f r om a contaminated surface may increase this f igure by a factor of 1. 5. The hands are the most likely part of the body to be exposed to contaminated surfaces and the maximum permiss ib le dose to these extremities is 7 5 r e m / y r o r l . 5 rem/week. Since it is desirable to restr ict the dose due to contamination alone to l / lOth of this, and assuming continuous exposure during a 40-hour working week, the derived working level f o r beta con-tamination should not exceed 4 X 10"4 p C i / c m 2 (i. e. ~ 3 mrad/h) .
A relaxation factor of 10 can be applied f o r some low-energy beta emitters (0. 2 MeV max. ).
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2. 4. Limits derived f r om ingestion r isk
As safety regulations governing procedures in active areas, e. g. banning the consumption of food, drink, and tobacco therein, and good supervision should reduce the risk of direct ingestion to negligible p r o -portions, derived working limits can be based on indirect ingestion risks. The most likely route f or indirect ingestion is via contaminated hands.
No quantitative data are available f o r relating skin contamination levels to the rate of ingestion but it has been assumed that a person may ingest all the contamination f r om 10 cm 2 of skin every day. F r o m this assumption, combined with the maximum permiss ib le daily intake f or radiation workers of radium-226, which is m o r e hazardous than plutonium-239 when ingested (mpcwater 2 2 6Ra = 4 X 10"7 /uCi/cm3 ; mpcw a t e r 2 3 9Pu = 10"4 AJCi/cm3), and of strontium-90 and lead-210, the most hazardous beta emitters likely to be ingested (mpcwater 90Sr = 10"5 MCi/cm3 ; mpcwater 2 1 0 Pb = 4 X 10"6 ,uCi/cm3) , the derived working limits for skin contamination have been set at 10"5 /uCi/cm2 and 10"4 |UCi/cm2 f o r alpha and beta contamination, respectively. At these levels , the intake should be about l / lOth of the maximum p e r m i s -sible concentration recommended by ICRP. Because of the additional risks of irradiating the basal layer of the skin and percutaneous absorption into the blood stream [16], careful consideration must be given be fore allowing any relaxation of these levels . In addition, the soc io logical and dietary habits of workers must be considered when determining derived working levels of hand contamination [17].
2. 5. Examples of derived working l imits currently in use
The above sections show that the greatest risk f r om alpha contami-nation is inhaling resuspended radioactive material , and f o r beta activity the limiting factor is the external radiation hazard. If the relevant derived working l imits are applied, then the risk due to ingestion of radioactive material f r om surface contamination is virtually eliminated. The levels f o r skin contamination coincide with those derived f or the inhalation and' external radiation r isks. Tables I and II give examples of derived working levels at present in use in the United Kingdom and the United States of Amer i ca , respectively . Other examples appear in ICRP Publication 5 [19] and in IAEA Safety Series No. 1 [20], Tables III and IV compare the permiss ib le levels of surface contamination in use in different atomic energy establishments [17-24],
It is recommended by the International Atomic Energy Agency [25] that radioactive contamination on the outside surfaces of containers used f or transporting radioactive material should not exceed the levels quoted in Table V.
Other national codes of pract ice require that the radioactive contami-nation on the outside of sealed therapeutic or industrial sources should not exceed 0. 5 ^Ci. .
The contamination on the inside surfaces of respirator f a ce -p i e ces , . air hoods, pressur ized suit helmets or other types of breathing apparatus should never exceed the derived working limit f o r skin. At these leve ls , and with intermittent use, the maximum permiss ib le inhaled daily intake , f o r plutonium-239 and strontium-90 should not be exceeded. F o r long-term and regular use it may be advisable ;to work to a, lower level.
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TABLE I. DERIVED WORKING LIMITS FOR SURFACE CONTAMINATION [ 1 8 ] a
1 2 3 4
' Type o f area Principal alpha
emitters ^ (|iCi /cm z )
Low-toxicity alpha emi t ters c
( f j C i / c m z )
' Beta^ emitters
( f j C i / c m 2 )
Low-energy beta emitters •
< 0 . 2 MeV- max. ( j rCi / cm 2 )
•Inactive and low activity areas 1 0 - 5 10"4 10"4 10" 3
1 Act ive areas 10~4 10" 3 10" 3 ' 10"2
Personal clothing 10"5 10"4 • 10"4 10" 3
Clothing not normally worn in inactive areas
1 0 - 4 10" 3 10"3 10"2
Skin 10"5 10" 5 10"4 10"3
a Averaging is permitted over inanimate areas of up to 300 cm 2 or, for floors, walls and cei l ing, 100 cm 2 . Averaging is permitted over 100 c m 2 for skin or, for the hands, over the whole area of the hand, nominally 300 cm 2 .
b A l l alpha emitters other than those listed under c. 0 Uranium isotopes (except separated isotopes o f mass numbers 230, 232 or 233) ; natural, enriched
and depleted uranium; natural thorium; thorium-232; thorium-228 and thorium-230 when diluted to a spec i f i c activity o f the same order as that o f natural uranium and natural thorium; - short-lived nuclides, such as astatine-211, radium-224 and daughters o f the isotopes o f radon.
d The relaxation for low-energy beta emitters does not apply to beta-emitting plutonium isotopes.
The resuspension factor of radioactive material f r om contaminated clothing during changing has been determined to be 10"4 /m,to 10"3 /m [12], The greatest inhalation risk to personnel f r om contaminated clothing occurs during removal of the clothing and during the process ing of clothing at a laundry be f o re it has been washed. Contaminated clothing should be laundered be fore being re -worn , when any residual contamination may be considered to be fixed. The contamination levels on clothing should not exceed those permitted f or skin contamination. A.relaxation factor of 10 may be applied to coveral ls and laboratory coats worn only in active areas.
2. 6; Relaxation of derived working limits
Some relaxations of these basic derived working limits may be allowed. In addition to the mitigating factors descr ibed in Section 2. 2 (second para -graph), others may be considered. The inhalation risks f r o m surface contamination so far considered assume l oose contamination uniformly distributed over at least several .square metres . In pract ice this rare ly occurs ; even in cases of widespread contamination it is usually patchy with a few l imited areas' or d iscrete spots per square metre. . If l imited • areas are defined as those which do not exceed 100 c m 2 / m 2 , the risk f o r this type of contamination is far l ess than general contamination [9, 18].
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If a relaxation factor of 10 is applied in these c i rcumstances , no appreciable increase in risk to personnel should occur . Such general relaxations are only applied in active areas; in other c ircumstances , each case requires individual assessment .
If the contamination is present as d iscrete partic les which may have a high spec i f i c activity resulting in high surface dose rates, further con -sideration is necessary be fore permitting relaxation of the derived working l imits , especial ly in the cases of skin or clothing contamination. Typical dose rates to 1 cm 2 of skin f r om particulate contamination on clothing [26] are approximately 2. 5 rad/h f r om 1. 0 ^Ci strontium/yttr ium-90, and 4 rad/h f r om about 1. 0 txCi of two -year -o ld f iss ion products. Because of the unfavourable geometry a contamination monitor may indicate that the activity of a spot of contamination is lower than it actually is.
TABLE II. PRESCRIBED VALUES OF SURFACE CONTAMINATION [18] (Oak Ridge National Laboratory)
Site Method Contamination value
Skin — hands -
Skin ~ general body
Clothing (other than contaminated zone)
Clothing (contaminated zone)
Direct reading
Direct reading -
Direct reading
•Direct reading
a 150 dpm per 100 cm 2 , or 6. 7 x 10"7 f iC i / cm 2
B-y 0. 3 mrad /h
a 150 dpm per 100 cm 2 , or 6. 7 X '10 - 7 f i C i / c m 2
8 - y 0 .06 mrad/h
a 150 dpm per ' lOO'cm2 , or' 6. 7 x - 1 0 " ' ( iC i / cm 2 '
B -y 0 .25 mrad/h
a 150 dpm per 100 cm 2 , or 6. 7 x 10"7 ( iC i / cm 2
B-y 0. 75 mrad /h
Non-contamination zone areas
Direct reading
Transferable (smear survey)
a 300 dpm per 100 cm 2 , or 1. 3 x 1 0 - 6 ( iC i / cm 2
B-y 0 .25 mrad/h '
a 30 dpm per 100 c m 2 , or 1. 3 x 10"7 ( j C i / c m 2
' 6 - y 1000 dpm per 100 c m 2 , or 4. 5 x 1 0 - 6 f i C i / c m 2
Tools and equipment for re-issue
Direct reading
Transferable (smear survey)
a 300 dpm per 100 cm 2 , or 1. 3 x 10"6 f j C i / c m 2
B-y 0 . 05 mrad /h
'•a 30 dpm per 100 c m 2 , or - 1. 3 X 10"7 ( iC i / cm 2
i-y 200 dpm per 100 cm 2 , or 9 x 10"7 p C i / c m 2 • .
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TABLE III. COMPARATIVE DATA ON MAXIMUM PERMISSIBLE LEVELS OF SURFACE CONTAMINATION USED IN DIFFERENT ATOMIC ENERGY ESTABLISHMENTS [17]
Location Level for
alpha emitters ( d p m / 1 0 0 cm 2 )
Level for be ta -gamma emitters
(mrad/h)
Atomic Energy Establishment, Trombay
Atomic Energy Research Establishment, Harwell (act ive areas)
Oak Ridge National Laboratory [ 2 0 ]
Los Alamos Scientific Laboratory [ 2 1 ]
Brookhaven National Laboratory [ 2 2 ]
USSR [ 2 3 ]
600
2 0 000
300
100
100
133
0 .3
7 . 5
0 .25
0. 05
0 . 1
1 . 4
Following a skin contamination incident it may be necessary to consider at which stage decontamination should stop, further decontamination possibly being m o r e injurious than the radiation dose f r o m the residual activity. The possibi l ity of percutaneous absorption into the blood stream, and irradiation of the basal layers of the skin as well as the surface dose rate and possibil ity of transference by ingestion or inhalation into the body must also be considered.
Although certain derived working levels and relaxations to these levels have been quoted, it is often possible to work to lower levels without too much additional expenditure of funds and time. The aim should be to reduce exposure of personnel to the minimum, therefore radioactive contamination should be removed at the earliest opportunity. The derived working levels are guide lines to which to work, and should be used as such, rather than as hard and fast rules which must be adhered to at all costs .
3. SURFACE MONITORING TECHNIQUES
3. 1. General
The aim of all radiological protection operations is to prevent persons being needlessly exposed to harmful radiation, and surface contamination monitoring must be planned with this aim in mind. Indiscriminate m o n i -toring is meaningless and therefore a waste of t ime and manpower.
The International Commiss ion on Radiological Protect ion in ICRP Publication 12 has subdivided monitoring of the working place into three distinct types, environmental monitoring, operational monitoring and special monitoring. The functions of each type respect ively are: to show that conditions in the working environment are satisfactory; to keep a check on a particular operation and f o r m a basis f o r immediate decisions on the conduct of the operation; to provide m o r e detailed information about a situation or operation to define problems and to f o r m a basis on which to plan future procedures. [4] ,
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T A B L E IV. EXAMPLES OF PRESCRIBED VALUES OF SURFACE CONTAMINATION [18]
NCRP Handbook 48
British Codes o f Practice ( j i C i / c m 2 )
A tomic Energy o f Canada Ltd. c
(liC i / c m 2 )
France, CEA
( / i C i / c m 2 )
1. Hands and other parts' 0 . 1 m r e p / h a
1. 0 mrep/h b
a 10"5
B 10-4
a 10"6
0 1 0 "4
a 5 x 10"6 a
0 5 x 10"5 a
0 5 x 10-s b
2. Personal c lothing Same as above a lO" 5
a 10-*
6 10-1
a lO - *
6 5 x 10"4
a 10"5
B 1.0-4
0 10"4
3. Protective clothing, glassware;' tools,
; objects to be handled with bare hands
4. Ac t ive areas
5. ' ' Inact ive ' areas
Same as above
Same .as above
Same as above
a 10"4
a "10"3 a
0 10" 3 b
•Same as above .
a 10"5
a 10"4 3
a 10" 6
6 10-3
a 10"5
0 ' c lean on swipe ' . Less than 2 mR/h on contact
a 10"6
8 10"5
a 10"4 a
0 10" 3 a
0 10" 3 . b
Same as above
a 10-5
6 10"4
6 1 0 - 4 .
Very hazardous radionuclides
Other radionuclides
In NCRP Handbook 92, the leve l for radionuclides corresponding to groups 1 and 2 o f T a b l e I is given as 0 . 1 mrad/h whilst that, for radionuclides corresponding to groups 3 and 4 o f Table I is given as 1. 0 mrad /h .
c The fundamental test is that the surface must be 'c lean on swipe ' , and 'swipe' tests cannot be expressed as ( i C i / c m 2 . T h e background in the active areas is such that lO" 4 ( i C i / c m 2 0 could not be measured with the instruments avai lable . With these provisos, the nearest values that could be indicated to conform with the others listed are those shown in this column.
TABLE V. MAXIMUM PERMISSIBLE LEVELS OF RADIOACTIVE CONTAMINATION ON OUTSIDE SURFACES OF TRANSPORT CONTAINERS [25]
Contaminant Maximum permissible leve l Contaminant (/iC i / c m 2 )
Beta or gamma emitters ' ' . 10"4
Alpha emitters 10"5
Note: The above levels are permissible when averaged over any area o f 300 cm 2
of any part of the surface.
A surface decontamination program should be planned with certain pr imary objects in view, and the limitations of such a program in the. general field of radiological protection should be realized. The program should be designed so that surface contamination does not exceed levels at which personnel might be exposed to radiation doses approaching the limits recommended by the ICRP. It should be able to detect failures of radioactive material containment and departures f r o m correc t operating procedures , and should provide information f r o m which it is poss ible to plan-personnel monitoring and air monitoring programs.
In pract ice it is difficult to correlate surface contamination in a working place with the exposure of workers , therefore surface contami-nation monitoring alone is inadequate. Additional radiation, air and individual monitoring programs .are required.
In addition to monitoring surfaces and artic les in radioactive working areas, neighbouring areas such as change rooms , o f f i ces and rooms in which food and drink are consumed should be monitored routinely. Presence "of radioactive material in these areas indicates a break-down in the control systems. . ,
F o r routine monitoring programs of large areas of f l oor , "benches, tables, etc. direct monitoring techniques are normally used. Indirect monitoring may be required in certain c ircumstances , and if direct monitoring shows the presence of contamination then indirect methods are' used to ascertain whether or not the contamination is fixed. \
To calibrate instruments so that meter readings can be converted into activity levels by applying the correc t convers ion ' factor , the energy of the radiation being measured, the geometry of :the radioactive source /de tec tor assembly, counter characterist ics and amount of sel f -absorption in the source must be considered. . ' . •
Laborator ies designed f o r radioactive work have, o r should have, relatively impervious surfaces making the quantitative estimation of the amount of activity present from, direct measurements fairly easy. Away f r om a laboratory or process area the problem is more difficult. Con-tamination on porous surfaces such as roadways, footpa.ths (tarmacadam, concrete) , br ickwork, soil or grassed areas, will penetrate below the surface and some of the radiations f rom the contamination will be fully attenuated. In these c ircumstances quantitative estimation of surface contamination f r om direct measurements made:with routine surface monitoring instruments is impossible .
10
All indirect methods re ly upon measuring the amount-of radioactivity removed f r om a surface. The greatest disadvantage of indirect monitoring l ies in the uncertainty as to the fraction of- surface contamination removed. The removal factor is affected by'the type of surface, physical f o rm of the contaminant, and in the case of smear monitoring, personal variations in sampling technique affecting the s ize of area smeared and pressure applied to the smear . ' Indirect monitoring will detect, the presence of only r e m o v -able radioactive contamination.- Calibration of instruments to measure the sample activity is necessary as in the case of direct monitoring.
3. 2. Selection of method , •. '
Several points must be considered be fore a choice between direct or indirect methods of monitoring is 'made. Most -important is the type of -• contamination which may be present, whether alpha emitters, beta emitters or - low-energy beta emitters, 'and whether they are present indi-vidually or col lect ively, so.that the appropriate detector is used. In laboratories or p r o c e s s areas ^handling only one speci f ic radionuclide, it will only be necessary to monitor f o r one type of contamination unless it is suspected that other, radionuclides have been spread f r om other areas. In some.places such as decontamination or.waste disposal areas,- the presence of any radionuclide used in the establishment must always be suspected.
Although alpha, beta and some low-energy beta emitters can all be detected by direct monitoring methods, certain interfering factors, may make direct monitoring impossible . . There is , generally speaking, little problem in monitoring smooth surfaces such as f l oors , bench tops and the outside of some apparatus, -except that.moisture or dirt may interfere with direct monitoring f or alpha emitters. A general high gamma back-ground, or even a high beta, background emanating f r om inside thin-walled vesse l s , will make it difficult to detect low levels -of beta, contamination by direct monitoring.- . , . . .
- It may.be geometrically- impossible to monitor directly. .Indentations • in.the surface of an article may be so deep that the distance f r om the bottom of the indentation to the 'detector ' is greater than the range of-the partic les emitted, and the inside of a-vessel or-tube may be inaccessible to the probe. In these c ircumstances indirect techniques must be employed.
Indirect monitoring will not detect the presence of fixed contamination, nor will direct monitoring show whether the contamination detected is fixed or loose . The two methods should be used, one to complement the other to obtain a complete assessment of the -state of a surface.being- examined.
4. DIRECT METHODS OF-.SURFACE MONITORING •
4. 1. Alpha contamination monitoring.-. '
Alpha monitoring raises special problems because of the short range of alpha partic les in air and their complete attenuation by thin f i lms of liquids :or solids; 5 -MeV alpha part ic les are completely attenuated in material with a thickness of .aD'out 5 ,mg/cm 2 . .
The transient response of a probe passing over a source is cons ider - . ably les;s than the response with the probe stationary above the same.
11
source [27], The t ime constant of the ratemeter is often longer than the transit time of the probe past the source , therefore the meter may not indicate the presence of the contamination.. Although a loudspeaker has a quicker response, the ear may not. detect small variations in the loud-speaker sound pulse-rate . .
To ensure detection of alpha contamination by direct monitoring methods, the probe should not be m o r e than about 0. 5 cm f r o m the surface under examination and the probe transit velocity across the surface should not exceed about 15 c m / s .
Although a variety of detectors is available f o r alpha monitoring, the technique f or using them is always similar .
4. 2. Beta contamination monitoring — general techniques
The detection of all but the lowest-energy beta radiation is easier than that of alpha contamination because of its longer range and greater pene-tration, but attention must be paid to-several points.
The response, of a beta probe is dependent on the energy of the beta radiation. Because all radionuclides do not emit beta radiation of the same energy, probes should be calibrated so that the count-rate f r om contami-nation by the relevant radionuclide at its derived working level is known.
During routine monitoring the area under examination is usually 'swept' by the probe, rather than a number of measurements made with a stationary probe over different parts of the area. The response of the probe varies with the distance between the surface and the probe but the effect is l ess than the change in response due to variable transit veloc it ies . The response of a Geiger tube with a 30 m g / c m 2 envelope varies by only a factor of about 2 at distances between 2. 5 and-10 cm f rom the surface with a probe transit velocity of about 15 c m / s . -For these reasons and those outlined.in Section 2. 1 (second paragraph), the transit velocity of the probe should not exceed about 10 to 15 c m / s at a distance of between 2. 5 and 5 cm f r o m the surface [27],
When high levels of contamination are being monitored the dead t ime of the detector will affect the count-rate and.may;lead to underestimation of the activity present. . . .
Radiation f r om very high levels of contamination may 'saturate' a Geiger tube, resulting in an erroneous nil reading being obtained on the ratemeter. Most modern instruments are designed to maintain fu l l - s ca le deflection on the ratemeter , and if fitted with a loudspeaker a distinctive note is heard should the tube be saturated.
Some probes are fitted with a shield, which when c losed prevents beta radiation f r om reaching the detector, enabling a gamma-only measurement to be made. Using this facility to identify whether the radiation detected is beta or gamma may help to determine whether or riot the emitting source is above or below a shielding surface, e. g. whether the radiation detected is f r o m contamination on a bench or from- a ,s,ource in a drawer under the . bench. Similarly, by taking measurements at varying distances f r o m a source and comparing the rate of change in count-rate it may be poss ib le to determine whether or not the radiations are beta or gamma. Use of this method over a porous surface may indicate the approximate depth of the contamination below the surface. • ;
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4. 3. Beta contamination monitoring — low-energy emitters
Because of the short range of l ow-energy beta radiations special prob lems s imi lar to those encountered in alpha monitoring are raised.
Geiger tubes or scintil lators with thin windows (about 2 m g / c m 2 ) o r . • gas - f l ow or a ir - f i l l ed proportional counters are most suitable. Indirect methods are usually employed to monitor for tritium [28] andplutonium-241.
, 5. INDIRECT METHODS OF SURFACE MONITORING.
5. 1. General .
Indirect methods of surface contamination monitoring are used when unsuitable geometry or interference f r om other radiations prevent direct monitoring and as a complementary method when direct monitoring has shown the presence of radioactive contamination. A wide range of tech-niques is available, but all depend upon the removal of radioactive material f r o m the surface being examined. It may be necessary to take samples f r o m the area being monitored to another area f or counting.
The fraction of material removed during routine indirect monitoring is not known accurately. A smear test may remove anything f r om 0 to 100% of the contamination present, but generally a removal fac tor of about'10 to 20% is assumed. The variations in the removal factor make quantitative estimation of the amount of contamination present on the surface uncertain. The removal factor is affected by varying the pressure applied during smearing, by the type of surface or smear , and by the f o r m of the contami-nation. Devices to ensure uniformity in pressures applied and area , • sampled have been descr ibed [29] and usually include some form' of spring loading of a pad to which the smear paper is attached.
When a smear paper is used manually, most of the contamination picked up on the paper is concentrated in an area of about 5 cm2 coincident with the area of the f inger tips [29], Because this area may be small compared to the area of the detector used to measure the smear activity, erroneous results may be obtained unless the probe is specially calibrated.
Most surfaces in radioactive laboratories and p r o c e s s areas can be smear-tested satisfactori ly. Outside these areas where rough brickwork, tarmacadam roadways, concrete paths and hardstandings, areas of gravel , soil or grassland may be encountered, the ability to obtain a satisfactory smear sample var ies f r o m the difficult to the almost impossible .
If success ive smears show decreasing amounts of activity, but it is suspected that some radioactive contamination is left on the surface being examined, further tests should be made after 24 h. Experience has shown that where direct and indirect monitoring has indicated that fixed contami-nation only is left after some decontamination procedure , after such a period loose contamination again may be detected. In certain cases the amount of removable activity detected on smears has reached a level approaching that originally found [29].
The validity of indirect methods' in which the amount of activity t rans-ferred to r inse solutions or wash-down l iquors is estimated depends upon the solubility of the contaminant in the liquid in question, or on the h o m o -geneity of part ic les dispersed in the liquor.
13
An artic le m a y b e declared f r e e f r om radioactive contamination with respect to indirect monitoring, but fixed contamination may still be present. Although adhering to the surface f i rmly enough not to have been removed and thus detected by the indirect method employed, it may become dislodged if the surface is m o r e vigorously attacked during work upon at, e; g. by grinding, filing or polishing.
5. 2. Dry smears
The technique of smear monitoring usually involves rubbing a c i r c l e of dry f i lter paper over about 100 cm2 of the suspected surface and then measuring the amount of activity transferred to the smear. This is repeated until the whole of the suspected area has been monitored. By applying a removal factor , the approximate amount of l oose radioactive contamination on the suspected surface can be estimated. This method obviously cannot be applied operationally and routinely when large surface areas or large p ieces of equipment are to be examined.
When taking smear samples f r om sealed sources or highly active surfaces the smear paper must be held by tongs or forceps . In this case, it is necessary to mount the smear paper on a pad such as a rubber bottle .stopper which can be more readily held by the remote handling equipment.
Dry smearing is a most inefficient way of removing radioactive material f r om a surface and serves only to indicate the presence or absence of material which may be transferred f r o m the .contaminated surface to a clean surface by casual contact or rubbing.
5..3. Wet smears •
The techniques of 'wet smearing are-s imi lar to those of dry smearing. The liquid chosen to moisten the smear papers should not attack the surface to be monitored but should be ef fect ive in removing the radioactive material involved. A low-toxicity organic degireasing agent such as tr ichloro-ethane, which is : non-inf lammable , is a good general solvent for this purpose. The commerc ia l ly available solvent, sold under various trade names, usually contains some corros ion inhibitors. • • •
Wet smears must be dried be fore monitoring f or alpha activity or l ow-energy beta activity. Attenuation of the radiations in the smear paper, due to the radioactive material having been carr ied down into the paper by the wetting liquid, may result in underestimation of the activity.
The techniques descr ibed in this section do not apply to monitoring f or tritium' (see Section 5. 9 in which m o r e appropriate methods are described) .
5. 4. Large area swabs
In many c ircumstances the. use of-small smears is uneconomic of time and labour, o r operational commitments may demand that large areas or art ic les must be examined quickly, e; g.. following suspected spills or large airborne re leases of radioactive material . It is acceptable pract i ce in these c ircumstances to use large area smears o r swabs. A pad of cotton waste or s imilar material is moistened with a suitable liquid and
M
an area up to 2 or 3 m 2 is wiped. The pad is then monitored. F loor mops or similar cleaning equipment can be used successfully in this'type of monitoring. This method is not satisfactory for alpha monitoring, unless a smooth pad or cloth of fairly impervious, easily dried material is used.
No attempt at quantitative estimation of the amount of activity present on surfaces should be made f rom large area swab results; direct moni-toring, if possible, or more detailed smear testing must be done if quantitative estimates are. required. .
5. 5. Adhesive tape samples.
A piece of impact adhesive tape (1 or 2 cm2) pressed against a surface will collect a sample of removable contamination. The contamination is f irmly held on the tape and can be examined without the risk of losing-it or cross-contaminating monitoring instruments.. This is a particularly useful method of obtaining samples f rom inside areas where access is restricted, such as hot cel ls .
As in the case.of conventional smear monitoring, removal factors depend upon the type of surface and physical form of the contamination, and the same reservations must be applied to results calculated from •measurements made in this way.
5. 6. 'Smair' sampling [30]
The smair sampling procedure employs air impingement to redisperse loose contamination f rom the surface under examination.
A smair sampler is a conventional air sampler fitted with a special sampling head. Through the head is a series of small holes, drilled at angles and so arranged that jets, of air will be directed at the surface to be examined. When the open end of the sampler is pressed against the surface, air intake is restricted to the series of holes. Contamination, resuspended by the air jets, is collected on the sample filter paper and counted like an ordinary air sample. •
. The smair technique shows a relatively constant removal factor f rom all types of surface and minimizes the inaccuracies introduced by var i -ations in personal techniques during smear testing. Variation in the air velocity through the impingement holes leads to associated variations in the resuspension factors.
5. 7. 'Sniffing' techniques'
These are c losely allied to the smair techniques, but are limited to the evaluation of radioactive gases or vapours sqrbed on surfaces. The sample head of a portable trit ium-in-air or similar type of monitor is passed over the suspected surface, at about 1-2 mm distance. The evolved radioactive gases or vapours pass through a detachable fi lter and e lectro -static ion trap into a compensated ionization chamber for counting.
Thoron and radon gases will decay inside the ion chamber contami-nating it with their radioactive daughters, making the instrument unservice-able until they have decayed.
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5.8. Examination of rinsing and washing solutions
Estimation of the amount of radioactivity in the r inse solutions f rom articles being decontaminated and f rom the washing machines in radio -active laundries will show whether or not radioactive contamination is still being removed. This method does not show the presence of fixed or insoluble contamination which may be oh the articles being cleaned.
Activity measured in a solution in which a radioactive source has been immersed will indicate either the presence of contamination on the source or leakage or leaching of radioactive material f r om the source [18, 24],
5. 9. Tritium smears
Metal foi ls or discs will remove tritium f rom surfaces . A f i l ter paper, moistened with glycerol , has proved to be a better smearing medium having a better transfer ratio, probably due to chemical exchange and better contact with the surface as it conforms m o r e readily to the surface geometry.-' The activity on the smear can be counted in a gas - f low p r o -portional counter. A more accurate method is to seal the smear sample immediately into a phial containing water and a liquid scintillator; the sample can then be counted later in any suitable counter.
Experiments [29] have shown that up to 50% of the tritium transferred f r o m a contaminated surface to the sampling material is lost by evaporation during the f irst 20 minutes after sampling.
5. 10. Skin and wound.monitoring
Although skin and wounds can usually be monitored directly, est i -mation of the activity on surgical swabs will check the ef f ic iency of the decontamination methods being used.
As a f i rst -a id check f o r radioactivity, samples of blood f r o m open wounds may be dried on glass or metal trays and counted. The blood ' should not be absorbed into swabs or f i l ter paper because alpha and l o w -energy beta particles may be fully attenuated. F o r detailed examination samples should be sent to a bioassay laboratory.
5. 11. Instruments used for indirect methods
Since the estimation of the amount of activity on a sample is a direct monitoring procedure, any portable monitor or installed counting equip-ment which fulfils the general characterist ics described in Section 8 may be used f or this purpose during the application of overall indirect methods of surface contamination monitoring.
6. SUPPLEMENTARY TECHNIQUES
Some supplementary specialized techniques available to assist in the evaluation of risk or dose to persons exposed to radioactive surface con -tamination are outlined in this section.
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6. 1.-' Autoradiography
An autoradiograph will locate the position of small sources or areas of beta and gamma radioactive contamination on skin or inanimate surfaces . The simplest method is to p lace an X - r a y plate in contact with the contami-nated surface f o r a per iod determined by the dose rate f r om the contami-nation. F o r beta activity good correlation between spot s ize arid activity is obtained. In the case of alpha contamination it is necessary to remove the wrappings f r om the X - r a y plate, therefore the operation must be done in a dark room. In this case the spot obtained on the X - r a y plate is more a measure of the range of the alpha emission than the activity or s ize of the particle . . . • • - . . - -
A more suitable method of autoradiography for alpha activity is f irst to convert the alpha partic le energy into visible light by a scintillator and then to record the light emission.on a l ight-sensitive photographic plate [31, 32], .. ; ..
6. 2. Thermoluminescent dosimetry
The surface dose f r o m some contaminated surfaces may be measured by exposing a thermoluminescent dosimeter in contact with it. There will be some under-reading with respect to low-energy beta particles due to their absorption in the sachet containing a thermoluminescent powder, therefore correct ion factors must be applied.
The distribution of the powder in a thermoluminescent powder sachet . will affect the dose estimation. When approximately 26 mg of LiF powder are l oose in a packet the powder thickness may vary between 20 m g / c m 2
and ~ 130 m g / c m 2 . Taking a sachet window thickness of 14 m g / c m 2 the mean depth will be 24 to 79 m g / c m 2 . Even f o r the relatively energetic beta particles f r om yttrium-90 the response of the dosimeter in the latter condition is only about 0. 65 of a s imi lar dosimeter having uniform powder distribution'. The use of lithium-fluoride-impregnated teflon discs would reduce this e r ror as the thermoluminescent powder is kept homogeneously distributed in the dosimeter . Some estimate of the dose to subcutaneous tissue f r om beta contamination'on skin is obtained by introducing appro-priate f i l ters between the contamination and the dosimeter.
6. 3. • Radiation spectrometry
Alpha, beta and gamma radiation spectrometry [33] identify radio -nuclides by the characterist ic energy of their radiation. .Such techniques may be extended to make quantitative measurements of radioactive mixtures. This information ass ists in determining the nature and degree of hazard arising f r om contamination and may also help to identify its source .
Gamma-ray spectrometry is particularly valuable because the pene-trating nature of gamma radiation precludes the need f or sample prepa-ration and it is usually possible to do measurements on samples in the. f o rm in which they are received. . The samples may be smears f rom contami-nated surfaces , bottled liquids, clothing, soi l , grass , • gravel, tools or small p ieces of apparatus. Similar measurements may be made on con-taminated humans or animals. Currently, two methods of gamma-ray
17
spectrometry are commonly used, by l ithium-drifted germanium solids, state detectors [34] and sodium iodide crystal scintillation detectors. The sol id-state detectors are capable of resolving gamma-ray energies differing by 2 or 3 keV, but have the disadvantage that they operate at liquid nitrogen temperatures only. Because of their small s ize , they have low sensitivity and samples should be not l ess than about a m i c r o c u r i e to obtain satisfactory results. The sodium iodide detectors have p o o r e r resolution (by normal techniques, energies differing by l ess than about . 50 keV cannot be resolved) , but their ease of operation and ability to detect about a p i cocur ie make them an attractive tool.
Since the energy spectrum of beta radiation is continuous, high r e s o -lution spectrometry is not possible . Scintillation spectrometry using a . sufficiently large plastic phosphor may be used although better resolution is obtained with a l ithium-drifted sil icon sol id-state detector in conjunction with a multi-channel pulse-height analyser. Variable distortion of the beta spectrum due to scattering of the radiation f r o m the detector housing, -sel f -absorption in thick sources , and interference by scattere'd gamma rays add to the problems of this f orm of spectrometry. Although not a very attractive form of spectrometry, when beta radiation only is being measured it is vastly superior to the use of aluminium absorbers .
Gridded ion chambers , scintillation 'detectors and sol id-state detectors are used f or alpha spectrometry. Because of the short range of alpha . part ic les the type of sample which may be examined is limited. Ideally, samples should be thin sources mounted on stainless-steel discs . O p e r -ational smears can be examined although the spectrum obtained is usually degraded.
Spectrometric methods are used f or in-vivo measurements in cases of either external or internal contamination of personnel.
Aluminium absorbers are commonly used to determine .maximum beta energy. Aluminium absorbers of increasing thicknesses are interposed between the source and the counter'window. The degree of absorption is determined by'counting the beta partic les penetrating each absorber . The observed count-rate, modif ied as necessary by counter dead-time c o r -rect ions, counter window thickness and air absorption factors are plotted against the absorber thickness (in m g / c m 2 ) on semi- logar i thmic paper. The maximum range is read by visual inspection, " The maximum' beta • ' energy is obtained by re ference to tables o r graphs.
The visual estimation of the maximum range is much less p r e c i s e when the beta radiation is accompanied by gamma radiation, s ince the latter produces a long 'tail ' to the curve, hiding the'end point of 'the beta radiation absorption curve;- In these cases , , the Feather method of analysis gives a m o r e accurate estimation of the range [35]. The long tail of the curve is ignored, and the f irst 70-80% of the beta absorption curve are c o m -pared with the corresponding portion of the absorption curve of a pure beta emitter. Originally Feather used RaE (2:10Bi), but phosphorus-32 is equally suitable.
The absorption curve f or the re ference source is plotted and the maximum range observed. The abscissa is then divided into ten equal fractions of the range. The activities corresponding to these fract ions, excluding the 9/10 and 10/10, are marked' in the ordinate. These marks , corresponding to the various fract ions of the range are transferred to a 'Feather analyser ' , which may simply be a strip of card, but can be made
18
f r om;more durable material. The Feather analyser may now.be used to determine the maximum range of beta particles f rom another radionuclide. The same experimental conditions must be used to obtain the absorption curve for identification and the results plotted with the same ordinate scale. The-Feather anaiyser is placed by the side of the graph, with the zero mark opposite the point corresponding to the count-rate for zero absorber. The values of absorber thickness, corresponding to the various fractions ar.e read .off the graph and divided by the corresponding fractions of. the range. Each is now an estimate of the total range.. These estimated values are plotted against their corresponding fraction, and the.curve is extra-polated to the full range. The energy corresponding to the full range is then obtained f rom reference tables or. graphs.
In areas where low-energy as well as higher-energy beta emitters can occur , all samples may be examined routinely as follows [36], The activity on the smear is measured by a thin window (2 mg/cm2) , detector^ with and without an aluminium absorber thick enough to absorb the low-energy betas between the sample and'the detector. The difference between the two count-rates will be that due to the low-energy component. If alpha activity is. • supposed also to.be present, a further count with tissue paper between the sample and the counter should be made. The relative amounts of each type of radiation present can then be calculated.
6.4.- A ir monitoring••. • ' ,
' Widespread contamination at derived working levels may lead to a ir -borne contamination .at or above maximum permissible concentrations'when movements associated with.normal work in an ar.ea are made. If air samples at this level are obtained, the possibility of widespread surface.contami-nation, should be considered, as well as the more obvious and usual con-clusion that airborne radioactivity is being released f rom an operation or process in the.area.
Surface contamination monitoring should be started when high air samples following a release of airborne activity f rom some accident or process ar.e .measured,, because surfaces may have become contaminated by radioactivity deposited f rom the airborne radioactivity.
6. 5. Deposition trays
The presence of a slow, long-term build-up.of surface contamination in areas not readily accessible for direct or indirect monitoring, such as building rafters, serv ice ducts, etc. , can be detected by placing deposition trays, in these areas and counting the activity on them at intervals of a few months. *
6. 6. Overshoe and.cleaning material monitoring • . ' . . . -
Routine monitoring of overshoes and shoes worn in radioactive areas often provides the f irst indication that radioactive contamination of the : f loor ..has occurred.
Mops, cleaning rags and vacuum cleaners used in radioactive areas should be checked frequently for two reasons. First , the'presence of radioactive material on .them, will indicate the presence of radioactive
19
surface contamination in the area. Second, the undetected presence of radioactive contamination on cleaning appliances can result in the spread of contamination to hitherto clean areas.
6. 7. Personnel dosimetry
An unusual density pattern on a developed personal f i lm badge d o s i -meter may be due to contamination on the badge. Alternatively, it may be due to unshielded radiation f r om surface contamination in the working area, or radiation f r om contaminated clothing.
The occurrence of doses higher than planned or expected on f inger or wrist dosimeters can be due to contaminated personal gloves or installed gloves, as in glove boxes at which the operator has worked.
6. 8. Biological monitoring
In-vivo, measurements in whole-body counting equipment and exami-nation f o r radioactivity in excreta will conf irm the effectiveness or other-wise of contamination control procedures .
6 .9 . Pinhole cameras
Mapping areas of beta [37] and gamma [38] contamination and the -distribution of activity on sources [39] can sometimes be done m o r e ef fectively by a pin-hole camera than by autoradiography. Some of the prob lems of exposure to radiation and contamination associated with-the latter method are eliminated because shielding and remote operation faci l it ies can be built into the camera.
A conventional picture and a 'radiation' picture [38] can be obtained if a l ight-sensit ive and a radiation-sensitive f i lm pinned together are exposed simultaneously. By superimposing one negative on the other the radiation, sources can be located.
Disadvantages of the system lie in the long exposure times and in the weight of the shielding which may be required to prevent interference f r o m extraneous sources of radiation.
7. CALIBRATION AND PERFORMANCE CHECKING OF SURFACE MONITORING EQUIPMENT
Surface contamination monitoring instruments as well as all health physics instruments should be calibrated and their per formance checked at certain tim'es so that their readings may be correc t ly interpreted. The frequency of calibration and its extent depend on the type of instrument and its usage but may be broadly divided into three categories .
(a) When a new instrument is f i rst put into serv ice , after an instru-ment has been repaired, and after predetermined periods in serv i ce it should be calibrated against standard radioactive sources and its p e r -f ormance at such a test recorded. The testing should be done by a person qualified to make simple adjustments to the instrument to make it serv iceable . In many countries this sort of testing and recording is a statutory requirement.
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(b) Routine checking on a daily o r weekly basis of the instruments' response to sources in a s imple calibration jig [40, 41] may be done by any person competent to use the instrument, interpret the results and make simple adjustments.
(c) Per formance checking in the f ield is necessary to conf irm that the instrument continues to work correc t ly . A small uranium disc is usually adequate f or this purpose.
The accuracy of calibration can be identified at four levels : pr imary , secondary, laboratory and field standards.
(a) Pr imary standards are those nationally or internationally established to ensure consistency in interpretation and measurement within the country or group of countries.
(b) Secondary standards are those established for use by laboratories to provide calibration faci l it ies f o r their own f ield and laboratory instru-mentation.
(c) Laboratory standards are those applied in prec is ion radiochemistry and physics work and in radiological protection laboratories supporting operational radiological protection teams.
(d) Fie ld standards are those applied to the per formance required in routine operational monitoring programs.
It may be necessary also to determine the response of the monitor to special c ircumstances . An instrument will respond to small smear samples (see Section 5. 1, third paragraph) and discrete spots of radioactive con -tamination [29] differently f rom the way in which it will respond to an extended source. To calibrate an instrument f o r this work, standard small area sources in s imple j igs designed to reproduce the geometry of the smear /de tec to r assembly are adequate.
The response of beta detectors varies with beta energy. F o r m e a s u r e -ments more prec i se than those f o r s imple routine surface contamination monitoring where the detection of the presence of contamination is all that may be required, and in areas where a number of different radionuclides is used, the detectors must be calibrated against standard sources of differing beta energies.
Whenever possible , points at the lower, middle and higher ends of the instruments' range should be checked. This is most important if switch-range instruments are being used.
In the case of contamination monitoring instruments in which the probe is not integral with the ratemeter, the latter may be calibrated e l e c t ro -nically by a pulse generator, after the detector probe has been disconnected. '
Internationally agreed per formance specif ications f o r radioactivity detecting instruments are in preparation. The standard specif ications will apply to the electronic reliability and the radiation detection eff ic iency.
To satisfy the calibration requirements outlined above, a number of calibration standards may be used. Most commonly these are ca l i -brated radionuclide sources or specially calibrated instruments [42], In the f irst case the response of the instrument is checked against the response it should give theoretically when exposed to the radioactive source . In the second case the response of the instrument under test to a . radio -active source is compared with that of the pr imary instrument to the same source .
21
I
FIG. l a . Combined alpha and beta ptobe checking j ig . Calibration j ig with alpha probe.
FIG. l b . Calibration j ig with standard beta probe (30 m g / c m 2 glass envelope Geiger-MDUer tube).
22
FIG. l c . Calibration j ig with end window (2 m g / c m 2 ) Geiger-Milller probe.
A simple jig, suitable f o r daily checking of instruments is illustrated in Fig. 1. Four americ ium-241 sources to check alpha detectors and three strontium/yttrium-90 sources to check beta detectors are mounted in a wooden box which f o rms the jig. The strength of each source i n p C i , and the expected count-rate in counts per second (cps) is given on a card attached to the inside of the lid of the jig. The j ig is designed so that the standard beta probe and the end-window probe are held in fixed positions with respect to the beta sources . Alpha probes may be moved free ly so that any non-uniformity in response over the detector area will be indicated.
Suitable calibration sources f o r beta contamination monitors are strontium-90 o r natural uranium in equilibrium with their respect ive daughters or phosphorus-32. Carbon-14 obtainable as thin sheets or discs of co lour less poly (methyl-1 4C) methacrylate may be used as solid re ference sources f or calibrating l ow-energy beta detectors. Plutonium-239, americ ium-241 or natural uranium in equilibrium with its daughters make suitable alpha calibration sources .
8. INSTRUMENTATION
8. 1. Criter ia f o r selection of instruments
A wide range of contamination monitoring instruments is available, varying in versatil ity and cost , f r o m the cheap single-purpose instrument to the m o r e expensive general -purpose monitor capable of being used with a variety of probes to detect the presence of most radionuclides. Figure 2
23
i l lustrates such an instrument. Great care should be exerc ised to select the most appropriate instrument f or the monitoring program to be under -taken. The cr i ter ia to be considered are summarized as fol lows:
(a) General cr i ter ia which should be considered when selecting instru-ments f o r surface contamination monitoring are:
(i) They must be reliable and give reproduceable data; (ii) The initial cost , and the costs of operating and maintaining the
instruments must not be excess ive with regard to other costs in the work to be controlled and the risks associated with that operation.
(b) F o r contamination monitoring all instruments should fulfil the following requirements:
(i) They should have adequate range; (ii) They should respond quickly to the presence of radiations;
(iii) When exposed to levels of radiation in excess of their full range, their meters must maintain fu l l - sca le deflection;
(iv) There should be an audible output either via a loudspeaker or an earphone system;
(v) Battery, low and high voltage checking systems should be in-corporated in the c ircuitry;
(vi) They should be robust enough to withstand a limited amount of rough treatment.
8. 2. Detailed assessment f or choice of instruments
The detailed assessment of an instrument may be made under three main headings: its mechanical , e lectrical and radiological per formance [43],
8. 2. 1. Mechanical per formance (a) The instrument should be rugged enough to withstand operational
use in the field. Special shock-proof ing is not usually required especially in the case of printed circuit , transistorized instruments, although a metal rather than a thin plastic case is often specified. The indicating meter movement is usually the most shock-sensit ive instrument component.
(b) The instrument components should be arranged so that they may be easily repaired or replaced. Meter scales and range change switches should be c learly marked to minimize the risk of an operator making an erroneous reading.
(c) Instruments should be sufficiently water-tight to meet the require -ments of the conditions under which they will be used. In some cl imatic conditions it may be necessary to keep the instrument in a desiccated, water-tight box, removing it only when measurements are actually being made.
(d) Especially in the case of instruments operating at mains voltage, the design and construction should be such that e lectr ical shock hazards are avoided.
(e) Controls should be easily manipulated and have position checks in each switch position so that operators wearing heavy protective clothing and breathing apparatus experience no difficulty in operating them.
25
(f) The batteries of battery-powered instruments should be readily access ib le f o r quick and easy change. Instruments should be designed so that only one type of battery is required to provide the operating power, and a battery check point should be incorporated to check the condition of the instrument batteries.
8. 2. 2. Electr ical per formance
(a) A mains electricity supply to instruments should have sufficient stability to avoid fluctuations in the instrument readings. Transient effects should be minimized so that steady readings are obtained when the instru-ment is operating in a uniform non-varying radiation field.
(b) Batteries should be capable of providing at least a hundred hours of continuous instrument use.
(c) Switching transients should be minimized, so that when range changing or when switching on the instrument the meter settles quickly to a stable reading.
(d) Instruments should be screened so that they are not affected by magnetic or radio_frequency f ields. Meter readings and changes in readings should result only f r om ionizing radiations entering the detector and not f r o m irradiation of the electronic c ircuitry or components.
(e) Electronic c ircuitry and components used should be stable in their response over the temperature range in which the instrument is to be used.
(f) The instrument should have a fast response time to ensure good per formance when monitoring non-uniformly contaminated surfaces .
8. 2. 3. Radiological per formance
(a) The instrument should have an adequate range and a sufficiently low detection limit to meet the needs f or which it is to be used. The lower detection limit required depends on the value of the derived working limit of surface contamination to be used in the areas in question. The range depends upon whether one has to measure the amount of radio -active contamination present, or whether one only wants to know whether or not it exceeds some predetermined acceptable action level.
(b) The degree of accuracy should be determined; f o r operational f ield work ± 20% may be adequate,, but for more detailed laboratory study ± 5% or less may be required.
(c) The .instrument should have good reproducibility within its accuracy range, so that when properly calibrated, meaningful estimates of the amount of surface contamination present can be made.
(d) Meters should always be driven off sca le at the high range when-ever they are exposed to radiation fields exceeding their maximum range.
(e) The response of the instrument to radiations of different energies must be known, and correct ion factors-calculated as necessary .
(f) The effect of temperature and atmospheric pressure changes and humidity on the instrument detector should be known and correct ion factors established if necessary.
(g) The response of the instrument to radiations other than those which •it is designed to measure should be minimized.
26
(h) The instrument should be easily calibrated and when adjusted to give a determined response should be sufficiently stable to maintain the response, without drift, during its period of use f r om one calibration to the next. •.
8. 3. Examples of instruments in-use \
An outline is given in this section of the types of detectors and monitors suitable f o r various surface monitoring programs.
F o r detection of alpha emitters, scintil lators, proportional counters or semi -conductor detectors are normally used. Gas - f l ow and a i r - f i l l ed proportional counters are m o r e readily affected by cl imatic conditions than scintil lators o r semi -conductors but are usually m o r e robust and less likely to become l ight-sensitive. Thin window Geiger tubes can be used, but their detection ef f ic iency is rather low, and the presence of beta or gamma radiation will interfere.
A Geiger tube with a window thickness' of about 30 m g / c m 2 is adequate to detect beta radiation with a maximum energy above about 200 keV, but the response varies with the energy of-the beta radiation [44], F o r l ower -energy beta radiation, either Geiger tubes with a thinner window (e. g. 2 m g / c m 2 ) , proportional counters or 'plast ic organic scintil lators behind a thin light-tight shield are suitable. The f irst and last will not detect the low-energy beta,radiation f r om plutonium-241 or tritium. Gamma radi -ation will interfere with all beta detectors and alpha radiations will also interfere with 2 - m g / c m 2 window Geige'r-Miiller detectors. Shielded Geiger tubes may be used in areas where the gamma background is likely to interfere with measurements , but they are heavy and are only really suitable f o r monitoring f l oor o r ground areas.
. Alpha and beta detectors in a range of s izes are available, f r om small area (2. 5 cm2) semi -conductor detectors through detectors with 100 cm 2
detection areas to the large area probes 500 cm2) suitable f o r f l oor monitoring. F o r routine monitoring, alpha probes with a sensitive area of 50 cm 2 , and standard betq, probes with sensitive areas of 30 cm2 or end window beta probes with 5 cm2 detectors are suitable.
Because of their small s ize , semi-conductor detectors are m o r e suitable f o r monitoring spots of contamination on apparatus or skin previously indicated by general monitoring..
F o r monitoring of high levels of alpha contamination which may be found in glove boxes , fume hoods and pressurized suit areas , either general purpose probes or specially developed probes incorporating small , thin-walled ion chambers may be used. The probe is connected to the rate-, meter , which is kept outside the area, via a special bulkhead fitting mounted in the side.of the box or the wall of the pressur ized suit area.
Special hand and clothing monitors and foot monitors may be installed at the exits f r om radioactive areas o r at strategic ,places within the areas. Doorpost monitors , which monitor personnel and their clothing; reduce the number of personal variables which arise during sel f -monitoring with standard probes [45] (Figs 3 and 4).
Automatic monitors which detect alpha and beta contamination on clothing may be installed in laundries serving radioactive establishments [46], Devices may be incorporated which will allow the clothing to be • sorted according to the degree of contamination.
27
FIG. 3. Doorpost monitor in use.
The detector system comprises twelve thin-walled Geiger -M filler tubes, e leven mounted vertically behind a thin protective cover , and one mounted horizontally in a foot plinth. T h e tubes are grouped in threes, and the output from each group is fed to its own alarm circuit. Counting is started when the user interrupts a l i ght -beam/photo -e lec t r i c c e l l assembly as he mounts the plinth. During the counting period o f ten seconds, the user must rotate himself to present the whole o f his clothing surfaces to the detectors. If the pre-set output leve l is .not e x c e e d e d , a green " N O T CONTAMINATED" sign is i l luminated. If the leve l from any group o f detectors is e x c e e d e d , an audible warning sounds, and a light indicates which group has triggered the alarm. The instrument wil l detect > 0. 04 |iCi 9 0Sr/9 0Y over 100 c m 2 in a gamma background of 50 /iR/h.
28
FIG. 4. Installed hand and clothing monitor, type H. C. M. 3.
The fully transistorized hand and clothing monitor, type H, C, M. 3, monitors both hands of personnel for alpha and beta contamination simultaneously. A pair o f dual phosphor probes are used for each hand, and automatic compensation against gamma backgrounds up to 1 . 5 mR/h is provided by an additional counter. The monitor provides meter indication o f up to two derived working limits (DWL) for both alpha and beta contamination on each hand. If one DWL is exceeded , a red 'Wash again' light comes on, and an alarm bel l rings at the end of the monitoring period which can be pre-set to any t ime between 2 and 20 s. If . contamination does not exceed one DWL a green 'Sa fe ' lamp lights and a two-tone ch ime sounds. A photo-e lectr ic c e l l interlock rings the alarm bell and lights a ' R e c h e c k ' lamp if the hands are withdrawn before, the monitoring c y c l e is completed. -Two separate single phosphor clothing probes are provided, one normally for alpha and the other for beta monitoring.
F o r monitoring alpha contamination on installed g love-box gloves a specially designed probe will give quicker and more reliable measurements than a general purpose instrument. .
F o r measurement of.high beta dose rates {up to 105 rad/h) a simple detector [47] incorporating a thin plastic scintillator (about 25 m g / c m 2 ) and a simple photodiode gives very good results. The gamma-ray ' response is small , below 100 R/h., and is a l inear function of dose - rate above this level .
Some gamma radiation monitors [48] have detachable beta shields which when removed expose a thin window of the ion chamber. ' These monitors can be used to detect the presence of high levels of beta radiation, but, because of their large variation in response to beta partic les of d i f f e r -ing energies, the dose - rate measurement indicated on the meter is in-accurate. The instruments must be calibrated over a range of beta energies and source ' 'areas, at varying distances f r om the source.
Instruments are available which will only record the presence of radiations in speci f i c energy bands peculiar to a spec i f i c radionuclide, e. g. one which will identify the presence of the plutonium-239 by its 17-keV X - r a y emission. Portable gamma-ray spectrometers are available which may be used to identify radionuclides present in contamination in situ.
29
Care must always be exerc ised when using radiation detectors. G lass -walled Geiger tubes,will implode when broken, causing glass fragments to f ly some considerable distance. Injuries have been sustained f rom this . type of accident, and it is recommended that eye protection is worn when replacing a tube in the detector holder.. A protective mesh screen may be used over the tube while in use; a medical -gauze s leeve on the tube has -proved effective, and it does not reduce the tube ef f ic iency significantly.
The short range of alpha particles means that any probe used f or alpha monitoring must have a very thin window. Alpha scintillation probes in general use have a light-tight foi l of thickness about 1 m g / c m 2 which, if damaged, will allow light to penetrate to the photomultiplier and give spurious counts. Because of the necessity to monitor c lose ' to the surface , alpha scintillation detector, fo i ls can easily be damaged; e.-'g. by small p ieces of glass,, wire or gravel and even-hair stubble. Probes must not touch contaminated surfaces as spurious results will be obtained should they b e c o m e contaminated. When preparing the instruments f or use, excess ive voltages must not be applied as this will either damage the detectors or lead to spurious counts being registered.
R E F E R E N C E S
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[ 1 5 ] . CHAMBERLAIN, A. C . , , Calculation of A lphaand Beta Dose'from RIane Surfaces, UKAEA Rep; AERE' '-1 / , - :M-1302. . •. • ' . : , ' ! : . ' ' ,
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[ 2 6 ] STEVENS,' D.'C. , STEPHENSON, J.', BRUCE, G. S.', A Practical Examination o f the Dose to Skin from Radioactive Particulate Contamination o f Clothing, 'UKAEA Rep. AERE R 5963.
[ 2 7 ] COOK,- J. E. , The'Response o f a 'Moving Geiger Probe'to Discrete Sources o f Beta Radiation, UKAE£ , Rep. AERE M 1671 (1965). . ' - . ' ': - . • - ,
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[ 3 1 ] SHERWOOD, R.J. , STEVENS, D . C . , A Phosphor-fi lm Technique to Determine the Activity o f Individual Particles on Air Sample Filters, UKAEA Rep. AERE R 4310. '• . . ..
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31
[44 ] , . WHITE,. D . F . , Response of the T 1 0 2 1 . 6 / Probe to Surface Contamination, UKAEA Rep. AHSB(RP) M 31. [ 4 5 ] COOK, J. E. , Some Observations Relating to the Eff ic iency o f Personal Monitoring for Beta Contami -
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B I B L I O G R A P H Y
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CASNATI, E. , BREUER, F. , "Crit ical consideration o f the methods o f calculation used in the evaluation o f the absorbed dose to the skin in cases of external contamination", ' Personnel Dosimetry for Radiation Accidents (Proc .Symp. Vienna, 1965), IAEA, Vienna (196 5) 525.
DUMMER, J. E . , Jr. , Evaluation of % Sr + 90Y Surface Contamination Using Radiation Survey Instruments, Rep. LA DC-4467 (1964). '
DUNSTER, H . I . , "Maximum permissible levels of surface contamination" , Radioactive Contamination of Workers (Proc. Symp. Munich, 1962), EURATOM, Brussels (1964). ' .
DL^JSTER, H . J . , GARNER, J. , "Derived maximum permissible levels" , Radiological Monitoring o f the Environment (Proc. Symp. Berkeley, 1963), CEGB, London (1965).
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FISH, B. R . , "Health physics technology" , Health Physics Div. Ann. Prog.Rep. for Period ending July 31, 1962, ORNL-3347 (1962). ..
GLAUBERMAN, H . , BOOTMANN, W. R. ,• BRESLIN, A . J . , " Studies o f the signif icance of surface c o n t a m i -nation" , ibid. •
INTERNATIONAL ATOMIC ENERGY AGENCY, Safe Handling o f Radioisotopes, Safety Series No. 1, IAEA, Vienna (1962).
INTERNATIONAL ATOMIC ENERGY AGENCY, Safe Handling o f Radioisotopes: Health Physics Addendum, Safety Series N o . 2 , IAEA, Vienna (1960). ' • '
INTERNATIONAL ATOMIC ENERGY AGENCY, Regulations for the Safe Transport o f Radioactive Materials (1967 edition), Safety Series No. 6, IAEA, Vienna (1967). .
INTERNATIONAL ATOMIC ENERGY AGENCY, Basic Safety Standards for Radiation Protection (1967 edition), Safety Series No. 9, IAEA, Vienna (1967).
INTERNATIONAL ATOMIC ENERGY AGENCY, .Manual on Environmental Monitoring in Normal Operation, Safety Series No. 16, -IAEA, Vienna (1966). .
INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Monitoring in Emergency Situations, Safety Series No. 18, IAEX, Vienna (1966).
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32
INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Principles of Environmental Monitoring Related to the Handling of Radioactive Materials, Publication 7, Pergamon Press (1965).
INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Recommendations of the International Commission on Radiological Protection, Publication 9, Pergamon Press (1966).
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KOENIG, L. A . , Limitation of the surface contamination for handling unprotected radioactive substances, Atompraxis 12̂ (1966). '
KRIEGER, G. L.', Principles o f Operation of the Indium Adhesion Tester Used for Surface Contamination Measurement, Rep. S C - D C - 6 7 - 2 0 0 1 , Sandia Corp . , Albuquerque, N. Mex. (1967).
LANZL, L . H . , PINGEL, J . H . , RUST, J .H. (Eds), Radiation Accidents and Emergencies in Medic ine , Research and Industry (Proc. Symp. Chicago, 1963), C. C. Thomas, Springfields, 111. (1965).
MATUEEV, V. V . , SELDYAKOV, Yu. P. , SOKOLOV, A. D . , Use o f semi-conductor detectors with e lec tron-hole transitions in nuclear instrumentation, Izmer. Tekk. 10 (1967) (in Russian).
MOLDENHAWER, H. F . , Contamination and decontamination of surfaces, Kemenergie 5 (1962) (in German).
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NEWBERY, G. R. , "Measurement and assessment o f skin doses from skin contamination" , Radiation and Skin (WRAY, E .T . , Ed.) , UKAEA Rep. AHSB (RP) R-39, H. M. Stationery Of f i ce , London (1964).
PRINCE, J. R. , WANG, C. H. , "A method for evaluating surface contamination o f soft beta emitters", ibid.
REES, D.J, , Health Physics - Principles o f Radiation Protection, Butterworths, London (1967).
RODIER, J , , "Organizing the fight against radioactive contamination in nuclear plants", Radioactive Contamination o f Workers (Proc. Symp. Munich, 1962), EURATOM, Brussels (1964).
ROYSTER, G. W. , FISH, B. R. , " Studies of surface contamination. II. Surface characteristics af fect ing the measurement and redispersion of particulates", Seventh Annual Meeting o f Health Physics Society (1962).
ROYSTER, G. W . , FISH, B. R. , "Techniques for assessing "removable" surface contamination" , ibid.
SPANGLER, G. W , , "Criteria for establishing appropriate limits for removable radioactive*surface c o n t a m i -nation", Sixth Annual Meeting o f Health Physics Society (1961).
SPANGLER, G . W . , WILLIS, C . A . , "Permissible contamination l imits" , ibid.
Strahlenschutzpraxis, Te i l II - Messtechnik, Karl Th iemig K. G . , Munich (1962).
SUMINO, T . , SEKITANI, K. , TSUJIMOTO, T. , KATSURAYAMA, Y. , Calibration o f Instruments Used to Monitor Surface Contamination Density, Rep. KURRI-TR-6 (1965) (in Japanese).
USSR Ministry o f Health and Counci l o f Ministers State Committee on the Utilization o f A tomic Energy, Health and Safety Regulations Governing Work with Radioactive Materials and Sources o f Ionizing Radiation, Gosatomizdat, Moscow (1960).
33
IAEA SALES AGENTS AND BOOKSELLERS
Orders for A g e n c y publ i cat ions c a n be. p l a c e d b e l o w :
ARGENTINA Comis i f in N a c i o n a l de Energla A t S m i c a A v e n i d a de l Libertador 8250 Buenos Aires . .
A U S T R A L I A Hunter Publ icat ions • 23 M c K i l l o p Street Me lbourne , . C . 1
A U S T R I A Publishing Sec t i on International A t o m i c Energy A g e n c y Karntner Ring 11 P . O . Box 590 ' - - i•' ' A - 1 0 1 1 Vienna
B E L G I U M O f f i c e International de Librairie 3 0 , A v e n u e Marnix Brussels 5
C A'N A D A ' . , • : ' • • Queen ' s Printer for C a n a d a . International Publ icat ions Ottawa, Ontario
C . S . S . R . ' S . N . T . L . Spo lena 51 NovS Mesto Prague 1
D E N M A R K Ejnar Munksgaard Ltd. 6 Norregade • .DK-1165 C o p e n h a g e n K
F R A N C E O f f i c e International de D o c u m e n t a t i o n et Librairie 4 8 , Rue Gay-Lussac F - 7 5 Paris 5e
i y o u r bookse l l er or any o f the addresses l isted
H U N G A R Y . . • ' • • Kultura .
. Hungarian Trading C o m p a n y for Books and Newspapers . P. O . Box 149 Budapest 62 •'
I N D I A -. Oxford Book & Stationery C o m p . 17, Park Street Calcut ta 16 - '
I S R A E L Hei l iger & C o . 3 , .Nathan Strauss Str. Jerusalem
I T A L Y 1 ' A g e n z i a Editoriale C o m m i s s i o n a r i a
. A . E. I . O . U . " "• -V ia M e r a v i g l i 16 • ' 1 -20123 Mi lan • - '•
J A P A N " .. • , j . M a r u z e n C o m p a n y , Ltd-. ,
P . O ; .Box 5 0 5 0 , ' • 1 0 0 - 3 1 Tokyo . Internat iona l .
M E X I C O ; Librerfa Internac iona l , S. A .
' " A v . ' So'nora 206 ' ' M e x i c o 11 , D . F .
N E T H E R L A N D S . . . Martinus Ni jho f f N. V . Lange Voorhout 9 P . O . ,Box 269 T h e Hague .
P A K I S T A N Mirza Book A g e n c y 65 , Shahrah Q u a i d - E - A z a m P . O . Box 729 Lahore - 3
P O L A N D Ars Polona Centrala Handlu Z a g r a n i c z n e g o Krakowskie P r z e d m i e s c i e 7 Warsaw
U . S . S . R . Mezhdunarodnaya Kniga S m o l e n s k a y a - S e n n a y a 3 2 - 3 4 M o s c o w G - 2 0 0
U. K . R O M A N I A
Cart imex 3 - 5 13 D e c e m b r i e Street P . O . Box 1 3 4 - 1 3 5 Bucarest
Her Majesty ' s Stationery O f f i c e P. O . Box 569 London S . E . I
S P A I N Libreria Bosch Ronda Universidad 11 Barcelona - 7
U . S. A . UNIPUB, I n c . P . O . Box 433 New York, N . Y . 10016
S W E D E N C . E. Fritzes Kungl . Hovbokl iandel Fredsgatan 2 S t o c k h o l m 16
Y U G O S L A V I A Jugoslovenska Knj iga T e r a z i j e 27 Belgrade
S W I T Z E R L A N D Librairie Payot Rue Grenus 6 C H - 1 2 1 1 Geneva 11
IAEA Publ icat ions can also b e purchased reta i l at the United Nations Bookshop at United Nations Headquarters, N e w York , f r o m the news-s tand at the Agency ' s Headquarters, V ienna , and at most c o n f e r e n c e s , sympos ia and seminars o r g a n i z e d by the A g e n c y . 1
In order to f a c i l i t a t e the distribution of its pub l i ca t ions , the A g e n c y is prepared to a c c e p t p a y m e n t in UNESCO coupons or in l o c a l currenc ies .
Orders and inquiries f r o m countries not l isted a b o v e m a y b e sent toi
Publishing Sect ion International A t o m i c Energy A g e n c y Kamtner Ring 11 P . O . Box 590 A - 1 0 1 1 V i e n n a , Austria
I N T E R N A T I O N A L A T O M I C E N E R G Y A G E N C Y V I E N N A , 1970
PRICE: U S $ 2 . 0 0 A u s t r i a n S c h i l l i n g s 52 , -(16/8 s tg ; F.Fr. 11,10; D M 7,50)
SUBJECT G R O U P : II Health, Safety a n d W a s t e M a n a g e m e n t / R a d i a t i o n P ro tect ion a n d S t a n d a r d s
