NEW METHOD FOR ESTIMATING GAMMA-RAY EXPOSURE SUSTAINED IN RADIATION ACCIDENTS
POSSIBILITIES OF USING ORGANIC SUBSTANCES AS MONITOR
TOSHIYUKI NAKAJIMA and SHIGUEO WATANABE
PUBLICAÇÃO lEA N / 393
Abril — 1975
I N S T I T U T O D E E N E R G I A ATÔMICA Caixa Postal 11049 (Pinheiros)
CIDADE UNIVERSITARIA "ARMANDO DE SALLES OLIVEIRA"
SAO PAULO — BRASIL
NEW METHOD FOR ESTIMATING GAMMA-RAY EXPOSURE SUSTAINED IN RADIATION ACCIDENTS
POSSIBILITIES OF USING ORGANIC SUBSTANCES AS MONITOR*
Toshiyuki Nakajima and Shigueo Watanabe
Coordenadoria de Ciência e Tecnologia de Materiais Instituto de Energia A t ô m i c a
São Paulo - Brasil
Publicação l E A N? 390 A b r i l - 1 9 7 5
' P u b l i c a d o na Journal of Nuclear Science and Technology, 1 1 ( 1 2 ) 5 7 5 - 8 2 , D e c . 1 9 7 4
Instituto de Energia A t ô m i c a
Conselho Superior
Eng9 Roberto N. Jafet - Presidente.
Prof. Dr. Emilio Mattar - Vice-Presidente Prof. Dr. Jose'Augusto Martins
Prof. Dr. M i l t o n Campos Eng? Helcio Modesto da Costa Superintendente
Prof. Dr. R ô m u l o Ribeiro Pieroni
NEW METHOD FOR ESTtMATING G A M M A R A Y EXPOSURE SUSTAINED IN RADIATION ACC8DENTS
POSSIBILITIES OF USING ORGANIC SUBSTANCES AS MONITOR
Toshiyuki Nakajima and Shigueo Watanabe
A B S T R A C T
»
A n e w m e t h o d is p r o p o s e d f o r e s t i m a t i n g t h e a m o u n t o f e x p o s u r e t o y - r a y s s u s t a i n e d b y v i c t i m s i n v o l v e d i n a r a d i a t i o n a c c i d e n t . T h e c o n c e p t is t o use as m e a s u r e o f e x p o s u r e t h e q u a n t i t y o f f r e e r a d i c a l s
^ g e n e r a t e d i n o r g a n i c m a t e r i a l s b y i r r a d i a t i o n , a n d t o d e t e r m i n e t h i s q u a n t i t y t h r o u g h i t s r e l a t i o n t o t h e e l e c t r o n s p i n r e s o n a n c e s h o w n b y s u b s t a n c e c o n t a i n i n g s u c h r a d i c a i s . A p p l y i n g t h i s m e t h o d , a m i n i m u m e x p o s u r e o f a b o u t 1 0 0 R c o u l d b e e s t i m a t e d w i t h m e a s u r e m e n t s a t r o o m t e m p e r a t u r e . T h e m o n i t o r s u b s t a n c e s t a k e n u p f o r t h e p r e s e n t series o f e x p e r i m e n t s i n c l u d e d n a i l s a n d h a i r t a k e n f r o m e x p o s e d p e r s o n s , p l a s t i c b u t t o n , p o l y e t h y l e n e p o w d e r , l u c i t e , p a p e r a n d w o o l . T h e e x p e r i m e n t s w e r e a i m e d a t d e t e r m i n i n g t h e s e n s i t i v i t y , t h e f a d i n g w i t h lapse o f s t o r a g e t i m e a f t e r i r r a d i a t i o n a n d t h e e f f e c t s o f d i f f e r e n c e s i n c o n d i t i o n s o f s t o r a g e s u c h as t e m p e r a t u r e , i l l u m i n a t i o n , ' a n d w a s h i n g b e f o r e m e a s u r e m e n t . T h e m e t h o d t h u s d e v e l o p e d s h o u l d p r o v e u s e f u l f o r e s t i m a t i n g t h e e x t e n t o f e x p o s u r e , t o serve i n d e c i d i n g t h e m e d i c a l t r e a t m e n t t o b e a p p l i e d .
L I n t r o d u c t i o n
^ A number of accidental exposures have occurred in the past in various countries. Notable i' examples of such accidents suffered by persons regularly occupied in w o r k involving radiation j (referred t o hereafter as "radiation-workers", as opposed t o " n o n - w o r k e r s " ) include the
^ Incidents that occurred at Oak Ridge, U . S . A . ' ' ' and in Yugoslavia*^' (these t w o cases involving exposure t o both neutrons and 7-rays) and in Australia'"^' (7-rays alone). Much e f f o r t has been applied t o the development o f medical treatments t o be applied t o patients to victims o f exposure t o radiation.
In the case of accidents suffered by radiation-workers, exposure sustained by t h e m can be estimated more easily and rapidly than in the case of non-workers, since they are constantly subjected t o systematic monitoring and c o n t r o l . Nevertheless, the personnel monitors currently used for this purpose are not adapted t o convenient and reliable estimation of exposures exceeding 100 R, w i t h the exception of thermoluminescence and glass dosimeters.
For non-workers exposed t o 7-rays, no efficacious method of measurement has so far
\ been developed, except the thermoluminescence dosimeter. The present study aims at filling i" the need felt f o r a practical and reliable means of gathering information t o serve in the
treatment of victims o f accidental exposure to radiation.
T w o approaches — biological and physical — are available for estimating the exposure or absorbed dose sustained by patients upon accidental exposure t o 7-rays. Brown &
M c N e i l ' ^ * have reported on a biological method f o r estimating the absorbed dose. One of the merits o f this method is that it permits direct estimation of the dose absorbed by the individual.
The method possesses, o n the other hand, the intrinsic disadvantage o f incurring fairly large
errors in estimating the extent of exposure or dose originally sustained, on account o f the differences existing between individuals in their capacity t o recover f r o m exposure and other factors such as dietary habits, all of which influence the relation between the observed data and the applied radiation. In view of these drawbacks inherent in the biological approach, we have directed o w n efforts t o the development of physical methods, which are not affected by the individual factors differing f r o m person t o person.
In Japan, there occurred in 1971 an accident involving the exposure of non-workers t o 7-rays, on which occasion the present authors establishedamethod f o r estimating the exposure received by the patients f r o m measurements of the thermoluminescence emitted by jewels taken f r o m wrist watches w o r n by the patients'^*. This method has the advantage of high sensitivity, but is on the other hand inevitably handicapped by the inherent limitations in the q u a n t i t y of sample available, and by the impossibility of repeating the measurements. Also, one cannot of course count upon people unexpectedly encountering exposure to be wearing a watch at the time of the eventuality.
In view of these shortcoming of the previously devised m e t h o d , the authors undertook a study t o assemble information that should contribute to the development of a new method f o r estimating 7-ray exposure, applicable to both radiation-workers and non-workers.
In the present paper, the authors will expose the basic concepts of the new method which utilizes the electron spin resonance of substances containing free radicals.
I I . Basic Concept and Measuring Technique
In discussing electron spin resonance, the first item of interest is the Zeeman effect. This takes the f o r m
JC = gpU.S (1) where 3C is the Hamiltonian, (3 the Bohr magneton, H the applied magnetic f i e l d , ,g the
spectroscopic splitting factor and S is the electron spin. The magnetic field is related t o the frequency v of the resonance of free electrons or of free radicals by the expression
hv = g|3H. (2)
We next consider the relation between the electron spin resonance and the q u a n t i t y present o f free radicals. It is k n o w n that the observable resonance intensity >4 of a substance is proportional t o
A=cX" {3) where c is a constant and X " the imaginary part of the susceptance of the substance. If the
applied magnetic field is f i x e d , and the frequency of the oscillating magneting field is varied, the intensity A becomes a f u n c t i o n of the frequency of w h i c h the resonance c o n d i t i o n is given by Eq. ( 2 ) , and is represented by
P = fA{v)dv, t4|
where P is defined as the absorption intensity.
If it is assumed that the w i d t h of the absorption is not wide, it can be proved that Eq. (4) becomes
P = cvoi~^dv, (5)
In the special case of Kramers-Kronig's relation, we obtain
X . = - i - . / ^ * (61
where XQ is the susceptance of the sample. T h e n , it. is possible t o apply Curie's law t o the sample:
Xo=/Vg^SrS + 1)|3V5A-r (7) Then, the relation between the number N of free electrons or o f free radicals and the
absorption intensity P becomes
P<^(/V/r)g'S(S + 1). (8) Hence it should be possible t o determine the number or quantity o f radicals contained in a
substance f r o m the absorption intensity o f electron spin resonance shown b y the substance.
Thus, if we can relate the q u a n t i t y of radicals t o irradiation, we see the possibility of estimating the exposure or dose received by a substance f r o m its electron spin resonance intensity.
N o w , it is k n o w n that when an organic material is exposed t o ionizing radiation, free radicals are created therein.
We therefore undertook t o determine the quantity of free radicals found after irradiation in such organic substance as polyethylene powder, lucite, ordinary plastic b u t t o n , paper, w o o l , cloth of cotton and of polyester, human hair and nail. A l l the samples, except the hair and nail, were unused materials freshly purchased. Prior t o irradiation, the ESR (electron spin resonance) absorption was measured o n t h e samples t o determine their natural absorption properties at r o o m temperature. The samples were then irradiated at room temperature w i t h y-rays f r o m ' " C s source.
The electron spin resonance spectrometer used in this experiment is a J E O L model JES-ME-3X equipped w i t h a 100 kHz modulation unit. The resonance cavity employed operates on the T E Q H mode, and has a slotted w i n d o w at one end t o permit direct illumination of the sample. The resonant frequency o f the loaded cavity was in the range o f 9,200 ~ 9,600 MHz. The ESR measurements were made at room temperature.
I I I . Experimental Results 1. Response of ESR Absorption
In the development of a monitoring method f o r application t o radiation-workers and non-workers, the most important properties of the substance t o serve as m o n i t o r are its sensitivity and fading behavior.
Our first experiments were undertaken t o determine tfie dosimetric properties such as the sensitivity and the iinearity of the response t o exposure. The substances chosen were lucite, polyethylene, paper^ clothing, and also human hair and nail. Samples of about 100 mg were used f o r the irradiation and the subsequent measurements of ESR (absorption.
Figure 1 reveals the linear relationship obtained between the exposure t o ' ^''Cs 7-rays and the ESR absorption intensity of polyethylene powder irradiated in the dark.
The vertical axis is the relative ESR absorption intensity due t o the free radicals in the sample irradiated w i t h y r a y s , observed at r o o m temperature w i t h i n 2 hr after irradiation. The Intensity was normalized in respect of b o t h the weight of the sample and the intensity of the standard sample of DPPH,
a
in
°102
a. 10
102 103
E x p o s u r e ( R ) 10^
F i g . 1
R e l a t i v e E S R a b s o r p t i o n i n t e n s i t y VS. fy-ray e x p o s u r e s h o w n i n p o l y e t h y l e n e
it-.seen f r o m ^ i g . . 1 t h a t good linearity was obtained between the ESR absorption intensity and the exposure, in the entire range of 180 ~ 5,000 R. While it is desirable that a m o n i t o r f o r use in radiation acciderits should be capable of measuring a m i n i m u m exposure of about 10 R, it. proved that under the present conditions, the ËSR methcid should o n l y be able t o estimate exposures d p w n t o 100 R, below Which the ESR signal level w o u l d be t o o l o w . It is however t o be noted in Eq. (8) that the ESR absorption intensity is inversely proportional t o t h e measuring temperature. I t follows f r o m - t h l s t h a t if the ESR absorption can be measured at either liquid nitrogen or liquid helium temperature, an exposure of about or even 1 R could be estimated by tl^e proposed ESR technique. T h | niiethod w o u l d then acquire the desired sensitivity f o r radiation accident m o n i t o r i n g . Measurements made during the present w o r k , o n polyethylene, plastic b u t t o n , lucite, w o o l and c o t t o n cloth showed t h a t the sensitivities o f all these substances were roughly similar t o each other.
2. Fading
The precise t i m i n g of an accidental exposure t o ' radiation is inherently d i f f i c u l t t o determine precisely. Thus it becomes important t o minimize fading or decay o f ' t h e free radicals generated by irradiation in the substances t o utilize as m o n i t o r , between the t i m e of their exposure and their measurement. It should of course be ideal t o use substances t h a t show no'
fading at all but this should be d i f f i c u l t t o f i n d on around person accidentally exposed t o radiation. The term fading or decay used in the present instance does not mean fading of defects f o r m e d by the exposure t o radiation, but the d i m i n u t i o n o f the amount o f free radicals contained in the irradiated samples w i t h lapse of t i m e after the irradiation during w h i c h the samples may be subjected t o various conditions, w h i c h should affect the fading of the generated radicals.
The fading properties under different conditions of storage were examined on samples of plastic b u t t o n , polyethylene and lucite, all irradiated at r o o m temperature w i t h 7-rays. A n example of the results of measurements on fading in the sense given above is shown in Fig, 2 , where the curve represents the fading of the free radicals in polyethylene, kept in the dark at r o o m temperature after irradiation t o 10^ R. It is seen f r o m Fig. 2 that the free radicals gradually decreased, dropping in 2 days t o about 88% of the initial ESR absorption intensity.
Even so, the decay shown by polyethylene was less prominent than in the case o f other organic materials such as lucite, b u t t o n and human hair.
14
V2
.Eiof
<a
•^.8
1 0 '
/
10^ 10' s t o r e d t i m e ( m i n )
F i g . 2
F a d i n g o f f r e e r a d i c a l s i n i r r a d i a t e d p o l y e t h y l e n e s t o r e d a t r o o m t e m p e r a t u r e
The decay observed in the case of w o o l and human hair and nail was very rapid, and f o r example, in human hair the ESR absorption intensity was reduced by one half between the first and the second observations, separated by a lapse of 5 m i n . These substances are therefore not suitable foi m o n i t o r i n g .
3. Effect o f Differences in Storing Temperature o n Decay of Free Radicals
. Figure 3 shows how the change brought by lapse of time t o the ESR absorption intensity of lucite is modified by the temperature at w h i c h the sample is stored after irradiation at r o o m temperature. The vertical axis is the ratio between the ESR absorption intensities of samples measured immediately after irradiation and measured after storing f o r various periods.
It is seen f r o m the figure t h a t , at r o o m temperature, the response gradually decreased w i t h storage t i m e , dropping d o w n t o about 9 0 % at 1,300 m i n after irradiation. When stored at 0 ° C , the 90% level was only reached at 10,000 m i n , while at 4 2 ° C , the response deteriorated very rapidly. When stored at 1 0 0 ° C (Fig. 4 ) , the response of the sample was reduced t o 0 . 0 1 %
b f the initial level in the space of 35 m i n .
10^ 10'' 10*
s t o r e d time(min)
Fig. 3
F a d i n g o f f r a a r a d i c a l s i n i r r a d i a t e d l u c i t e s t o r e d a t v a r i o u s t e m p e r a t u r e s
10^
Id"
\
10^
10 0 5 15 20 25 30 35
S t o r e d t i m e ( m i n )
Fig. 4
o F a d i n g o f f r e e r a d i c a l s i n i r r a d i a t e d l u c i t e s t o r e d a t 1 0 0 C
The mechanism o f the decrease o f the free radicals at high temperature is not y e t understood f u l l y , but it is indicated f r o m the foregoing result that certain organic materials should be utilizable in radiation dosimetry, provided they are stored at room temperature or below after irradiation, while after measurement, storing the sample at high temperature f o r 30 min or more should restore the substance t o a state suitable for re-use or for recalibration.
A. Effect o f Illumination
A study was made t o determine the effect o f illumination on the ESR response o f irradiated lucite. A sample irradiated t o 10" R was Illuminated w i t h the condensed light of a 100 W super-high-pressure mercury lamp placed at a distance of 52 c m . The illumination was applied at room temperature.
Figure 5 shows the results obtained w i t h (B) and w i t h o u t (A) illumination at r o o m temperature o n irradiated plastic buttons. It is indicated t h a t the samples are sensitive t o some extent t o i l l u m i n a t i o n , but in practical applications this property could be neglected because of relatively short duration of illumination t o w h i c h the samples w o u l d be subjected t o after Irradiation. Further, the degree of Influence should depend on the wavelength of the light, since it may be considered t h a t the cause of the increase seen in ESR response upon illumination is the generation of additional free radicals b y ultra-violet light.
1.0
-6
10' lO'^ 10-^
I l l u m i n a t i n g t i m e ( m i n )
C u r v e A : w i t h o u t i l i u m i n a t i o n C u r v e B : w i t h i l i u m i n a t i o n B o t h at r o o m t e m p e r a t u r e
F i g . 5
E f f e c t o f i l l u m i n a t i o n o n f a d i n g o f f r e e r a d i c a l s i n i r r a d i a t e d p l a s t i c b u t t o n
As already mentioned, the free radicals in many organic materials are not stable at room temperature in air. It is well established that the decay of the free radicals is due t o their reaction w i t h oxygen contained in the air. Thus, if such a sample is coated w i t h some protective material before irradiation, the decay might be less rapid than in the case o f an un-coated sample. Figure 6 shows the results of a test w i t h carbon paint coating on the lucite t o see its effect o n the decay o f the free radicals in lucite. It is seen t h a t the coating d i d not bring any beneficial effect. On the other hand, i t has been reported that irradiated samples stored in vacuum were preserved free of d e c a y ' ^ ' . T h e n , in case where m o n i t o r samples subject t o rapid decay of the free radicals contained therein must be stored before measurement, preservation in vacuum may possibly be an effective measure f o r using t h e m as m o n i t o r or dosimeter.
1 0 ' 10' s t o r e d l i m e (min)
C u r v e A : u n - c o a t e d , C u r v e B : c o a t e d F i g . 6
E f f e c t o f c a r b o n c o a t i n g o n f a d i n g o f f r e e r a d i c a l s i n i r r m J i a t ^ l u c i t e s t o r e d a t r o o m t e m p e r a t u r e
5. Washing Effact o n Decay of Free Radicals
Our aim being t o utilize samples of organic materials f o u n d on the patients' body and among their personal effects and belongings such as hair, nail, plastic accessories and part o f clothing as monitor substance f o r the estimation of exposure in a radiation accident, i t must be expected that the samples made available in actual application w o u l d generally be soiled arui d i r t y . Since d j r t ^È^Jld become source of noise in the subsequent measurements of ESR absorption, the samt>les collected f r o m the patients should first have t o ba cleaned. In view o f this circumstance, a study was made of the effect of cleaning on irradiated lucite.
Table 1 gives the effect o f various solvents on the decay of free radicals in the lucite.
Table 1
Effects o n decay o f fraa radicals shown b y some solvents f o r d i r t and soil
Alcohol Aceton Benzen
Shap
and Water Mineral
water oil
Mineral oil
0.995 0.993 0.996 0.996 1.00 0.972
±0.0042 ±0.0428 ±0.0023 ±0.0030 ±0.001 ±0,0026
The irradiated samples were washed f o r 5 min at r o o m temperature w i t h each solvent.
The results given in Table 1 , indicate that water and organic solvents scarcely affect the decay of the free radicals, while mineral oil is seen tQ;,have a distinct influence. Differences in the duration of washing, as seen in Fgg.7, produced no consistent effect, w i t h i n the range covered, f r o m 0.5 t o 21 m i n . In the experiment shown Fig. 7, the irradiated lucite was washed w i t h benzene. The foregoing results w o u l d indicate the u t i l i t y o f washing soiled samples w i t h water
and w i t h organic solvents, t o eliminate d i r t , w h i c h tends t o accelerate decay of the sample.
1.01
- Q99,
0.37
0 5 10 15 20 W a s h i n g t i m e (min)
F i g . 7
D e c a y o f f r e e r a d i c a l s i n i r r a d i a t e d l u c i t e b y w a s h i n g w i t h b e n z e n e
6. Application t o Dating of Accident
The main purpose of the present study was t o estimate the exposure or the absorbed dose. But if a m o n i t o r could further estimate also the period elapsed since the accidental irradiation, this w o u l d add t o the u t i l i t y of the m o n i t o r . It has been seen in the preceding sections t h a t , after irradiation of organic substance, the free radicals produced decay inevitably t o some extent w i t h lapse o f time. The rate at w h i c h this decay proceeds w o u l d o f course be expcted t o differ according t o type of the free radical.
Figure 8 shows the ESR spectrum o f lucite irradiated at room temperature w i t h -y-rays f r o m ' ^''Cs source. Analysis of this spectrum indicates the presence of t w o kinds of free radical
H — C — H and
I
H
H 1
H
1
1— c — c -
1
1 - c — 1
1H
1
H
C O O C H 3
Thus, if one of the free radicals in the sample has faded markedly, and the other only very little, the ratio between the ESR absorption intensities due t o the t w o different radicals could be a f u n c t i o n of the t i m e elapsed after the exposure. A n d if this is true, the ratio could be measured t o give an estimation o f the t i m i n g of the exposure.
T o ascertain this possibility, experiments were perfomed w i t h lucite and w i t h polyethylene powder sample containing a few free radicals.
10
F i g . 8
E S R s p e c t r u m o f i r r a d i a t e d l u c i t e o b s e r v e d a t r o o m t e m p e r a t u r e
F j g u r e 9 shows the result obtained w i t h polyethylene, and reveals the changes observed in the ratio of ESR absorption intensities between at the position of the free electrons and at that of other radicals in the vicinity of the position occupied by the free electrons, as a f u n c t i o n o f t i m e after exposure. This ratio is independent of both extent o f exposure and energy o f radiation. These results c o n f i r m the possibility of estimating the dating of the accidental exposure through ESR measurements.
Stored
10
t i m e ( m i n )
F i g . 9
C h a n g e s i n t i m e o f r a t i o b e t w e e n E S R a b s o r p t i o n p e a k d u e t o f r e e e l e c t r o n s a n d t h a t d u e t o o t h e r f r e e r a d i c a l s - p o l y e t h y l e n e s t o r e d a t r o o m t e m p e r a t u r e
11
In this experiment, the fading ratio was determined f r o m many measurements o f the ESR absorption using t w o or more samples containing free radicals possessing different fading rates.
The dating can be cross-checked among t w o or more samples of different substances.
IV. Discussion
In this experiment, we have undertaken t o obtain some information that could contribute t o the development of a method for estimating the exposure or the absorbed dose received by radiation-workers and non-workers in a radiation accident. I t is proposed t o utilize measurements of the electron spin resonance of the free radicals generated by irradiation in organic materials. We shall here consider the differences between the present and other current methods.
Since non-workers cannot be expected t o be carrying personnel monitors at the time of an accidental exposure t o radiation, the exposure data required f o r the ensuring medical treatment must be sought either among their personal belongings or in their o w n body. We have previously developed a thermoluminescence method utilizing jewels in the wrist watch as monitor substance*^'. The drawbacks of this method are as mentioned above in the
" I n t r o d u c t i o n " .
The ESR method, proposed w i t h t h e aim of covering the shortcomings of the watch-jewel m e t h o d , possesses, in t u r n , the inherent drawback o f poor sensitivity compared w i t h the normally used dosimeters such as f i l m badge, pocket chamber, glass dosimeter and thermoluminescence dosimeter. I t is, however, t o be noted that radiological i n f o r m a t i o n can already be of use to doctors treating patients involved in a radiation accident, even if can only distinguish and classify the dose received into the very rough ranges of (a) below 100 rad, (b) between 100 and 300 rad, and (c) above 300 rad. This order of precision should be well w i t h i n the scope practical realization w i t h the proposed approach based o n ESR measurements, considering the large quantity and choice of m o n i t o r substance available for this m e t h o d .
Another useful property t o be borne in mind is that the ESR absorption intensity is proportional t o the reciprocal of the measuring temperature, as indicated by Eq. (8). Thus the sensitivity of the method should be markedly improved by performing the ESR measurements at very low temperatures. While it may not be practical t o attempt such measurements at liquid helium temperature, that of liquid nitrogen should be relatively easy. A n d the present experimental results would indicate that it should be quite possible t o detect exposures of about 10 rad at liquid nitrogen temperature, which should f u l l y qualify the proposed method as means of obtaining exposure information of use in medical treatment.
The expense involved in equipment for ESR measurements is offset by the ease w i t h w h i c h the monitor substances could be collected f r o m the personal effects o f the patients. This abundant availability of monitor substance is the strongest advantage o f the proposed method, and in this respect it is the converse of the method utilizing the thermoluminescence of watch jewels reported in the previous paper*'**.,,
Vd/Conclusion
We have examined the possibilities of utilizing the electron spin resonance absorption
12
intensity as a measure of the number of free radicals generated in a substance by exposure t o radiation. The results o f the study can be summarized as follows.
(1) The method promises t o possess the sensitivity demanded of monitors t o provide information of use t o medical doctors treating radiation-workers and non-workers involved in accidental exposure t o radiation.
(2) If the monitoring is effected w i t h i n 1 week after the accident, i t should be quite possible t o estimate the exposure, before effacement b y fading o f the effects o f radiation sustained b y the monitors substances.
(3) Illumination of irradiated m o n i t o r sample was f o u n d t o increase rather than decay the free radicals, but o n l y b y a small amount.
(4) Heat distinctly accelerated the decay of free radicals in the irradiated monitor sample. It is hence desirable that the monitor substance be maintained at a temperature as low as possible during the period between exposure and measurement.
(5) There is possibility of dating the t i m e of exposure by this method.
Effects similar t o those observed in the present experiment should be f o u n d also in the case of other kinds of the organic material. Thus we have a large choice o f m o n i t o r substances t o select among the organic materials around exposed persons, on w h i c h t o apply the proposed method o f radiation dosimetry for accidental exposure t o radiation.
The authors intend t o f o l l o w the present study w i t h the development o f a practical dosimeter based on the principles described above.
Acknowledgment
The authors wish t o express their thanks t o Prof. R. R. Pieroni, Director o f the Institute of A t o m i c Energy, São Paulo, f o r his encouragement o f the w o r k , w h i c h was supported by grants f r o m CNEN and FAPESP in Brazil.
RESUMO
F o i a q u i p r o p o s t o u m m é t o d o n o v o p a r a f a z e r a e s t i m a t i v a d a e x p o s i ç ã o d e r a i o s g a m a a q u e v í t i m a s f o r a m s u j e i t o s n u m a c i d e n t e d e r a d i a ç ã o . A i d é i a c o n s i s t e e m usar c o m o m e d i d a d e e x p o s i ç ã o a q u a n t i d a d e d e r a d i c a i s livr::s p r o d u z i d o s e m m a t e r i a i s o r g â n i c o s p o r i r r a d i a ç ã o , e d e t e r m i n a r e s t a q u a n t i d a d e p e l a m e d i d a d a r e s s o n â n c i a p a r a m a g n á t i c a e l e t r ô n i c a . A p l i c a n d o este m é t o d o , u m a e x p o s i ç ã o m í n i m a d e c e r c a d e 1 0 0 R p o d e ser d e t e r m i n a d a c o m m e d i d a s e m t e m p e r a t u r a a m b i e n t e . A s s u b s t â n c i a s m o n i t o r a s usadas n a p r e s e n t e s é r i e d e e x p e r i ê n c i a s são u n h a s e c a b e l o s d e pessoas e x p o s t a s , b o t ã o p l á s t i c o , p ó p o l i e t i l e n o , l u c i t e , p e p e l e l ã . A s e x p e r i ê n c i a s f o r a m d e s t i n a d a s à d e t e r m i n a ç ã o d e s e n s i b i l i d a d e d o d e s v a n e c i m e n t o c o m o t e m p o d e a r m a z e n a m e n t o a p ó s a i r r a d i a ç ã o e d o s e f e i t o s d a s d i f e r e n ç a s nas c o n d i ç õ e s d e a r m a z e n a m e n t o , t a i s c o m o a t e m p e r a t u r a , i l u m i n a ç ã o e l a v a g e m a n t e s d a m e d i d a . O m é t o d o a s s i m d e s e n v o l v i d o será ú t i l n a e s t i m a t i v a d o g r a u d a e x p o s i ç ã o p a r a a u x i l i a r n a d e c i s ã o d o t r a t a m e n t o m é d i c o a ser a p l i c a d o .
RÉSUMÉ
U n e n o u v e l l e m é t h o d e d ' é v a l u a t i o n d e l ' e x p o s i t i o n d e r a y o n n e m e n t s g a m m a à l e s q u e l s des v i c t i m e s o n t é t é a s s u j e t i s d a n s u n a c c i d e n t d e r a d i a t i o n . L ' i d é e c o n s i s t e d ' u t i l i s e r c o m m e l a m e s u r e d ' e x p o s i t i o n l a q u a n t i t é des r a d i c a u x l i b r e s crées d a n s les m a t é r i a u x o r g a n i q u e s p o u r i r r a d i a t i o n , e t d é t e r m i n e r c e t t e q u a n t i t é p o u r la m e s u r e d e l a r e s o n a n c e p a r a m a g n é t i q u e é l e c t r o n i q u e . P o u r l ' a p p l i c a t i o n d e c e t t e m é t h o d e , u n e e x p o s i t i o n m i n i m u m d ' e n v i r o n 1 0 0 R p e u t ê t r e d é t e r m i n é e avec les m e s u r e s à la t e m p e r a t u r e a m b i a n t e . L e s s u b s t a n c e s
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m o n i t e u r e s u t i l i s é e s d a n s c e t t e série d ' e x p é r i e n c e s s o n t les o n g l e s , les c h e v e u x des p e r s o n n e s e x p o s é e s , le b o u t t o n p l a s t i q u e , p o u d r e p o l i e t i l è n e , l u c i t e , p a p i e r e t l a i n e . Les e x p é r i e n c e s s o n t d e s t i n é e s à la d é t e r m i n a t i o n d e la s e n s i b i l i t é , d e la d i s s i p a t i o n a u c o u r s d u t e m p s d e la m a g a s i n a g e a p r è s r i r r a d i a t i o n , e t des e f f e c t s des d i f f e r e n c e s d a n s les c o n d i t i o n s d e la m a g a s i n a g e , c o m m e l a t e m p e r a t u r e , l ' i l l u m i n a t i o n e t la lavage a v a n t d e la m e s u r e . L a m é t h o d e i c i d e v e l o p é e , sera u t i l e p o u r l ' é v a l u a t i o n d u d e g r é d ' e x p o s i t i o n p o u r a i d e r la d e c i s i o n d u t r a i t e m e n t m é d i c a l p o u r ê t r e a p p l i q u é .
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