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UNITED STATES ATOMIC ENERGY COMMISSION
PHOTOGRAPHIC FILM AS A POCKET RADIATION DOSIMETER
L. A. Pardue
E. O. Wollan
Argonne National Laboratory
This document consists of 7 pages.
Date of Manuscript: April 8, 1944
Date Declassified: June 23, 1947
This document is for official use.
Its issuance does not constitute authority
for declassification of classified copies
of the same or similar content and title
and by the same authors.
Technical Information Division, Oak Ridge Directed Operations
Printed in United States of America
AEC, Oak Ridge, Tenn.-8-25-48-1,500
Price 10 cents
PHOTOGRAPHIC FILM AS A POCKET RADIATION DOSIMETER
By L. A. Pardue, N. Goldstein, and E. O. Wollan
An air ionization chamber is theoretically the ideal type of radiation dosimeter since the roent-
gen is defined in terms of the ionization in air. When it comes to the continuous monitoring of a
large number of people for radiation exposure, however, the practical problems of handling and serv-
icing the meters must be considered in the choice of a method for carrying out such monitoring.
Dental films have been used for a long time as a means of obtaining qualitative information re-
garding individual exposure to x-rays and gamma rays. The blackening of a film, however, varies
by such a large factor for radiations of various quantum energies that this method could not be con-
sidered satisfactory unless some means of compensation for this energy dependence of response
were achieved. A good pocket dosimeter should have the following properties.
1) Response for equal exposures in roentgens should be, as far as possible, independent of
the quantum energy of the radiation.
2) The range of the meter preferably should be such that doses from less than 0.1 r to 10 or
20 r should be accurately measurable.
3) The response should not be producible by agents other than the radiations to be measured.
4) It should be small and light and should permit of being handled in large numbers by tech-
ENERGY DEPENDENCE OF FILM BLACKENING
Photographic blackening is defined as the negative logarithm of the ratio of the intensity trans-
mitted through the exposed film to that transmitted by a similarly developed unexposed film: or
B =-log I 0= log lo I
Plotting blackening against energy of quantum radiation for fixed exposure in roentgens we get the
curves shown in Figure 1. The solid curve shows the response of a standard dental film when ex-
posed to 0.5 r of quantum radiation without a filter. The fact that the blackening increases rapidly
as one goes to radiation of lower quantum energy suggests that the response for a given dose would
be more nearly independent of the quantum energy if an absorber were placed over the film. The
broken curve gives the response when 1 mm of Cd is used as a filter without changing the intensity
of the primary beam. The dashed curve gives the response one calculates for the same absorber.
If one calculates the response for a Cu filter 2 mm thick the results are not altered appreciably ex-
eept in the range 50 kv to 60 kv. The useful lower limit would remain at 60 kv approximately. Below
this value the film response cannot be satisfactorily compensated by a single filter.
EXPOSED FILM RESPONSE WITHOJJT FILTER
EXPOSED FILM RESPONSE WITH FILTER
CALCULATED FILM RESPONSE WITH FILTER
0 )00 200 300 400 500
600 700 800 900 1000
Figure 1. Blackemnig versus energy for fixed exposure.
THICKNESS OF Cd(MM)
Figure 2. DuPont No. 502 film exposed at 0.5 r.
In the many studies made to find the best filter material and thickness, Pb, Ag, and Cd were tried.
Pb was considered bad because absorption becomes very large at 87 kv; i.e., the K absorption edge
comes at this energy. Ag and Cd are better in this respect since their K absorption edges come at
25.5 kv and 26.6 kv respectively. As we did not use monochromatic radiation this feature was not
brought out very clearly in our results. Ag and Cd are equally satisfactory as filters. Cd was cho-
sen, however, because it is not expensive and it is easy to work.
The method of finding the thickness of absorber was as follows. Step absorbers from 0 to 1 or
2 mm in thickness, depending on the material, were used. Films were exposed through these absorbers
to a given dose of quantum radiation of energies ranging from 50 kv x-rays to the gamma rays for
each energy. For duPont film No. 502 one obtains a family of curves as shown in Figure 2 from which
the proper filter thickness may be chosen. It is seen that for this film a filter thickness slightly in
excess of 1 mm would be optimuna; 1 mm was chosen because there is an additional 0.5 mm of iron
in the badge. Figure 3 shows the energy dependence of this film when 1 mm of Cd is used as filter.
For duPont film No. 351 the energy dependence for a 1 mm Cd filter is shown in Figure 4. Both films
give a satisfactory response from hard gamrr.a rays down to 100 kv quantum energy and are useful
down to about 60 kv.
EXPOSURE RANGE CF FILMS
duPont No. 502 is typical of the most sensitive films obtainable for this use from Eastman and
duPont. Figure 5 gives a curve of blackening versus exposure in roentgens up to 3 r, over which
range the curvature is not bad. A Ra source was used. The smallest blackening that can be meas-
ured with any reliability corresponds to about 0.03 r. The total useful range of a film meter can be
extended by the-use of a second less sensitive film. For this duPont No. 351 has been chosen. Its
blackening versus exposure curve is shown in Figure 6. With these two films exposures from 0.03 r
to 20 r can be measured reliably. Fortunately, with x-rays and gamma rays there is no significant
departure from the reciprocity law in the useful range. With visible light however there is an inertia
region followed by a linear part if blackening is plotted versus the logarithm of exposure, then by
saturation and perhaps reversal.
MONITORING RADIATION WITH FILM
The plan in use requires the worker to carry an x-ray film of dental packet size in a badge of
light weight designed for the purpose. The photographs in Figure 7 show the parts of the badge. Be-
sides protecting the film while it is worn, it divided it into three parts. The middle portion has the
necessary filter (1 mm of Cd) to give a fairly uniform response over the range of wavelengths indi-
On opposite sides of the center part are regions for detecting soft radiation and for numbering
the film by means of x-rays. The section for detectingesoft radiation exposes the film packet through
windows in the badge. This section will certainly be useful in indicating the absence of beta rays and
quantum radiation. It may be possible to calibrate this unfiltered area when the nature of the radi-
ation is known. One of these windows carries the badge number printed on paper covered by cello-
phane. The same number is perforated in the Cd in the lower part of the badge. This makes it pos-
sible to use x-rays to number the film before it is removed from its badge.
Development can then be carried out without delay involved in keeping large numbers of films
in identifiable order during the developing process. Except for angular dependence at long wave-
lengths, it is immaterial which way the badge is turned toward the beam. This accomplished in the
design of the badge and by using dental packages without the usual metal foil.
0 00 200 300 400 500
600 700 B00 900
Figure 3. DuPont film No. 502 exposed to 0.5 r- 1 mm Cd filter.
Figure 4. DuPont film No. 351 exposed to 10 r- 1 mm Cd filter.
Figure 5. Blackening versus exposure for No. 502 duPont film.
*: -- -----------7----
0 1 2 3 4 5 6 7 8 9 10 12 13 14 i5 16 17 1B 19
Figure 6. Blackening versus exposure for No. 351 duPont film.
Figure 7. Parts of the film badge.
Figure 8. Two duPont No. 502 films exposed to 0.5 y, (a) by gamma rays from Ra and (b) by 200 kv
As in all photography the conditions of developing are important. Temperature must be held
constant and the tithe in the developer always must be the same for comparable results. Fixing and
washing are not so important but it does not hurt to use controlled and reproducible conditions here.
Still another factor must be watched. There is a decided dependence on depth in the developer if it
is old and is not agitated. Developer should be replaced frequently and it should be stirred while in
use. Accuracy is increased if films of known exposure are carried through the development simul-
taneously with the unknowns.
Figure 8 shows two No. 502 films exposed to 0.5 r, (a) by gamma rays from Ra and (b) by 200
kv x-rays. These are prints of the original negatives. While the photography does not preserve all
the features of these negatives, it is evident that the films are permanently marked. Also it can be
seen that the regions of interest in the middle of the films have comparable blackening. Measure-
ments on the original films confirm this.
Measurements on blackening are made on a model No. 500 Photovolt densitometer which is
found to be quite satisfactory if operating voltages are held constant.
In deciding the conditions to be met by a large scale radiation monitoring program, the things
to be accomplished have to be considered. Obviously, the primary purpose of any such program is
to eliminate as far as possible radiation injury to personnel. Such injuries can result from large
single exposures or they may arise from the accumulation of small doses. In either case the effects
may not be apparent for some time; in some instances of the accumulation type, many years may
elapse before the injury of the first doses are felt.
On account of the nature of the hazard one can add two corollary objectives: (a) a reasonably
diligent radiation monitoring program would tend to take responsibility from those providing it for
injuries traceable to radiation exposure at some earlier time; (b) a good record of exposures may
be useful in reducing the limits of uncertainty of the tolerance dose. To accomplish the chief ob-
jective it is desirable to keep exposures whenever possible below the best estimate of the tolerance
upper limit, but when an exposure goes outside this range its value should be known. A film meter
with its range up to about 20 r will lose very few exposures. Its accuracy in routine measurements
is believed to be better than 25%. In this latter regard it compares quite favorably with pocket cham-
bers and it does not have the present range limitation (0.2 r)of these instruments. There should not
be so many spurious readings as with pocket chambers where leakage, jars, and tampering produce
the same effect as radiation.
The reliability of film meters will not change with time, and film blackening almost certainly
will be the result of radiation. Pocket chambers excel in allowing the integrated dose to be read
quickly. This is an important advantage when one wishes to follow an exposure during the course
of an experiment or any other job where the material handled is known to be very active. Until a sin-
gle method with the advantages of both films and ion chambers is developed there will be a need for
using both types. At the present time no routine method is available which measures exposures to
a high degree of accuracy. If the worst were true, however, and the accuracy were no better than
50%, the effort would still be justified.
Pocket meters can, of course, not be expected to replace radiation surveys which should be
made regularly in order to establish the general radiation conditions which prevail in the areas.
These are necessary for finding high local intensities such as beams from imperfect shields, and
beta ray sources for which the present pocket meters are not designed. Special surveys must take
care of fast neutron radiation and atmospheric contamination.
UNIVERSITY OF FLORIDA
3 1262 08909 7579
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