PB 151388

NBS

ecknical <ylote 29

IOTOGRAPHIC DOSIMETRY AT

ITAL EXPOSURE LEVELS BELOW 20 mr

DEPARTMENT OF COMMERCE

IONAL BUREAU OF STANDARDS
LTIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS

technical ^lote

29

OCTOBER 1959

PHOTOGRAPHIC DOSIMETRY AT

TOTAL EXPOSURE LEVELS BELOW 20 mr

Margarete Ehrlich and William L. McLaughlin

The work described in this Report was sponsored in part by the Division of Biology and Medicine, U. S. Atomic Energy Commission.

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FOREWORD

This Note covers the material presented by the authors at

the 1959 annual meeting of the Health Physics Society. Work on

some of the phases covered in the report will be continued; for

this reason, it is not contemplated to publish the present results

in a more formal way.

Scott W. Smith, Chief H.O. Wyckoff , Chief

Radiological Equipment Section Radiation Physics Laboratory

iii
ABSTRACT

Assemblies of commercial photographic material sandwiched between two plastic scintillators can be used to measure high-energy

X- or gamma-ray exposures down to 1 mr and less. The energy dependence of the assemblies 1 response is much less than that of a conventional photographic dosimeter, in some instances allowing an exposure interpretation with an accuracy of ± 25 percent over the

energy range from about 0.1 Mev to 1.25 Mev. However, low- intensity

reciprocity failure limits the range of applicability of the system.

Conventional photographic dosimeters, not incorporating scintillators, are usually preferable for routine personnel dosimetry. By

extending the monitoring period, it may be possible to avoid personnel dosimetry below 20 mr entirely. However, by doing this, one

introduces additional difficulties because of instabilities in the

photographic image. In some instances, an increase in effective

emulsion thickness, achieved by using stacks of identical films, may

lead to an increase in emulsion sensitivity sufficient to extend the

useful range of a film badge well below 20 mr, without the use of a

scintillator.

iv
PHOTOGRAPHIC DOSIMETRY AT TOTAL EXPOSURE LEVELS BELOW 20 mr.

Margarete Ehrlich and William L. McLaughlin

1. INTRODUCTION

Photographic dosimetry at exposure levels below 20 mr is difficult

because of insufficient sensitivity of commercial photographic emulsions, instabilities in the latent photographic image, and inaccuracies in sensitometric procedure. Some of the endeavors to cover the

range below 20 mr more adequately, both with film-scintillator combinations and with conventional photographic dosimeters , will be briefly

discussed.

2. FILM-SCINTILLATOR COMBINATIONS

2.1 Discussion of Method; History

One way to intensify the photographic effect of X-radiation, is to

use the photographic material in contact with a scintillator. In such

a system, a considerable fraction of the film response stems from the

effect on the film of the visible light emitted by the scintillator,

rather than from the direct effect of the primary radiation. In fact,

if film and scintillator are suitably chosen, the response of the film

to the primary radiation is so small in comparison with that to the

visible light, that--essentially--the scintillator becomes the radiation detector, while the film merely takes the place of the recording

apparatus (similar to a photomultiplier and associated electronic circuitry in the more elaborate scintillation- type survey instruments).
- 2 -

Since the intrinsic efficiency of the luminescence process depends

only very little on the energy of the incident photons , the total amount

of light produced in the scintillator is roughly proportional to the

radiation energy absorbed in the scintillator. Therefore, for airequivalent scintillators of the order of equilibrium thickness, one can

expect that--as a function of radiation energy—the scintillator response is proportional to the ionization in air, as measured in roentgens. Unfortunately, the scintillators used in practice are not completely air equivalent, and, in addition, the fraction of the light

produced that is incident on the film varies with photon energy. As

a consequence, the response of the film-scintillator system is energy

dependent, although much less so than the response of a photographic

detector used without a scintillator. Also, with proper choice of

scintillator shape, the film-scintillator system may be made independent of the direction of the incident radiation- -which is usually impossible with film alone. The only severe handicap of the film-scintillator

detector is its rate dependence, a phenomenon that will be dicussed later,

In the photon energy range under consideration, the fraction of the

visible light produced close to the film increases with increasing

photon energy, and, as a consequence, the fraction of the light registered by the film also increases. On the other hand, at any given photon

energy, increasing the scintillator size beyond a certain optimum, de- creases the amount of light registered, because of the attenuation of

the primary radiation and of the visible light on its travel to the film.

This decrease is especially pronounced for photons of low energy, which

are strongly absorbed in the surface layers of the scintillator.
- 3 -

Table 1 shows the important features of some of the published

work on dosimeters using film-scintillator combinations. While the

Ansco dosimeter was specifically designed as a personnel dosimeter and

is useful for total exposures down to about 20 mr, and for rates down

to about 5 mr/hr, the dosimeter of the British Atomic Weapons Research

Establishment apparently was developed specifically for use with low

total exposures in radiation fields of high intensities. Its response is essentially independent of the direction of incidence of

the primary radiation. The Russian device is the most sensitive; no

information is available on its rate dependence.

About two years ago, we were asked by the AEC Division of Biology

and Medicine to investigate whether the use of the film-scintillator

system could be successfully extended to exposure rates close to

natural background, and total exposures down to 1 mr or less. Thus,

the idea was to extend the usefulness of the method to rates lower

than the ones limiting the response of the previous systems, and to

total exposures comparable to those recorded with the Russian Instru-

, 2

ment.

2. 2 Rate Dependence of the Photographic Process

The reason for the rate dependence of film-scintillator dosimeters is to be found in the peculiarities of the photographic action

of visible light. Photographic emulsions consist of AgBr crystallites

2 The results of a similar endeavor, also initiated by the AEC Division of Biology and Medicine, were reported by O'Brian, K. , Solon, L.R.

,

Lowder, W.M. , in Rev. Sci. Ins trum. , 29, 1097, 1958.
- 4 -

(grains) embedded in a gelatinous matrix. Upon absorption of photons or

of their corpuscular secondaries by the emulsion, some of the AgBr is

reduced to free silver, which- -when clustered in aggregates of several

silver atoms- -forms units that are fairly stable at room temperature and

not too high a relative humidity. These aggregates are the smallest units

of the so-called latent image, which, according to modern theory, requires

3

for its formation at least three silver atoms. During development,

these units serve as centers for the further reduction of AgBr in the

crystal, a process that eventually leads to the visible silver aggregates

known as the photographic image.

Rate dependence of photographic material, referred to as reciprocity failure, i.e., a failure of the photographic effect to be proportional to the product of the intensity of the exposing radiation and the

exposure time, occurs when more than one ionizing event is required to

make one of the photographic emulsion's silver halide grains developable.

A photon of visible light creates only one photoelectron upon its absorption in a silver halide crystal. This is not sufficient for the

production of a latent image and for grain developability. As a consequence, if the illuminance is so low that the time between ionizing

events in any one grain approaches the lifetime of the tiny, very

unstable, precursors of the latent image, the number of grains actually

made developable depends on the rate of incidence of the photons, and

the photographic process shows what is known as low- intensity reciprocity

failure. There exists also a high-intensity failure, but it is not of

interest to the present discussion.

3 Mitchell, J.W., Photographic Sensitivity, p. 433 of Report on Progress

in Physics, Vol 20, The Physical Society, London.
- 5

-

When a photographic emulsion of sufficiently high sensitivity is

exposed to X- or gamma radiation, the number of electrons produced by

one single absorbed photon usually is large enough to make a grain developable. Therefore, the photographic effect of X- or gamma rays as a

rule shows no rate dependence. However, if a photographic material is

exposed to X- or gamma rays in conjunction with scintillators (which

usually emit visible light of a wavelength in the neighborhood of

4000 A) the assembly is rate dependent—at least at very low and very

high radiation intensities. Therefore, the design of dosimeters embodying film- scintillator assemblies is actually more straightforward for

the intermediate rate range than for the very low range of exposure

rates. The present attempt to go to background rates was only parti4

ally successful.

2.3 Experiment

The Pilot B plastic scintillator, which consists of diphenylstilbene

in a 100$ hydrocarbon plastic, was chosen as the intensifier. Figure 1

shows the smallest and the largest assembly used. The scintillators

were painted with Tygon paint on all but the surfaces in contact with

4

E.T. Larson, and H.A. Levine of Ansco reported very interesting results

on the reduction of reciprocity failure with mercaptoozazoline development retarders (Phot. Sci. and Eng. , 1, 59, 1957). It is likely that,

with the aid of such retarders , most of the rate dependence of the system

could be removed. However, the associated decrease in sensitivity would

probably make it impossible to use the present assembly for total ex- posures of less than 20 mr.
- 6 -

the- photographic material, and then wrapped in black tape. The surface

upon which the radiation was incident was covered only with a thin layer

of Bakelite (in practical use, a Bakelite container, rather than tape,

could be used to enclose the entire system). Black tape also shielded

one-half of the photographic emulsion from the visible light emitted by

the scintillator. This provided a comparison between photographic densities due to high-energy photons alone and due to light photons plus

high-energy photons. In the exposure range covered in this study, the

area shielded from the scintillator light showed no density above that

due to base and emulsion fog.

Experiments with several different photographic materials finally

led to the decision to investigate more extensively the Eastman spectroscopic plates, type 103a-0, usually employed for astronomical photography. In this material, the onset of low- intensity reciprocity

failure is shifted to lower intensities than in other commercial emulsions. Emulsion chemists achieve this partially by increasing the

number of silver ions on the emulsion surface, which facilitates the

formation of precursors of the latent image, which then grow into

latent images upon addition of only very little energy. This explains

why such emulsions are less stable than other commercially available

photographic emulsions. In spite of their relatively low stability,

it was possible to use the Eastman 103a-0 plates at room temperature

and at 40 percent to 50 percent relative humidity over at least a

period of one week, without any appreciable changes in background and

in sensitivity. Stored in the refrigerator, the plates remained usable
- 7

-

for more than a year. We obtained the commercial product, consisting

of a single emulsion layer coated on glass, and made a plate-scintillator sandwich, whenever possible employing a plastic spacer to avoid

undue pressure on the emulsion. When ordered in bulk, the same emulsion can also be obtained coated on one or both sides of a film base.

The response of the plate-scintillator assemblies was studied as

a function of total exposure, exposure rate, and photon energy, with

the radiation incident perpendicularly to the surface of the photographic plate. The response was not studied as a function of the

direction of the incident radiation. Because of the similarity in

shape, it is likely that the response of the assemblies as a function

of the direction of radiation incidence is quite similar to that of

the AWRE assemblies (see table 1 for reference).

2.4 Results

a. Characteristic curves

Figure 2 shows the response of the large scintillator assembly to

gamma radiation at several different rates from about 12 mr/hr down

to 0.08 mr/hr, and to the laboratory background rate of 0.023 mr/hr.

The assembly records exposures as low as about 0.5 mr and less;

however, the characteristic curves shift and change shape with exposure rate. The change in shape is most pronounced for the background

rate.

Figure 3 shows the same phenomenon for the small scintillator.

This assembly makes possible an exposure interpretation down to about

2 mr, but the rate dependence is seen to be even more pronounced;
- 8 -

this is to be expected, since the smaller scintillator emits less light

per incident gamma photon than the larger scintillator. Note that

while rates down to about 0.02 mr/hr were included in figure 2, the

lowest rate shown here is 1 mr/hr.

b. Limitations due to rate dependence

Figure 4 shows--for both scintillators —the exposures required

for three different densities as a function of exposure rate. With the

small scintillator, the useful rate range only extends down to about

15 to 20 mr/hr. With the large scintillator, at a density of 0.5,

corresponding to about 0.5 mr, there is little rate dependence all the

way down to laboratory background rates. For higher densities, the exposure interpretation is accurate to within ± 25 percent from about

0.1 mr/hr up. It is to be expected that, with scintillators larger

than our 2

M x 2" specimens, and with double-coated photographic material, it will be possible to obtain an adequate interpretation of

exposures as low as 1 mr for rates all the way down to natural background.

c. Energy dependence

There is, however, one disadvantage in employing large scintillators. As the scintillator size is increased, the energy dependence

of the assembly's response increases due to the change in spatial

distribution of the attenuation of the primary radiation and of the

luminescence light, as discussed in footnote 1. Figure 5 shows that,

over the energy range from about 100 kev to Co energies and higher,

the sensitivity of the assembly employing the 1" x 1/2" scintillator
- 9 -

varies by only ± 25 percent, while for the large scintillator, the

variation over the same range is nearly ± 50 percent.

It may be noted that, wherever possible, the same exposure rate

was used to obtain all energy- dependence data. Because of the change

in curve shape with exposure rate, the shape of the energy dependence

curve changes slightly with exposure rate- -and also with the density

level for which the sensitivity is calculated. However, the rates here

employed are high enough to make these variations unimportant for the

present study.

d. Summary

The results of this study indicate that it is quite feasible to

use large film-scintillator assemblies to measure very low total exposures at very low rates, but only over a limited range of rather low

film densities. This requirement can be met by limiting the total

time over which the assembly is exposed. Smaller film-scintillator

assemblies can be used only to interpret exposures at rates above a

certain minimum value. Such a complicated procedure cannot be recommended for routine personnel dosimetry.

3. LIMITATIONS IN CONVENTIONAL PHOTOGRAPHIC

DOSIMETRY DUE TO ENVIRONMENTAL EFFECTS

In personnel dosimetry, it is still advisable to use conventional

film packets in suitable holders, and to extend their useful range by

extending the period over which the holders are carried. This procedure is gaining in popularity, particularly in view of the change
-lOin thought on how to specify maximum permissible exposure in a biologically meaningful way. As a consequence, we have been asked on several occasions for advice on the stability of the latent image in the time

interval between its formation and processing.

Results on studies of changes in photographic response with storage

time and with temperature during and after exposure have been obtained

earlier at the National Bureau of Standards , but only over periods of

one week or less. Data over periods up to thirteen weeks were more

recently obtained by Ziegler and Chleck under controlled laboratory

conditions. A more realistic study over a period of thirteen weeks is

now being performed at the National Bureau of Standards. In general,

it may be said that the interplay of a variation in temperature and in

the relative humidity of the atmosphere produces a complicated pattern

of behavior of the latent image. According to the particular experimental conditions employed, the result may be either an increase or a

decrease in the photographic response.

In personnel dosimetry, there is little one can do to protect the

monitoring films from large changes in ambient temperature. However,

the influence of humidity and of changes in the chemical composition of

the atmosphere can be readily eliminated. A simple hermetically sealed

plastic container provides adequate protection.

Handbook 59, Addendum, U.S. Department of Commerce, National Bureau

of Standards, 1958.

Handbook 57, U.S. Department of Commerce, National Bureau of Standards,

1954.

Ziegler, C.A., Chleck, D.J. , Latent-image fading in film badge dosimeters, to be published.
- 11 -

4. INCREASE IN SENSITIVITY WITH EFFECTIVE EMULSION THICKNESS

In conclusion, it will be demonstrated that, in some instances, an

increase in effective emulsion thickness, achieved by stacking a number

of identical films during exposure and densitometry, may lead to an extension of the useful range of a film badge below 20 mr, without the use

of a scintillator.

Figure 6 shows a characteristic curve of DuPont film type 502, obtained with narrow bands of X-radiation of about 210-kev effective

energy, once with single films, and the other time with three films exposed and densitometered on top of each other. Percentagewise, the

limitation in precision of the exposure interpretation resulting from

variations in emulsion coating and uneven processing is the same for the

two curves. However, if one uses a densitometer whose precision limit

is, roughly, ±0.02 density units (such as, for instance, the Ansco Sweet

Color Densitometer) , tripling the density for a given exposure improves

the accuracy of the exposure interpretation considerably.

Table 2 shows the extent of improvement in the precision of the

exposure interpretation with DuPont film type 502, under the assumption

that the density produced by a given exposure is reproducible to

within ± 5 percent or ±0.02 density units, whichever is greater.

Notice 'that, whereas with a single film the precision in the interpretation of an exposure of, for example, about 100 mr is only ± 40 percent,

it is ± 14 percent with a stack of three films.
- 12 -

Figure 7 shows the results of triple versus single densitometry on

the more sensitive DuPont film type 555 (packaged without lead backing

as personnel monitoring film type 554) , exposed to Co gamma radiation.

While, with a single film, the low-exposure limit for a ± 25 percent precision in exposure interpretation is 61 mr, it is 22 mr for triple stacks.

Figure 8 shows that for the even more sensitive Kodak film type KK,

(packaged as personnel monitoring film type 1) , the low-exposure limit

for a ± 25 percent precision is 23 mr for the single film, and 8.2 mr

for the triple stack. Actually, an exposure of about 4 mr can still be

interpreted with an accuracy of about ± 50 percent with three films

stacked, while no reasonable information at all could have been obtained

for such a low exposure with one single KK film.

5. SUMMARY AND CONCLUSIONS

Because of insufficient sensitivity of available photographic materials , photographic X- and gamma-ray dosimetry at total exposures in the

range from 1 to 20 mr requires special procedures, which, in turn, may

introduce new problems.

If one chooses to intensify the high-energy photon image through

the action of visible light from a scintillator, one reduces the energy

dependence of the film device without introducing metallic filters, but,

at the same time, one restricts its applicability because of the presence

of low- intensity reciprocity failure. In some instances, an increase in

effective emulsion thickness, achieved by using stacks of identical

films, may lead to an increase in emulsion sensitivity sufficient to

extend the useful range of a film badge well below 20 mr, without the

use of a scintillator.
- 13 -

In applications to personnel dosimetry, it may be possible to avoid

dosimetry below 20 mr entirely, by extending the monitoring period. However, by doing this, one introduces additional difficulties because of

instabilities in the photographic latent image . Realistic long-term

studies of the effect on the latent image of variations in relative

humidity and temperature are scarce. More work in this direction is

under way at the National Bureau of Standards.
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U.S. DEPARTMENT OF COMMERCE

Krnlrritl H. Muillfr. Srrrrlary

\ \ TIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its headquarters in Washington, D.C., and its major

laboratories in Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work. In general, each section carries out specialized research, development, and engineering in the

field indicated by its title. A brief description of the activities, and of the resultant publications, appears on the

inside of the front cover. WASHINGTON, D.C.

Electricity and Electronics. Resistance and Reactance. Electron Devices. Electrical Instruments. Magnetic Measurements. Dielectrics. Engineering Electronics. Electronic Instrumentation. Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Optical Instruments. Photographic Technology.

Length. Engineering Metrology.

^^HL Temperature Physics.. Thermodynamics. Cryogenic Physics. Rheology. Engine Fuels. Free Radicals Research. \tomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid State Physics.

Electron Physics. Atomic Physics. Neutron Physics. Radiation Theory. Radioactivity. X-rays. High

Energy Radiation. Nucleonic Instrumentation. Radiological Equipment.

Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry. Inorganic

Ministry. Electrodeposition. Molecular Structure and Properties of Gases. Physical Chemistry. Thermoistry. Spectrochemistry. Pure Substances.

echanics. Sound. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics. Mass and Scale,

ipacity, Density, and Fluid Meters. Combustion Controls.

$anic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. Polymer

acture. Plastics. Dental Research.

tallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion. Metal Physics.

iiH-ral Products. Engineering Ceramics. Glass. Refractories. Enameled Metals. Concreting Materials,

institution and Microstructure.

folding Technology. Structural Engineering. Fire Protection. Air Conditioning, Heating, and Refrigeration. Floor, Roof, and Wall Coverings. Codes and Safety Standards. Heat Transfer.

lied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics.

Data Processing Systems. SEAC Engineering Group. Components and Techniques. Digital Circuitry.

^^Hkl Systems. Analog Systems. Application Engineering.

• < >ffice of Basic Instrumentation. • Office of Weights and Measures. BOULDER, COLORADO

C/j ogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials. Gas Liquefaction. ^Hf° Propagation Physics. Upper Atmosphere Research. Ionospheric Research. Regular Propagation

Services. Sun-Earth Relationships. VHF Research. Radio Warning Services. Airglow and Aurora. Radio

Astronomy and Arctic Propagation.

Radio Propagation Engineering. Data Reduction Instrumentation. Modulation Systems. Radio Noise. rropospheric Measurements. Tropospheric Analysis. Radio Systems Application Engineering. RadioMeteorolog) Ixwer Atmosphere Physics.

Radio Standards. High Frequency Electrical Standards. Radio Broadcast Service. High Frequency Im-

[w-dance Standards. Electronic Calibration Center. Microwave Physics. Microwave Circuit Standards.

Radio Communication and Systems. Low Frequency and Very Low Frequency Research. High Fre- quency and Very High Frequency Research. Ultra High Frequency and Super High Frequency Research. Modulation Research. Antenna Research. Navigation Systems. Systems Analysis. Field Operations.
NBS