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OCR for page 10
Basic Principles of Film Badge Dosimetry
For those readers who are familiar with the use of the film badge as a device for
the measurement of radiation in potentially exposed workers, this chapter may be
superfluous. For others, it will provide background material helpful in under-
standing the rest of the report.
A. HISTORICALINTRODUCTION
Photographic emulsions have long been used for detection and measurement
of ionizing radiations. Even before Roentgen's discovery of x-rays in 1895,
fogging of unknown origin was observed in photographic emulsions by research-
ers who were unknowingly producing x-rays during their research with evacuated
discharge tubes. Among the first to apply photographic emulsions to radiation
protection was William H. Rollins, a Boston dentist and x-ray protection pioneer
who in 1902 described a protective housing for x-ray tubes (Rollins 1902~. As a
test of the efficacy of the shielding, Rollins recommended placing an unexposed
photographic plate against the exterior of the housing, noting that the housing was
satisfactory if the plate was not fogged by an exposure of seven minutes duration.
Perhaps the first application of photographic film, rather than plates, to radia-
ton protection came the following year when an American dermatologist, S.
Stern, proposed its use to quantify the dose received by patients undergoing
radiologic procedures (Stern 1903~. Fundamental work carried out a decade later
established the suitability of photographic emulsions for dose measurements. In
Germany, Kronke (1914), Friedrich and Koch (1914) and Clocker and Traub
10
OCR for page 11
2 BASIC PRINCIPLES
11
(1921), along with Allen and Lafy (1919) and Bloch and Renwick (1920) in
Britain, demonstrated that for a given x-ray spectrum, the blackening or density of
the film could be correlated with the exposure, producing a characteristic dose-
response curve.
Routine monitoring of personnel exposures to x-rays and radium with photos
graphic films for protection purposes was first suggested in 1922 by George
Pfahler, a prominent American radiologist. Pfahler recommended that x-ray and
radium workers routinely carry an unexposed dental radiographic film packet in
their breast pocket. After two weeks, this film was to be developed and the degree
of blackening correlated with radiation exposure in terms of skin erythema dose
(Pfahler 1922~. Four years later, Edith Quimby, a New York medical physicist,
proposed the first true film badge, incorporating a system of metallic filters to
compensate for the energy dependence of the film sensitivity (Quimby 1926) i.e.,
the propensity of the photographic emulsion to over-respond or produce excessive
darkening to certain energies of x radiation. A few months later, Robert S.
Landauer Sr., a physicist at Cook County Hospital in Chicago, suggested the use
of easily obtained and reasonably constant quality dental x-ray film packets
(Landauer 1927).
In 1928, the roentgen unit for radiation exposure was formally adopted by the
Second International Congress on Radiology. This unit, which was defined in
terms of air ionization, thus became the primary standard for radiological meas-
urements, replacing other units based on biological effects (such as the skin
erythema dose) or Induced calorimetric change in chemicals. The degree of film
blackening or optical density, essentially a chemical effect, was correlated with
the exposure measured in roentgens (R), a physical effect, by Franke (1928) in
Germany. In Holland, Bouwers and van der Tuuk (1930) extended the work of
-Franke to a lower level of detection, below the then-current daily exposure limit
of 0.2 R. and described a sophisticated film badge for personnel monitoring that
utilized multiple metallic filters.
Despite the correlations established under laboratory conditions, and the film
badge of Quimby, practical difficulties were encountered with dose detennina-
tions in the field because the response of photographic film was dependent on
photon energy. Photographic films were accordingly considered unreliable and
hence not always used for monitoring exposure of x-ray workers, although they
were considered satisfactory for monitoring exposure of radium workers (Ha-
mann 1932; Holthausen and Hamann 1932~.
The work of the Manhattan Dismct in the early 1940's created a need for a
reliable and sufficiently sensitive personnel monitoring device capable of applica-
tion to the protection program for a large and diverse work force. Commercially
available x ray films were tried and found to be well suited to this task if used with
filtration to compensate for energy dependence. The standard holder or badge
OCR for page 12
12
FILM BADGE DOSIMEI RY 11V ATMOSPHERIC NUCLEAR TESTS
contained two pieces of dental x ray film-one low range (20 mR - 20 R) and one
high-range (1 R - 400 R) in a holder made of silver or cadmium, 1 mm thick, with
a window to admit beta radiation (Figure 2-l)~Iorgan 1947; Pardue et al. 1944;
Parker 1980~. The metal filter provided compensation, albeit imperfect, for the
over-response of film to photons with energies between about 25 and 100 keV. It
was also in the Manhattan District that the basic techniques for large scale
personnel monitoring with films evolved, including quantity purchasing (and
hence uniformity of large batches), storage under controlled conditions to en-
hance shelf life, batch calibration and development techniques with suitable
controls, improved densitometry, and controlled distribution, recovery, and de-
velopment (Auxier 1980; Pardue et al. 1944~.
B. PERSONNELDOSIMETRY FILMS
Films used for personnel dosimetry are basically the same as ordinary black
and white photographic film or x-ray films, consisting of a layer of gelatin
emulsion containing a specified quantity of silver halide laid on top of a sheet of
supporting structure known as the film base (Figure 2-29.
The film base is typically made from a nonflammable inert material such as
cellulose acetate, and is relatively thick, usually on the order of 100-200 microme-
ters ~m). The base serves both to protect and to support the emulsion.
The response of a photographic emulsion to a given exposure to radiation is
dependent on a number of factors, including the presence or absence of various
chemicals which may act as sensitizers or retardants, and grain size. Generally,
the larger the grain size, the more sensitive the film is to a given exposure to
radiation. Thus, the so-called fine-grain films typically will have less radiation
Silver or cadmium
holder 1 mm thick
Window to admit
beta radiation
(both sides of
holder)
FIGURE 2-1 Standard Film Badge with Silver or Cadmium Holder.
Dental x-ray film
A. 20 mR - 20 R
B. 1 R - 400 R
(paper wrapping
not shown)
OCR for page 13
2 BASIC PRINCIPLES
A. . · - , O ,0, " . ~ ~ o · A'. ' ~ O' ~ ., ·
W////////////////~//i~
FIGURE 2-2 Cross-seciion of a Typical Photographic Film (not to scale).
13
T-Coat (~1,um)
Emulsion (~20 ,u m)
Film Base (~100 - 200,u m)
sensitivity than those with coarser grains. Depending on the intended use of the
film, the emulsion thickness may range from a few to several hundred ~m. In
films used for personnel monitoring of beta and photon radiations, the emulsion is
typically a few tens of film in thickness. Grains of silver bromide (AgBr) typically
ranging from 0.1 to 10 Em in diameter are distributed more or less uniformly
throughout the emulsion. These constitute the sensitive portion of the film;
exposure to ionizing radiation, light or other forms of electromagnetic energy,
such as infrared, induce a physico-chemical change which is a function of the
exposure.
Photographic emulsions are produced by a complex series of well controlled
manufacturing operations. The first step is the precipitation of silver halide in a
gelatin solution. This is accomplished by addition of an aqueous solution of silver
salts, primarily silver nitrate, to a gelatin solution containing an excess of alkali
halide under controlled conditions. Grain size is increased by subsequent heating
to 50-70°C for up to an hour. The grains are not uniform in size and shape but do
have a reasonably consistent distribution. The emulsion is then washed with
water to remove the remaining soluble salt, and heated to the melting point.
Additional gelatin and various sensitizers and stabilizers are added, and the
emulsion is held at temperature for a suitable time to produce the desired sensitiv-
ity characteristics and to minimize background darkening (fog). It is then spread
in a uniform layer on the film base and allowed to cool and dry. It may be coated
with a thin protective layer about a micrometer in thickness known as the T-coat
(Figure 2-2~.
A dosimeter film may be single-coated (i.e., the base has the emulsion on one
side only) or double-coated. If double-coated, the same emulsion may be on each
side of the base, or two different emulsions may be used. Dual coating with the
same emulsion was originally used primarily to enhance sensitivity. Dual coating
with emulsions of different sensitivity is now used to enhance the overall range of
the film. A typical photographic emulsion for personnel dosimetry purposes
contains about 50% by weight of AgBr (including a few per cent of silver iodide)
and 50% gelatin. The thickness of AgBr in the emulsion layer is a few mg/cm2,
and the grain density of AgBr is in the range 109-10~2 grains/cm2.
OCR for page 14
14
FILM BADGE DOSIMETRY 11V ATMOSPHERIC NUCLEAR TESTS
Manufacture of photographic film is carried out in darkness, as visible light
will expose the film. It is fabricated in large sheets which are cut into the desired
size and packaged in light-tight paper or plastic wrappings. Dosimeter firms have
traditionally been sized and wrapped like dental x-ray films, although smaller
sizes have been produced.
C. PHYSICAL AND CHEMICAL BASIS OF FILM DOSIMETRY
When a film is exposed to radiation, a complex series of interactions takes
place. The basic theory of the photographic process was described a half century
ago by Gurney and Mott (1938) and can be expressed in terms of solid state
quantum theory (Mees 1967~. Basically, the Gurney-Mott theory proposes that
all or a portion of the incident energy of a photon or charged particle is transferred
to one or more valence band electrons in the silver halide crystal, raising them into
the conduction band, where they are free to migrate through the crystal. These
electrons will either recombine with positive holes (i.e., a deficiency of electrons)
within the valence band or will be captured by electron traps (also known as
sensitivity centers) elsewhere within the crystal. Deep electron traps result from
lattice imperfections within the crystal due to structural defects or to the inclusion
of certain impurities such as ions with a greater net positive charge than the silver.
Once captured, electrons in these traps have little chance of escape.
The negatively charged electrons are attracted to the positively charged traps.
As electrons accumulate in traps, a region of slight negative charge is produced,
which serves to attract a small mobile fraction of the interstitial silver ions,
reducing them to metallic silver according to the relationship Ag+ + e- = Ago.
The reduced silver atoms constitute the latent image which serves as the focal
point for the development process. Only a few of the very large number of silver
atoms in a single grain of AgBr are directly reduced to atomic silver by the
. . .
radiation exposure.
D. THE DEVELOPMENT PROCESS
Film development is a multi-stage process that may be thought of as a chemical
amplification process. In a darkroom, the film is removed from its wrappings and
dipped into a solution containing a reducing agent such as methyl p-amino phenol
sulfate, hydroxyquinone, 1-phenyl, 3-pyrazolidone, or other pare-substituted ben-
zene derivatives, which reduces the silver halide in the emulsion to metallic silver.
The developer also contains alkali buffers to maintain constant pH (because the
rate of development is pH-dependent) and sulfites to retard oxidation by air. The
development process occurs very rapidly in those grains in which there is a latent
image, being initiated at the point of the latent image. These grains are fully
OCR for page 15
2 BASIC PRINCIPLES
15
developed long before the unexposed grains i.e., those with no latent image.
The film is thus held in the developing solution only long enough to develop those
grains in which a latent image has been formed, typically on the order of 3-5
minutes.
The degree of blackening or response of a film is dependent upon the fraction
of grains in a film that is developed, which in turn is dependent upon the number
of grains In which a suitable latent image has been formed. A minimum of about
four silver atoms is required to render a grain developable, which is equivalent to
an energy deposition of about 10 electron volts (eV). The number of silver ions
reduced to metallic silver in the development process is on the order of 10~2 times
greater than that in the latent image.
The development process is a chemical reaction and as such is affected by the
amount of reducing agent present. The developer needs to be replenished or
replaced from time to time, as the reducing agent is consumed by the development
process or is oxidized by dissolved oxygen or by contact with the air. As is true of
most chemical reactions, the reaction rate is temperature~ependent, and develop
ment is normally carried out at a constant controlled temperature of 68 i 0.5°F (20
+ 0.3°C). To ensure continued contact of the film with fresh developer, the
developer is agitated mechanically during the development process. This can be
done by stirring or by bubbling an inert gas such as nitrogen through the developer
solution.
After chemical development, the film is washed in water or in a suitable
chemical "stop bath", such as a weak solution of acetic acid, which serves to halt
the action of the developer by physically removing the residual developer from
the film or by lowering the pH. This stage is brief, usually lasting only a minute or
so. The film is then transferred to a chemical bath containing sodium thiosulfate,
sodium metabisulfite, or similar materials which dissolve the undeveloped silver
halide grains, leaving behind the developed grains. This is the fixing procedure,
and typically requires 15-20 minutes for completion. After final washing and
drying, the film is ready for readout and interpretation. The final washing is
usually carried out for an hour in running water, perhaps containing a wetting
agent, to ensure complete removal of chemical residues. The wetting agent helps
prevent the occurrence of water marks which may affect subsequent optical
density measurement.
E. DENSITOMETRY
Transmission of light through the developed film is largely a function of the
amount of elemental silver remaining on the developed film base. The process by
which transmission of light through the developed film is measured is known as
densitome~y (or, alternatively, sensitometry) and is accomplished with a device
OCR for page 16
16
FILM BADGE DOSIMETRY mr ATMOSPHERIC NUCLEAR TESTS
known as a densitometer. Light transmission is measured in terms of the optical
density (OD) which is defined as the logarithm of the intensity of the light
incident on the film (Io) divided by the intensity of the light passing through the
film (I), or
0D = log(IO /1).
2-1
The light absorption attributable to background fog ~bka ), determined from meas-
urement of control films processed simultaneously with the exposed group, is
subtracted from the OD to obtain the net optical density MODS. Thus,
NOD = log(IO /I) - log(IO Ike ~
= logging in.
2-2
From Equation 2-2 it is clear that only the optical density of the control film
and the exposed film need be measured. In actual practice, only a single measure-
ment is required, as many densitometers are equipped with a potentiometric
adjustment to zero out the contribution from background.
F. RESPONSE CHARACTERISTICS OF FILM
The optical density of an exposed film is usually plotted as a semilogarithmic
function of the radiation exposure and is characterized by a curve of the form
shown in Figure 2-3. This characteristic response curve is known as a Hurter and
Driffield (H and D) curve, and has five distinct identifiable regions, but with no
shark boundaries.
Region I is the toe of the curve in which the density does not increase
appreciably with exposure; this so-called base density and background fog define
the lower limit of detectability of the film. In Region II, the response as deter-
mined by the OD is approximately proportional to exposure, and film becomes
useful for dosimetry. In Region III the film response is proportional to the
logarithm of the exposure; hence this region is most useful for dosimetry. Region
IV is the shoulder of the curve, and the film response or increase in density per
unit exposure declines with increasing exposure until some maximum OD value
is reached. The final portion of the curve, Region V, shows a decline in density
with increasing dose. This is the region of reversal, technically known as solari-
zation, a phenomenon attributable to a reduction in the number of sensitivity
centers in the AgBr caused by the escape of bromine from the surface of the AgBr
grains. For any given film emulsion, the onset of solarization is controlled by a
complex combination of many factors, including the exposure rate, development
conditions, and the energy and type of the exposing radiation. However, in
OCR for page 17
2 BASIC PRINCIPLES
CD
Ad
o
I ~
17
IV
1 11 1 ~
/
1~
-
LOG EXPOSURE
FIGURE 2-3 Charactensiic Response Culve (H & D) for a Photographic Emulsion Exposed to
Ionizing Radiation.
personnel monitoring films, solarization does not occur except at doses well
beyond the defined usable range of the film.
In general, film response depends on the total exposure (Ehrlich 1956; Herz
1969~. In other words, the response of a film to a given exposure level is
independent of the rate of exposure. However, at extremely high exposure rates
(10~° R/s), a diminution in the response per unit exposure i.e., a reduction in the
sensitivity of the film- has been observed (Dudley 19669. This is known as the
Schwartzchild effect, or reciprocity failure.
The response or degree of blackening per unit exposure is a measure of the
sensitivity of the film and is analogous to film speed as used in the context of
photography. More rigorously, film sensitivity is defined as the reciprocal of the
dose required to produce a specified NOD. For photographic emulsions used for
personnel dosimetry in the normally expected occupational exposure range, a
typical film sensitivity is 0.5 NOD units per 400 mR exposure. For films with this
sensitivity, the lower limit of detection is about 10-20 mR for photon energies
above a few hundred keV.
This type of film sensitivity is determined by a number of factors, including the
energy and type of exposing radiation, inclusion of impurities or sensitizers in the
emulsion, the development process, quantity of silver halide in the emulsion, and
OCR for page 18
18
FILM BADGE DOSIMEIRY17V ATMOSPHERIC NUCLEAR TESTS
grain size and density. In general, the greater the grain density (i.e., the number of
grains per unit area), the greater the sensitivity. Similarly, sensitivity is a function
of grain size; as only about four reduced silver atoms in a grain will result in
development of the entire grain, the larger the grain, the greater the sensitivity.
G. ENERGY DEPENDENCE AND FILM BADGE DESIGN
Because the atomic numbers (Z) of both silver (Z = 48) and bromine (Z = 35),
which constitute the sensitive portion of the film, are significantly greater than the
atoms in air or soft tissue, film sensitivity to photons relative to that of air (Z =
7.78) and tissue (Z = 7.64) is strongly energy dependent. This follows because the
probability of photoelectric interactions (and hence energy absorption) is a func-
tion of both photon energy and the atomic number of the absorbing medium.
Simply stated, the response of film relative to the dose received by tissue is not
constant, but rather varies with photon energy. In Figure 2~, the energy depend-
ence relative to exposure in air is shown; this is similar to the soft tissue response
curve. In other words, the sensitivity of the film is highly dependent on the energy
of the exposing photons. The effect is most pronounced in the photon energy
region below a few hundred kilovolts, peaking as shown in Figure 2~.
A reasonable solution to the problem of photon energy dependence is to use
filters to obtain a response for the film that is reasonably independent of photon
energy and approximates that of soft tissue. A photon filter is simply an appropri-
ate thickness of a suitable material (usually a metallic element) placed over the
film to selectively absorb a greater proportion of the lower-energy photons and
thus compensate for the over-response at these energies. No single filter will
provide a perfectly flat response, and typically several filters are used.
Reasonably good results for both beta and photon radiations can be obtained
with a film badge having three filters a high-,, a medium-, and a low-,-in
addition to an unshielded or"open window" portion. The low-, filter is selected
to absorb all or most of the beta radiation, but a minimal amount of photons. A
low-, material such as polyethylene or other plastic with an a real density of 1 g-
cm~2 is sufficient to attenuate beta particles with energies < 2 MeV, and has little
effect on photon transmission. Thus, the photon response under the low-, shield
and on the unshielded portion of the film will be essentially the same. However,
only the open window portion will be affected by the beta radiation. Hence, by
subtracting the response under the low-, portion from that of the open window
portion, the response attributable to beta radiation will be obtained, and the beta
dose can be evaluated. The NOD under each filter must be converted to a
common calibration exposure before subtraction to assure linear relations among
the values.
OCR for page 19
2 BASIC PRINCIPLES
LL
cn
o
x
LL
Or:
CD
Ad
100
10
~ 1.0
c:
o
>
_
\
-
10 100 1000
PHOTON ENERGY, keV
FIGURE 2-4 Energy Dependence Curve for Unshielded Personnel Monitonng Film.
19
-
The measured and converted NOD values under each of the three hllters can be
used to determine the dose from photons over a wide energy range. If the filters
are judiciously selected, the combination of responses under the three filters will
uniquely correspond to an effective energy and thus the sensitivity of the film to
the unknown exposing spectrum can be dete~,nined and the appropriate exposure/
density relationship obtained. This may be done by computerized techniques or
manually.
On a practical level, the high-, filter is selected to provide an essentially flat
response over the widest possible energy range. An appropriate thickness, e.g.,
0.5 mm (0.020 inch) of tantalum (Z = 83), will provide an essentially flat or
constant sensitivity to photons with energies in the range of approximately 50 keV
to about 2 Mev (Figure 2-5), and if the exposure is wholly due to photons in this
energy region, only a single NOD is needed to dete~n~ine the dose. Similar results
can be obtained with other high-, materials. If the exposure includes photons
OCR for page 20
20
1.4
1.3
1.2
LL 1.1 _
oh
z ~ 1.0 _
~ 45: 0.9 _
" 0.8 _
~ LL 0.7 _
co ~
Ox ~ 0.6 _
~ ~ 0.5 _
111 Z
> ~ 0 4 ~
5 A 0.3 _
A:
0.2 _
0.1
FILM BADGE DOSIMETRY IV ATMOSPHERIC NUCLEAR TESTS
~ , ,_
l
l
O
r
f
1o1 1o2 103 104
PHOTON ENERGY - keV
FIGURE 2-5 FjLrn Response With 0.020-Inch Tantalum Filter (adapted from Brady and Iverson, 1968).
below the effective energy cutoff range of the high-, filter (e.g., 50 keV in the
case of the tantalum filter mentioned), the NOD values under the other two filters
will be greater than the NOD under the tantalum, and the interpretation of the low-
energy component must be made using the densities under the other filters.
At high photon energies, dose interpretation is complicated by the lack of
charged particle equilibrium. Exposure to photons with quantum energies above
2 MeV may result in a situation in which the density under the filters is greater
than the density in the open window area, with the greatest density occurring
under the high-, filter. Additional filters may be required to facilitate interpreta-
tion of doses in mixed radiation fields involving high-energy photons. Note that
there is no theoretical limit on the number of filters that can be used; in fact, the
greater the number and sophistication of filters, the more quantitative the evalu-
ation (Storm and Shlaer 1965~.
H. OTHER SOURCES OF ERROR IN FILM BADGE DOSIMETRY
Although the intrinsic accuracy of personnel dosimetry films to suitable refer-
ence levels of radiation is quite good (Brodsky 1963; Brodsky and Kathren 1963;
OCR for page 21
2 BASIC PRINCIPLES
21
Brodsky et al. 1965; Herz 1969), films are subject to a variety of influences which
may adversely affect subsequent dose interpretation. Because the planar geome-
try of film and badge-f~lter combination cause angular dependence, the angle of
incidence of the exposing radiation will affect the response. Photons or beta
particles incident at oblique angles will pass through a proportionately larger
thickness of overlying filter. This produces a variable response, an effect particu-
larly pronounced for the lower-energy photons and beta particles (Ehrlich 1954,
1962; Heard et al. 1960~.
Environmental conditions may affect film response in a variety of ways.
Numerous studies have documented the complex effects of temperature and
humidity on personnel dosimetry films and have been summarized in the litera-
ture (Becker 1966, 1973; Kathren 1987~. The numerous and varied effects noted
also may be time dependent and reversal of the effect may occur with time. Latent
image fading will result from high humidity, but condensation of water on the
film emulsion may cause fogging. Heat-induced fogging may occur, and is most
pronounced in the relative humidity range 40-60%. Chemicals such as mercury
or sulfur present in the atmosphere can act as either sensitizers or inhibitors of the
photographic response. Protective packaging in polyethylene or other hermeti-
cally sealed pouches has been recommended to minimize or obviate effects
induced by humidity or chemicals (Kathren et al. 1966~.
Static charge will produce characteristic discharge "trees" on the developed
film. These are usually insufficient to interfere with sensitometry and dose
interpretation. Pressure may result in increased density, as may exposure of the
film to light. Light-struck films are characterized by areas of high density at the
points of light exposure. These latter effects are readily recognizable to the
experienced observer, although they may produce spurious results in automated
readout systems.
I. CALIBRATION AND STANDARDIZATION
Film calibration procedure involves the exposure of a number of film badges to
suitable levels of reference radiation. For a typical sensitive personnel dosimeter
film, ten to fifteen points over an exposure range of three to four decades is
adequate. It is important to determine the specific energy and angular dependence
characteristics of He particular film and film badge-filter combination. Sources
providing specific photon energies and spectra suitable for calibration have been
describedintheliterature(IAEA 1971;ISO 1983;Kathrenet al. 1965~. Because
these characteristics are constant, it usually is unnecessary to repeat the determi-
nation unless the film or film badge-filter combination has been altered. Once the
specific energy and directional dependence have been determined, it is possible to
obtain adequate calibration with a single or a few specific calibrated sources; a
OCR for page 22
22
FILM BADGE DOSIMFIRY IN ATMOSPlIERIC NUCLEAR TESTS
high-energy photon source, such as Cs 137, is well suited to this purpose (IAEA
1971).
Suitable film badge calibrations can be obtained by exposure in free air,
without a backing phantom, and this is the traditional calibration procedure. In
some instances, use of a phantom may be necessary to determine the backscatter
contribution (Figure 2-5) (IAEA 1970~. Calibrations are specific for each unique
combination of source, badge, and geometry conditions. In all cases, the source
output at the specific locations at which the calibration is performed should be
determined and should be relatable to one at the National Institute of Standards
and Technology, or similar recognized primary standards laboratory.
Calibration films and controls should be developed along with each processing
batch as a quality-control measure and to compensate for variations associated
with the processing. Slight changes in the temperature and strength of processing
solutions or temporal factors may introduce a shift in the dose response curve
which will be detectable by calibration films processed with each batch of do-
simeters. The number of calibration films developed with each batch will depend
on the specific dosimetry operation. Usually, a few percent of the processing
batch should be unexposed controls to establish the background fog level for that
particular processing batch; similarly, each batch should contain one or more
films exposed to a predetermined level in the usable portion of the H and D curve
(e.g., 100 mR to 1 R referenced to air for a typical personnel dosimetry film).
Although fUm manufacture is well controlled, variations in response and
background fog may occur from batch to batch, necessitating individual calibra-
tion of each manufacturing batch. Energy and directional dependence should
remain constant from batch to batch, unless there have been changes in the
composition or geometry of the emulsion or film base. An American Standards
Association report (ASA 1956) gives procedures for evaluating films for monitor-
ing x rays and gamma rays with energies up to 2 MeV.
J. NEUTRON DOSI1VIETRY
Photographic emulsions also have been applied to personnel dosimetry of both
thermal and fast neutrons, although they were not often used for this purpose
during aunospheric testing. Thermal neutrons may be measured with the aid of a
filter made from a material with a high thermal-neutron capture cross-section,
such as cadmium or rhodium. When exposed to neutrons, these elements will be
activated, and the film will be exposed from both the beta and gamma rays
produced in the reaction or by the activated matenal. The NOD attributable to the
neutron activation is determined by subtracting the NOD produced by photon
radiation. This is accomplished by use of a filter with a very small thermal
neutron cross-section but with similar photon absorption properties. Two ele
OCR for page 23
2 BASIC PRINCIPLES
23
meets win nearly equal atomic numbers are suitable; tin and cadmium or tin and
rhodium have been used successfully (Kocher et al. 1963~. Again, subtraction
must be after conversion of NOD to a calibration exposure.
Thick emulsions so-called nuclear track emulsions-are used for fast neu-
tron dosimetry. Such emulsions are 100 to several hundred micrometers in
thickness. The most common track inducing process is from proton recoils
produced by the (n,p) reaction in the emulsion, film base, and low-, material
(e.g., paper wrappings) around the film (Cheka 1954~. There is also the poten-
tially significant 14N(n,p)14C reaction with thermal neutrons (Lehman 1961~.
Quantification is accomplished by direct counting of proton recoil tracks.
Nuclear emulsions have a fairly limited dynamic range and are subject to large
errors from statistical uncertainties associated with counting. Different persons
counting tracks on the same film will come up with widely divergent results.
Tracks may be lost through latent image fading, which is more pronounced In
nuclear-track emulsions, and may be obscured by concomitant exposure to pho-
tons which produce a general darkening of the film. Nuclear-track emulsions are
also sensitive to all the environmental effects associated with films used for beta
and photon monitoring.
Representative terms from entire chapter:
latent image
