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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
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
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)
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.
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
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
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
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
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.
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
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;
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
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
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.
