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4 Use of Film Badges in Atmospheric Nuclear Testing A. FISSION AND ACTIVATION-PRODUCT MONITORING WITH FILM BADGES Radiation produced by fission and activation products contain mixtures of beta particles, gamma rays, and x rays. The relative proportion and energies of these radiations will change with time and location. Such changes pose special prom lems for film badge dosimetry. These problems include: The need to compensate for rapidly changing sensitivity of film to photons with energies less than 100 keV (see Section 2.G). The selection of an appropriate calibration source, representing field expo sure conditions, with which to produce the characteristic response curve relating density and exposure. · The need to distinguish beta from photon exposures. The prime radiological concern is exposure from photons with energies ranging from several hundred keV to a few MeV. These photons are the most significant radiation emitted by fission and activation products because of their abundance per disintegration range in air, and their ability to irradiate the deeper radiosensi- tive organs of the body. As described in Section 2.G, the response of film per roentgen of exposure to these energies of photons is relatively uniform. This allows the same characteris- tic response curve to be used over a wide range of photon energies and also allows any one of several radionuclides that emit photons in the higher portion of this 35 .
36 FILM BADGE DOSlMETRY IN ATMOSPHERIC NUCLEAR TESTS energy range to be used as a calibration source. These features allow film badges to be used to reliably monitor the most important radiations contributing to exposure from weapons-test-related photon fields. The accuracy of monitoring exposure from photons with film badges is ad- versely affected by the presence of photons with energies less than 250 keV. These lower-energy photons cause a disproportionate amount of film darkening relative to their contribution to exposure. A lead filter, covering part of the film, was generally used during the atmospheric testing period to minimize the effect of the lower-energy photons. Use of filters to flatten the energy response of film was discussed in Section 2.G. When exposure was assessed from the optical density of the film underneath the filter, the same characteristic curve developed for high energies could be used for mixtures of low- and high-energy photons encountered by test participants. A 0.020-inch-thick lead filter was used during operations CROSSROADS through IVY. This was not totally effective in correcting the over-response caused by photons of lower energy (Storm and Bemis 1950; Storm 1951~. As a result of research performed at the National Bureau of Standards (NBS), begin- ning with the TUMBLER-SNAPPER operation and continuing throughout the atmospheric testing program, a 0.028-inch-thick lead filter was used. With this filter, the response to photons above 120 keV varied by 6%. The maximum sensitivity of this film badge occurred at 70 keV and was only 20% higher than the response at 1 MeV (AEC 1952~. Because the experts in film monitoring at that time believed that the predominant energy of the troublesome low-energy photons was 100 keV, the 0.028-inch-thick lead filter was felt to be the most appropriate. Small changes in lead thickness can alter the film badge response to low- energy photons. A 10% change in the thickness of the 0.028-inch-lead filter caused a 20% change in the response to 120 and 70 keV photons (Servis 1954~. This variability was considered acceptable. Variations in lead thickness had little influence on film response at higher energies. Because the abundance of low- energy photons was small and variable, the true effect of changing lead thick- nesses should be negligible in the presence of all the other factors known to influence film response. As implied above, determination of the characteristic curve of exposure versus film density underneath the photon filter can be accomplished with any radionu- clide that emits high-energy photons. Radium 226 in equilibrium with its daugh- ters and cobalt 60 were used during the nuclear testing period. Radium 226 (1600-year half-life) and its daughters emit photons of many energies and best approximate the primary distribution of energies that led to exposures of test participants. Radium 226 was a principal standard in radiation measurements and was a useful laboratory source.
4 FILM BADGES IN ATMOSPHERIC TESTING 37 Another calibration source was cobalt 60 (5.3-year half-life) which emits two high-energy photons that represent higher photon energies in the test environ- ment. A disadvantage is the relatively short half-life that limits the useful time for using the source to several years. The effects of low-energy photons and beta particles present in the radiological field are not properly addressed by radium or cobalt sources used in the labora- tory. The film response to unfiltered radiation cannot be evaluated because the composition of the radiation causing the density is unknown. For the open- window portion of the film it is not possible to duplicate field radiation conditions in the laboratory. The film in the open window area responds to all radiations penetrating the wrapper and any other overlying material. When low-energy photons are present, beta-particle exposures cannot be assessed because the increased sensitivity of film to low-energy photons masks response to beta par- ticles (see Section 4.B for further discussion). More than one film emulsion is normally required to measure the range of exposures sometimes encountered in atmospheric testing. Early test operations employed Kodak Type K film to measure lower exposures and Kodak Type A to measure exposures of several roentgens or more. Later operations used Du Pont Type 502 or 508 film for lower exposures and Du Pont Types 606, 1290, or 834 for higher ranges. All emulsions had similar energy-response curves, with the maximum sensitivity occurring for photons of about 40 to 50 keV (Storm 1951; Storm and Bemis 1950; Storm and Shlaer 1965~. The shape of the characteristic curve was similar for all of the emulsions. The Du Pont emulsions exhibited an effect in which the slope of the curse depended on the ionization density of the radiation. The slope of the curve for photons decreased with decreasing energy at optical densities less than 2.0. No effect was observed for densities greater than 2.0. Neither Kodak emulsion demonstrated this phenomenon (Golden and Tochilin 1959~. For weapons testing dosimetry this effect is not likely to be of any consequence. The use of the optical density under the filter assumes that a single characteris- tic curve is applicable for all energies. For the Du Pont emulsions, the exposure from low-energy photons evaluated with a characteristic curve for cobalt 60 could be underestimated by 10% to 20%. Because the 0.028 inch lead filter was not totally effective in reducing the over-response to these energies, the effect of an energy-dependent characteristic curve appeared minimal as no compensating corrections were proposed. B. BETA PARTICLE MONITORING Personnel film badge dosimeters were used for beta radiation monitoring during underground nuclear testing operations at the NTS from 1966 until 1987
38 FILM BADGE DOSIMETRY IN ATMOSPHERIC NUCLEAR TESTS (Brady and Iverson 1968~. Film badges used for monitoring beta radiation at NTS and other locations where mixtures of beta and photon radiations were encoun- tered, had at least three unfiltered and filtered film packet areas. As discussed in Section 2.B, unfettered film responds to a given exposure from low-energy pho- tons by more than 20 times the response to the same exposure from high-energy photons (Hine and Brownell 1956~. For example, the NOD of films exposed to one roentgen of 40 keV x rays will be more than 20 times the NOD of films exposed to one roentgen of cobalt 60, an emitter of high-energy photons. The unfiltered or unshielded area of a film packet is referred to as the "open area" or "open window." As also discussed in Section 2.B, a metallic filter with a high atomic number is used to provide a relatively uniform film response under this filter to photons from low to high energies. Thus, even if exposure is to low-energy photons, the amount of darkening under an optimum photon filter is not greatly different from the darkening which will result after the same amount of exposure to high-energy photons. A third film area employs a filter either to attenuate beta radiation preferen- tially or to provide a different photon response. By discriminating against beta radiation, the photon contribution to the open area NOD can be determined. Photon-energy information can be obtained with a second photon filter response as a ratio with the primary photon filter response plotted against effective photon energy. Both methods can be used to subtract the NOD caused by photons in the open area. The subtraction must be performed, however, in terms of exposure, not NOD, because the function of NOD versus exposure is not linear, i.e., an increment of NOD represents a different amount of exposure at different locations on a calibra- tion curve. After subtraction, the remaining open-area NOD can be used to evaluate beta dose, provided that qualification is made and uncertainties provided regarding the film response variations at different beta-particle energies. The response of Du Pont 502 double-coated emulsion in a paper wrapper (a typical low-range film component used during atmospheric testing) changes for maximum beta-particle energies between 0.5 and 3 MeV by almost a factor of ten (Hine and Brownell 1956~. The energy distribution of beta particles from fission products changes with time. Uncertainties introduced by the film response to different beta-pariicle energies can be large when monitoring fission products with unknown beta-particle energies. Optimum materials for a beta-discriminating filter are those with a suitably high mass density to maximize the attenuation of beta particles and a low atomic number to minimize photon attenuation (Brady and Iverson 1968; NAS 1986~. One of the earliest beta-discriminating filters used was aluminum (atomic number 13 and density 2.7 g-cm-3~. For comparison, the most recent NTS film dosimeter
4 FILM BADGES IN ATMOSPHERIC TESTING 39 utilized Teflon (1~) (effective atomic number 8, density 2.15 g-cm-3). The most recent beta-discnminating filter developed is tetraboron carbide (effective atomic number 5.45, density 2.5 g-cm-3) (NAS 1986). The filter system used in most atmospheric testing operations included a lead (atomic number X2, density 11.34 g-cm~3) filter and open areas (wrapped with paper and plastic). Only very high energy beta particles could penetrate the lead fluter. As a result, contribution of beta particles to the NOD under the lead filter was small, and had little effect on the evaluation of photon exposures. NODs in open areas, however, were affected by high-energy photons, low-energy photons to a much greater degree, and beta particles, to an extent dependent on beta- particle energy. When a film badge with only a lead filter and an open area is exposed to unknown mixtures of beta and photon energies, it is not possible to determine contributions from each component to NOD in the film open area. At one extreme, an excess NOD in the open window area may be the result of only photons. At the other extreme, it may be the result of only beta radiation. The fast attempt to monitor beta exposures with personnel film badges during atmospheric nuclear testing was at Operation CROSSROADS in 1946 at Bikini Atoll in the Pacific. Double emulsion Eastman Kodak Type K film was used with a 0.02~inch-thick lead cross on one side of the packet; the tips of each cross leg bent around the four edges of the packet about 0.25 inch (see Figure 4-1~. All of the NOD in the open areas (the four corners of the packet) was assumed to be caused by beta radiation exposure. This assumption did not allow for exposure to high and, particularly, low-energy photons contributing to the NOD in the open areas. It is likely that the NOD of some films attributed to beta exposure was in fact caused entirely by photon exposure. For these reasons, beta exposure results determined with fUm badges at Operation CROSSROADS are unreliable. The next test operation with reported beta exposures was RANGER which took place at NTS during January and February 1951. The film badge used was a Los Alamos badge with brass and cadmium filters. Both the brass and cadmium filters were 0.020 inches thick. Ratios of the responses under these filters were used to determine photon energies and photon-caused NOD in the open area. The same film badge design was used in the BUSTER-JANGLE test operation during October and November of 1951 in Nevada, but beta dosimetry was not attempted. Communication with the person responsible for dosimetry at Los Alamos and at Nevada during this time period established that the methodology used to deter- mine beta exposure with the brass-cadmium badge was successful with laboratory calibration sources, but was not successful in the field (Littlejohn l98Sa). Operation WIGWAM was a single nuclear detonation deep in the Pacific Ocean about 500 miles from San Diego, California, and contamination which
40 FILM BADGE DOSIMETRY IN ATMOSPlIERIC NUCLEAR TESTS reached the surface rapidly dispersed. The WIGWAM radiation safety report stated that a cadmium filter and a vinyl filter "intended to facilitate the measure- ment of beta radiation" were used (Baietti 1957~. There is no evidence in the records that beta dosimetry was performed during WIGWAM. The final attempt to evaluate and report beta exposure with film badges during atmospheric testing was at Camp Desert Rock, outside NTS, during Operation PLUMBBOB in 1957. The U.S. Army Lexington Bluegrass Signal Depot pro- vided film badges which were processed at Desert Rock by Signal Corps person- nel. Most military personnel entering NTS in convoy for maneuvers during tests wore these badges. Other military personnel wore He standard NTS film badge with a lead filter. This badge had four filter areas: lead-tin laminate, open window, copper, and aluminum. This combination was thought to be capable of providing beta expo sure, but the analytical procedures used were faulty. The NOD measurements were improperly incorporated into certain equations, when converted exposure data should have been used instead. As stated previously, the function of NOD versus exposure is not linear, and NODs from a film must be converted to exposure with a common calibration curve because an increment of NOD can represent a different amount of exposure at different locations on a calibration curve. Each of the film badge types used to monitor beta dose at the three test operations discussed could have been used to adequately monitor exposure to Black cover papers (2) and paper wrapping hi. ~/~ 0.02-inch-lead cross (legs bent over 1/4 inch on back side) . . \ a/////////////// ~ ~ '$ $ ,~ ~ Double emulsion Kodak Type K film FIGURE 4-1 Film Badge Used in Operation CROSSROADS First attempt to monitor beta exposure).
4 FILM BADGES IN ATMOSPHERIC TESTING 41 photons. Use of these badges to monitor beta dose, however, was unsuccessful. Either the firm badge used did not have the capabilities for monitoring beta dose, or procedures used for evaluating beta exposures were incorrect. Thus, beta- particle monitoring with personnel film badges was not successful during atmos- pheric nuclear testing series. / C. CALIBRATION The response of a fUm badge emulsion to ionizing radiation is measured by the darkening of the film that results after chemical processing (development) of the exposed firm. This darkening is sensitive to the specific batch of emulsion from which the films were prepared by the manufacturer, conditions and length of storage before use, and conditions during the development process. To minimize uncertainties from all of these contributing factors during the nuclear test series, calibrations of films were made using gamma-ray sources, usually radium 226 in equilibrium with daughters, or cobalt 60, to establish the NOD versus log- exposure relationship for a f~lm-development combination. Either of two calibra- tion procedures was used: the gamma source was used to expose a number of different films simultaneously at different well-defined distances from the source for a well defined single time, or at a number of individual films for a well defined single distance for a set of well-defined times. Using the inverse-square-law dependence of gamma-ray intensity on distance from a physically small source, and a knowledge of source strength (relatable to an NBS calibration), the exposures of the calibration films were calculated. The NOD's of films thus exposed were measured after development and plotted as a function of log-e~osure to produce a continuous calibration curve. Comparison of film darkening for a film badge exposed while worn in the field with this curve enabled the unknown film to be assigned a value indicating its exposure. In most of the test series, one or a few films that had been exposed as calibration films to a radioactive source in a standard way ~ ~ ocessed with each batch of films from the field. This provided an additional internal check on the reproducibility of the chemical processing. It was the practice during some early test series to calibrate each new batch of film from the manufacturer and to use the calibration thus derived to interpret all field-exposed films from that batch. These calibrations were earned out only every other day and resulted in some loss of accuracy in the calibration. This was not severe if processing conditions were carefully controlled. D. FILM BADGE RANGE AND THE PROBLEM OF OVERLAP Films of the types used for personnel dosimetry during He atmospheric tests had limited exposure ranges over which their responses changed in a useful way.
42 FILM BADGE DOSIMETRY IN ATMOSPlIERIC NUCLEAR TESTS From the least exposure at which a reliably measurable NOD is produced to the highest measurable exposure is a factor of only a few hundred. Furthermore, the change in NOD per unit exposure, and hence the accuracy of dose determination, is much greater in the middle of the range than at either end. For different types of films, the most useful middle portion of the exposure range occurs at different exposures (see Section 2.F). The film badge used at CROSSROADS had a Type K film component for which the useful exposure range was small, only from 0.04 to 2 R with the densitometer used. One way of extending the useful measurement range of a film badge is to include more than one type of film in the packet. After CROSSROADS, multiple films were used in all film badges. The choice of films had an important impact on the accuracy of the exposure determination in the exposure regions where responses overlapped. Dunng the test series in 1951, a Du Pont 553 packet containing Type 502 (0.02-10 R), Type 510 (5-50 R) and Type 606 (10-300 R) components was used. This combination was adequate to determine exposures from 0.02 R to as much as 300 R for the photon energy spectra encountered in the tests. During 1952, however, the Du Pont 558 packet with Type 508 and Type 1290 components was used. Figure 4-2 shows typical calibration curves for the upper range of the 508 component and the lower range of the 1290 component. Calibra- tion data show that the useful upper limit of the Type 508 exposure range was 10 R. and there is little change in the NOD from 10 to 20 R. Similarly, the NOD of the Type 1290 component changes very little in the exposure range between 10 and 20 R. For this combination of film components there is inadequate overlap in the 10-20 R range, because the NOD changes are small and the calibration curves are relatively flat. The Du Pont 559 packet with Type 502 low-range (0.02-10 R) and Type 606 high-range (10-300 R) components was used in each test series from 1953 until 1958. Figure 4-3 shows that this packet can achieve better results in the 10-20 R range than the 558 packet used in 1952, because NOD changes are greater and the curves accordingly are steeper. For test operations from 1958 through the end of atmospheric tests in 1962, a modified Du Pont 559 packet (later called a 556 packet) with Type 502 and Type 834 components was used. Figure 44 shows the overlap region of this packet and illustrates that the exposure uncertainty in the overlap region was also reduced considerably compared to Figure 4-2. E. EFFECTS OF SOLARIZATION ON FILM BADGE MONITORING Solanzation is the reduction of film OD with increasing exposures. As related to film badge dosimetry, reduction (known as reversal) of OD may occur when a
4 FILM BADGES IN ATMOSPHERIC TESTING o ._ In 0 lo - CO z 2 o z 1 10 EXPOSURE (Roentgens) FIGURE 4-2 Overlap of Types 508 and 1290 Film Canpanents. 43 Upper Limit ~3000 R No GO - z 1 - ~po~ ., ~ ' 100 1 In cat _ O z LU o of film component is exposed well beyond its useful or saturation range (see Section 2.E). Definitive research in this area of film dosimetry has been done by Ehrlich and McLaughlin (1961). A typical low-range bum component used for film badging during atmospheric test series was the Du Pont Type 502, which had a maximum useful exposure range of about 10 R. At exposures to ionizing radiation between 100 and 300 R. the 502 film characteristic curve of NOD versus log-exposure reached its peak and descended, under certain exposure rate and film-development conditions. This reversal of NOD with increasing exposure could have caused serious underestimates of exposure to the wearer of a reversed film component were it not for other compensating factors. First, the film badge, a passive integrating device, was used to determine an exposure of record, and could not serve as an indication of how long a person should stay in a radiation area or how much exposure was being accumulated before leaving. Radiation monitoring instruments were used to determine exposure rates and to estimate how long to remain in a radiation area. Self-reading pocket dosimeters were used to approximate how much exposure was being received while in a radiation area. Because film badge results were not available until after an individual left a radiation area, film badges were not used
44 - o ~ · - v LL At lo ~ 2 In Z LL ' ~ 1 o A FILM BADGE DOSIMETRY IN ATMOSPHERIC NUCLEAR TESTS / / o 10-2 10-1 EXPOSURE (Roentgens) FIGURE 4-3 Overlap of Types 502 and 606 Film Components. - o ._ 0.5 E lo 0.1 cr. of IL o of 10 100 to control time spent in a radiation area, i.e., to control exposure being received during that time. Secondly, at least one additional higher range film component was included with the Type 502 in the fUm packet during each test series when the Type 502 was used. The additional film usually had a useful exposure range that began at about the maximum exposure measurable with the more sensitive film component (see Section 4.D). If the Type-502 component indicated an exposure approaching its limit of 10 R. then exposure evaluation was performed with results from the high-range film. As previously stated, reversal of the Type 502 begins at 100 to 300 R. but reversal to a density indicating 10 R or less would require an exposure of more than 600 R. An acute personnel exposure of this magnitude is considered lethal, and radiation-sickness symptoms would be obvious if a person received such an exposure over a few days or weeks. When high-range film component exposures of several hundred R are applied to film packets to establish calibration curves, or for testing purposes, developing the low-range film components sometimes shows that reversal has occurred. Film-packet numbers were stamped (embossed) with impression dots on film packets used in most atmospheric test senes. The colored dots were readable on the outside paper wrapping and, because film emulsions are sensitive to pressure,
4 FILM BADGES IN ATMOSPHERIC TESTING 45 the dots of developed films were usually much darker than the remaining fUm areas. If a film was very dark, the dots could still be read as numbers because the impression dies caused indentations on one side of the film and raised portions on the other. If a film was exposed beyond its range, the first indication of reversal would be the dots, which had a greater optical density to begin with. Thus, impression dots lighter than the remaining film indicated an exposure between the maximum usable range of the film component and the minimum required for reversal. in, Another useful characteristic of reversal is as an indication and verification of light damage. As discussed in the next section, several types of environmental damage affect film, and knowing the cause or causes of emulsion damage is an aid to evaluating a film. Type 502 film OD does not reverse completely to the density of an unexposed film after cobalt 60 exposures up to 10,000 R or more. Light leaks occur in film packets after damage to the wrapping causes a pinhole or tear. Typical light leaks show dark streaks radiating from the damage point (typically the edge or corner) on the developed films. More extensive light leaks may cause the entire film to be dark, but NOD measurements will show a - .0 00 E LL cat 3 lo In z C) Upper Limits / ~800 R / - 1 1 On ~ AO~O~/ 1 1 1 1 1 1 11 1 1 1 1 1 1 1 1 100 10 EXPOSURE (Roentgens) FIGURE 4-4 Overlap of Types 502 and 834 Film Components. to 1 In co of z LL O ~ o As
46 FILM BADGE DOSIMETRY TV ATMOSPHERIC NUCLEAR TESTS decreasing NOD gradient away from the damaged area. If a light leak is sufficient to darken the entire film, the light exposure may be sufficient to cause a small area of the film adjacent to the leak to reverse. Thus, a small, clear area is an immediate visual indication to the dosimetry technician that a light leak has occurred and that a careful evaluation of density gradient and minimum measur- able density is necessary before any exposure assignment is made or investigation is conducted. In summary, use of portable radiation-detection instruments and self-reading pocket dosimeters can usually avoid film badge exposures in the range that will result in solarization. A high-range film component, used in all atmospheric test series except one, could be evaluated to determine exposures above the range of the low-range component. If personnel exposures of the magnitude necessary to reverse a typical low-range film occurred, acute radiation syndrome symptoms most certainly would have been exhibited by the exposed person. The dosimetry technician was alerted to the approach to or the actual solarization by reversal of impression dots or other usually higher-density film areas. Finally, reversal is an aid to recognizing extensively light-damaged films. F. RADIOLOGICAL AND OPERATIONAL EFFECTS Interpretation of film badges is based on performance of the badges under controlled laboratory conditions, which are chosen to reveal the variations and uncertainties that can occur under various field situations. It is assumed that the film badges used in the laboratory are no different than those used in the field. This way, laboratory experiences can be transferred to evaluation and understand- ing of films used in the field. This assumption must be seriously examined for film badge evaluations made during the weapons testing series. The conditions of radiation exposure of a film badge will be different between the field and the laboratory. This section identi- fies those differences between field and laboratory conditions which may require consideration of additional uncertainties in interpreting exposures of film badges worn in the field. Conversion of the OD of an exposed film to exposure is accomplished by comparison with ODs of films exposed to known amounts of radiation. These latter films provide a calibration of film response to exposure . Different radiation sources were used to deliver the known exposures to calibration films during various test series. References have been made for radium 226 in equilibrium with daughters, for radium-beryllium and and cobalt 60 (see 5.D). Calibrations were performed in air without any deliberate attempt to provide a backscatter contribution (not truly representative of the real situation in which photons are backscattered to a film badge by the wearer's body). Variations in
4 FILM BADGES IN ATMOSPHERIC TOTING 47 calibration conditions occurred among the test series with regard to the control of other sources of scattered radiation (e.g., from nearby walls, bulkheads of ships, etc.~. Films exposed in the field differ from calibrated films in the following ways with regard to exposure conditions: 1. Personnel film badges were worn on a person, not freely suspended in air. 2. Sources described for the weapons tests are better characterized as area or volume sources as compared to the calibration sources which are consid- ered to be point sources. 3. Scattered radiation was present in uncontrolled ways based on the objects that were present to generate the scatter. The body can be effective in scattering radiation into the rear or sides of a film badge worn on a person. This backscatter contribution can range from 10 to 50 percent of the response from unscattered radiation, resulting in more darkening per unit of exposure for film worn on the body compared to film freely suspended in air. Based on calibration techniques used during atmospheric weapons tests, results in terms of exposure to personnel film badges worn by individuals might be overstated. No evidence exists that corrections for backscatter were made. Film badge dosimeters with different filtration on front and back may be affected more by backscatter than those with equal filtration on both faces. Interpretation of film results using calibration data assumes that the radiation is normally incident on the film and has passed through the expected filtration. The film may have a different response to unfiltered radiation passing under the filter and affecting the accuracy of the evaluation. This problem can be very severe when low-energy photons are present. The purpose of filtration is to compensate for film response to such photons and to prevent significant overstatement of exposure. Wide-area distribution of radiation sources and the presence of large objects which create scatter cause a film badge to be exposed from many angles. For the same level of exposure, film badges may exhibit large differences in response for different angles of irradiation. This angular dependence also changes with radia- tion energy, which may vary over space and time. The actual response of films worn in the field is the cumulative response to all the different combinations of exposures and angles at which radiation entered the badge. Under field conditions, the angular distribution of radiation entering the badge is unknown and largely uncontrolled. Angular dependence is one of the most important contributors to uncertainty in the evaluation of dosimeters in the field. Residual radionuclides associated with atmospheric weapons tests generally emit photons with energies from a few hundred keV to a few MeV. At these
48 FILM BADGE DOSIMETRY IN ATMOSPHERIC NUCLEAR TESTS energies, the calibration sources used could have been adequate representations for the field environment. However, the interaction of radiation fields with various objects (e.g., ships and machinery) could alter the photon energy spec- trum. The resulting spectrum could have a larger low-energy component than would be expected based on the original energy spectrum after a nuclear test. Film badge response could be changed, most likely resulting in an overestimate of exposure . Radioactive contamination of a film badge is another problem. Fallout in areas around a detonation could have been directly deposited, resuspended, or other- wise transferred to the surface of a badge. Contaminated badges result in expel sure of the films that are not characteristic of those received by the individual; solarization may occur in the center. Such contamination is rarely deposited. Results of contamination normally appear as spots or blotches of intense black- ness. In addition, the exposure rate close to a speck of contamination changes very rapidly over fractions of an inch. These variations make it possible to identify most contaminated film. If the contamination is localized to a small area of the badge, an estimate of the radiation from other sources could be attempted, using the unaffected regions. This is not advisable if the contamination affected a significant part of the film, as an erroneously high value is likely to result. In such cases it may be advisable to disregard the film data, considering the potential for error. The pressures of developing and analyzing large numbers of films in short time frames can increase the likelihood of human error. Processing was often per- formed at night. Results were needed by morning so that those personnel approaching or exceeding exposure limits could be identified and work reassign- ments implemented. Films were developed in batches. Vanations in darkening of similarly exposed films among processed batches should be expected from differences in solution temperatures, developing time, and developer concentration. Standard operating procedures required that these factors be controlled to minimize these variations. The effect of batch-to-batch developing variations can be minimized if one or more films exposed to a known level are developed in each batch; in effect, this enables each batch to be separately calibrated. This technique was used in most of the later test series. Gross variations between batches could be identified in these earlier tests through the use of unexposed control films; however, unexposed films are not as sensitive as calibrated film for indicating changes during develop- ~ng. Clerical errors can become more frequent under the stress of large-volume processing. Clerical mistakes are unpredictable, isolated, and usually noticed if inconsistent with expectations. The large numbers of films necessitated manual readout by several persons
4 FILM BADGES IN ATMOSPHERIC TESTING 49 using a number of densitometers. This required the densitometers to be intercom- pared to assure that each film would indicate the same density irrespective of which densitometer was used. This was accomplished by cross-calibration of the densitometers with films of different densities to assure appropriate agreement across the range of densities to be measured. The extent to which this procedure was performed is unknown. G. ENVIRONMENTAL EFFECTS IN FIELD USE Extreme or harsh environmental conditions may alter the response or affect the dose interpretation of film dosimeters. A variety of such environments was encountered during atmospheric testing. High temperatures and humidities along with salt water spray characterized the Pacific tests, while very dry, hot, and physically abusive conditions were found in the Nevada desert. The potential effects can be conveniently grouped into five categories: 1. Heat-laduced Fog. Film is susceptible to fogging from exposure to heat and humidity. The film darkening due to such fogging can result in erroneously high estimates of radiation exposure if undetected. Even when detected, subjec- tive judgment is required to assess whether any darkening might be due to radiation. When radiation is suspected, conservatism often results in a higher- than-actual exposure estimate. There appears to be a temperature threshold of about 130°F (50°C) below which fogging does not occur (Kathren et al. 1966~. Several days at this tempera- ture are required to induce measurable density, even in the more sensitive emulsions. At higher temperatures, shorter times are needed as little as several hours at temperatures above 150°F (70°C). Several factors minimized the effects of heat fogging during atmospheric testing. Prior to use, films were stored in controlled environments where the effects of heat or humidity are not a concern. Secondly, the times to which film might be subjected to high heat were restricted by the short wearing intervals and the limits of human physical endurance at the temperatures necessary for rapid fogging. Therefore, it is unlikely that heat fogging is an important source of error, except in an extraordinary situation in which a film was placed near an infrared radiation source such as a hot metal bulkhead or engine. The non-uniform densities across a film and the inconsistent density relations among different emulsions make such cases identifiable. For example, radiation would not be indicated by a slight darkening of the sensitive Type 502 emulsion when accom- panied by a darkened, less-sensitive Type 606. 2. Latent-Image Instability. Latent-image instability refers to the fading of the latent image and the resultant decrease in the expected density of exposed film.
so FILM BADGE DOSIMETRY TV ATMOSPHERIC NUCLEAR TESTS Also dependent on temperature and humidity, fading results in underestimates of radiation dose. High relative humidities have been shown in numerous studies to cause fading, with the greatest effect occurring when humidities approach 100%. Minimal effects are observed at relative humidities below 75% (Kathren et al. 1966~. In the low-humidity desert climate of Nevada, latent-image fading can be eliminated as a contributor to uncertainties in radiation-dose estimates. If unprotected, paper-wrapped films must be subjected to high humidities for one or (more likely) two weeks after exposure before fading becomes measurable. At the Pacific Proving Ground, where high relative humidities were the norm, the short times during which films were worn greatly lessened if not totally elimi- nated humidity-induced fading. Further protection from potential high-humidity effects was realized when firm packets were sealed with wax or in plastic cases. Such efforts could extend the usable wearing interval to 2-3 months (Kathren 1987~. These protective actions also reduced the damaging effects of water dampened film packets which increased film density, a much more prevalent problem than either latent-image fading or heat fogging. For unprotected film badges worn for intervals greater than a week in relative humidities exceeding 70%, some fading can be postulated. The amount of fading depends on the time between radiation exposure and development. The amount of fading exponentially declines with time in reaching a maximum loss of 50% of the expected net optical density after six weeks. This represents an upper boundary to the error in the dose estimate. It is unreasonable to expect this amount of error as all of the radiation exposure would need to have occurred on the first day of use, followed by six weeks of constant high humidity. More realistically, exposures would have occurred at various times during the wearing interval, and the necessary humidity to produce fading would not always exist. Therefore, a suggested correction might be to increase the net optical density by one-third for films with positive readings and with documented potential for fading. This approach would result in an underestimate of 25% for the unrealistic upper boundary condition and an overestimate of about 30% for film that suffered no fading. Another problem related to heat and humidity is the degradation of the film- packet integrity. During Operation REDWING, operation or series badges were initially issued for 4- to 6-week intervals. When unprotected, those badges used for longer periods showed frequent evidence of light leaks and water damage. Failure of adhesives holding the packet together is suspected to have resulted from the prolonged exposure to the weakening effects of heat and humidity. Fortunately, light leaks can be visually detected. 3. Water Damage. Water-damaged films were frequently encountered during the atmospheric tests. Decontamination activities, salt water sea spray, and
4 FILM BADGESIN ATMOSPHERIC TESTING 51 clothing wet with perspiration offered ample opportunities for damaging films. Early efforts to protect the film with plastic pouches were sometimes ineffective because water vapor would condense inside the pouch. Better protection was afforded by coating the badges in ceresin wax or encasing them in sealed plastic cases. The latter technique, while successful in one test series, required a saw to open the case, and this led to light leaks in another test series. Water-damaged film usually can be visually identified. The damaged area appears as an irregularly shaped, unevenly darkened image, sometimes resem- bling a dried water drop. Often having a mottled appearance, the damage can be localized or involve most of the film. When localized, radiation exposure can be estimated by evaluating the undamaged area. Damage to radiation-exposed fUm may not be visually recognized when the exposure results in densities exceeding 2.5 or so. If the range of densities evaluated across the fUm is greater than expected, damage might be indicated. No one limit can be established for the amount of uncertainty or error intro- duced by water damage. Subjectivity is almost always involved in deciding whether to attribute darkening to radiation or to water. Radiological safety reports and film reexaminations suggest that conservative decisions were made which resulted in overestimates of radiation exposure (Cooney 1951~. 4. Exposure to Light. Exposure to visible light manifests itself as an area of intense darkening. Small breaches in the light-tight packaging will produce streaked areas or dark lines, usually radiating outward. Large openings can cause the entire film to become black with some areas possibly exhibiting density reversal from solanzation (Section 4.E). Light-struck films were experienced during many of the tests. Physical abuse was not the only reason for cracks or tears in the film packet. Embossing identifying numbers as dots on packets sometimes resulted in small holes through which light could strike the film. A source of damage in one series was the sawing open of protective plastic cases. The saw blade sometimes would nick the corner of the packet, producing a light leak. The influence of light damage on exposure estimates cannot be predicted. If localized, the damage may have no adverse effect, and the exposure can be determined from an undamaged area. Uncertainty occurs when deciding how much damage can be tolerated before a significant assessment error results. Extensive damage can preclude any meaningful dose assessment. Light-damaged film can mask darkening due to radiation. In those badges containing more than one emulsion, the possibility exists that the emulsions were not equally affected. The less-affected emulsion might have been used to estab- lish boundaries on the amount of radiation that had been received. 5. Otherfactors. Other environmental factors with the potential for affecting the response or interpretation of a film include pressure and other mechanical
52 FILM BADGE DOSIMEI RY TV ATMOSPHERIC NUCLEAR TESTS effects, chemical sensitization, static electricity, and radioactive contamination. With the exception of chemical sensitization, each of the effects listed produces a clearly identifiable anomaly. Pressure effects were noted in some of the earlier test-series badges which used the metal cross-shaped filter; these, however, were minor and should not have interfered significantly with densitometry or subse- quent exposure interpretation. Static electricity can produce a characteristic tree- like pattern on the developed film. The effect is usually associated with clothing made from nylon or other synthetic fabrics and is unlikely in humid environ- ments. If severe, the effect can result in increased density readings. However, static discharge effects were rare and could easily be identified so that an undam- aged film area could be used for evaluation. Certain chemicals (such as mercury vapor in air) may cause a chemical sensitization or desensitization which produces a generalized increase or decrease in firm density. However, there is no reason to suspect that films were exposed to sensitizing chemicals, and corrections are therefore not indicated. Radioactive contamination in He form of pariiculates on the exterior of the film badge will produce what is basically an autoradiograph on the developed film, and has been discussed in Section 5.F. H. FILM BADGE EXPOSURE VERSUS DOSE This section presents a brief summary of the basic quantities used in the measurement of ionizing radiation and the units in which these quantities were expressed throughout the atmospheric test series period. The concepts of primary importance are (1) "exposure" or "exposure dose", (2) "absorbed dose" or simply "dose", and (3) "dose equivalent". These concepts and their units are discussed below. The traditional "special units" (the roentgen, the red and the rem) were used exclusively during the subject period. The new International System of Units (SI) was not adopted until 1975, and is now in common use outside the United States. For the precise technical definitions of radiation quantities and units, see ICRU Report 33 (ICRU 1980~. The term "exposure" has several meanings which depend upon the context in which it is used. In the generic sense it frequently means the condition of being exposed to something such as the elements, or light, or radiation. It also has a specific technical definition as a measure of the amount of x-rays and/or gamma rays at some point, as described below. When used in this latter sense in this report, it will be italicized. Exposure, E, is a measure of the intensity of x or gamma rays reflecting the amount of ionization such radiation produces in air under standard conditions of temperature and pressure. When the air molecules (mostly oxygen and nitrogen) are ionized by radiation, some of the radiation energy is absorbed, releasing
4 FILM BADGES L7V ATMOSPHERIC TESTING 53 electrons. The original unit of exposure was the roentgen named after the discoverer of x rays. The roentgen, with the symbol "R", was defined as the quantity of radiation which would release sufficient electrons to produce at a specified point in air one electrostatic unit of electric charge per cubic centimeter under standard conditions. Thus the exposure is an indirect measure of the intensity of x or gamma rays. It should be stressed that exposure refers only to x or gamma rays in air. Thus, strictly speaking, one cannot refer to the dose to a person in units or in terms of the roentgen. Its value in R units is determined not only by the number of x or gamma rays incident per unit area but also by the energy of the x or gamma rays. The SI unit of exposure is coulomb per kilogram (of air) and is equal to 3876 R. This conversion factor takes into account the mass of one cm3 of air under standard conditions. Because absorption of photons is a complex function of the atomic number of the absorber and the photon energy, the measurement of exposure or exposure rate at a given point in air provides only the first step in the determination of how much radiation energy would be absorbed by an object placed at that point in the radiation field. The absorbed dose, D, is the amount of energy absorbed from any kind of ionizing radiation per unit mass of absorbing material at a specified point. The previous special unit of absorbed dose was the rad which was defined as 100 ergs of radiation energy absorbed per gram of material. The SI unit for absorbed dose is joule per kilogram and its special name is the gray (Gy). One gray is equal to 100 red. One millirad is 0.001 red and 0.00001 Gy. Note that the concept of absorbed dose applies to all kinds of ionizing radia- tion, not only to x and gamma rays. It also applies to any kind of absorbing material and is not limited to air as is exposure . Absorbed dose is the most commonly used concept in radiation dosimetry. However, absorbed dose is difficult to measure in practice, whereas exposure is relatively easily measured by the use of air ionization chambers. Therefore absorbed dose at a given point in a specified material was often calculated from a measurement of exposure in air at or near the point of interest. Such calculations require knowledge of other dose- dependent factors such as the energy spectrum of the radiation field, density and effective atomic number of the absorbing material, attenuation of the incident radiation, and geometric orientation of the absorber relative to the radiation field. Radiation-measuring devices such as hums (film badges) and thermoluminescent dosimeters in the past have been calibrated in terms of exposure (i.e. roentgen) for a given energy spectrum. Conversion of this calibration to dose has special limitations which are dependent on the instrument used, the characteristics of the radiation, and the conditions of exposure. Equal absorbed doses of different radiations and energies may produce bio- logical effects that differ in severity or frequency of occurrence if the doses are
54 FILM BADGE DOSIMETRY IN ATMOSPHERIC NUCLEAR TESTS high enough for such effects to be observed. For radiation-protection purposes, where absorbed doses are usually very low, presumed differences in biological effectiveness has led to the development of the concept of dose equivalent, H. which is the absorbed dose modified by a "quality factor", Q. The dose equivalent at a specified point in tissue is defined as: H=DQ, where D is the absorbed dose at the point and Q is the "quality factor" which takes into account differences in biological effectiveness. In the SI, the unit of dose equivalent is given the special name sievert (Sv). The traditional special unit used throughout this report is the rem. One sievert is equal to 100 rem. For a more comprehensive discussion on dose equivalent and quality factor, see ICRU (1980~. Note that for x rays, gamma rays, and electrons (the so called low-LET radiations) the Q factor is 1.0. Therefore for these radiations the dose equivalent is numeri- cally the same as the absorbed dose. In this report, the traditional units are used throughout because the SI units were not in use during the time period of atmospheric testing. If the exposure, E, at a specified point is known or can be calculated from a knowledge of the relevant parameters, then the absorbed dose, D, also can be calculated by taking into account differences in the absorption coefficients for air and the medium at the point of interest and in the energy required to produce ionization in air. These parameters can be combined into one factor called the "I factor." The f factor for air itself is about 0.88. Hence for air, D = 0.88E. Thus an exposure of one R produces in air an absorbed dose of 0.88 red ~ 8.8 mGy) (ICRU 19731. It should be noted that radiation dosimetry concepts are widely misunderstood by the public and radiation units are often used incorrectly even by the experts in radiation protection. For example, the traditional units "roentgen, red, and rem" are often used interchangeably. In the case of x and gamma rays, the three units are numerically about the same (within 13%) for an accurately identified point in soft tissue and, because the uncertainties in absorbed dose measurements are often very much larger at very low levels (less than 1 red), many experts ignore the distinction. In addition, the point or points where the absorbed dose is measured or calculated often is not accurately identified, even though the absorbed dose can and usually does vary widely from point to point throughout the body. If the dose to any point is below a level that can be considered biologically significant, then the failure to be specific about the dosimetry points of interest is of no practical consequence. This is usually the case in personnel dosimetry. A simple statement of exposure in roentgen provides only very limited infor- mation about the absorbed dose to organs at risk. Such is the case when no information is given about the location where the measurement was made, or specifying the orientation of the person with respect to the measurement point, or the type of radiation and its energy, or the uniformity and extent of the radiation field. The organ of biological significance, the so-called "critical organ", also is
4 FILM BADGESIN ATMOSPHERIC TESTING 55 usually not specified. Finally, a simple statement of the exposure gives no information about the reliability of the exposure measurement itself nor the time period over which the exposure (and hence the dose) was delivered. Neverthe- less, when the reported exposure is low, an estimate of the upper limit of the absorbed dose to critical organs of interest may be sufficient, and certain plausible conclusions are possible. When the entire body is in a penetrating x or gamma radiation field (such as during weapons testing), the critical organ is usually the bone marrow, which is relatively sensitive to ionizing radiation and is the source of radiogenic leukemia. If a dosimeter, such as a film badge, on or near an exposed person produces a response consistent with an exposure of one R. then it is likely that the biologi- cally significant dose, (i.e., the mean dose to the bone marrow, is less than one red (0.01 Gy), perhaps around 0.7 red. If any part of the body was shielded, the mean bone marrow dose could be considerably less. In any case, when the absorbed dose is low (less than 1 red to any critical organ), the lifetime risk for future cancer induction is also very low so that efforts to carry out further refinements in dose reconstruction are usually not justified. Such refinements, if made, are likely to reduce the estimated dose even further. Thus the error made by using exposure as a substitute for absorbed dose to a critical organ is of little consequence when the exposure values are low (less than the allowable exposure limits). I. TEST SERIES EXPOSURE LIMITS Recommended exposure (dose) limits for individuals who are exposed to ionizing radiations in the course of their work (radiation workers) have been reduced over the years from about 30 R per year in the 1930s and 1940s to 5 rem per year in recent years. Dose limits recommended by the National Council on Radiation Protection and Measurements (NCRP) in the United States generally have been adopted by various governmental agencies from time to time with only minor modifications. In addition to these limits, there is now a general policy that all doses should be kept as low as is reasonably achievable (the ALARA prin- ciple). The prospective exposure limits adopted for the various U.S. nuclear test series were generally consistent with NCRP (and/or ICRP) standards for occupational exposure at the time. These are summarized in Table 4-1. There were several reasons for wearing personnel dosimeters (such as film badges). The first was to monitor the radiation environment to provide reasonable assurance that exposures to individuals would remain below the applicable limits and to take corrective action if those limits were approached. The second purpose was to make possible rough estimates of absorbed doses to critical organs of any individuals who might be inadvertently subjected to exposures considerably greater than the prescribed limits.
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