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7 Conclusions A. TRACTABILITY OF THE PROBLEM There is extensive documentation of the U.S. atmospheric nuclear tests that were conducted from 1945 through 1962. A repository of these documents is maintained for the Department of Energy by Reynolds Electrical & Engineering Company, Inc. It includes archives of records made at the time of the tests, correspondence and reports dating from that same period, detailed summaries of each individual test series written by Defense Nuclear Agency contractors after 1980, and numerous other reports, critiques, and criticisms that have appeared after the atmospheric test series. The archival records are voluminous but not complete. For example, not all the original developed films from film badges are available, particularly for some of the earliest test series. Incomplete records and poor penmanship in some original film badge records in archives have left ambiguities in assignment of some badge readings to particular individuals. Incomplete records and inaccurate plotting of data for film badge calibration experiments produced uncertainties in the quality of some calibrations. In spite of their deficiencies, the documents are sufficiently complete to pro- vide a clear picture of He way film badges were used to record and determine x- and gamma-radiation exposure of participants who wore them. Records of film badge procedures leave no doubt that radiation safety of participants was a major concern in all of the tests. From the first atmospheric test, film badges were recognized as the most reliable means for documenting cumulative radiation 186
7 CONCLUSIONS 187 exposure that participants received. The very large number of participants in aunosphenc nuclear tests and the widely different test conditions made this a formidable undertaking. The specific film badge dosimet~y methodology that was used evolved with time through the different test series. Special circumstances in the field made individual tests unique and produced special problems. Nevertheless, the general approach to film badge dosimetry remained the same throughout the atmospheric testing period. This commonality has made the Committee's task of evaluating the reliability of results drawn from archival records a tractable problem. It enabled the Committee to develop a relatively simple means for expressing the most probable radiation exposure received by a single film badge and the limits within which the exposure can be determined with 95% confidence. Applying methodology recently developed by the International Commission on Radiation Units and Measurements (ICRU 1985), the Committee was able to translate a film badge exposure and its associated uncertainties into a best estimate of deep-dose equivalent for a person wearing that badge. The uncertainty assessment for this report was based on careful quantification of bias and uncertainty from each of several sources, followed by evaluation of their combined effects based on statistical principles. In conducting this assess- ment, the Committee carefully evaluated the available evidence, and used their collective expertise to obtain factors for bias and uncertainty. However, the available evidence was not adequate to allow a rigorous statistical treatment of all uncertainty sources, and therefore the assessment necessarily had a subjective component. Nevertheless, the quantification of bias and uncertainty provided in this report is based on specific assumptions that are discussed in Chapter 5 and justified for individual test series in Chapter 6. B. GAMMA RADIATION FROM FISSION PRODUCTS AND ACTIVATION PRODUCTS Personnel exposure from atmospheric weapons testing was largely from x and gamma radiation in the energy region from 0.1 to 2 MeV associated with decay of fission and activation products (See Section 3.D). Less than 10% of the overall photon energy spectrum was below this energy range, and was primarily attribut- able to the scattering of photons from large area sources. With few exceptions, neutrons did not contribute significantly to personnel exposure (Section 3.B). Unfissioned uranium, plutonium and other transuranium elements produced by the detonation were alpha-radiation emitters (Section 3.C). However, alpha radiation was not measurable by film badges used in these tests. A significant beta-radiation component is associated with residual radiation fields (Section 3.C). Beta radiation is non-penetrating radiation and does not
188 FILM BADGE DOSIMEIRY IN ATMOSPHERIC NUCLEAR TESTS contribute as such to the deep-dose equivalent. Beta radiation interactions give rise to bremsstrahlung (x rays), but the contribution of this source to the overall photon field is small (Section 3.D). Therefore, film badges provided a reasonable basis for estimating deep-dose equivalent to participants in atmospheric testing operations. C. CAPABILITIES AND LIMITATIONS OF FILM BADGE DOSIMETERS Film packets utilized in film badge dosimeters during the period 1945-1962 changed only in the exposure ranges of emulsions and the number of components per packet (See Section 4.D for examples). Improvement in the range of exposures measured occurred as operational experience was gained with different film- component combinations (Section 4.D). Very few personnel exposures, however, were affected by film-emulsion range limitations. Metallic filters used over film packets to establish more uniform film response varied during early test operations and generally were standardized in 1953 to have a 0.028-inch-thick lead filter. This filter was adequate for monitoring fission and activation-product photons over a wide range of energies (Section 4.A). Other filters used in earlier tests had the effect of overestimating exposure from photons below 100 keV. Attempts were made during several test operations to estimate beta exposure. These attempts were not successful. Beta-dose results reported during atmos- pheric testing are therefore not reliable (Section 4.B). Densitometry capability was a limiting factor only in CROSSROADS where the measurement range limited exposure determination to a maximum of 2 R. Only a few participants, however, exceeded this limit during one badge-wearing period. The minimum detectable limit (MDL) of a particular film badge component type is a limitation in measuring low exposures. Conclusion F discusses this . . . . .lmltatlon. D. BIAS AND UNCERTAINTY Best estimates of the x- and gamma-radiation exposure of a single film badge and the 95% confidence limits were evaluated by combining uncertainties from a number of different origins. These have been grouped into laboratory, radiologi- cal, and environmental categories as described in detail in Section 5.B. Each source has been characterized by a lognormal distribution with a bias and an uncertainty as discussed in Section 5.A. The overall bias and uncertainty result- ing from all sources were deduced by a combination of individual bias and
7 CONCLUSIONS 189 uncertainties as also discussed in Section 5.A. The composite results vary considerably from one test series to another, as detailed for individual series in Chapter 6. In all cases, the relative uncertainty increases at very low exposures (less than 0.2 R) because exposed optical density approaches optical density of unexposed film, which also vanes to some degree. In the exposure range between 0.2 and 2 R. where this contribution to uncertainty is small, and for a well controlled test series such as PLUMBBOB, the net exposures are typically found to be unbiased and to have uncertainties within a factor of 1.4 above or 0.7 below the best estimate of exposure. Conversion of exposure to deep-dose equivalent yields a deep-dose equivalent in rem which is 0.8 times the exposure in R. Because there is an additional uncertainty of 1.2 in this conversion, the 95% confidence limits on the final deep-dose equivalent for PLUMB BOB would have an uncertainty of 1.5 for the best estimate. Somewhat larger values are obtained for less well controlled series. Numerical values of reported estimates of exposure obtained from film badges are always larger than the corresponding calculated numerical values of the deep- dose equivalent. E. METHODOLOGY FOR ASSESSING BIAS AND UNCERTAINTY In this report (Section 5.A), the approach developed is a reasonable method for combining biases and uncertainties from several different sources and for estimat- ing the deep-dose equivalent from film badge readings. Although the specific values for bias and uncertainty given in this report are strictly applicable only to atmospheric test series participants, the methods used to obtain these values could be applied to other personnel monitoring situations such as underground testing at NTS after 1962 and similar monitoring under field conditions, or with revised uncertainties, to monitoring for reactor and hospital radiation workers. F. MINIMUM DETECTABLE EXPOSURE LEVEL As defined in Section 5.C, the minimum detectable level of radiation exposure measured with a film badge is generally established as the point where the uncertainty of the reading at the 95% confidence level is i 100% in normal distribution terms. Application of this concept to film dosimetry during the atmospheric tests generally results in an MDL of approximately 40 mR, indicat- ing that 95% of a series of exposures at 40 mR would yield readings between 0 and 80 mR. Readings below the MDL appear in the records for some of the test series. The general practice in film badge dosimetry is to make the best possible interpretation of the exposures in the region between zero and the MDL, reporting
190 FILM BADGE DOSIMETRY IN ATMOSPHERIC NUCLEAR TESTS zero for those that favor that end of the range and a positive reading for those approaching the MDL, bearing in mind that there is no statistical difference between the two. This practice appears to have been followed in a majority of the test series. G. CONVERSION FROM EXPOSURE TO DEEP-DOSE EQUIVALENT The film badges used throughout the atmospheric weapons tests were de- signed, calibrated, and used to measure gamma and X-My exposures. As men- tioned in Chapter 4, occasional attempts to measure beta radiation in various series were generally unsuccessful, and were not considered in this study. The re- lationship of exposure to deep-dose equivalent is described in Section S.E. Deep- dose equivalent is the quantity of interest in evaluating the potential for biological effects in an individual involved in the weapons test genes. Therefore, each of the individual series evaluations in Chapter 6 include an overall bias and uncer- tainty that will effectively provide a mechanism for conversion from exposure, as reported on the film badges, to deep-dose equivalent. The Committee concludes that this conversion is a necessary element in the evaluation of an individual participant's radiation-exposure history.