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OCR for page 186
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
OCR for page 187
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
OCR for page 188
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
OCR for page 189
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
OCR for page 190
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.
Representative terms from entire chapter:
test series
