Doses from external irradiation @ A. BouviLLe Er AL.

exposure rate were calculated from classified and declassified data available to Hicks on the amountof fissionable nuclides in the device and the measured fission
neutron spectra. The “zero time” activation product
values were the results of measurements made byaircraft
surveillance within | to 4 h post detonation. Hicks made
assumptions regarding fractionation effects from which
he developed his tables for unfractionated debris (designated as R/V = 1, where R stands for refractory radionuclides and V for volatile radionuclides), as well as for

debris with 50 and 90% of the refractory elements
removed (designated as R/V = 0.5 and R/V = 0.1,
respectively). As described in Beck et al. (2010), we
modified Hicks’ calculated activity ratios for unfraction-

ated fallout (R/V = 1) to estimate the activity ratios for

various degrees of fractionation. For all tests except the
Bravotest, available data support our assumption of an
R/Vratio of 0.5 at all atolls. In contrast, however, there

were some significant variations in the degree of fractionation for Bravo fallout at someatolls: 0.7 for Likiep,
0.9 for Mejit Island, 1.3 for Ailinginae, 1.4 for Rongelap,

1.5 for Rongerik, and 0.5 at all other inhabited locations.

The high fractionation conditions (R/V > 1) for test

Bravoat atolls close to the Bikini Atoll test site reflect
the preferential deposition of large particles at early times
of arrival, in which the activity of refractory radionuclides is greater than that of volatile radionuclides.
Hicks calculated nuclide composition as a function
of time for six thermonuclear tests in the 1954 Castle
series (Mike, Bravo, Romeo, Yankee, Zuni, and Tewa);

the data from the other 14 thermonuclear tests that
deposited fallout in the Marshall Islands are still classified. As described below, Hicks’ data were used in two

different ways in our calculations according to the
information that wasavailable for each test and location:

1. If the exposure rate was measured or inferred at any
time after the test, then only information on the

temporal variation of the exposure rate was required
to correct exposure-rate measurements madeatdifferent times to H+12, and, as described later in this

paper, to integrate the estimated exposure rates to
obtain total exposure. This is the method that was
generally used for the atolls and tests where exposure
rates were measured by airplane surveys or ground
surveys conducted soon after the test. In our method,
corrections were also made for the gradual decrease of
radionuclide activities in the upper layers of soil
resulting from environmental loss processes (termed
“weathering effects” in this paper), which are not
taken into account in Hicks’ calculations. Those
corrections, describedlater in this paper, are trivial for


the first week or monthafter the test, but are substan-

tial when calculations of exposure rate are made for
years or decadesafter thetest.
2. If the exposure rate had not been measured, but rather

the '’Cs deposition density was estimated for a given

test 7 and at an atoll j, then £12(i, 7) was estimated

from Hicks’ predicted ratios of '*’Cs to E12, modified
to account for our best estimate of fractionation. Eqn
(1) presents the form of this calculation:

where A(i, j), in Bq m~,is the '*’Csactivity deposited

per unit area of groundatatoll j after test i (Becket al.

2010), and ND(i, j) is the normalized '’’Cs deposition
density, expressed in Bg m* per mR h| at H+12,

and inferred from the work of Hicks (1981, 1984) for
the selected value of R/V for test i at atoll 7.

The method described above would be appropriate if

the '"’Cs deposition density was measured within about
one month after the test and if it could be unequivocally
assumed to have been a result of fallout from thattest.
However, as a rule, '’Cs was measured in soil many
years later in the 1970’s and the 1990’s. In that case, we

first decay-corrected the measurements of '*’Cs deposi-

tion back to the time of the testing in order to obtain a
preliminary estimate of E12 for further refinement.
In practice, as described by Becketal. (2010), both

methods were used to estimate both E12 and '’’Cs

deposition, often in an iterative manner in order to
obtain: (1) credible fallout patterns over the territory of
the Marshall Islands; (2) reasonable sets of E12 and
fallout TOA values; and (3) in some cases, estimates of


As shownbyBecketal. (2010), the ratio of '*’Cs to

E12 decreases as the degree of fractionation increases,
from 31.8 Bg m~ per mR h| at H+12 for R/V = 0.5 to
7.8 Bq m~ per mR h' at H+12 for R/V = 1.5. As
previously indicated in this paper, the fallout from Bravo
at some of the more northern atolls was enriched in
refractory nuclides (i.e., R/V > 1) resulting in a reduced

ratio of '’Cs to E12 relative to fallout deposited at

further distances from the test site where typical values of

R/V were 0.5. Although the dependence of the '’’Cs to

E12 ratio on fractionation is substantial, it had only a

minor impact on the exposure-rate estimates madein this
study since actual exposure-rate measurements were
available for most of the atolls impacted by fractionated

Bravofallout. Thus, in practice, only the Hicks’ data for

R/V = 0.5, typical of fallout at distant sites from a
detonation point (Hicks 1982), were used to estimate E12

Select target paragraph3