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 145 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: (1) 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 fractionation. 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