Fallout deposition in the Marshall Islands @ H. L. Beck er AL. Ujelang, Utrik, Wotho, and Rongelap during the 1958 tests (PHS 1956, 1958), reporting the maximum daily exposure rates measured with survey meters. Except for test Yoke at Kwajalein, we could not locate any exposure rate data for tests conducted before 1952. All reported exposure rate survey data for the Castle series were multiplied by a factor of 1.3 to correct for deficiencies in the energy and angular response of the survey meters used at that time that caused readingsto be 127 a large numberof radionuclides, normalized to 1 mR h! at H+12, for various degrees of fractionation and for several nuclear tests. The ratios of '°’Cs to exposure rate at H+12 (°"Cs/E12) were usedto estimate '°’Cs deposition density from exposure rate data. Theratio of '*’Cs to exposure rate at H+ 12 dependsstrongly on the degree of fractionation [estimated as a ratio of refractory (R) to volatile (V) nuclides relative to no fractionation, (R/V = 1)] as shown in Table 1. Our estimates of fractionation at low by 20-30% (Sondhaus and Bond 1959). each atoll are discussedlater in this section. For example, (1955) was found to be in reasonable agreement with Bq m~* per mR bh! at H+12 for all non-TNtests and to equal 31.8 Bg m* per mR h' at H+12 for all TN tests. The correction factor to convert measurements made at altitude to ground level used by Breslin and Cassidy model calculations made later of the variation of exposure rate with height above the ground for surface deposits of radioactivity (Beck and dePlanque 1968). In some cases, however, the exposure rate estimates made in the 1950’s at altitude may not have accurately reflected the true amount of fallout deposited. This was likely true for small islands because the relatively large field of view at the flight altitude would include some open ocean. Thus, estimated exposurerates at atolls from the test sites where the net signal was on the order of the average backgroundsignal (0.05 to 0.1 mR h') are very uncertain and assumed to have been generally biased low. This relatively high background signal, determined by measurements over open ocean in the vicinity of the atoll, and due primarily to airplane and detector background, limited the ability to accurately detect small amounts of fallout on the atolls. The exposure rates, corrected for all known defi- clencies, were normalized to the same time postdetonation (12 h afterwards, termed H+12) using the temporal variation reported by Hicks (1982) for a number of nuclear tests. The variation with time reported for Bravo wasused for all TN tests, while the variation with time reported for Tesla was used for the non-TNtests. In a companion paper (Bouville et al. 2010), we show that the decay rate as a function of time after detonation does not vary substantially from one TNtest to another and, for that reason, we assumedthe Bravo decay rate forall TN tests and the Tesla decay rate for non-TNtests to be an acceptable approximation. We also show in Bouville et al. (2010) that the decay rate during the first few weeks after the test does not vary substantially with the degree of fractionation of the fallout or the likely degree of weathering. In both cases, sums of 10 exponential terms were usedto accurately fit the reported time-dependence. In order to estimate '*’Cs deposition density from exposure rate measurements we used the model calcula- tions reported by Hicks (1981, 1982, 1984). Besides the variation in exposure rate as a function of time, Hicks also reported the values of relative deposition density for for distant atolls (where R/V = 0.5), the '*’Cs deposition density at the time of fallout was taken to be equal to 43.7 Historical measurements of daily deposition density The daily deposition density could be estimated, in some cases, from historical measurements of beta activ- ity collected on gummed film used to monitor fallout (Bouville and Beck 2000; Harley et al. 1960). Gummed film wasa passive collector of atmospheric aerosols that was mounted horizontally above ground at 1 m height and that would collect particles through their adherence to its sticky surface. Gummedfilm was usually exposed for 24 hours, after which the total beta activity of the radioactive material collected on the film was measured. In the Marshall Islands, routine gummed film sampling was carried out only on Kwajalein (1952-1959) and Majuro (1952 and 1954 only) Atolls, and further away but still relatively close-by at two locations in Micronesia, Pohnpei and Kosrae as indicated by archival gummedfilm data (List 1955; Heidt et al. 1952). However, some additional gummed film data for other atolls (unpublished) were also foundin the archives of the HASL in New York City (NYC). Those gummed film data were used to help infer fallout patterns and confirm ground and aerial survey measurements. In order to estimate the '°’Cs deposition or the exposure rate at H+12 from the beta activity measured on the film, the gummed film data were corrected for collection effi- ciency, loss of volatiles, and decayed in a mannersimilar to that used to analyze the gummedfilm data from the nuclear tests that were conducted at the NTS (Becketal. 1990). Table 1. Estimated ratios of °’Cs/E12 (Bq m~ per mR h”! at H+12) as a function of R/V for TN tests. RIV BICS/E12 0.5 1.0 1.5 2.0 31.8 20.7 78 52