e. 26. Activity and Exposure Rate History Based on Bikini Ash i. Areal Activity and Exposure Rate from Nuclides Observed on Day We used the Yamatera and Tsuzuki results to estimate BRAVO fallout activity on the ground and exposure rate prior to evacuation. We accounted for the fractionation of fission products, the presence of transuranic and activation products observed in Bikini ash, and a buildup followed by a decline of exposure rate. The Japanese results, summarized in Table 9, were used to generate the percent of fallout beta activity represented by each nuclide in Bikini ash (Table 12, column 2). The values in this column were based on the mean value of the Yamatera and Tsuzuki result if two values of a nuclide's beta activity in Bikini ash were reported. If not, only the one value was used. We calculated the day-26 exposure rates, at 1 m above the surface of a planar source of a unit area of Bikini ash activity (Table 12, column 3), for each nuclide. We based the estimate on data of Beck (Be80) or Kocher (Ko80) and results recorded in Table 12, column 2. Beck recorded factors to convert activity to exposure-rate for a number of particulate gamma-emitting fission products and for a number of particulate activation products and residual nuclear materials on the ground, as a result of weapons tests (Be80). By summing each nuclide's exposure rate relative to total Bikini ash activity per unit area, we estimated an exposure-rate conversion factor for Bikini ash to be 1.12 x 107!’ ¢ kg7l st Bq! m2 (5.8 x 1073 uk h7! mein! «m2), By inverting this factor and multiplying by the frac~ tion of each nuclide's beta activity in Bikini ash (see Table 12, column 2), we estimated the beta activity of each nuclide per unit area, which was relative to a unit fallout exposure rate from Bikini ash. Held (He65) reported a mean exposure-rate at Rongelap Island of about 2.9 x 1079 C kg s~! (40 mR h7!) at 26 days post-detonation. He also reported a storm with heavy rain two weeks post-detonation (He65). This was followed by a reduction in exposure rate greater than he would have expected from decay of BRAVO fallout. Glasstone (G162) reported a 40% reduction after 25 days for the BRAVO exposure rate which he attributed to weathering in certain areas of the Marshall Islands. We estimated the reduction in exposure rate due to weathering at Rongelap Island on the basis of the survey taken by the USS PHILIP radsafe team. We assumed the survey at this early time post-detonation to be a measure- ment of unweathered fallout and assumed a decay exponent m= -1.4 from day 2.2 to day 26. This value for m was the mean value calculated for the decay of the nuclide mixture present at Rongelap 2.2 to 26 days post-detonation, which we based on the gamma decay of 142 nuclides tabulated in Table 13. Specifically, we accounted for the contribution to exposure rate from 1) the transuranic nuclides 237uU and 239ND, 2) the neutron-induced nuclides, 35S and 45ca, 3) the day-26 fission products which had fractionated according to Japanese results (Ya56, Ts55), and 4) the fission product and transuranic product precursors initially present on day 2.2. The day-26 value of the exposure rate which we extrapolated from the measurement made by the radsafe team on day 2.2 was 18% greater than that reported by Held. Thus, we estimated that, had the rainstorm not occurred, the mean unweathered exposure rate on Rongelap on day-26 would have been 3.4 x 1079 C kg 7! s7! (47 mR ho!). - 30 =