5. Activity and Exposure Rate History Based On Bikini Ash a. Areal Activity of Nuclides Observed in Bikini Ash The Yamatera and Tsuzuki data were used to estimate BRAVO fall-~ out activity on the ground and exposure rate prior to the evacuation. This esti- mate accounts for the fractionation of fission products, the presence of transuranic products observed in Bikini Ash and the build-up of exposure rate. In order to make this estimate, the data in Table 1 were first used to generate the per cent of fallout beta activity represented by each nuclide's beta activity in Bikini Ash (recorded in Table 4 colum 2). This column of data was calculated based on the mean value of the Yamatera and Tsuzuki data if two values for the measurement of beta activity in Bikini Ash were available for each nuclide. The day 26 exposure rate, at one meter above the surface of a planar source of a unit area of Bikini Ash activity (Table 4 column 3), was calculated for each nuclide based on the data of Kocher (Ko80), Beck (Be80) and Table 4 colum 2, By summing the exposure rate relative to fallout beta activ- ity per unit area from each nuclide in Bikini Ash and using Table 4 colum2, the beta activity of each nuclide per unit area relative to a unit fallout exposure rate from Bikini Ash was determined for day 26 post detonation and was listed in Table 4 colum 4. The summed fallout exposure rate per unit Bikini Ash activity per unit area was 5 .8x107> uR per hour per mCi per km? at 26 days post detonation. Held indicated a mean exposure rate at Rongelap Island of about 40 mR h_ i at 26 days post detonation (He65). Held reported after the initial contamination there was a storm with heavy rain about two weeks post detonation (He65). This was followed by a reduction in exposure rate greater than what would have been expected from decay alone. 15 Glasstone reports a 40% reduc- Bn