out activity from several hours to a few weeks post-BRAVO apparently excluded the decay characteristics of non-fission-fragment nuclides. This would affect surface activity estimates which we derived from Bikini ash results since these estimates relied in part on extrapolated decay rates. The estimated thyroid dose from internal sources would be affected by decay characteristics because it relied in part on these derived surface activity estimates. In order to derive ground activity estimates at times close to BRAVO detonation and to derive external and internal thyroid dose, the gamma- or. beta-decay-rate decline over short periods of time was assumed to have followed the relationship x where 2x at , (1) xX, = gauma- or beta-decay rate at time t,, and X9 = gamma- or beta-decay rate at time t4. m = decay exponent. In the early post-BRAVO period and for time intervals of a few hours, Miller's estimate of decay exponents may have departed significantly from the standard value used for planning fallout activity decline (m= -1.2). His values for m at different times post-detonation of BRAVO are listed in Table 10. At short time intervals there was a departure from m= -1.2. However, the overall decay exponent calculated from Miller's declining activity results, for the interval one hour to sixty days post-detonation, was -1.2. This was in agreement with the standard value used for decay of unfractionated fission products. Thus, it seemed to us that the impact on activity decline due to non-fission-fragment nu- clides may not have been folded into Miller's extrapolation. Further study was done to establish actual BRAVO activity and exposure-rate decline. Surveys performed by the radsafe team of the USS PHILIP, the ship dispatched to evacuate people from Rongelap Island, have recorded an exposure-rate level for Ronge lap village of 3.8 x 107% C kg! no! (1473 mR ht) average and 4.9 x 107% c kg! n@! (1900 mR h7!) maximum at 2.2 days post- detonation ( COMTASK GROUP 7.3 Diep 020848Z of March 1954, 0C68). A similar but less precise statement of the exposure rate at the time of evacuation was given by Sharp (Sh57). In order to reconstruct the BRAVO exposure-rate decline prior to evacuation and not use the standard decay exponent, we derived additional information about the arrival time and nuclide composition of BRAVO fallout from Bikini ash measurements. ¢. The Buildup of BRAVO Fallout on the Ground. The studies by Suito, Takiyama, and Uyeda (Su56) indicated that Bikini ash consisted of irregularly shaped white granules. Bikini ash, taken from the deck of the 5th Lucky Dragon, was deposited while the ship was located about 150 km (90 miles) from the detonation site (Ts55). Suito defined the mean volume diameter to be the diameter corresponding to the mean volume. From the size and shape distributions, Suito determined | the mean volume diameter of Bikini ash granules to be 320 + 70 \m (1.3 x 1072 + 2.8 x 1073 in.). The mean mass of a granule was 0.039 mg (8.7 x 10-7 lb). “re specific gravity was 2.4, less than the specific gravity of wete A Qys tae 22 -