Freiling (Fr61) indicated that the degree of fractionation from a surface burst could be significant. The extent of the fractionation throughout the debris was another variable he observed to be significant. Freiling emphasized the high degree of fractionation between nuclides classified as volatile and refractory for coral atoll surface bursts. Generalizations were made, to be used with-much caution. Freiling indicated that, in general, fractionation would decrease as device yield decreased and would increase with depth, that is, air bursts would be less fractionated than surface bursts which would be less fractionated than subsurface bursts. From Freiling's studies, we cautiously expect that the high yield surface burst creating the BRAVO fallout caused a moderate-to-high degree of fractionation which occurred moderately to extensively throughout the debris. For the coral surface burst, Freiling observed that the ratio of 957r to 89sr activity could be chosen as a representative measure of the overall degree of fractionation between refractory and volatile elements. This ratio had a value of 5 for a deep water surface burst of megaton range and a value of 100 for a coral surface burst (Fr61). The unfractionated value for this ratio, for day 26 post~detonation and for thermonuclear neutron fission of 238y, was calculated to be 1.6 from data given by Crocker (Cr65)._ From the average of Yamatera and Tsuzuki data, we calculated the ratio of 9°zr activity to Isr activity measured on day 25 to be 4.8. This measured value for the degree of fractionation was characteristic of a deep water surface burst of the megaton range, moderately but not highly fractionated. This moderate fractionation probably occurred moderately to extensively throughout the fallout cloud because of the large yield and surface location of the device (Fr6l). The effect of fractionation on decay rate is very complex, and simple observation of overall radioactive decay does not yield significant informa- tion. Even so, the decay rates from widely distributed samples obtained out to 480 km (300 miles) from the BRAVO detonation site were similar. The decay rates from activity on different-size fallout granules collected at the same site were similar (0C68). These facts alone do not indicate that the same fractionation was common to all granule sizes. In fact, small granules traveled with the cloud for longer periods of time and possibly adsorbed more longer-lived nuclides than did the very large granules. In the analysis, we assume that the fractionation observed for Bikini ash granules was similar for granules at Rongelap, Sifo, and Utirik Islands. With the possible exception of Utirik Island, this assumption was considered valid owing to the proximity of Rongelap and Sifo Islands to the 5th Lucky Dragon. b. The Decay of Fallout. The gamma and beta decay of the BRAVO radioactivity after the first 10 days post-detonation was measured by several researchers (e.g., Miller, Servis, Tomkins, Wilsey, and Stetson, see 0C68). Decay data from measurements made between 0 and 10 days were not found in the literature. Fallout samples, taken weeks after the BRAVO event, were from Bikini Atoll, Rongelap Atoll, the 5th Lucky Dragon, and the surface of US Navy ships in the area. The measured decay exponent after two weeks was used by many researchers to extrapolate exposure rate back to times prior to sample collec- tion and in one case was used to estimate activity decline every hour post- detonation (Miller, 0C68). These extrapolations by Miller for the decay of fall- -21-