of a given size were spread throughout the stem, the cloud top at 40 km. the base of the cloud and up to In fact an entire distribution of granule sizes wou ld reach -the surface at any point in time not just one size. The simple model (Eq. 2) was adequate for the purpose of estimating the rate of rise of exposure rate and the rate of accummlation of activity at the surface. The approximation was sufficient to estimate external exposure during the period of rising exposure rates. The measurement of rate of rise of exposure rate for weapons tests dur- ing the Hardtack Series in 1957 were in agreement with the rate of rise of the exposure rate for BRAVO calculated from using Eq. 2 (USPHS59). These estimates of granule fall time, granule diameter and activity versus granule diameter were combined in a straight forward manner to determine the cumulative per cent of activity deposited on the surface of the 5th Lucky Dragon as a function of time after the BRAVO explosion. plotted in Fig. 2. curve on Fig. 2. This estimate was The rate of activity build up was taken as the slope of the The result indicated the bulk of the activity had fallen on the fishing vessel by three and a half hours post BRAVO detonation due to the greater amount of activity carried by the more abundant large diameter granules which fall first. Granules could no longer be seen falling be the crew of the fishing vessel by about 8 hours post BRAVO (Ts55). Interviews with Rongelap people indicated the granules were et Egat Tre noticed first at 5 hours post detonation (Sh57). At the time of the BRAVO deto~ mation people were located at Rongelap Island, Rongelap Atoll, about 210 km from Namu Island, Bikini Atoll, the original location of BRAVO fallout. of the fallout was observed for about 7 hours (Sh57). The duration Equation 2 yielded gran- ule diameter information for Rongelap Island based on the observed fallout arrival and cessation times. The Rongelap granule diameter distribution was il