source energies, mainly by Compton ‘scattering ‘in air. The dose from each of (he resus inp energy intervals was calculated and plotted as a fraction of the total dose. This: was seen to yroup roughly info three regions, with maxima at 100, 700, and 1500 kev. An exposure to such a source was thus the resultant effect of partial doses from each region, making the exposure enerpy conditions quite different from those of the clinte or Laboratory. Pipure 1.2 iustrates the dose speetrum of 4-day -old fallout from a cloud sample. In the absence of other data to the contrary, this had to be taken as representative of the tallout on all of the islands, At this time the proportion of low energy component wis at tts nuoximun, During the several days before and after this Gime, the general shape of the spectrum appar ently did not vary grossly from that illustrated here, since the observed flux decay rates closely followed that of the observed gamma dose rate. For the period between fallout and surveys, therefore, a knowledge of instrument response to each energy region allowed a total correction factor to be calculated. The instruments used were calibrated just prior to the surveys, and their readings have been corrected for the spectrum shape here illustrated, 1.4.2 Rate of Decay of the Fallout Mixture Decay rates of fallout samples were measured in the field and in the laboratory, where a fairly consistent pattern was observed among various locations and samples. In addition, theoretical considerations based on the radiochemical composition of the fallout mixture permitted decay rates to be calculated for different intervals between the times of initial exposure and later survey readings. These agreed well with the experimental data, and were used both in the dose calculations during the exposure intervals and in extrapolating the later survey readings to earher times. 1.4.3 Time of Arrival of the Radioactive Cloud, Duration of the Fallout, and Time of Evacuation for Fach Case Only the time of evacuation is known accurately for all the islands. On Rongerik, however, the time of arrival of the radioactive cloud was determined precisely by the continuously re- cording dose rate monitor at the v:ather station. The fallout becamevisible at the time the instrument first indicated the presence of a radiation field above background. The material had the appearance of snow. The times of beginning of fallout on Rongelap and Ailinginae were estimated from similar visual observations, combined with knowledge of the relative distances of these atolls from Bikini and the wind velocities in the area. Fallout was not observed on Utirik, hence the estimate of arrival time there was made onthe basis of the Rongerik fallout time, wind, and distance factors. Two extreme possibilities exist relative to the duration of the fallout: the first, that the fallout occurred entirely within a short time; the second, that it was gradual and extended over a period of many hours. The monitoring instrument on Rongerik went off-scale at 100 mr-hr, y hour after the dose rate began to rise above background. If this rate of increase is extrapolated to a point for which subsequent decay would reduce the dose rate to the values found at later times, a long fallout is implied. This was taken as one limiting case, und corresponding doses were calculated. However, the possibility does not seem great that this actually oc eurred. Existing data are inconclusive, but several indications tending to favor the short time hypothesis arc summarized below. First: a fong fallout probably would not be uniformly heavy throughout, the first portion being the most intense and the balance tailing off. The total phenomenon thus tends toward the effect of a shorter fallout. This is supported by monitor data from other nuclear events. Second: the estimated durations of fallout, of about 18 hours, which result from the above extrapolation for Rongerik and Rongelap, appear too long to have occurred at the distances of these atolls from Bikini, since the wind velocity in the area was high enoughfor the cloud to pass over the islands in a considerably shorter time. Third: the accounts of the visibility of the fallout, although conflicting, do not seem to indicate such late cessation.