source energies, mainly by Compton ‘Seatterung ‘Tn ar. ‘The dose trom each of the census wage energy raleryals was caleulated and plotted as a fraction of the total dose. ‘This was seen to croup roughly onto three regions, with maxim at 100, 700. and 1500 key. An exposure to such dosouree was thas fap resultant effect of partial doses from each reyron, making the exposure cnerpy conditions quile different from those of the clinte oe laboratory. Fipare t.2 cllastrates the dose spectrum of 4-day -old tatlout froma cloud sample. Tn the vbsence of ather datailo the contrary, (his had to be Giken us representative of the Gallout on allof the istiids, At tlis (ime the proportion of low energy component wis itt tOs maximum Durtapy the several days before and after this time, the general shape of the spectrum appar ently did net vary grossly from that dlustrated here, since the observed flux decay rates closely followed that af the observed gamma dose rate. For the period between fallout and surveys, therefore, a kKnowledze of instrument response to each energy region allowed a total correction fitctar to be caleuhiuted. The instruments used were calibrated just prior to the surveys, and (heir 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 und in the laboratory, where a fairly consistent pattern was observed among various locations and samples. In addition, theuretical considerations based on the radiochemical composition of the Cillout mixture permitted decay rutes 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 miervals and in extrapolating the later survey readings to earlier 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 ishinds. On Rongerik, however, the time of arrival of the radioactive cloud was determined precisely by the continuously recording dose rate monitor at the vvather station, The fallout became visible 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 on the basis of the Ronge- rik fallout time, wind, and distance factors. Two extreme possibilities exist relative to the duration of the fallout: the first, that the tallout 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, ', hour after the dose rate began to rise above background, If this rate of increase is extrapoluted to a poimt for which subsequent decay would reduce the dose rite to the values found at later (imes, a long fallout is implied. This was taken as one limiting case, and corresponding doses were calculated. However, the possibility does not seem great that this actually oc curred, Existine data are inconclusive, bul several indications tending to favor the short time hypothesis are summarized below, First: a lon fallout probably would not be uniformly heavy throughout, the fiest portion bemp 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 enough for 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. 19