ACL EFS . enews oo soe * wei . . sos ee. we # ‘ - oe . aoe ear . . 2 * a4 a ' 5 na 4 =o source energies, mainly by Campton ‘Scattering ‘in air. The dose from each of the resul.ing enervy intervals was calculated and platted as a fraction of the total dose. This was seen to yroup roughly into three regions, with maxima at 100, 700, and 1500 kev. An exposure to such a source was thus the resultant effect of partial doses frorn each region, making the exposure energy conditions quite different from those of the clinic or laboratory. Figure 1.2 illustrates the dose spectrum of 4-day-old fallout {rom a cloud sample. In the absence of other data to the contrary, this had to be taken as representative of the fallout on all of the islands. At this time the proportion of low energy component was at its maximum, During the several days before and after this time, the general shape of the spectrum apparently 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 amony 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 earlier times. a 1.4.3 STATUS VERIFIEDUNCLASSIFIED > x Li : =~ ~ i >» fe | , \ ‘ Qj & wot 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 recording dose rate monitor at the weather 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 knowledye 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 fallout occurred entirely within a short time; the second, thal il was gradual and extended over a period of many hours, The monitoring instrument on Rongerik went off-scale at 100 mryhr, %, hour after the dose rate bepan lo rise above backpround. 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, and corresponding doses were calculated. However, the possibility does not seem yreat that this actually occurred. Existing data are inconclusive, but several indications tending to favor the short time hypothesis are summarized below. First: a long fallout probably would not be uniformly heavy throughout, the first portion by Ma Pe wa | Denn S (a se . S Time of Arrival of the Radioactive Cloud, Duration of the Fallout, and Time of Evacuation for EachCase being the most intense and the balance tailing off. the effect of a shorter fallout. The total phenomenon thes tends toward 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 Ronuyerik and Rongelap, appear too long to have occurred at the distances of ‘ these atolls from Bikini, since the wind velovily in the area was high enough for the cloud lo pass over the islands .na considerably shorter time. Third: the accounts of the visibtlity of the fatlout, although conflicting, de not seem to indicate such late cessation. 19 5009525