6 EFFECTS OF IONIZING diation as it emanated from the material itself made possible an approximate calculation of the proportion of total dose delivered in each of several energy regions. Such a caleulation, using spectrometric data on the source material of mixed fission products and taking into account this energy degradation by Comptonscattering along the path in air, (1) led to the dose-energy histogram shown in Figure 1.2. Roughly there were three regions, with maxima at 100, 700 and 1500 KEV. Thetotal exposure was thus the resultant effect of partial doses from each energy region, making the exposure energy condition significantly different from those of radiation therapy or experimental radiobiology. The data in Figure 1.2 are based on the spectrum of 4 day old fission products from a fallout sample. In the absence of other data, this was taken as representative of the fallout on all of the islands to which the individuals were exposed. An energy correction factor for the radiation measuring instrument was calculated by weighting the dose from each energy interval by an average meter response factor for that energy (2). A geometry correction factor was also calculated. The total correctionresulting from this procedure was found to be about twenty percent. Using this correction, the dose rates on the islands at the time of survey were determined. Since radioactive decay of the fission products had occurred between the start of the exposure and this time, it was necessary to obtain a value for this decay rate during the exposure period in order to calculate a total dose in each case. A large numberof radioisotopes are present in varying proportionsin the fission product mixture, and the total rate of change of radiation intensity resulting from them maydiffer somewhat with place and time. The best data available in this case came from fallout samples taken soon after the detonation at points some distance from the contaminatedatolls. Decay rates of these samples were measured in the field and in the laboratory, and afairly consistent pattern was observed among various lo- RADIATION cations and samples. In addition, theoretical radiochemical composition of the fallout mi ure permitted clecay rates to be calculated for ifferent intervals between the timeofinitial e3 posure and later survey readings (3). ‘These agree well with the experimental data, and were used both in the dose calculations during t le exposure intervals and in extrapolating the ater survey readings to earlier times. considerations based ontl 1.22 Duration of the Exg OsUures The time of evacuation] is known accurately for all the islands; howeve r, the time of arrival of the radioactive cloud was determined precisely only for Rongerik means of a continuously recording dose rate onitor located at the weather station on that at DIT. As the radiation mtensity rose above the background, a material The times of beginning of fall put for Rongelap and with a misty appearance began to fall. Ailinginae atolls were est imated from similar visual observations. Thes b estimates were consistent with the relative d stances from the site of detonation and the kn pwn wind velocities. Fallout was not observed on Utirik, hence the estimate of arrival time T° made on the busts of windvelocity and dist: ice. Two extreme possibiliti¢ s exist relative to the duration of the fallouts: t 1e first, that the fall- out occurred entirely within a short time: the second, that it was gradua and extended overa longer period. The moni oring instrument on Rongerik went off scale at 100 mr/hr, one-half hour after the dose rate began to rise above background. If this rate f increase is taken as constant, and is extrapo ited to a point for which sabsequent decay would reduce the dose rate to the values found dt later times. the assumption of a long falloug of about 16 hours is found to be necessary. TPhis slow rate of fall and late maximum time pf dose rate was one limiting case; however tis situation was not considered likely. Existigg data are inconclusive, but several indicatipns favor a shorter “effective fallout time hyppthesis” and are summarized below,