6 EFFECTS OF IONIZING RADIATION 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 calculation, using spectrometric data on the source material of mixedfission products and taking into ac- count 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. The total exposure was thus the resultant effect of partial doses from each energy region, making the exposure energy condition significantly different from those of radiation therapyor experimental radiobiology. The data in Figure 1.2 are based on the spectrumof + 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 eachenergy interval by an average meter response factor for that energy (2). A geometry correction factor wasalso calculated. The total correction resulting 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 betweenthe 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 proportions in the fission product mixture, and the total rate of change of radiation intensity resulting from them may differ 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 contaminated atolls. Decay rates of these samples were measured in the field and in the laboratory, and a fairly consistent pattern was observed among various lo- cations 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 timeof initial exposure and later survey readings (3). These agree 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. 1.22 Duration of the Exposures The time of evacuation is knownaccurately for all the islands; however, the time of arrival of the radioactive cloud was determined precisely only for Rongerik by means of a continu- ously recording dose rate monitorlocated at the weather station on that atoll. As the radiation intensity rose above the background, a material with a misty appearance beganto fall. The times of beginning of fallout for Rongelap and Ailinginae atolls were estimated from similar visual observations. These estimates were consistent with the relative distances from thesite of detonation and the known wind velocities. Fallout was not observed on Utirik, hence the estimate of arrival time was madeonthe basis of wind velocity and distance. Two extremepossibilities exist relative to the duration of the fallouts: the first, that the fallout occurred entirely within a short time; the second, that it was gradual and extended over a longer period. The monitoring 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 of increase is taken as constant, and is extrapolated to a point for which subsequent decay would reduce the dose rate to the values found at later times, the assumption of a long fallout of about 16 hoursis found to be necessary. This slow rate of fall and late maximum time of dose rate was one limiting case; however this situation was not considered likely. Existing data are inconclusive, but several indications favor a shorter “effective fallout time hypothesis” and are summarized below.