6 EFFECTS OF IONIZING diation as jt emanated fromthe 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 Compton scattering along the path in air, (1) led to the dose-energy histogram shown in Figure Lz. Roughly there were three recions. with maxima at 100, 700 and 1400 KEV. The total exposure was thus the resnitant effect of partial doses from each energy refion, making the exposure energy condition significantly different fromthose of ridintion therapyor experimental radiobiologs. The data in Figure 1.2 are based on the spec- trum 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: instrament was calculated by weighting the dose from each energy interval by an average meter response factor for that enerey (2). A geometry correction factor was also calculated. The total correction resulting from this procedure was found to he about twenty percent. Using this correction, the dose rates on the ishinds 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 number of radioisotopes are present in varying proportions in the fisston product mixture, and the total rate of change of radiation intensity resulting from them may difler somewhat with place and ‘ime. The best data available in this case came from fallout samples taken soon after the detonation at points some distance from the contaminuted atolls. Decay rates of these samples were measured in fhe held and in the laboratory, and a fairly consistent pattern was observed amene various lo- I0027b4y RADIATION cations and samples. In addition, theoretical considerations based on the radiochemical con.- position of the fallout mixture permitted deeuy rates to be calculated for different intervals between the time of 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 known accurately for all the islands: however, the time of arrival! of the rachoactive cloud was determined precisely only for Rongerik by means of a continuously 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 began to 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 the site of detonation and the known wind velocities. Fallout was not observed on Utirik. hence the estimate of arrival time was made on the basis of wind velocity and distance. Two extreme possibilities exist relative ta the duration of the fallouts: the first, that the fallout occurred entirely within a short thne: the second, that it was gradual and extended over a longer periad. The monitoring instrument on Rongertk went off scale at 100 mr/hr, one-half hour after the dose rate bezan to rise above backeroumd. Tf this rate of increase is taken as constunt. and is extrapolated (9 a point for which subsequent decay would reduce the doze rate to the values found at later times. the as- sumption of a long fallout of about 16 hours is found to be necessary. This slow rate of fall and Jate maximinn time of dose rate was one limiting case; however this situation was not considered likely. Existing data are inconcla- sive, but several indications favor a shorter “effective fallout tine hypothesiz” and are summarized below.