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 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. 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 therapy or experimental radiobiology. The data in Figure 1.2 are based on the spectrum of + 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. Thetotal 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 proportionsin 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 avail- able in this case came from fallout samples taken soon after the detonation at points some distance from the contuminated 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, theor considerations based on the radiochemical position of the fallout mixture permitted« rates to be calculated for ditferent interva tween the time of initial exposure andlate! vey rendings (3). ‘These agree well wit. experimental data, and were used both i dose calculations during the exposure inte and in extrapolating the later survey rea to earlier times. 1.22 Duration of the Exposures The time of evacuation is known accut for all the islands; however, the time of a of the radioactive cloud was «letermined cisely only for Rongerik by means of a con ously recording dose rate monitor located: weather station on that atoll. .\s the radi intensity rose above the background. a ma with a misty appearance began to fall. times of beginning of fallout for Rongela| Ailinginae atolls were estimated from si visual observations. These estimates were sistent with the relative distances from th of detonation and the known wind velo Fallout was not observed on Utirik, henc estimate of arrival time was made on the of windvelocity and distance. Two extreme possibilities exist relative | duration of the fallouts: the first, that the out oecurred entirely within a short time second, that it was gradual and extended « longer period. The monitoring instrume Rongerik went off scale at 100 mr/hr, one hour after the dose rate began to rise background. If this rate of increase is tal constant, and is extrapolated to a poin which subsequent decay would reduce the rate to the values found at later times. tl sumption of a long fallout of about 16 ho found to be necessary. This siow rate o and Jate maximum time of dose rate wa limiting case; however this situation wa considered likely. Existing data are ince sive, but several indications favor asi “effective fallout time hypothesis” and are marized below.