10. In the period immediately following the shot, the activity in the organisms wold be expected to vary greatly within short periods owing to changes in geographical distribution of the radioactive materials and to length of exposure time of the organisms. The curve from which the correction factors were determined was the decay curve for a sand sample that had been dredged from Rojoa and Aaraanbiru on November7 at a depth of 30 ft. The principal reason for selecting this curve was that, by inspection, the composite of 91 decay curves from various types of organisms and tissues closely resembled the sand curve, and, of the two curves, the data for the sand curve were more extensive and fitted more closely to a curve of low degree (Fig. 3.2). However, there were a few curves that departed significantly from the sand curve; those are also shown in Fig. 3.2. The decay correction factor was determined by dividing the value on the sand curve for December 1 by the value on the sand curve for the day the sample was counted. The range of these factors was from 0.68 for November 24 to 1.51 for December 12. The samples from the pretest collections were not corrected for decay since the change ia counting rate during the period the samples were counted was slight. Maximum correction factors would have been about | per cent. For differences between pre- and postshot decay curves, see Fig. 3.2. 3.4.6 Seif-absorption This year sample counts were corrected for self-absorption. In 1949 no correction for self-absorption was determined, but an attempt was made to keep the ash on the plate thin and constant in amount. Although it was recognized that the types and the proportions of isotopes varied from sample to sample, the decay and mass-absorption curves (Figs. 3.2 and 3.3) indicate that the sand sample approximates the mean of all the curves. Hence the same sand that was used to determine the correction factor for decay was also used to determine the self-absorption correction factor. A few of the actual values, based on the total weight of ash on 144-in. plate, are as follows: 100 mg, 2.6; 300 mg, 4.0; 600 mg, 5.2; and 1100 mg, 6.9. The average self-absorption correction factor from 135 randomly selected plates was 2.23 + 0.023. The total correction factor for 68 pretest samples was 3.93 + 0.21, and for 64 posttest samples it was 5.98 + 0.62.