88 KREY AND FRIED RESULTS Gamma-decay Measurements The gross gamma-decay rate of a Sedan air-filter sample is plotted as individual points in Fig. 6. The decay rate of similar Small Boy debris is normalized to these points and drawn as the solid and 19,000 _— S r Oo = T_T > OU 1000 a S — ve | a a ~“ 4 Po 100 300 500 700 900 1100 HR Fig. 6—Gross gamma decay of Sedan and Small Boy air-filter sam- ples. (July 6, 1962.) dashed line in Fig. 6. It is clear that the gross gamma-decay rates of the samples from both events are identical during the period of measurement. Plotted on log paper, the data follow the expression A= At where A is the activity at time t after fission and A, is the activity at unit time after fission. The exponent of the time is greater than the —1.2 usually employed. It is interesting that the debris from two such widely different events decay at exactly the same rate. Gross Gamma Analyses The plot of the gross gammaactivity vs. site location (from north to south) for the Sedan mobile line is given in Fig. 7. The activity for each postshot sample was corrected to 1000 hr on D+ 1 by uSing an extrapolation of the Sedan gross gamma-decay curve (Fig. 6). The background levels reflected in Fig. 7 are the activity of a preshot air-filter sample assayed on D + 7 and the activity of a preshot fallout collector assayed on D+ 10. Since the background samples were not followed for decay, it is impossible to correct the data to a previous time. The background may constitute a major part of the activity in the first day of air sampling, but it is of little consequence in sub-