SMALL BOY SHOT FALLOUT RESEARCH PROGRAM 55 ip(1) = 435 i)(100) (17) i9(1) = 246 if(100) (18) and Thus _ i(100) ry, = 0.575 i,(100) (19) i(100) a rip 0.813 i,(100) (20) and Actually the decay factors for i and i’ should vary with r,, or with the particle size of the fallout. Thus, for samples in which the degree of fractionation (*y products) is small, the decay factor from 100 to 1 hr should approach 435. In this preliminary analysis, the previously assumed decay factors (Eqs. 15 to 18) were used for all samples; neither observed nor specially computed decay factors for all fallout samples were available for application in Eq. 19. The assumption that all samples had the same decay factor could result in an error ofa factor of 2 in the Trp value for some of the fallout samples. The variation of i(100), r;,(100), and ry, with particle size was investigated by using ion-chamber and radiochemical-analysis data’ on sieved fractions of collected fallout samples. Two sets of the ionchamber measurements were reported. In addition to the field measurements, which were corrected to absolute ion-current values, an uncorrected second set of measurements was reported by the radiochemical-analysis project.° These data were corrected to 100 hr after fission by using (t/100)'-"’ for computing the decay factors. The decay-corrected data were then correlated with the field measurements to obtain a correction factor for conversion of the data to absolute units. This was done by computing the ratio of the two measurements (corrected to 100 hr) on each sample. The median value of this correction factor was calculated to be 0.788, which is close to the value of 0.78 obtained in the past with a radium standard.’ After application of this second correction factor to the second set of measure- ments, the geometric mean value of i(100) was calculated from the two sets of the ion-chamber measurements and the reported® fission-yield content of each sample. The i(100) values thus obtained are given in Table 6 and are plotted in Fig. 1 as a function of the midrange particle diameter, d, for each sieve fraction. Subsieve analyses indicated the