ation in penetration into the ground; i.e. all the isotopes have roughly the same local depth distribution. If these two assumptions hold, one can calculate the percentage of the total exposure rate due to each emitter in the vicinity of the soil sample site since the ratios of exposure rates per unit concentration for the various isotopes in question vary hardly at all with assumed depth distribution in the soil for relaxation lengths of 1 to 4 cm?’°. Thus, to determine the percentage of the total exposure rate due to a given isotope one need not know the average isotopic depth distribution over the area as a whole, from what depth the soil sample was taken, or whether the total activity of the sample is representative. The first assumption is probably quite reasonable. We tested the second assumption by using the gamma spectrometric analyses of the soils taken at the three sites where profiles were obtained on Bikini Island. We give below the percentage of the total sample activity due to each isotope as a function of depth for these sites. data is given in Table 2. The actual concentration PERCENTAGE OF TOTALy-RAY ACTIVITY* Depth **" Cs °° Co **®* Sb Other** Location 7 - Bikini 0-1" 1-2" 2-3" 84% 82% 71% 3% 2% 1% 8% 12% 23% 5% 4% 5% 16% 18% 14% 7% 4% 10% Location 5 - Bikini 0-1" 1-2" 2-6% 65% 66% 71% 12% 12% 5% - ll -