‘distributed on the interface, the differences for both the unscattered and total exposure rates at h = 1 meter were less than 1%, while at h = 100 meters, the differences were less than 0.5%. The differences were slightly larger when the source was distributed in the soil. For a uniformly distributed 1.0 MeV source, the unscattered aluminum exposure rate was found to be about 4% greater than the corresponding soil value at h = 1 meter and about 3.5% greater at h = 100 meters. The total exposure rates were 3.4% greater for aluminum than for soil at both heights. A further comparison was made by calculating the exposure rates for soils whose ratios of constituents other than water were the same as our mock soil, but whose moisture contents were 0% and 25% as compared to the 10% of our standard soil. In all cases the density was kept constant at 1.6 gm/cm® and the source was 1.0 MeV, uniformly distributed, since the aluminum calculations indicated the greatest differences occur for a distributed source. The 0% and 10% moisture soils gave almost identical results while the 25% moisture SsOll gave exposure rates only 1.7% lower than the 10% moisture soil at both 1 meter and 100 meters. This small difference is in the direction expected since the increased moisture content would cause slightly more Compton scattering relative to photoelectric absorption. The (Z/A) for the 0% moisture content soil was .497, while the (Z/A) for the 25% case was .512. Thus, the calculations are relatively insensitive to minor differences in soil composition and moisture content and should be valid for a wide range of soils. Corrections would, of course, have to be made when the density changes due to rainfall. These corrections are discussed under Section ITI, A. D. Error Estimates Exposure rates, in this report, are given in units of MeV/gm-sec and represent the energy absorbed in a gram of 4 ay: ‘ - see gee