Our previous calculations of 7°*u, *3*7h and *°xK exposure rates based on the infinite medium buildup factors are also given in Table 3. These calculations for an infinite medium assumed that the energy absorbed at the earth-air interface per gram of air would be % the energy emitted per gram of soil 1) Since for low energies the absorption in soil is greater than in air this tended to overestimate the exposure rate. By comparing the “°K results (Table 3), we see that this overestimate was only about 4% for 1.46 MeV. It would have been greater for a lower energy source. The much larger differences in the old and new calculated 7*°u exposure rates, however, are due not as much to the more accurate method (which accounts for only about 5~8% of the difference) as to the different source relative intensities used in the present work. In any case, the small changes in the values of 23°y, 232 ah and *°K exposure rates per unit concentration of these elements do not significantly alter any of the results given in any of our previous reports on environmental radiation’?°®?., The present report, of course, is much more detailed than our previous work, since it provides data on energy and angle distributions and detector height dependence rather than just exposure rates at one meter above the interface. B. . Differential Energy Spectra The relative contributions of various energy photons to the total scattered energy spectrum is determined by examining the differential energy spectra. The differential energy spectra of the scattered energy flux (flux x energy) for three source energies, .364 MeV, 1.0 MeV, and 2.5 MeV are shown in Figure 2 for h = 1.0 meter and h = 100 meters. The effect of the increased scattering in air relative to soil at the lower energies is illustrated in the figure by the shift to lower energy and buildup of the Compton peak - 10-