at around 50-100 keV as the detector height is increased. The relatively high photoelectric cross section in soil compared to air causes it to act as a sink for low energy y-rays, an effect which is enhanced the lower the source energy. To further illustrate the magnitude of this effect, we have also shown in Figure 2 the spectrum for a l MeV source at 1 meter in an infinite air medium instead of a soil-air medium. For this source energy the difference in the total exposure rate is only about 5% in all since about 50% of the exposure rate is due to unscattered y-rays. At lower source energies, where the scattered y-rays contribute a larger portion of the total expcsure rate, the effect on the total exposure rate of using infinite air calculations would be more significant. Cc. Integral Exposure Rate Spectra Another way to examine the effect of different regions of the energy spectrum on the total exposure rate is to examine the integral exposure rate spectrum, i.e. the fraction of the total exposure rate due to y-rays of energy less than E. This approach can be extended easily to the analysis of the composite *3°y and 7°*Th spectra. Figure 3 illustrates that as the detector height increases the fraction of the total exposure rate due to low energy photons increases. The effect is more pronounced the lower the initial source energy since the percentage of unscattered y~-rays is lower and the fractional change in energy due to a collision is smaller. For the 2.5 MeV source only 2% of the exposure rate is from y~-rays of less than 100 keV at h = 1 meter and only 5% at h = 100 meters, while for a .364 MeV source, the corresponding values are 13% and 30%. The values at the source energy in Figure 3 indicate the fraction of the exposure rate due to-scattered y-rays. fraction, of course, decreases with increasing energy. Figure 4 shows the integral spectra for *°K, and **°7Th series. This #3°U series The U and Th spectra are quite similar.