largest portion of the exposure rate is due to unscattered y-rays (Figure 13). The direct beam or unscattered y-ray angular distributions at such source depths rise to a very sharp peak in the angular region 6 = 89° - 90°, and this peak dominates the scattered component and results in a very skewed total exposure rate angular distribution (Figure 18a). The shape of this distribution is slightly dependent on source energy becoming less peaked as the source energy decreases. The sharpness of the peak is only slightly diminished as the source becomes more deeply distributed in the ground and is still very acute even for a = .33. The angular distributions flatten out considerably as the detector height is increased but even at h = 100 meters (Figures 18a and 18b) these are still more peaked than the corresponding distributions for uniformly distri- buted sources (Figures 5 and 6). The peaks of the curves for a = .33 (Figure 18b) are slightly reduced and shifted toward the vertical (@ = 0°) compared to the corresponding curves for the plane source. This fact might be useful in estimating the depth distribution of a given isotope from an angular distribution measurement taken in an airplane or helicopter. Since the response of almost all instruments has some angular dependence, our observations are significant for interpreting field measurements. It is clear from the data presented here that careful instrument calibrations are essential in order to properly interpret measurements made at different detector heights.