— Secon pas APS (HPGe) detector having a surface area of 19 em? and a thickness of 1.6 cm. aclu 6 The detector was suspended from a retractable pneumatic boom 740 cm above ground. The boom was mounted to the rear of the Thiokol IMP - a small, lightweight, tracked vehicle--modified and equipped to be a fully self-contained, mobile, data-acquisition and reduction system. Quantitative data can be obtained from in situ measurement by combining a theoretical calculation of the flux at the detector as a function of source and source distribution with an experimental calibration of the detector response to a given incident flux. The un- scattered flux of game rays of energy E at a height h above a smooth -alr-ground interface resulting from en emitter distributed in the soil (Fig. 2) is given by: « a S exp PL go [-(u_h sec 6 + 2% sece )7 a 47 (h sece + 2 sec 6)? 2> rdrdz, (1) where S, = the source activity per unit volune ( . Va, ¥o = the air and soil tetal photons), Cm ang Sec lineéar attenuetion coefficients (cn!). This expression assumes a source distribution that varies only with depth. For fallout activity the distribution after a period of time can be reasonably approximated by an exponentia] distribution given by s = sf «7% (2)