, 2 Detector response defined in terms of an effective detector area, A, given by where Np = net photopeak count rate The detector response, in general, varies as a function of the gamma ray angle of incidence and is normally written as A= AOR (G) (5) where A. = detector photopeak count rate for unit flux incident perpendicular to detector face (*) R’{6) = ratio of detector response at angle G to that at B= 0° Determine A. and R (6) experimentally for a given detector. Combining Eq’s 4 and 5 with Eq. 3 leads to an expression relating measured photopeak count rate to source activity in the soil.