where oO vs . sy = the activity per unit volume at the surface s : Eatans , cm and sec a = the reciprocal of the relaxation length (cm7!). The detector response to a given flux of gamma rays of energy E, incident at an angle , can be given in terms of an effective detector area, A, defined by N Aza. The effective area, (3) in general, varics as a function of the gamma ray angle of incidence and is normally written as A= AR(E), (4) , where Ay = the detector photopeak-count-rate for a unit perpendicular to the detector face flux incident aan y /cm” - sec R(@) = the ratio of tne detector response at an angle » and to that at © = 0°, Both A, and R(:) are determined experimentally as a function of energy; calibrated reference sources are used with the detector mountce in its standard field configuration. Wa soiiiss