Rewriting Equation (1) in terms of © and z, combining with Equation (2) and integrating over z leads
to:
s?
tan 6 exp{- (L/p), pg h secé) de
2
(3)
a+ (H/p). P, sec €
Detector Calibration. 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:
>
(4)
where No is the net photopeak countrate (sec™! ).
The effective area, in general, varies as a function of the gamma ray angle of incidence and is
normally written as:
A = A, R (6)
(5)
where
A, = the detector photopeak countrate for a unit flux incident perpendicular to the detector face
(——) and
Yiem2 * SEC
oO
R(@) = the ratio of the detector response at an angle 6 to that at 6 = OY.
Both A, and R (6) can be determined experimentally.
_ Conversion Factor, Combining Equations (4) and (5) with Equation (3) leads to an expression which
relates the measured photopeak count rate to source activity at the surface. This is given by:
Np
A
nf2
|
R (6) tan 6 exp [-(u/p), pg h sec 6] dé
Oo
(6)
o + (u/p), p, sec
The conversion factor N,/Sy given by Equation (6) is in units of
cps
¥/em3. sec
For a specific isotope the conversion factor is normally changed to units of
89
cps
pCi/em®?
.