superimposition of the suitable Gaussian on the pulse height
spectrum with the aid of a frosted-glass light table, the
area of each total absorption peak was determined without
tedious measuring with a planimeter.
For errors incurred
by this measuring system see the section on errors.
.
The ratio of absolute photon intensity to total absorption peak area ts some function of the energy of the gamma
ray, I/A = T{E).
For the determination of the graphical
relationship between I/A and E, it mas first necessary to
¥
ascertain further machine parameters and other energydependent factors.
(yz)
These were:
Crystal
interaction probability, v.
This
1s simply the fraction of incident ghotons of a particular energy which interacts in the crystal and is
given by:.
vel = exp(-1,X)
where p, is the total linear absorption coefficient
for gamma rays in sodium todide® and X is the crystal
length.
For @ crystal 20.16 cm (4 inches) in length,
‘vy versus E is shown plotted in Fig. 5.
_
(2). Peak-to-total ratio, w. This Tactoriis deter-
mined experimentally byrecording the spectra of several
sources of known gamma energy and measuring areas of the
total absorptiton peaks and the areas under the total spectrum,
The rretilos of these areas plotted against energy
14