Page 141 - Radiochemistry and nuclear chemistry

126 Radiochemistry and Nuclear Chemistry

FIG. 6.1. Geometrical arrangement for measuring absorption curves.

6.2. Absorption curves

In order to measure the absorption of nuclear radiation, the experiments must be
performed in such a manner as to eliminate as many of the interfering factors as possible.
Usually a well-collimated beam is used. This is illustrated in Figure 6.1 for a point
radioactive source. The relation between the disintegration rate A and the count rate R is
given by (4.45):

R=6A

The counting efficiency ~k includes a number of factors:

"- ~sample l~abs ~det ~geom (6.1)

If conditions were ideal, there would be no self-absorption or scattering in the sample (in
which case ~k~mple = 1), no absorption of radiation between the sample and the detector
window (~'abs = 1), and the detector would have a 100% efficiency (sensitivity) to a
"count" for each particle reaching its window (~kdet = 1).
The geometric efficiency ~bgeo m, is 1 for 47r-geometry, i.e. for a spherical detector
subtending a 360 ~ solid angle about the sample. Although such detectors exist (Ch. 8),
more commonly the sample is counted outside the detector at some distance r, as indicated
in Figure 6.1. If the detector window offers an area of Sac t perpendicular to the radiation,
the geometrical efficiency is approximated by (for small ~geom)

~bgeo m ~ Sde t/(47rr 2 ) (6.2)

If a detector with a circular window of radius s is at a distance r from a source of radius
d, the geometrical efficiency is given by

~bgeo m = 1/2[1 - (1 + s 21r 2 )-'h ] k (6.3)

When the sample is a point source, k = 1, else k can be read from the series of curves
in Figure 6.2.