absolute activity, the photopeak efficiency must be determined for each
photon energy.
When multiple g-rays, either from a single radionuclide or from many
radionuclides, are present in a radioactive sample, then the energy spectrum
becomes complicated by the overlapping of different photopeaks and also
by Compton contributions from the high-energy photons to the low-energy
photopeaks. The latter contributions are termed the spillover, or crosstalk
contributions.
Figure 8.12 illustrates an energy spectrum of the 140-keV peak of 99mTc
and 364-keV peak of^131 I, in which the Compton contribution from the
364-keV peak to the 140-keV peak is shown. Corrections must be made for
this spillover to the 140-keV peak. This is accomplished by counting a
sample of pure^131 I in both 140-keV and 364-keV discriminator settings
and determining the percentage of spillover from the ratio of the counts in
the 140-keV photopeak to those in the 364-keV photopeak.
Effects of Sample Volume
The sample volume affects the counting efficiency of well counters. As the
sample volume for a given activity is increased, more radiations are lost
through the opening of the well without interacting in the detector, and
hence, the counting efficiency drops. Therefore, correction factors should be
determined for different sample volumes and applied to the measured
activity.
Gamma Well Counters 103Fig. 8.12. A combined spectrum of the 140-keV g-ray of 99mTc and 364-keV g-ray
of^131 I. The dotted line under the 140-keV photopeak is the spillover, or crosstalk,
contribution from the 364-keV photon.