200 keV, the iodine escape peak would fall within the width of the photo-
peak, because of the small differences between the two peaks.
Annihilation Peak
g-rays with energy greater than 1.02 MeV may undergo pair production in
the detector in which a positive-negative electron pair is produced. The b+-
particles are annihilated to produce two 511-keV photons, which appear as
photopeaks in the g-ray spectrum. If, however, one of the 511-keV photons
escapes from the detector, then a peak, called the single-escape peak, cor-
responding to the primary photon energy minus 511 keV, will appear in the
spectrum. If both annihilation photons escape, then a double-escape peak
results, corresponding to the primary photon energy minus 1.02 MeV.
Larger detectors can prevent the escape of the annihilation radiations.
Coincidence Peak
A coincidenceor sum peakresults when more than one photon is absorbed
simultaneously in the detector to be considered as a single event. The peak
equals the sum of the energies of the photons. Such situations occur with
radionuclides that have short-lived isomeric states and thus emit g-rays in
cascade. For example,^111 In emits 171- and 245-keV photons, which can
result in a sum peak of 416 keV (Fig. 8.5). Sum peaks are also caused by
counting high-activity samples in which two photons may strike the detec-
tor at the same time. These peaks can be reduced by counting the samples
at larger distances between the source and the detector or by using smaller
92 8. Scintillation and Semiconductor Detectors
Fig. 8.5. A spectrum of^111 In with 171- and 245-keV photons showing a coincidence
(sum) peak at 416 keV.