in pulse heights that are smaller than that of the photopeak. The Compton
electrons, however, can have variable energies from zero to Emax, where Emax
is the kinetic energy of those electrons that are produced by the 180°
Compton backscattering of the g-ray photons in the detector. At relatively
high photon energy,Emaxis given by the photon energy minus 256 keV (Eq.
6.3). Thus, the g-ray spectrum will show a continuum of pulses correspond-
ing to Compton electron energies between zero and Emax. The peak at Emax
is called the Compton edge, and the portion of the spectrum below the
Compton edge down to about zero energy is called the Compton plateau
(see Fig. 8.3). The portion of the spectrum between the photopeak and the
Compton edge is called the Compton valley, which results from multiple
Compton scattering of a g-ray in the detector yielding a narrow range of
pulses in this region.
The relative heights of the photopeak and the Compton edge depend on
the photon energy as well as the size of the NaI(Tl) detector. At low ener-
gies, photoelectric effect predominates over Compton scattering, whereas
at higher energies the latter becomes predominant. In larger detectors,g-
rays may undergo multiple Compton scattering, which can add up to the
absorption of the total photon energy identical to the photoelectric effect.
This increases the contribution to the photopeak and decreases to the
Compton plateau.
Characteristic X-Ray Peak
Photoelectric interactions of the g-ray photons in the lead shield around the
detector can lead to the ejection of the K-shell electrons, followed by tran-
90 8. Scintillation and Semiconductor Detectors
Fig. 8.3. A typical spectrum of the 662-keV g-ray of^137 Cs illustrating the photopeak,
Compton plateau, Compton edge, Compton valley, backscatter, characteristic lead
Kx-ray, and barium Kx-ray peaks.