Photopeak
In an ideal situation, if the g-ray photon energy is absorbed by the photo-
electric mechanism and each g-ray photon yields a pulse of the same height,
then each g-ray would be seen as a line on the g-ray spectrum (Fig. 8.2A).
In reality, the photopeak is broader, which is due to various statistical vari-
ations in the process of forming the pulses. These random fluctuations arise
from the following conditions:
- Because 20–30 light photons are produced for every keV of g-ray energy
absorbed, there is a statistical variation in the number of light photons
produced by the absorption of a given g-ray energy in the detector. Also,
statistically all light photons produced may not strike the photocathode. - As already stated, 7 to 10 light photons are required to release 1 to 3
photoelectrons from the photocathode. Therefore, the number of pho-
toelectrons that one g-ray will produce may vary from one event to
another. - The number of electrons released from the successive dynodes by
impingement of each electron from the previous dynode varies from 2
to 4, and therefore pulse heights from the PM tube will vary from one
g-ray to the next of the same energy.
All of the preceding statistical fluctuations in generating a pulse cause a
spread in the photopeak (see Fig. 8.2B). A typical spectrum of the 662-keV
g-ray of^137 Cs is shown in Figure 8.3.
Compton Valley, Edge, and Plateau
When g-rays interact with the NaI(Tl) detector via Compton scattering and
scattered photons escape from the detector, the Compton electrons result
Gamma-Ray Spectrometry 89Fig. 8.2.g-ray spectra. (A) An ideal spectrum would represent the different g-rays
as lines. (B) An actual spectrum showing the spread of the photopeak that is due
to statistical fluctuations in the pulse formation.