Physics and Engineering of Radiation Detection

12.1. Spectroscopy of Photons 675

Photoelectron

PMT

Beta Absorber

Source

Photon

Electron/Positron

Reflector

x−Ray

x−Ray

PP

An

An

PP:Pair Production

An:Annihilation

Scintillator

Lead Shielding

Photocathode

PP

Figure 12.1.2: Photon interaction mechanisms in a γ-ray spec-

troscopy setup.

of the system at that particular energy. In a typicalγ-ray source spectrum, other
peaks are also visible. In chapter 2, during the discussion on Compton scattering, we
saw that the two other peaks generally found are Compton edge and the backscatter
peak. The Compton edge appears at approximatelyEγ− 255 keV and corresponds
to the Compton scattering of electrons at an angle such that the electrons carry
away the maximum possible energy. This scattering can actually be visualized as
a head-on collision of an incoming photon with an electron at rest such that the
electron starts moving forward and the photon scatters back at 180^0 .Thereader
is encouraged to review the Compton scattering section in chapter 2 for detailed
discussion of the Compton edge.