
Compton edge

In spectrophotometry, the Compton edge is a feature of the spectrograph that results from the Compton scattering in the scintillator or detector. When a gamma-ray scatters off the scintillator but escapes, only some fraction of its energy is registered by the detector. The amount of energy deposited in the detector depends on the scattering angle of the photon, leading to a spectrum of energies each corresponding to a different scattering angle. The highest energy that can be deposited, corresponding to full back-scatter, is called the Compton edge.
Background.
In a Compton scattering process, an incident photon collides with an electron in a material. The amount of energy exchanged varies with angle, and is given by the formula:
or
The amount of energy transferred to the material varies with the angle of deflection. As formula_4 approaches zero, none of the energy is transferred. The maximum amount of energy is transferred when formula_4 approaches 180 degrees.
It is impossible for the photon to transfer any more energy via this process, hence there is a sharp cutoff at this energy giving rise to the name "Compton edge".
The region between zero energy transfer and the Compton edge is known as the "Compton continuum".