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 a fraction of its energy is registered by the detector. This leads to a spectrum of gamma-rays in the data that is not really there. The highest energy that occurs from this process is 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:

 \frac{1}{E^\prime} - \frac{1}{E} = \frac{1}{m_{\text{e}} c^2}\left(1-\cos \theta \right)

or

 E^\prime = \frac{E}{1 %2B \frac{(1 - \cos \theta)E}{m_{\text{e}} c^2}}

The amount of energy transferred to the material varies with the angle of deflection. As \theta approaches zero, none of the energy is transferred. The maximum amount of energy is transferred when \theta approaches 180 degrees.

 E_T = E - E^\prime
 E_{\text{Compton}} = E_T (\text{max}) = \frac{2E^2}{m_{\text{e}} c^2 %2B 2E}

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.

See also