Compton edge

In gamma-ray spectrometry, the Compton edge is a feature of the measured gamma-ray energy spectrum that results from Compton scattering in a scintillator or a semiconductor detector. It is a measurement phenomenom introduced by scattering within the detector, and is not present in the incident radiation.

When a gamma ray scatters within the detector and the scattered photon escapes from the detector's volume, only a fraction of the incident energy is deposited in the detector. This fraction depends on the scattering angle of the photon, leading to a spectrum of energies corresponding to the entire range of possible scattering angles. The highest energy that can be deposited, corresponding to full backscatter, is called the Compton edge. In mathematical terms, the Compton edge is the inflection point of the high-energy side of the Compton region.

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


 * $$ E^\prime = \frac{E}{1 + \frac{E}{m_{\text{e}} c^2}(1-\cos\theta)} $$


 * E is the energy of the incident photon.
 * E'  is the energy of the outgoing photon.
 * $$m_{\text{e}}$$ is the mass of the electron.
 * c is the speed of light.
 * $$\theta$$ is the angle of deflection for the photon.

The amount of energy transferred to the electron 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.


 * $$ E_T = E - E^\prime $$


 * $$ E_{\text{Compton}} = E_T (\text{max}) = E \left(1-\frac{1}{1 + \frac{2E}{m_{\text{e}} c^2}} \right)$$

In a single scattering act, is impossible for the photon to transfer any more energy via this process; thus, there is a sharp cutoff at this energy, leading to the name Compton edge. If multiple photopeaks are present in the spectrum, each of them will have its own Compton edge. The part of the spectrum between the Compton edge and the photopeak is due to multiple subsequent Compton-scattering processes.

The continuum of energies corresponding to Compton scattered electrons is known as the Compton continuum.