# Physics:Compton edge

__: Greatest energy a photon can deposit when scattering off of a detector__

**Short description**

In gamma spectrometry, the **Compton edge** is a feature of a detector output spectrum that results from Compton scattering in such as a scintillation detector or Photodiode detector. It occurs when a gamma-ray scatters within the detector and some of the interaction energy escapes so that only a fraction is detected. 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 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.^{[1]}

## 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:

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

or

- [math]\displaystyle{ E^\prime = \frac{E}{1 + \frac{E}{m_{\text{e}} c^2}(1-\cos\theta)} }[/math]
^{[2]}

*E*is the energy of the incident photon.*E'*is the energy of the outgoing photon, which escapes the material.- [math]\displaystyle{ m_{\text{e}} }[/math] is the mass of the electron.
*c*is the speed of light.- [math]\displaystyle{ \theta }[/math] is the angle of deflection for the photon.

The amount of energy transferred to the material varies with the angle of deflection. As [math]\displaystyle{ \theta }[/math] approaches zero, none of the energy is transferred. The maximum amount of energy is transferred when [math]\displaystyle{ \theta }[/math] approaches 180 degrees.

- [math]\displaystyle{ E_T = E - E^\prime }[/math]
^{[1]}

- [math]\displaystyle{ E_{\text{Compton}} = E_T (\text{max}) = E \left(1-\frac{1}{1 + \frac{2E}{m_{\text{e}} c^2}} \right) }[/math]

It 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 an isotope has multiple photopeaks, each inflection point will have its own Compton edge.^{[1]}

The region between zero energy transfer and the Compton edge is known as the *Compton continuum*.

## References

- ↑
^{1.0}^{1.1}^{1.2}Prekeges, Jennifer (2010).*Nuclear medicine instrumentation*. Sudbury, Massachusetts: Jones and Bartlett Publishers. p. 42. ISBN 9781449611125. - ↑ Knoll, Glenn F.
*Radiation Detection and Measurement*2000. John Wiley & Sons, Inc.

## See also

Original source: https://en.wikipedia.org/wiki/Compton edge.
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