Misplaced Pages

This article may be too technical for most readers to understand. Please help improve it to make it understandable to non-experts, without removing the technical details. (September 2024) (Learn how and when to remove this message)

In gamma-ray spectrometry, the Compton edge is a feature of the measured gamma-ray energy spectrum that results from Compton scattering in the detector material. It corresponds to the highest energy that can be transferred to a weakly bound electron of a detector's atom by an incident photon in a single scattering process, and manifests itself as a ridge in the measured gamma-ray energy spectrum. It is a measurement phenomenon (meaning that the incident radiation does not possess this feature), which is particularly evident in gamma-ray energy spectra of monoenergetic photons.

When a gamma ray scatters within a scintillator or a semiconductor 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 a weakly bound electron, leading to its release from the atomic shell. The energy of the outgoing photon, E' , is given by the formula:

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

• E is the energy of the incident photon.
• ${\displaystyle m_{\text{e}}}$ is the mass of the electron.
• c is the speed of light.
• ${\displaystyle \theta }$ is the angle of deflection of the photon.

(note that the above formula does not account for the electron binding energy, which can play a non-negligible role for low-energy gamma rays).

The energy transferred to the electron, ${\displaystyle E_{T}}$, varies with the photon's scattering angle. For ${\displaystyle \theta }$ equal to zero there is no energy transfer, while the maximum energy transfer occurs for ${\displaystyle \theta }$ equal to 180 degrees (backscattering).

${\displaystyle E_{T}=E-E^{\prime }}$

${\displaystyle E_{\text{ComptonEdge}}=E_{T}({\text{max}})=E\left(1-{\frac {1}{1+{\frac {2E}{m_{\text{e}}c^{2}}}}}\right)}$

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.

References

1. ^ Knoll, Glenn F. Radiation Detection and Measurement 2000. John Wiley & Sons, Inc.
2. ^ Prekeges, Jennifer (2010). Nuclear medicine instrumentation. Sudbury, Massachusetts: Jones and Bartlett Publishers. p. 42. ISBN 9781449611125.

See also

• Gamma spectroscopy
• Compton suppression

• Spectroscopy