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 occurs when a gamma ray scatters within the detector and the scattered photon escapes from the detector's volume so that only a fraction of the incident energy is deposited in the detector^[1]. 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.^[2]

Background[edit]

Gamma spectrum of radioactive Am-Be source. The Compton Spectrum or Continuum is due to scattering effects within the detector material. The highest energy that can be transferred by an incident photon in a single scattering process is referred to as the Compton edge. The photopeak after the Compton edge corresponds to the full deposition of the incident gamma-ray's energy (through a single process such as the photoelectric effect, or a sequence of various processes).

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 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 }

^[2]

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.^[2] The part of the spectrum between the Compton edge and the photopeak is due to multiple subsequent Compton-scattering processes.

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

References[edit]

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

Background[edit]

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