KJ 3055: X Ray Spectrometry and Radiochemical Methods

The Compton Effect

Fig. 1

Fig. 2

In physics, Compton scattering or the Compton effect (Fig. 1) is the decrease in energy (or increase in wavelength, > - Fig. 3) of an X-ray or gamma ray photon, when it interacts with electrons in matter (see also Fig. 3).

The interaction between electrons and high energy photons results in the electron being given part of the energy (making it recoil), and a photon containing the remaining energy being emitted in a different direction from the original position as a recoil electron. This is an inelastic (or incoherent) scattering. If the photon still has enough energy left, the process may be repeated. By contrast, in elastic scattering the photon change the direction but conserves its energy.

Fig. 3. A photon (yellow) collides with an electron (blue), the electron gains energy and leave as a recoil electron, while the photon loses energy to change into a lower energy photon with longer wavelength (red).

The Compton Effect was observed with X ray by Arthur Holly Compton in 1923 and further verified by his graduate student Y. H. Woo in the years following. Arthur Compton earned the 1927 Nobel Prize in Physics for the discovery.

The Compton scattering has an effect on the response of scintillation detectors. The amount of energy transferred to the scintillator varies with the angle of deflection. As θ approaches zero, none of the energy is transferred. The maximum amount of energy is transferred when θ approaches 180 degrees. 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. As an example, Fig. 4 displays the X ray spectrum of Rh recorded from a Rh X ray tube. In addition to characteristic Rh-Kα and Rh-Kb, lines due to Compton scattering appear as background signals at a lower energy compared with the pertinent characteristic line.

Fig. 4

In X or gamma ray spectrometry, 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.

F.G. Banica, 09-03-20