The Compton effect is the name given by physicists to the collision between a photon and an electron. The photon bounces off a target electron and loses energy. These collisions referred as elastic compete with the photoelectric effect when gamma pass through matter. It contributes to their attenuation.
The effect was discovered in 1922 by the amercan physicist Arthur H. Compton. Compton received the Nobel Prize in Physics in 1927. He demonstrated the particle nature of electromagnetic radiation. It was a sensational discovery at the time.
Compton collisions can be viewed as elastic collisions between a photon and an electron. These elastic collisions become predominant when the photon energy becomes large compared to the energy that holds the electron in an atom, its binding energy. For a light atom such as carbon, the Compton effect prevails on the photoelectric effect above 20 keV. For copper it is above 130 keV, and for lead 600 keV.
In this gamma energy range, which is rather extended , the phenomenon concerns all the electrons of the atom, whereas in the photoelectric effect these are the two electrons from the innermost K shell, which play a role. For an absorber, it is the density of electrons that is crucial in the range where Compton effect dominates. Lead has thus also an advantage over lighter materials, even if less important than for the photoelectric effect, which came at the fourth power of the high electrical charge of its nucleus.
The scattered photon emerges usually in a different direction than the incident photon one. It can even go backward ( backscattering). On average it scatters with an angle of 30 to 45 degrees. Gamma of hundreds of keV can undergo multiple Compton scattering before being absorbed by photoelectric effect.
When the gamma energy exceeds 1 MeV, which is rarely the case for the gamma rays emitted by nuclei, Compton scattering begins to be challenged by a new phenomenon: the transformation of a gamma into an electron and its antiparticle, a positron. This phenomenon becomes prominent with the high-energy gamma produced by example with particle accelerators.
NEXT: Photoelectric Effect
Access to page in french.