The Compton Effect

INTRODUCTION Compton scattering is the inelastic scattering of an X or gamma-ray photon by a (virtually) free charged particle (usually an electron), in which some of the photon`s energy is transferred to the recoiling particle. (If the scattered photon has enough energy left after the first interaction, the process may be repeated.)

The term Compton effect is virtually synonymous with Compton scattering; perhaps one might feel inclined to use the former term when alluding mainly to the wavelength change aspect (the amount by which the wavelength changes is called the Compton shift.), and the latter term (scattering) when alluding mainly to the photon`s change of path.

Inverse Compton scattering (important in astronomy) also exists, in which it is the charged particle that transfers part of its energy to the photon.

PHOTONS IMPINGING ON ATOMS Although nuclear Compton scattering (where protons are the scattering particles) exists, the term Compton scattering mostly refers to interactions involving the atomic electrons. X-ray photon energies are very much larger than the binding energy of loosely bound electrons, so the latter can be treated as being free. . A lower energy photon - a few eV to a few keV, i.e. visible light through soft X-rays - will eject an electron from its host atom (the photoelectric effect), instead of undergoing Compton scattering. . A photon of energy comparable to the electron`s rest energy of 511 keV, i.e. within the hard X ray range, may lose part of its energy to the electron, making it recoil (Compton scattering). . A higher energy photon of 1.022 MeV or more, i.e. gamma rays or super hard X rays, may cause the formation of an electron plus a positron (pair production). Compton scattering is of prime importance to radiobiology, as it is the most probable interaction of gamma rays and high energy X-rays with atoms in living beings

HISTORY The Compton effect was discovered in 1923 by Arthur Holly Compton, who earned the Nobel Prize in Physics for this work. The effect is historically important because it corroborated existing evidence that light cannot be explained purely as a wave phenomenon. Thomson scattering, the classical theory of an electromagnetic wave scattered by charged particles, cannot explain low intensity shifts in wavelength.

APPLICATIONS Compton scattering is used in, amongst other things, radiobiology, radiation therapy, Mmaterial physics and gamma spectroscopy.