Gamma rays constitute the highest frequency, shortest wavelength end of the Electromagnetic Spectrum. They are emitted following transitions between excited states, or between an excited state and the ground state, of atomic nuclei. Atomic nuclei are often left in excited states following a radioactive decay process such as alpha decay or beta decay. The emitted gamma ray energy is equal to the change in excitation energy of the nucleus, apart from a small and usually insignificant recoil energy.
High energy gamma rays can travel significant distances through solid material and, in doing so, they interact with electrons, or nuclei, of the material producing ionization. Gamma rays are therefore included with other types of radiation under the title Ionizing Radiation. There are three main types of gamma ray interaction with matter:
These involve the scattering of the gamma ray by a free electron. The laws of conservation of energy and momentum determine the relationship between the scattered gamma ray energy, the electron energy and the scattering angles to the energy of the initial gamma ray. The probability of Compton scattering per atom of absorber material depends on the number of electrons available as scattering targets, and therefore increases linearly with the atomic number (Z) of the material. The probability of Compton scattering generally falls off as the gamma ray energy increases. The angular distribution of the scattered gamma ray is such that there is a strong preference for forward scattering at high gamma ray energies (~10 MeV), while at 1 keV the scattering approaches isotropic.
In the photoelectric process an incoming gamma ray undergoes an interaction with an absorber atom in which the gamma ray completely disappears. In its place, an energetic photoelectron is ejected by the atom from one of its bound shells. The interaction is with the atom as a whole and cannot take place with free electrons. The photoelectric process is the predominant mode of interaction for low energy gamma rays. The photoelectric effect strongly enhanced in materials of high atomic number. In gamma ray spectroscopy the sharp peaks (photopeaks) in the spectra arise from photoelectric interactions in the detector. These photopeaks may be used to identify the radioisotopes which emitted the gamma rays.
In the field of an atomic nucleus a gamma ray energy above 1 MeV can interact to produce an electron-positron pair. In this process the gamma ray energy is converted into the mass of the electron-positron pair. The electron mass corresponds to an energy of 511 keV so pair production requires gamma rays of at least 2 × 511 keV = 1.022 MeV energy. Any energy in excess of this appears as kinetic energy of the electron-positron pair which move off in opposite directions.
Glenn F. Knoll (1989) Radiation Detection and Measurement, 2nd edn., John Wiley and Sons.