1. n. [Formation Evaluation]

A gamma ray interaction in which the gamma ray, or photon, is converted into an electron and a positron when the gamma ray enters the strong electric field near the nucleus of an atom. The gamma ray energy must equal at least the rest mass of an electron and a positron (1.022 MeV) for the interaction to be possible. Following pair production, the positron will annihilate with an electron, emitting two gamma rays of 0.511 MeV. The highest probability of occurrence is at high gamma ray energy, above 10 MeV, and in a material of high atomic number.

See: Compton scattering,  photoelectric effect,  scintillation detector