Phenomena resulting from the transfer of energy from a gamma ray to matter, usually to an electron. The probability of a specific interaction occurring depends on the atomic number of the material and the energy of the gamma ray. In formation evaluation, there are two types of interactions of interest: the photoelectric effect, which indicates lithology, and Compton scattering, which depends on formation density. The third type of interaction, pair production, occurs at energies above those used for logging.