The conversion of a photon (gamma ray), which has more than twice the rest mass energy of an electron (about 0.51 MeV per electron), into an electron and a positron when the incident photon passes through the strong electric field surrounding an atomic nucleus and vanishes. This is an example of creation of matter (the electron pair, one negative and one positive) from energy (the photon) according to Einstein's law: E = mc2. Relatively unimportant in density logging because of the high threshold energy (greater than 1.02 MeV) required for the incident gamma ray. Important in the detection of gamma rays in the ionization chamber and Geiger-Mueller counter. One of the three interactions of gamma rays with matter. Compare photoelectric effect and Compton scattering.
