Pair production

In the third type of absorption, especially evident when elements of high atomic weight are irradiated with extremely high-energy X rays, the phenomenon of pair production occurs. When a high-energy photon penetrates the electron shell close to the nucleus, it may create a pair of electrons, one of negative charge and the other positive; a positively charged electron is also known as a POSITRON, (q.v.). This pair production is an example of the conversion of energy into MASS, (q.v.). The photon requires at least 1.2 MeV of energy to yield the mass of the pair. If the incident photon possesses more energy than is required for pair production, the excess energy is imparted to the electron pair as kinetic energy. The paths of the two particles are divergent.