energy of photon

Photon-nucleus pair production can only occur if the photons have an energy exceeding twice the rest energy (mec2) of an electron (1.022 MeV), photon-photon pair production may occur at 511 KeV; the same conservation laws apply for the generation of other higher energy leptons such as the muon and tauon (for two photons each should have the one-particle energy in the center of momentum frame, for one photon and a heavy nucleus, the photon needs the entire pair rest energy). These interactions were first observed in Patrick Blackett's counter-controlled cloud chamber, leading to the 1948 Nobel Prize in Physics.

In semiclassical general relativity, pair production is also invoked to explain the Hawking radiation effect. According to quantum mechanics, at short scales short-lived particle-pairs are constantly appearing and disappearing (see quantum foam); in a region of strong gravitational tidal forces, the two particles in a pair may sometimes be wrenched apart before they have a chance to mutually annihilate. When this happens in the region around a black hole, one particle may escape, with its antiparticle being captured by the hole.

Pair production is also the hypothesized mechanism behind the Pair instability supernova type of stellar explosions, where pair production suddenly lowers pressure inside a supergiant star, leading to a partial implosion, and then explosive thermonuclear burning. Supernova SN 2006gy is hypothesized to have been a pair production type supernova.

In 2008 the Titan laser aimed at a 1-millimeter-thick gold target was used to generate positron electron pairs in large numbers.