|Low energy phenomena||Photoelectric effect|
|Mid-energy phenomena||Compton scattering|
|High energy phenomena||Pair production|
Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon (or another neutral boson). This is allowed, provided there is enough energy available to create the pair – at least the total rest mass energy of the two particles – and that the situation allows both energy and momentum to be conserved (though not necessarily on shell). All other conserved quantum numbers (angular momentum, electric charge) of the produced particles must sum to zero — thus the created particles shall have opposite values of each (for instance, if one particle has strangeness +1 then another one must have strangeness −1).
In nuclear physics, this occurs when a high-energy photon interacts in the vicinity of a nucleus, allowing the production of an electron and a positron pair without violating conservation of momentum. Since the momentum of the initial photon must be absorbed by something, pair production cannot occur in empty space out of a single photon; the nucleus (or another photon) is needed to conserve both momentum and energy (consider the time reversal of Electron-positron annihilation).
Photon-nucleus pair production can only occur if the photons have an energy exceeding twice the rest mass (me) 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 tau (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 physics, pair production is what happens when a photon interacts with the nucleus of an atom, instead of the electron like in lower level x-ray interactions. The photon gives its energy to the nucleus and then creates a pair of positively and negatively charged electrons. The positive electron (positron) ionizes until it combines with a free electron. The two photons scatter in opposite directions.
The probability of pair production is proportional to the energy of the incoming photon and is affected by the material's atomic number. Pair production usually happens with energy levels over 25 MeV. Pair production happens sometimes in radiation therapy treatments with high-energy photon beams.