parproduktion

Parproduktion, or pair production, is a physical process in which energy is converted into a particle and its antiparticle. The most common case involves a high-energy gamma-ray photon transforming in the presence of an electromagnetic field—typically the field of a nucleus—into an electron and a positron. This conversion requires sufficient energy to create the rest mass of the pair.

A threshold exists: the total energy must exceed 2 m_e c^2, about 1.022 MeV, for the electron-positron pair to be produced. In vacuum the process cannot conserve both energy and momentum, so a third body or strong external field is required. The nucleus or other charged particle acts as the partner that absorbs recoil momentum, making the reaction possible. The probability (cross section) of pair production in nuclei scales roughly with the square of the nuclear charge, Z^2, and becomes significant for heavy elements.

One well-known mechanism is the Bethe-Heitler process, where a gamma photon converts in the Coulomb field of a nucleus to produce an e+e− pair. At higher gamma energies, the cross section exhibits gradual energy dependence and remains substantial in materials with large Z, making pair production a dominant interaction for photons above a few MeV in dense media. At still higher energies, multiple pair production can occur.

Parproduktion also has other contexts, such as two-photon pair production (photon-photon collisions) and, in extremely strong electric fields, the Schwinger effect, which predicts spontaneous pair creation from vacuum. In practice, produced positrons eventually annihilate with electrons, emitting characteristic 511 keV photons, a signature exploited in gamma-ray astronomy and particle detectors.