![]() The photon must be near a nucleus in order to satisfy conservation of momentum, as an electron–positron pair produced in free space cannot satisfy conservation of both energy and momentum. (Thus, pair production does not occur in medical X-ray imaging because these X-rays only contain ~150 keV.) The photon must have higher energy than the sum of the rest mass energies of an electron and positron (2 ⋅ 511 keV = 1.022 MeV, resulting in a photon-wavelength of 1.2132 picometer) for the production to occur. The photon's energy is converted to particle mass in accordance with Einstein's equation, E = m ⋅ c 2 where E is energy, m is mass and c is the speed of light. The probability of pair production in photon–matter interactions increases with photon energy and also increases approximately as the square of atomic number of (hence, number of protons in) the nearby atom. For instance, if one particle has electric charge of +1 the other must have electric charge of −1, or if one particle has strangeness of +1 then another one must have strangeness of −1. Īll other conserved quantum numbers ( angular momentum, electric charge, lepton number) of the produced particles must sum to zero – thus the created particles shall have opposite values of each other. (As the electron is the lightest, hence, lowest mass/energy, elementary particle, it requires the least energetic photons of all possible pair-production processes.) Conservation of energy and momentum are the principal constraints on the process. As energy must be conserved, for pair production to occur, the incoming energy of the photon must be above a threshold of at least the total rest mass energy of the two particles created. Pair production often refers specifically to a photon creating an electron–positron pair near a nucleus. Examples include creating an electron and a positron, a muon and an antimuon, or a proton and an antiproton. ![]() Pair production is the creation of a subatomic particle and its antiparticle from a neutral boson. ![]()
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