In pair production, a high-energy photon is converted into a particle and its antiparticle. The most common example produces an electron and a positron:γ → e- + e+ Energy is converted into mass, so the photon must have at least enough energy to supply the total rest energy of the two particles:Emin = 2mc2
For an electron-positron pair, Emin = 2 × 0.511 = 1.022 MeV. Any extra photon energy becomes the kinetic energy of the pair.Charge (0 = –1 + 1) and lepton number (0 = +1 + (–1)) are conserved. Pair production happens close to a nucleus, which recoils and takes some momentum, so that momentum is conserved as well as energy.In a detector with a magnetic field, the photon leaves no track. The pair appears as two tracks starting from the same point and curving in opposite directions, often spiralling as they lose energy.
Worked example: find the minimum frequency of a photon that can produce an electron-positron pair.
E = 1.022 × 106 × 1.6 × 10-19 = 1.64 × 10-13 J
f = E/h = 1.64 × 10-13 / 6.63 × 10-34 = 2.5 × 1020 Hz (a gamma ray)Heavier pairs need more energy: a proton-antiproton pair needs at least 2 × 938 = 1876 MeV.
For an electron-positron pair, Emin = 2 × 0.511 = 1.022 MeV. Any extra photon energy becomes the kinetic energy of the pair.Charge (0 = –1 + 1) and lepton number (0 = +1 + (–1)) are conserved. Pair production happens close to a nucleus, which recoils and takes some momentum, so that momentum is conserved as well as energy.In a detector with a magnetic field, the photon leaves no track. The pair appears as two tracks starting from the same point and curving in opposite directions, often spiralling as they lose energy.
Worked example: find the minimum frequency of a photon that can produce an electron-positron pair.E = 1.022 × 106 × 1.6 × 10-19 = 1.64 × 10-13 J
f = E/h = 1.64 × 10-13 / 6.63 × 10-34 = 2.5 × 1020 Hz (a gamma ray)Heavier pairs need more energy: a proton-antiproton pair needs at least 2 × 938 = 1876 MeV.