γ-ray near a nucleus → electron + positron: mass from energy, E = 2m_e c²
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KE each (MeV)
1.022 MeV
Threshold 2m_e c²
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Radius of curvature (cm)
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Above threshold
Pair Production: A gamma-ray photon with E_γ ≥ 2m_e c² = 1.022 MeV can convert to an electron-positron pair in the Coulomb field of a nucleus (needed for 4-momentum conservation). The excess energy becomes kinetic: KE = (E_γ − 1.022)/2 MeV each. In a magnetic field B, both particles travel in circular arcs with the same radius r = p/(eB), but curve in opposite directions. This is the signature seen in cloud chambers and pair spectrometers. Click "Fire" to launch a photon.