Talk:Pair production
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[edit] Revert regarding pair production in matter
This is a note explaning my revert of the edit that was just made. The paragraph beginning with this text is not about pair production in the vacuum:
In nuclear physics, this occurs when a high-energy photon interacts with an atomic nucleus, allowing it to decay into an electron and a positron without violating conservation of momentum. It is the chief method by which energy from gamma rays is observed in condensed matter.
It refers to a real photon decaying to an electron and a positron. Although there is no nuclear decay involved, a nucleus is required to conserve momentum. If that were not true, photons with E > 1.022 MeV would decay extremely rapidly while moving through space; instead, it happens only in matter. -- SCZenz 14:49, 13 September 2005 (UTC)
[edit] Info from anon that can be properly formatted and added
• Usually occurs near a nucleus (only a negligible amount of energy is transferred to the nucleus, but momentum is not conserved.).
• hν – 1.022 MeV = E+ + E-, that is the energy of the positron plus the energy of the electron is equal to the energy of the original photon minus the rest mass of the two created particles.
• The attenuation cooeficient is proportional to Z-squared, and has a logarithmic dependence on the photon energy
• Because the probability increases with rapidly with energy above the threshold level (1.022MeV), higher energy megavoltage beams can, in fact, be less penetrating than lower energy beams, particularly in high Z materials.
• Nuclear attraction and repulsion tend to give the positron slightly more energy than the electron.
• If pair production occurs in the field of an electron, it is called Triplet Production: o Three particles appear, the original electron, the created electron and the created positron. o The threshold for triplet production is 2.04 MeV o Small probability compared to pair production (for Pb, Z = 82, triplet probability ~ 0.01 x pair probability.
• Significant compared to Compton effect when hν → 10 MeV and Z ≥ 10
• In radiotherapy, pair production can occur in bones, increasing the dose to bone.
