Pair production and annihilation
Pair production is the creation of a particle-antiparticle pair (typically electron-positron) from a high-energy photon in the presence of a nucleus. Annihilation is the reverse: a particle and antiparticle collide and convert into photons or other particles. Both processes conserve energy and momentum.
Formula
E_threshold = 2m₀c²
Real World
PET scanners in hospitals detect the two 0.511 MeV gamma photons produced when a positron from a radioactive tracer annihilates with an electron inside a patient's body.
Exam Focus
In annihilation problems, always produce two photons travelling in opposite directions to conserve momentum.
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