Pair production

photon, generally interacting with an atomic nucleus, produces a particle and an antiparticle. It is the chief method by which energy from gamma rays is observed in condensed matter.

Electrons and positrons are frequently produced by this process because the light weight of the electron/positron (0.51MeV) allows relatively low-energy gamma rays (>1.02MeV) to create particle pairs.

These interactions were first observed in Patrick Maynard Stuart Blackett's counter-controlled bubble chamber, leading to the 1948 nobel prize for physics.