See also: [[Compton Scattering]] and [[Photoelectric Effect]] # Pair Production Motivation: [[Lecture 2 - NERS 579]] and just a good review / refresher Pair production is a type of radiation interaction with matter that can occur when a photon has an energy greater than twice the rest mass-energy of an electron (1.022 MeV). At energies a few keV above this threshold, the probability of Pair Production is small, but as the initial photon increases in energy, Pair production becomes significantly more likely [^1]. [^1]: Knoll Radiation 4th Edition In the process, the gamma-ray will enter the target material and then vanish, with an electron-positron pair appearing in its place (this is where the 1.022 MeV requirement comes from). The positron will then annihilate in the material after slowing down, generating more annihilation photons, which can also be measured, and is important when considering use in [[Scintillator Detectors]]. In spectroscopy, we can see pair productions occurring from a very significant 511 keV peak (Na-22 for example). The remaining photon energy will then deposit in a second peak, and we can therefore identify the original energy of the photon by summing the two energies together.