Pair production

Pair production (PP), like the photoelectric absorptioneffect results in the complete attenuation of the incident photon. Pair production can only occur if the incident photon energy is at least 1.022 MeV. As the photon interacts with the strong electric field around the nucleus it undergoes a change of state and is transformed into two particles; an electron and its antimatter equivalent the positron, therefore matter is created from energy. These two particles form the pair referred to in the name of the process. The reason at least 1.022 MeV is necessary is because the resting mass (E=MC² ) of the electron and positron expressed in units of energy is 0.511 MeV each, therefore unless there is at least 0.511 MeV *2 (i.e., 1.022 MeV) it is not possible for the electron-positron pair to be created. If the energy of the incident photon is greater than 1.022 MeV, the excess is shared (although not always equally) between the electron and positron as kinetic energy. 

PP is related to the atomic number (Z) of attenuator, incident photon energy (E) and physical density (p) by ZEpZ E p. The electron and positronspositron, once liberated within the medium are dissipated through successive interactions within the medium. The electron is quickly absorbed, however the fate of the positron is not so straight forward. As it comes to a rest, it combines with a neighbouring electron and the two particles neutralise each other in a phenomenon known as annihilation radiation. Here, the two particles are converted back into two photons of electromagnetic radiation, each of 0.511 MeV energy travelling at 180 degrees to each other (a concept utilised in PET scans). These photons are then absorbed or scattered within the medium. Pair production in reality does not become the dominant process in water below about 30 MeV and is therefore of less importance in the low atomic number soft tissue elements. In industrial radiography where high atomic number elements are irradiated, pair production can become the major attenuation process assuming the incident radiation energy exceeds 1.022 MeV.