Pair production (PP), like the photoelectric effect, 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 (essentially creating matter from energy):
- one electron
- one positron (antimatter equivalent of the electron)
These two particles form the pair referred to in the name of the process. It is noteworthy that other 'pairs' of leptons (of which the electron is a type) can be created such as muon - antimuon and tau - antitau pairs, however the type of lepton pair would dictate the energy of the incident photon necessary to create them as both have far higher resting energy masses (1776 MeV for the tau and 105 MeV for the muon) than the electron and positron.
The reason at least 1.022 MeV of photon energy is necessary is because the resting mass (using E=MC² ) of the electron and positron expressed in units of energy is 0.511 MeV (or 9.1 x 10-31 kg) 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 Z E (- 1.022) p.
The electron and positron, 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 positron emission tomography - PET). 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 (due to its dependence on the 'Z' of absorber) 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.
- 1. Bomford, K (2003) Walter and Miller's Textbook of Radiotherapy. 6th Ed. London: Churchill Livingstone.
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