Understanding Neutron Radiography Post Exam Reading VIII-Part 2a of 2A

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.
Charlie Chong/ Fion Zhang
http://radiopaedia.org/articles/pair-production