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

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