Complex sentence: For incident photon energies E larger than two times the rest mass of the electron (1.022 MeV), pair production can occur and the hereby produced positron annihilates with one of the surrounding electrons, typically producing two photons with 511 keV.
Split into simpler sentences: For incident photon energies E larger than two times the rest mass of the electron (1.022 MeV), pair production can occur. The resulting positron annihilates with one of the surrounding electrons, typically producing two photons with 511 keV.