Answer:
0.51 1 MeV
Step-by-step explanation:
-mec2 is the rest energy of the electron (0.511 MeV)
-Pair production and triplet production are followed by the annihilation of the positron with a ‘free’ electron producing two annihilation quanta, most commonly with an energy of 0.511 MeV each and emitted at 180º from each other to satisfy conservation of energy, momentum and charge.