Attenuation Mechanisms (Pair Production) (d)Pair Production A very high energy photon interacts with a nucleus to create an electron/positron pair.The mass of each particle is 9.11 x 10-31 kg.So the minimum photon energy is: E=2x9.11x10kg×6x10'm%c》 =1.64×10J =1.02MeV Both the electron and the positron lose energy via ionization until an annihilation event takes place yielding two photons of 0.51 MeV moving in opposite directions