A gamma ray photon creates an electron- positron pair. If the rest mass energy of an electron is 0.5 MeV and the total K.E. of the electron- positron pair is 0.78 MeV, then the energy of the gamma ray photon must be

0.78 MeV 1.78 MeV 1.28 MeV 0.28 MeV

1.78 MeV

Energy of γ - ray photon = 0.5 + 0.5 + 0.78 = 1.78 MeV.