Q: A gamma ray photon creates an electron-positron pair. If the rest of mass of electron is 0.5 MeV and the total kinetic energy of the electron-positron pair is 0.78 MeV, then the energy of the gamma ray photon must be
(a) 0.28 MeV
(b) 0.7 MeV
(c) 1.28 MeV
(d) 1.78 MeV
Ans: (d)
Sol: γ → e+ + e–
Rest energy of electron = 9 .1 × 10-31 × (3 × 108)2 J
= 81.9 × 10-15 J
= 81.9 × 10-15/(1.6 × 10-13)
= 0.510 Me V
As mass of electron = mass of positron , hence rest mass energy is also equal
Energy of photon = 2 × 0.51 + 0.78
= 1.78 Me V