Question

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.

• A. 0.78 MeV
• B. 1.78 MeV
• C. 1.28 MeV
• D. 0.28 MeV

Answer 1

Answer: (B)

1.78 MeV

Answer 2

Energy of γ-ray photon =$0.5 + 0.5 + 0.78 = 1.78\mspace{6mu} MeV$.