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 kinetic energy 0.7 MeV, then the energy of the gamma ray Photon must be 

(A) 3.9 MeV 

(B) 1.78 MeV 

(C) 0.78 MeV 

(D) 0.16 MeV

Answer 1

The correct option is (B) 1.78 MeV.

Explanation:

γ photon → ₁β⁰(positron) + _–1β⁰(electron)

The energy of γ photon = rest mass energy + kinetic energy

= 0.5 + 0.5 + 0.78 ----- (rest mass of positron = rest mass of electron)

 = 1.78 MeV.