Gamma photon — UPSC Physics Optional CSE 1991 Paper II

A nuclear transition in a nucleus of mass 24 a.m.u. emits a gamma photon of energy E_1 if the transition occurs when the nucleus was bound in a crystal, and of energy E_1 if the transition occurs when the nucleus were free. Deduce the difference [E_1 - E_2] in keV units if E is in the range 7 meV.