Suppose that the energy of the emitted gamma ray photon is given the symbol "e" in units of Kev
Now, we use Einstein's equation (E = mc² ) where c is the speed of light (3 x 10^8 m/sec)
substituting in this equation, we find that:
e = mass x c²
mass = e / c²
Now to get the corresponding value in atomic mass unit:
1 eV/c² = 1/(931.46 * 10^6) amu
Therefore,
The mass in amu = e x 1/(931.46 * 10^6) atomic mass unit