The answer is - 2mc^2(1+m/M)
We know that mass of stationary nucleus is 2mc^2(1+m/M)
Here c is the light speed and m mass of electron in rest
The total energy in the system would be the sum of all rest energy and kinetic energy.
It can be written as;
The expression for the minimum energy a photon must have is given by,
Now the minimum energy a photon should posses to create an electron positron pair, should be greater than
Minimum energy > 2mc^2(1+m/M)