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;

˙M=((2m+M)c^2+KE)^2

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)