1. The problem statement, all variables and given/known data To show that the minimum energy a photon must have to create an electron-positron pair in the presence of a stationary nucleus of mass M is 2mc^2[1 + (m/M)], m is the electron rest mass. 2. Relevant equations Conservation of energy and momentum.Also the minimum energy a photon should have is 1.02MeV to form an electron and a positron ( wont be needing the figures here though) 3. The attempt at a solution I tried doing it in the center of mass frame ( coz the CM frame happens to give the minimum energy) but I'm getting nowhere. Should I be using the lorentz invariance E^2 - p^2c^2 in CM and lab frame?