1. The problem statement, all variables and given/known data How much energy is required to produce the electron-positron pair in the filed of nucleus (Z=20) if electron is captured to the ground 1s state of the system. 3. The attempt at a solution I can calculate the energy of the captured electron from E = - εZ2/(2n2) where ε=me4/hbar2≈27.2eV E = - 5442eV Now I have to add this energy to the energy of created positron to have total required energy, right ? But I do not know how to find energy of a positron.