This expression can be derived by using a quantum mechanical symmetry between pair production and Bremsstrahlung. Z is the atomic number, α α f i n e ≈ ≈ 1 / 137 the fine structure constant, ℏ ℏ the reduced Planck constant and c the speed of light. The kinetic energies E k i n , + / − − of the positron and electron relate to their total energies E + , − − and momenta p + , − − via