where p is the final momentum, Γ the Gamma function, and (if α is the fine-structure constant and r N the radius of the final state nucleus) S = 1 − − α α 2 Z 2 , η η = ± ± Z e 2 E / ( ℏ ℏ c p ) (+ for electrons, − for positrons), and ρ ρ = r N / ℏ ℏ .