where k = E γ / E e, and where E γ = hν is the photon energy given in eV and E e = m e c ≈ 5,11∙10 eV is the electron rest mass energy, Z is an atomic number of the absorber's element, α = e /(ħc) ≈ 1/137 is the fine structure constant, and r e = e /E e ≈ 0.07941 b is the square of the classical electron radius in barns.