Klein-Nishina formula

A formula for the differential cross section of photons scattered from a single free electron, calculated in the lowest order of quantum electrodynamics: (dσ)/(dΩ)=1/2r_e²(λ/(λ'))²[λ/(λ')+(λ')/λ- sin ²(θ)], where r_e is the classical electron radius, λ/λ' is the ratio of the wavelengths of the incident and scattered photons, and θ is the scattering angle (0 for an undeflected photon).