The theory of the double Compton effect

Using the methods of Feynman, the exact expression is derived for the differential crosssection that two photons of given energy are emitted into given directions, as the result of an unpolarized γ-ray striking an electron at rest. The probability for this process contains an extra factor e2/ħc compared with ordinary Compton scattering. There is great preference for one soft and one hard photon to be emitted, rather than two photons of approximately equal energy. The hard photon is most likely to be ejected into a narrow cone in the forward direction and, as the bombarding energy increases, this cone becomes narrower. All these results were to be expected. However, the soft photon also has a considerable preference for small-angle scattering, and its angular distribution is not roughly isotropic as one might have expected.