Double atomic-field bremsstrahlung

We report the first measurement of the absolute cross section for the fourth-order quantum electrodynamic process of double atomic-field bremsstrahlung. The experiment observed two photons radiated in coincidence at 90° and 270° with respect to a 75-keV electron beam incident on targets of silver, terbium, gold, and uranium. The measured cross section disagrees with the predictions of the first Born approximation in both magnitude and photon energy dependence. The atomic number dependence, however, is consistent with $Z^2$.