Review of bremsstrahlung experiments with free gas atom targets

Absolute doubly differential bremsstrahlung cross-sections from Ne, Ar, Kr and Xe have been measured for electron bombarding energies of 28 and 50 keV. Bremsstrahlung photons have been detected at 90° to the incident electron beam at energies ranging from 5 keV up to the kinematic endpoint. The data have been compared with two theoretical models: ordinary bremsstrahlung and total bremsstrahlung, which includes polarizational bremsstrahlung from the target atom, calculated in the stripping approximation. The data disagree with ordinary bremsstrahlung and provide definitive evidence for a contribution of polarizational bremsstrahlung. The results suggest the importance of interference between the ordinary and polarization amplitudes that is neglected in the stripping approximation. Recently, an additional background has been recognized that can affect the data for gas targets particularly at higher energy. Estimates of the effect of this background on the present data will be discussed.