ELECTRON SCREENING IN BACKWARD ELASTIC-SCATTERING

The elastic scattering cross sections, sigma(E, theta), for the system s He+Ta and He+W have been measured at theta(lab) = 165 degrees and E- lab = 76.1 keV to 3.988 MeV using targets with a thickness of a few at omic layers. The results are smaller than the results given by the Rut herford scattering law, sigma(R)(E, theta), due to the effects of elec tron screening and can be described by sigma(E, theta)/sigma(R)(E, the ta) = (1 + U-e/E)(-1), where U-e is an atomic screening potential ener gy. The deduced average value, U-e = 28 +/- 3 keV, is consistent with the Moliere- and Lenz-Jensen-models as well as electron binding energi es.