Search for the density effect in inner-shell ionization by ultra relativistic electron impact

We have extended the measurements of K- and L-shell ionization cross sections by electron impact into the ultra relativistic energy region, 0.9 <= E <= 2.0 GeV, in order to search for a saturation of the cross section. The phenomenon, which is due to the target medium, is called density effect. It is predicted to occur at several hundred MeV impact energy and preferentially for low Z target elements. Theoretical calculations are presented, based on the one-photon exchange approximation. The absolute measurements of the K- and L-shell cross sections for Ni(K), Cu(K), Ag(K,L) and Au(L) performed at the 2.5 GeV electron synchrotron of the University Bonn, however, exhibit that the cross sections show no saturation but are still increasing. Furthermore, from the KX-ray yields, obtained at 0.9 and 2.0 GeV by bombarding the low Z elements S, Ca, Mn, Ni and Ge, we obtain for the corresponding cross section ratio sigmasub(K)(2 GeV)/sigmasub(K)(0.9 GeV) = 1.08 +- 0.012 on the average. (orig./WL)