Influence of transition radiation and density effect on atomic K-shell excitation

A theoretical and experimental investigation is presented for the process of atomic K-shell excitation for GeV particles with special emphasis on the influence from the polarization of the surrounding target medium. It is shown that the transition radiation emitted upon entrance into the target compensates partly the reduction due to the density effect. In this way inner-shell excitation yields are obtained which depend on penetrated depth into the target, e.g., with the net result that polarization effects may be neglected for thin samples. A simple relation is constructed which explains all existing experimental data. The large spread in γ values (1-104) for a GeV secondary beam has been exploited to measure the above relativistic effects in one single experiment. Here the simple model has proven to stand all tests including variation of target thickness, comparison of X-ray yields from the front and back sides of the target, and inclusion of a stack of foils to produce extra transition radiation upstream with respect to the target. (orig.)