Density effect in Cu K-shell ionization by 5.1-GeV electrons.

We have made an absolute measurement of the Cu {ital K}-shell impact ionization cross section by 5.1-GeV electrons, which demonstrates directly a density effect predicted by Fermi in 1940. By determining the ratio of the {ital K} x-ray yield from a thin front and back layer of the target by a novel grazing emission method, we have verified the effect of transition radiation on the x-ray production, suggested by Sorensen and reported by Bak {ital et} {ital al}.