The large spread in $\gamma$ values (${1-10}^5$) for gigaelectronvolt secondary beams has been used in a novel technique to measure the relativistic effects on inner-shell excitation in Cu and Al and energy loss in Si and Ge. In $K$-shell excitation, a pronounced influence of density effect is found for the first time, in apparent contradiction to earlier measurements, where the lack of density effects has been a great mystery. These problems are explained as target-thickness effects in connection with transition radiation.

• Received 18 April 1983

DOI:https://doi.org/10.1103/PhysRevLett.51.1163