Direct and Fluorescence Excitation of theK-Level in Thick Targets of Copper

Abstract

The ratio $P$ of the probability of exciting the $K$ shell of copper by direct cathode-ray impact to the probability of exciting the same shell by fluorescence in a thick target, is measured with normally emergent rays over a range of voltages extending from 2 to 17.4 times the $K$ excitation voltage. $P$ is found to vary from 6.63 at $U=2\mathrm{} \mathrm{to} 7.2$ at $U=17.4\mathrm{}$, where $U$ is the ratio of tube voltage to $K$ excitation voltage. The ratio $Q$ of the probability of a cathode ray ionizing the $K$ shell directly to the probability of a cathode ray exciting a continuous quantum of frequency greater than the $K$-limit frequency is found to be very nearly constant at 3.5 for the same voltage range. The variation of $P$ with $Z$ is examined theoretically and compared with experiment showing a lack of good agreement.