Direct and Fluorescence Excitation of theKLevel in Thick Targets of Gold

The ratio $P$ of the number of $K\alpha$ line quanta produced by direct cathode electron impact to the number of quanta produced by fluorescence as observed in the radiation from a thick gold target, is measured with normally emergent x-rays over a range of voltages extending from 1.3 to 2.2 times the $K$ excitation voltage. $P$ is found to vary from 0.195 at $U=1.3\mathrm{} \mathrm{to} 0.27$ at $U=2.2\mathrm{}$, where $U$ is the ratio of tube voltage to $K$ excitation voltage. The ratio $Q$ of the probability of a cathode electron ionizing the $K$ shell directly to the probability of a cathode electron exciting a continuous quantum of frequency greater than the $K$ limit frequency is found to vary from 0.07 at $U=1.3\mathrm{} \mathrm{to} 0.11$ at $U=2.2\mathrm{}$. A discussion of the variation of $P$ with atomic number found experimentally and that to be expected theoretically is given, still indicating a lack of agreement between theory and experiment.