Direct and Fluorescence Excitation of theKLevel in Thick Targets of Gold

The ratio P of the number of Kα 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 to 0.27 at $U=2.2$, 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 to 0.11 at $U=2.2$. 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.