Indirectly and Directly Produced X-Ray Line Radiation

A direct comparison has been made of the number of x-ray quanta in the $K$ line spectrum to the number of quanta in the continuous spectrum having energies greater than the critical value for $K$ excitation. From this, one can deduce that, for a copper target operating at low voltages, only about 10% of the line radiation is indirectly produced. A further result is that the ratio of the cross section for direct ionization to the cross section for bremsstrahlung production is about $\frac{4}{1}$, a value which is at least an order of magnitude less than theory predicts.