Radiative Transition Probabilities in Heavy Nuclei. Excitation of Nuclei by X-Rays

From experimental absorption cross sections for excitation of nuclei by x-rays, radiation widths, i.e., emission probabilities are deduced. For the lowest activation levels of ${\mathrm{In}}^{115}$ and Pb widths of the order of a few millivolts are obtained. These widths are of the same order as those of the low levels of the natural radioactive nuclei. The quadrupole radiation widths obtained are within the upper limits set by a sum formula. Formulae for the transition probabilities are given depending both upon energy and spin. A numerical constant entering into the formulae, which is usually calculated from assumed models, is obtained from experiment. A simple expression is given, valid in the neighborhood of the short wave limit, for the intensity of x-rays produced by fast electrons. At the short wave limit the intensity is finite and changes but little both with energy and frequency in the neighborhood of the limit. Therefrom an excitation curve with many flat plateaus follows, each plateau indicating another nuclear level. These flat plateaus reveal directly the shape of isochromats, thus providing a qualitative check on the theory of production of x-rays by fast electrons.