The excitation of soft X-rays

Abstract

1. In a paper (1) by one of us (O. W. R.) and Prof. Bazzoni it was shown that it was possible to detect the excitation of characteristic soft X-rays, such as the K X-rays of carbon, by a photoelectric method. The substance under investigation was bombarded with electrons in a highly-evacuated bulb of quartz glass, and the radiation generated, after passing between two parallel plates of a vacuum condenser to filter out ions and electrons, was received on a metal plate from which the photoelectric emission could be measured. The photoelectric current increases with the thermionic electron current, and with the potential difference driving the latter. If the photoelectric current divided by the thermionic current is plotted against this potential difference the excitation of characteristic X-rays setting in at certain voltages could be detected by the existence of discontinuous changes of slope in the curves so obtained. Similar observations were made almost simultaneously by A. Ll. Hughes (2), E. H. Kurth (3), P. Holweck (4), and Mohler and Foote (5), and since that time a very considerable number of papers on this subject has appeared (6)—(16). This paper gives an account of measurements made since the autumn of 1923 on the elements carbon, tungsten, nickel and iron, starting with carbon and proceeding in the order named. It is believed that they represent a progressive improvement in this branch of experimenting so that the accuracy of the results probably increases from carbon to iron. It should also be made clear that the present communication does not represent an immediate continuation of the work done with Prof. Bazzoni and referred to above. There was an intervening period (1921-1923) in which some experiments were made by one of us (O. W. R.) with the collaboration of Prof. U. Yoshida and later of Dr. H. H. Potter. The experiments made with Prof. Yoshida, on carbon and nickel, showed that the photoelectric current due to the general radiation, divided by the primary thermionic electron current, as measured in our apparatus, was nearly proportional to the voltage driving the primary electron current over the range 0-600 volts. Apart from this, which was quite definite, some of the other results obtained seemed rather indecisive, and they were not published as it seemed, at the time, that publication might tend to retard rather than to advance progress. A considerable number of the measurements with carbon included in the present paper were made with the collaboration of Dr. Potter.