Critical potentials for soft X-ray excitation

Abstract

In the ‘ Proceedings of the Royal Society,’ A, vol. 110, p. 64 (1926), the authors have described an investigation of the critical potentials corresponding to the values of the electron energy associated with some of the softer X-ray levels of the elements chromium, manganese, iron, cobalt, nickel, copper, and zinc. In this investigation a greater number of critical voltages were found for each element than had been anticipated. An attempt was made to deter­mine which of the critical potentials corresponded to the M I and M II. III levels for the element in question, by extrapolating the appropriate Moseley curves, and also to account for all the critical potentials in terms of single or multiple electron displacements from the M group. The significance of the various critical potentials obtained experimentally has also been further discussed in a subsequent paper by two of the authors. The results recorded in the present paper have been obtained from a continuation of the experiments, in which more convenient methods of measuring the photoelectric current produced by the soft X-rays have been employed. The experimental tube in which the electron bombardment of the various metals took place, and in which the resulting radiation was detected, has been described in previous papers. In the present research, the photoelectric current, which served as a measure of the radiation produced by the bombard­ment, was usually measured by observing the steady deflexion of an electro­meter needle instead of by timing its motion. This method has the advantage that it allows of a series of observations being taken more rapidly than is possible by the timing method, so that the chances of variations occurring in the con­ditions of the apparatus during the experiment are considerably reduced, and series of observations can be made over larger voltage ranges and at smaller voltage intervals. Moreover, it was found possible to make the electrometer measurements more sensitive by this method, an advantage which was especially useful when the slowest bombarding streams were being used.