An Attempt to Obtain Nuclear Excitation by Means of X-Rays

Abstract

WHEN a substance is irradiated with γ-rays of an energy less than that necessary for disintegration of the nuclei (for most of the elements about 8 MeV.), only a nuclear excitation can take place. If radioactivity is obtained by such an irradiation, it is necessary to assume the existence of isomeric metastable states to which the nuclei have been excited and from which transitions to the ground-level take place by emission of (highly converted) γ-rays. Experiments in this direction were performed in 1939 by Pontecorvo¹ and Collins². They succeeded in activating indium by means of X-rays with an energy of more than 1 MeV. By direct irradiation of eleven elements, Trumpy³ later claimed to have ascertained the existence of thirty-five new isomeric nuclei with characteristic half-life periods. By varying the tube voltage from 300 to 1,800 kV., Trumpy was able to determine the critical excitation energy with an accuracy of 10 per cent. The values were surprisingly low and in fourteen cases less than 0.4 MeV.