Nuclear Photo-effects

Abstract

THE beautiful experiments of Bothe and Gentner¹ on the ejection of neutrons from heavier nuclei by means of γ-rays with energy of about 17 M.v. resulting from impact of protons on lithium, have revealed a remarkable selectivity of these nuclear photo-effects. Thus a few elements, apparently irregularly distributed, were found to possess cross-sections for such effects of the order 10^â26 cm.², while for the large majority of the elements investigated no measurable effects could so far be detected. At first sight this selectivity might, as pointed out from various sides, seem difficult to reconcile with the views on the mechanism of nuclear reactions to which one has been led through the study of the effects initiated by neutron impacts². In fact, the distribution of the energy levels of the compound nucleus formed in such collisions must for all heavier elements be expected to be practically continuous for excitation energies above 10 M.v., while the nuclear photo-effects obviously demand a special sensitivity for much higher excitations confined within well-separated energy regions.