Direct Neutron Capture and Related Mechanisms

Abstract

We consider the evidence for the role of direct and related mechanisms in neutron capture at low and medium energies. Firstly, we compare the experimental data on the thermal neutron cross sections for El transitions in light nuclei with careful estimates of direct capture. Over the full range of light nuclei with small cross sections direct capture is found to be the predominant mechanism, in some cases being remarkably accurate, but in a few showing evidence for collective effects. When resonance effects become substantial there is evidence for an important contribution from the closely related valence mechanism, but full agreement with the data in such cases appears to require the introduction of a more generalized valence model. The possibility of direct and valence mechanisms playing a role in Ml capture is studied, and it is concluded that in light nuclei at relatively low gamma ray energies it does indeed play some role. In heavier nuclei it appears that the evidence, especially from the correlations between El and Ml transitions to the same final states, favors the hypothesis that the main transition strength is governed by the Ml giant resonance.