Nuclear Level Densities and Reaction Mechanisms from Inelastic Neutron Scattering

Abstract

The continuous spectrum of fast neutrons scattered inelastically by each of twenty elements has been observed at an incident energy of 7.0 MeV. The elements range in mass number from 26 to 209. The neutron spectra were observed in the range of 0.5 to 4.0 MeV. Similar observations were made for many of the same elements at one or more lower values of incident energy ranging from 4.0 to 6.0 MeV. Angular distributions of the continuous spectra of 5.0 MeV neutrons scattered inelastically by niobium and indium were also measured. The results of the investigation are as follows: (1) Compound-nucleus formation is the predominant reaction mechanism, but direct interaction plays a significant role. (2) The inferred variations of nuclear level density indicate that the nuclear entropy varies with the excitation energy either linearly, as the square root, or in some intermediate fashion, and there is some evidence that the character of this result depends on the shell structure of the nucleus in question. (3) The inferred values of Fermi-gas single-particle level density increase with mass number in the regions between closed shells and go through marked minima near and at closed shells. (4) There is evidence that the effect of pairing is significant at the excitation energies studied.