Inelastic Neutron Scattering Near Threshold

Cross sections for the production of excited states by neutron inelastic scattering were measured as a function of bombarding energy from 0 to 300 keV above threshold for the first two excited states of Fe⁵⁶, the first two excited states of Cu⁶³, the first excited state of Zr⁹⁰, the first two excited states of Ba¹³⁸, and the first three excited states of Ce¹⁴⁰. Neutron-beam energy resolution was 10-20 keV. Inelastic-scattering events were counted by detecting gamma rays from the excited states. When the number of open channels for compound-nucleus decay is small, (a condition which exists near threshold for exciting low-lying states) fluctuations of reaction widths about their average values reduce the inelastic-scattering cross section from that predicted by the Hauser-Feshbach equation by as much as 50%. This correction was evaluated, assuming a Porter-Thomas distribution of widths. Good agreement was found between experimental cross sections and the modified Hauser-Feshbach equation in all but two cases.