High-Resolution Cross-Section Measurements for Production of the 0.478-MeV Gamma Ray from the ⁹Be(n,x)⁷Li Reaction for Neutron Energies Between 12 and 200 MeV with Insight into the ¹⁰Be Level Scheme

Because of the low-energy threshold and relatively large cross section of the ⁹Be(n,2n) reaction, beryllium is a very attractive neutron multiplier for some fast-neutron systems, such as those used in the production of fusion energy. But, tritium is also produced when beryllium is irradiated with 14-MeV neutrons emitted from the fusion of deuterium and tritium ions. Among the two exit channels of the ⁹Be(n,t)⁷Li reaction of ~14-MeV incident-neutron energy, the ⁹Be(n,t₁)⁷Li channel also emits a 0.478-MeV gamma ray. The purpose of the present study is to measure the cross section for the ⁹Be(n,t₁)⁷Li reaction and also that of the more general ⁹Be(n,x)⁷Li reaction with the production of the same 0.478-MeV gamma ray for incident-neutron energies from the 12-MeV threshold to 200 MeV. Because the ⁷Li levels excited above 0.478 MeV are unstable against particle emission, the study of the ⁹Be(n,t₁)⁷Li reaction gives direct access to the cross section for the formation of ⁷Li* in its 0.478-MeV excited state. The few previous experimental data for this reaction are restricted to incident-neutron energies of ~14 MeV with large discrepancies between the results. The present data are obtained with a BeO sample, using the pulsed source of high-energy neutrons of the Weapons Neutron Research Facility (WNR) at the Los Alamos Neutron Science Center (LANSCE). The 478-keV gamma rays emitted in ⁹Be(n,x)⁷Li reactions are detected with two high-resolution Ge detectors. The data thus obtained are presented and compared with previous data on the ⁹Be(n,t₁)⁷Li and the ⁹Be(n,t)⁷Li reactions. Examination of the present data also provides insight into the ¹⁰Be level scheme.