Elastic and Nonelastic Neutron Cross Sections for Beryllium

A time-of-flight technique has been used to measure angular distributions for the elastic scattering of neutrons from beryllium in the energy range from 2.6 to 6.0 Mev. Total neutron cross sections were also measured and the following nonelastic (total minus elastic) cross sections obtained: 2.60 Mev (0.27±0.13 barn), 3.50 (0.43±0.10), 4.10 (0.51±0.08), 5.00 (0.60±0.08), and 6.00 (0.73±0.07). Differential cross sections were also obtained for the inelastic neutrons leaving ${\mathrm{Be}}^9$ in the 2.43-Mev excited state. Integration of these angular distributions yielded the following inelastic cross sections: 3.50 Mev (0.23±0.04 barn), 4.10 (0.25±0.04), and 5.00 (0.34±0.05). A continuous spectrum of neutrons was observed which corresponds to the direct ($n, 2n$) reaction and/or the excitation of the ${\mathrm{Be}}^9$ "level" at 1.7 Mev. This continuum was found to have an angular distribution roughly symmetric about 90°. The 2.43-Mev state of ${\mathrm{Be}}^9$ was found to decay by neutron emission to the ground state of ${\mathrm{Be}}^8$ with a branching ratio of 12±5%.