The capture cross sections for unmoderated fission neutrons, effective energy one Mev, are determined by the average properties of nuclear levels at excitation energies equal to neutron binding plus one Mev. The level spacings determined from measured cross sections for 61 isotopes are discussed in connection with the statistical and shell nuclear models. Whereas the discontinuity in neutron binding energy at magic numbers accounts for most of the abnormality of the cross sections of the magic-N nuclei, the effect of shell structure is evident in level spacing even at high (5-7 Mev) excitation energy. The level spacings obtained for nonmagic-number nuclei, on the other hand, are in reasonable agreement with the statistical nuclear model, and in addition are the same for levels of widely different spin.

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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=b0.10), 4.10 (0.51=fc0.08), 5.00 (0.60=b0.08), and 6.00 (0.73=b0.07). Differential cross sections were also obtained for the inelastic neutrons leaving 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 (0.34dz0.05). A continuous spectrum of neutrons was observed which corresponds to the direct (n,2n) reaction and/or the excitation of the 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 Be 9 was found to decay by neutron emission to the ground state of Be 8 with a branching ratio of 12±5%.

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