Fast Neutron Cross Sections and Nuclear Level Density

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.