Spacings and Neutron Widths of Nuclear Energy Levels

The parameters of nuclear energy levels at excitation energy just above neutron binding have been studied for about 20 heavy nuclides. Total cross sections for neutrons of energy 0 to 700 ev were made utilizing the Brookhaven fast chopper and the transmissions were analyzed to obtain the resonance parameters. The reduced neutron widths, {₍}^0, show a wide variation among the resonances in single nuclides relative to radiation widths, _. Within experimental error, the size distribution of the reduced neutron widths is exponential, the most probable width being zero. The experimental level spacings, D, exhibit discontinuities at closed shells, an effect that remains after correction of the spacings for differences in excitation energy. The ratio {{{}₍}^0}D, of particular significance to the "cloudy crystal ball" nuclear model, has a maximum about A=160, as expected from theory, but of a much smaller magnitude than the computed peak.