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, ${{\Gamma }_n}^0$, show a wide variation among the resonances in single nuclides relative to radiation widths, ${\Gamma }_{\gamma }$. 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 $\frac{{{\overline{\Gamma }}_n}^0}{D}$, of particular significance to the "cloudy crystal ball" nuclear model, has a maximum about $A=160\mathrm{}$, as expected from theory, but of a much smaller magnitude than the computed peak.

• Received 28 February 1955

DOI:https://doi.org/10.1103/PhysRev.99.10