Neutron Resonance Spectroscopy. V. Nb, Ag, I, and Cs

Total neutron cross-section measurements for niobium, natural silver, iodine, and cesium have been made using a neutron velocity spectrometer with a resolution of about 0.5 nsec/m near the highest energy. The measurements were performed in the energy interval of 35 eV to 8 keV for niobium and 100 eV to about 4 keV for the other elements. The energies and neutron widths of about one thousand levels observed in these nuclei have been determined. The level-spacing distributions of these nuclei were investigated and found to be compatible with the random-matrix model of Wigner. These elements have $A$ values in the neighborhood of an expected peak in the $p$-wave strength function and relative minimum in the $s$-wave strength function. Histograms of the distribution of values for $y={〈g{{\Gamma }_n}^0〉}^{\frac{1}{2}}$ show a considerable excess of small widths over the number expected from the Porter-Thomas $\nu =1\mathrm{}$ distribution. This excess is interpreted as being due to the observation of many $p$-wave levels. In the case of Nb, about $\frac{2}{3}$ of the observed levels are probably $p$ wave, while the percentages were much smaller for the other elements. The numerical values of the correlation coefficients between level spacings, neutron reduced widths, and local spacing have been determined for all these nuclei. The correlation coefficients, except for the spacing correlation coefficients, are found to be consistent with zero within the statistical accuracy of the measurements. The $l=0\mathrm{}$ strength-function values averaged over spin states and isotopes of Nb, Ag, I, and Cs have been determined as (0.36±0.06)×${10}^{-4\mathrm{}}$, (0.48±0.04)×${10}^{-4\mathrm{}}$, (0.69±0.08)×${10}^{-4\mathrm{}}$, and (0.70±0.10)×${10}^{-4\mathrm{}}$, respectively.