58Ni+n transmission, differential elastic scattering, and capture measurements and analysis up to 813 keV.

Abstract

High-resolution neutron cross section measurements for $^{58}\mathrm{enriched}$ targets were made at the Oak Ridge Electron Linear Accelerator of transmission below 20 MeV, of differential elastic scattering from 10 keV to 5 MeV, and of capture from 2.6 keV to 2.5 MeV. The transmission data were analyzed from 0.1 to 813 keV with a multilevel R-matrix code which uses Bayes' theorem for the fitting process. This code provides energies and neutron widths of resonances within the analyzed region, as well as a possible parametrization for resonances external to that region, as a way of describing the smooth cross section over the entire energy range. The differential elastic data at different scattering angles were compared to theoretical calculations from 30 to 813 keV using an R-function code which is based on the Blatt-Biedenharn formalism. Various combinations of spin and parity were tested to predict cross sections for the well-defined lg0 resonances, and comparison with the data then provided spin and parity assignments for most of these resonances.The capture data were analyzed from 5 to 450 keV with a least-squares fitting code using the Breit-Wigner formula. The resulting set of resonance parameters yields values for the thermal total and capture cross sections within experimental uncertainties. A total of 482 resonances are reported, of which five are fictitious s-wave resonances outside the analyzed energy region and 61 are seen and analyzed only in the capture data. The reduced widths of the 61 s-wave resonances follow the Porter-Thomas distribution and their nearest neighbor spacings agree with the Wigner distribution. The average s-wave level spacing is 13.1\ifmmode\pm\else\textpm\fi{}0.9 keV and the s-wave strength function is (3.2\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$. Since most of the large non-s-wave resonances have their angular momentum assigned with confidence, the strength functions for the p- and d-wave resonances could be determined; values are (1.3\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ and (3.0\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$, respectively. The level densities calculated with the Fermi-gas model for l=0 and lg0 resonances are compared with the cumulative number of observed resonances. The average radiation widths were deduced from resonances analyzed in the three data sets below 450 keV. The mean values and standard deviations of the distributions of the radiation widths are 2.3\ifmmode\pm\else\textpm\fi{}1.7 eV for the s-wave resonances, 0.77\ifmmode\pm\else\textpm\fi{}0.32 eV for the p-wave resonances, and 1.4\ifmmode\pm\else\textpm\fi{}0.5 eV for the d-wave resonances. The average capture cross section as a function of the incident neutron energy is compared to a prediction based on the tail of the giant electric dipole resonance.