Nonstatistical effects observed with 52Cr+n resonances.

Abstract

The neutron total and neutron capture cross sections of $^{52}\mathrm{Cr}$ have been measured using the neutron time-of-flight technique at a pulsed electron linear accelerator. Data analyses have been performed in the energy ranges 1 to 500 keV and 1 keV to 1 MeV, respectively, for capture and transmission, with R-matrix multilevel multichannel codes and with resonance shape fitting procedures, to determine the resonance parameters ${E}_{0}$, g${\ensuremath{\Gamma}}_{n}$, g${\ensuremath{\Gamma}}_{Y}$, J, and l. Subsequent values for the average resonance parameters for s-wave and p-wave neutron resonances are ${D}_{0}$=(43.4\ifmmode\pm\else\textpm\fi{}4.7) keV and ${S}_{0}$=(2.85\ifmmode\pm\else\textpm\fi{}0.25)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ up to 1 MeV, and ${D}_{1}$=14.7 keV and ${S}_{1}$=(0.30\ifmmode\pm\else\textpm\fi{}0.05)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ up to 200 keV. The following nonstatistical effects are indicated in the resonance parameter set: two gaps are observed in the s-wave level distribution, where at least two resonances for each gap are missing; a strong discontinuity in the level spacing is observed for p-wave resonances whereby three energy ranges, up to 500 keV, with different level spacings may be distinguished. This energy-dependent behavior of the p-wave level density shows that the level density parameter (a) strongly depends on the excitation energy and causes parity dependence of nuclear states in the neutron energy range (200--500) keV. These deviations of the resonance parameters from statistical behavior may be explained by doorway structures with a small energy spread of states, as has been observed for $^{28}\mathrm{Si}$ and $^{32}\mathrm{S}$ which, like $^{52}\mathrm{Cr}$, have a multiple of four nucleons in the target nucleus.