Nonstatistical effects observed with Cr52+n resonances

The neutron total and neutron capture cross sections of Cr52 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 E0, gn, gY, J, and l. Subsequent values for the average resonance parameters for s-wave and p-wave neutron resonances are D0=(43.4±4.7) keV and S0=(2.85±0.25)×10-4 up to 1 MeV, and D1=14.7 keV and S1=(0.30±0.05)×10-4 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 (200500) 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 Si28 and S32 which, like Cr52, have a multiple of four nucleons in the target nucleus.