Neutron resonances and parity-dependence of nuclear level density at high excitation

Average s- and p-wave neutron resonance spacings (Do, D1) have long been one of the main sources of nuclear level density information. This paper is centered on the high resolution neutron total cross section measurements on 52Cr, 54Cr, 67Zn and 89Y isotopes made from a few eV to 1 MeV using pulsed neutron time of flight spectroscopy at Oak Ridge Electron Linear Accelerator (ORELA). R-matrix analysis of the data resulted in individual resonance parameters (Eo,Jπ,gτn) and average quantities like So, S1, Do, D1. The experimental ratios of s- to p- wave level spacings were found to be in disagreement with the theoretical predictions. This indicates that the underlying assumptions ρ (l,J) α (2J+1) and ρ(l+1, J) ρ(l,J) are not valid in the theoretical expression. However, the (2J+1) spin dependence of level density has been checked over the years and found valid. Recently the data on Do and D1, for various nuclei have been compiled and the analysis indicates compelling evidence that level density depends on parity. The possible implication of the parity dependence of level density at high excitation energy in the calculation of compound and pre-compound contributions to the nuclear reactions along with theoretical aspects is presented