Influence of pairing correlations on the excitation energy, angular momentum, and parity dependence of nuclear level densities

We have studied the effects of pairing correlations on the excitation energy (E*) dependence of the total nuclear level density, parity-projected level density, and spin cutoff factor for two heavy systems Sm-152 and Yb-160, within a microscopic approach based on static path approximation to a grand canonical partition function. For both systems considered, we find that pairing has significant effects on the E* dependence of total level densities. At moderate excitation energies (E*similar to 10 MeV) it supresses the total level density nearly by two orders of magnitude. By fitting the total level density to a Bethe-type formula with energy backshift (Delta(BS)), we find that Delta(BS) approximate to 2.5 MeV for the excitation energy beyond 15 MeV. At lower excitation energies, however, Delta(BS) is strongly E* dependent. We find that the effects of pairing on the spin cutoff factor and the parity-projected level densities (or parity asymmetry) is somewhat less important. We also compare our results for the spin cutoff factor with the ones conventionally employed in statistical model codes.