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

Abstract

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 ${}^{152}\mathrm{Sm}$ and ${}^{160}\mathrm{Yb},$ 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 ${(E}^{*}\ensuremath{\sim}10 \mathrm{MeV})$ 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 $({\ensuremath{\Delta}}_{\mathrm{BS}}),$ we find that ${\ensuremath{\Delta}}_{\mathrm{BS}}\ensuremath{\approx}2.5 \mathrm{MeV}$ for the excitation energy beyond $15 \mathrm{MeV}.$ At lower excitation energies, however, ${\ensuremath{\Delta}}_{\mathrm{BS}}$ 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.