Excitation energy and angular momentum dependence of nuclear level densities and spin cut-off factor in SPA and SPA+RPA approaches

We investigate the excitation energy (E*) and angular momentum (J) dependence of nuclear level density and spin cut-off factor (sigma) within microscopic approaches based on SPA and its extension SPA+RPA representation of the grand partition function for quadrupole-quadrupole interaction model Hamiltonian. For Sn-110, we find that excitation energy dependence of the total level density obtained within these approaches is significantly different. On the other hand, these approaches yield similar behaviour for J-dependence of the level density at fixed values of E*. Values of sigma(SPA+RPA) at low E*are found to be slightly smaller than sigma(SPA) but they tend to become almost the same at higher E*(> 30 MeV). We also find that Bethe's formula for fixed-J level density based on the spin cut-off approximation can be used to compute rho(E*, J) near the yrast line provided one uses an appropriate value of the spin cut-off factor. (C) 1998 Elsevier Science B.V.