Realistic few-quasiparticle level densities in spherical nuclei

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Abstract
State and level densities with fixed exciton numbers were estimated using combinatorial calculations. Shell-model orbitals were used as a basis from which all possible configurations were generated. The pairing interaction was taken into account by applying the BCS theory to each configuration. The spin and parity distributions were obtained. The role of the pairing interaction was understood and a method of treating pairing effects in practical reaction calculations is given. The spin and parity distributions were obtained and analyzed. The assumption of a Gaussian distribution for the spin projections was found to be valid for states containing at least four quasiparticles. The energy, spin, and parity distributions of states with low exciton numbers reveal strong fluctuations which are not predicted by closed formulae derived from the equidistant single-particle-level model. Effects of the restriction limiting configurations to only bound orbitals are discussed. The restriction decreases the possible spins of highly excited ‘‘bound’’ states compared to those of the unrestricted states.