Parity-Projected Shell Model Monte Carlo Level Densities for fp-shell Nuclei

Abstract

We calculate parity-dependent level densities for the even-even isotopes 58,62,66Fe and 58Ni and the odd-A nuclei 59Ni and 65Fe using the Shell Model Monte Carlo method. We perform these calculations in the complete fp-gds shell-model space using a pairing+quadrupole residual interaction. We find that, due to pairing of identical nucleons, the low-energy spectrum is dominated by positive parity states. Although these pairs break at around the same excitation energy in all nuclei, the energy dependence of the ratio of negative-to-positive parity level densities depends strongly on the particular nucleus of interest. We find equilibration of both parities at noticeably lower excitation energies for the odd-A nuclei 59Ni and 65Fe than for the neighboring even-even nuclei 58Ni and 66Fe.