Nuclear level density and the determination of thermonuclear rates for astrophysics

The prediction of cross sections for nuclei far off stability is crucial in the field of nuclear astrophysics. In recent calculations the nuclear level density—as an important ingredient to the statistical model (Hauser-Feshbach)—has shown the highest uncertainties. We present a global parametrization of nuclear level densities within the back-shifted Fermi-gas formalism. Employment of an energy-dependent level density parameter $a$, based on microscopic corrections from a recent finite range droplet model mass formula, and a backshift $\delta$, based on pairing and shell corrections, leads to a highly improved fit of level densities at the neutron-separation energy in the mass range $20<~A<~245$. The importance of using proper microscopic corrections from mass formulas is emphasized. The resulting level description is well suited for astrophysical applications. The level density can also provide clues to the applicability of the statistical model which is only correct for a high density of excited states. Using the above description, one can derive a “map” for the applicability of the model to reactions of stable and unstable nuclei with neutral and charged particles.