M1 strength functions from large-scale shell-model calculations and their effect on astrophysical neutron capture cross-sections

We have computed magnetic dipole strength distributions for iron isotopes within shell-model calculations based on model spaces with Ca-40 and Ca-48 cores, respectively. These distributions have been incorporated into statistical model calculations of neutron capture cross-sections. We find significant differences if the cross-sections are compared to those obtained with empirical parametrizations of the M1 strength distributions, the latter being commonly used in applications of the statistical model to astrophysically important capture reactions. As this is traditionally done, these studies are based on the hypothesis that the strength functions for all excited states are the same as for the ground state. Using neutron capture on Fe-68 as an example we investigate the validity of this hypothesis and calculate the capture cross-section on the basis of individual strength distributions calculated within the shell model for the lowest 30 states in the compound nucleus Fe-69. Finally we explore which effect the scissors mode, a fundamental orbital M1 excitation observed in deformed nuclei at rather low excitation energies, might have on capture cross-sections for nuclei with low neutron thresholds, a situation which typically occurs for r-process nuclei. The appendix compares the spin-and parity-dependent level densities for Fe-69 with those obtained with other models.