https://doi.org/10.1140/epja/i2012-12034-5
Regular Article - Theoretical Physics
M1 strength functions from large-scale shell-model calculations and their effect on astrophysical neutron capture cross-sections
1
GSI Helmholtzzentrum für Schwerionenforschung, Planckstr. 1, 64291, Darmstadt, Germany
2
Technische Universität Darmstadt, Institut für Kernphysik, Schlossgartenstr. 9, 64289, Darmstadt, Germany
3
Frankfurt Institute of Advanced Studies, Ruth-Moufang Str. 1, 60438, Frankfurt, Germany
4
IPHC, IN2P3-CNRS et Université de Strasbourg, 23 rue du Loess, Strasbourg, France
* e-mail: G.Martinez@gsi.de
Received:
31
August
2011
Revised:
11
February
2012
Accepted:
13
February
2012
Published online:
21
March
2012
We have computed magnetic dipole strength distributions for iron isotopes within shell-model calculations based on model spaces with 40Ca and 48Ca 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 68Fe 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 69Fe. 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 69Fe with those obtained with other models.
© SIF, Springer-Verlag Berlin Heidelberg, 2012
