Structures of exotic 131 133Sn isotopes and effect on r-process nucleosynthesis

Background: Four strong single-particle bound levels with strikingly similar level spacings have recently been measured in 131Sn and 133Sn. This similarity has not yet been addressed by a theoretical nuclear structure model. Information on these single-particle bound levels, as well as on resonant levels above the neutron capture threshold, is also needed to determine neutron capture cross sections-and corresponding capture reaction rates-on 130 ^,132Sn. The 130Sn(n,γ) rate was shown in a recent sensitivity study to significantly impact the synthesis of heavy elements in the r-process in supernovae. Purpose: Understand the structure of bound and resonant levels in 131^,133Sn, and determine if the densities of unbound resonant levels are sufficiently high to warrant statistical model treatments of neutron capture on 130^,132Sn. Method: Single-particle bound and resonant levels for 131^,133Sn are self-consistently calculated by the analytical continuation of the coupling constant (ACCC) method based on a relativistic mean field (RMF) theory with BCS approximation. Results: We obtain four strong single-particle bound levels in both 131^,133Sn with an ordering that agrees with experiments and spacings that, while differing from experiment, are consistent between the Sn isotopes. We also find at most one single-particle level in the effective energy range for neutron captures in the r-process. Conclusions: Our RMF+ACCC+BCS model successfully reproduces observed single-particle bound levels in 131^,133Sn and self-consistently predicts single-particle resonant levels with densities too low for widely used traditional statistical model treatments of neutron capture cross sections on 130^,132Sn employing Fermi gas level density formulations.