High-resolution neutron capture and transmission measurements for ^116,120Sn, and their stellar neutron-capture cross sections at s-process temperatures

We have made high-resolution measurements of the neutron capture and total cross sections for ¹¹⁶Sn and the neutron capture cross section for ¹²⁰Sn from 50 eV to 500 keV at the Oak Ridge Electron Linear Accelerator (ORELA). There have been no previously reported high-resolution measurements of these cross sections in this energy range. We performed an R-matrix analysis of our new data as well as previous total cross section data for ¹²⁰Sn to extract parameters for 216 resonances in ¹¹⁷Sn between 100 eV and 30 keV, and 187 resonances in ¹²¹Sn between 350 eV and 55 keV. These resonance parameters were used to calculate the average radiation widths, average level spacings, and strength functions for s- and p-wave neutron resonances. The ^{116, 120}Sn(n, γ) astrophysical reaction rates were determined, to approximately 3% accuracy across the entire range of temperatures needed by the latest stellar models of the s process, from these resonance parameters together with our neutron capture measurements in the unresolved region. At the canonical s-process temperature of kT=30 keV, our ¹¹⁶Sn(n, γ) rate agrees to within the experimental uncertainties with the most recent high-precision measurement, and our ¹²⁰Sn(n, γ) rate resolves the discrepancy between two recent high-precision measurements. However, our measurements show that the most recently recommended reaction rates (which are based on extrapolations from previous measurements at higher energies) at kT=6-8 keV, where most of the neutron exposure occurs in stellar s-process models, are too high by approximately 10-20 %. We discuss the astrophysical impact of our new rates.