High-resolution neutron capture and transmission measurements for116,120Sn,and their stellar neutron-capture cross sections ats-process temperatures

Abstract

We have made high-resolution measurements of the neutron capture and total cross sections for ${}^{116}\mathrm{Sn}$ and the neutron capture cross section for ${}^{120}\mathrm{Sn}$ 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 $\mathcal{R}$-matrix analysis of our new data as well as previous total cross section data for ${}^{120}\mathrm{Sn}$ to extract parameters for 216 resonances in ${}^{117}\mathrm{Sn}$ between 100 eV and 30 keV, and 187 resonances in ${}^{121}\mathrm{Sn}$ 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 ${}^{116,120}\mathrm{Sn}(n,\ensuremath{\gamma})$ 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 $kT=30 \mathrm{keV},$ our ${}^{116}\mathrm{Sn}(n,\ensuremath{\gamma})$ rate agrees to within the experimental uncertainties with the most recent high-precision measurement, and our ${}^{120}\mathrm{Sn}(n,\ensuremath{\gamma})$ 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 $kT=6--8 \mathrm{keV},$ 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.