High-resolution neutron capture and transmission measurements, and the stellar neutron-capture cross section of Sr-88 - art. no. 055803

We have made new and improved measurements of the neutron capture and total cross sections for Sr-88 at the Oak Ridge Electron Linear Accelerator (ORE LA). Improvements over previous measurements include a wider incident neutr on energy range, better resolution, the use of metallic rather than carbona te samples, better background subtraction, reduced sensitivity to sample-de pendent backgrounds, and better pulse-height weighting functions. Because o f its small cross section, the Sr-88(n,gamma) reaction is an important bott leneck during s-process nucleosynthesis. Hence, an accurate determination o f this rate is needed to better constrain the neutron exposure in s-process models and to better understand the recently discovered isotopic anomalies in certain meteorites. We performed an R-matrix analysis of our capture an d transmission data to extract parameters for 101 resonances between 100 eV and 350 keV. In addition, we fitted our transmission data alone to extract parameters for 342 additional resonances between 350 and 950 keV. We used this information to calculate average properties of the Sr-88+ n system for comparison to previous work. Although previous data and resonance analyses were much less extensive, they are, in general, in good agreement with our results except that the average radiation widths as well as the p-wave cor relation coefficients we determined are significantly smaller, and the s-wa ve correlation coefficient we determined has opposite sign from that report ed in previous work. We used these resonance parameters together with a cal culation of the small, but significant direct-capture contribution to deter mine the astrophysical reaction rate for the Sr-88(n,gamma) reaction to app roximately 3% accuracy across the entire range of temperatures needed by s- process models. Our new rate is in good agreement with the results from a h igh-precision activation measurement at kT=25 keV, but it is approximately 9.5% lower than the rate used in most previous nucleosynthesis calculations in the temperature range (kT=6-8 keV), where most of the neutron exposure occurs in current stellar models of the s process. We discuss the possible astrophysical impact of this new, lower rate.