RESONANCE NEUTRON-CAPTURE AND TRANSMISSION MEASUREMENTS AND THE STELLAR NEUTRON-CAPTURE CROSS-SECTIONS OF BA-134 AND BA-136

We have made high-resolution neutron capture and transmission measurem ents on isotopically enriched samples of Ba-134 and Ba-136 at the Oak Ridge Electron Linear Accelerator (ORELA) in the energy range from 20 eV to 500 keV. Previous measurements had a lower energy limit of 3-5 k eV, which is too high to determine accurately the Maxwellian-averaged capture cross section at the low temperatures (kT approximate to 8-12 keV) favored by the most recent stellar models of the s process. By fi tting the data with a multilevel R-matrix code, we determined paramete rs for 86 resonances in Ba-134 below 11 keV and 92 resonances in Ba-13 6 below 35 keV. Astrophysical reaction rates were calculated using the se parameters together with our cross section data for the unresolved resonance region. Our results for the astrophysical reaction rates are in good agreement with the most recent previous measurement at the cl assical s-process temperature kT=30 keV, but show significant differen ces at lower temperatures. We determined that these differences were d ue to the effect of resonances below the energy range of previous expe riments and to the use of incorrect neutron widths in a previous reson ance analysis. Our data show that the ratio of reaction rates for thes e two isotopes depends more strongly on temperature than previous meas urements indicated. One result of this temperature dependence is that the mean s-process temperature we derived from a classical analysis of the branching at Cs-134 is too low to be consistent with the temperat ure derived from other branching points. This inconsistency is evidenc e for the need for more sophisticated models of the s process beyond t he classical model. We used a reaction network code to explore the cha nges in the calculated isotopic abundances resulting from our new reac tion rates for an s-process scenario based on a stellar model. These c alculations indicate that the previously observed 20% discrepancy with respect to the solar barium abundance is reduced but not resolved by our new reaction rates.