STELLAR NEUTRON-CAPTURE CROSS-SECTIONS OF THE BA ISOTOPES

The neutron capture cross sections of Ba-134, Ba-135, Ba-138, and Ba-1 37 were measured in the energy range from 5 to 225 keV at the Karlsruh e 3.75 MV Van de Graaff accelerator. Neutrons were produced via the Li -7(p, n)Be-7 reaction by bombarding metallic Li targets with a pulsed proton beam. Capture events were registered with the Karlsruhe 4 pi ba rium fluoride detector. The cross section ratios were determined with an overall uncertainty of similar to 3%, an improvement by factors of 5 to 8 compared to existing data. Severe discrepancies were found with respect to previous results. As a new possibility in time of flight e xperiments, isomeric cross section ratios could be determined for Ba-1 35, Ba-136, and Ba-137. Maxwellian averaged neutron capture cross sect ions were calculated for thermal energies between kT = 10 keV and 100 keV. These stellar cross sections were used in an s-process analysis. For the s-only isotopes Ba-134 and Ba-138 the N-s(sigma) ratio was det ermined to 0.875 +/- 0.025. Hence, a significant branching of the s-pr ocess path at Cs-134 can be claimed for the first time, in contrast to predictions from the classical approach. This branching yields inform ation on the s-process temperature, indicating values around T-8 = 2. The new cross sections are also important for the interpretation of ba rium isotopic anomalies, which were recently discovered in SiC grains of carbonaceous chondrite meteorites. Together with the results from p revious experiments on tellurium and samarium, a general improvement o f the N-s(sigma) systematics in the mass range A = 120-150 is achieved . This yields a more reliable separation of s- and r-process contribut ions for comparison with stellar observations, but reveals a 20% discr epancy with respect to the solar barium abundance.