Stellar neutron capture cross sections of the Ba isotopes

The neutron capture cross sections of ¹³⁴Ba, ¹³⁵Ba, ¹³⁶Ba, and ¹³⁷Ba were measured in the energy range from 5 to 225 keV at the Karlsruhe 3.75 MV Van de Graaff accelerator. Neutrons were produced via the ⁷Li (p,n)⁷ Be reaction by bombarding metallic Li targets with a pulsed proton beam. Capture events were registered with the Karlsruhe 4π barium fluoride detector. The cross section ratios were determined with an overall uncertainty of ∼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 experiments, isomeric cross section ratios could be determined for ¹³⁵Ba, ¹³⁶Ba, and ¹³⁷Ba. Maxwellian averaged neutron capture cross sections 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 ¹³⁴Ba and ¹³⁶Ba the N_s〈σ〉 ratio was determined to 0.875±0.025. Hence, a significant branching of the s-process path at ¹³⁴Cs can be claimed for the first time, in contrast to predictions from the classical approach. This branching yields information on the s-process temperature, indicating values around T₈=2. The new cross sections are also important for the interpretation of barium isotopic anomalies, which were recently discovered in SiC grains of carbonaceous chondrite meteorites. Together with the results from previous experiments on tellurium and samarium, a general improvement of the N_s〈σ〉 systematics in the mass range A=120–150 is achieved. This yields a more reliable separation of s- and r-process contributions for comparison with stellar observations, but reveals a 20% discrepancy with respect to the solar barium abundance.