STELLAR NEUTRON-CAPTURE CROSS-SECTIONS OF THE TIN ISOTOPES

The neutron capture cross sections of Sn-114, Sn-115, Sn-116, Sn-117, Sn-118, and Sn-120 were measured in the energy range from 3 to 225 keV at the Karlsruhe 3.75 MV Van de Graaff accelerator. Neutrons were pro duced via the Li-7(p,n)Be-7 reaction using a pulsed proton beam. Captu re events were registered with the Karlsruhe 4 pi barium fluoride dete ctor. The experiment was complicated by the small (n,gamma) cross sect ions of the proton magic tin isotopes and by the comparably low enrich ment of the rare isotopes Sn-114 and Sn-115. Despite significant corre ctions for capture of scattered neutrons and for isotopic impurities, the high efficiency and the spectroscopic quality of the BaF2 detector allowed the determination of the cross-section ratios with overall un certainties of 1-2%, five times smaller compared to existing data. Bas ed on these results, Maxwellian averaged (n,gamma) cross sections were calculated for thermal energies between kT-10 and 100 keV. These data are used for a discussion of the solar tin abundance and for an impro ved determination of the isotopic s- and r-process components.