STELLAR NEUTRON-CAPTURE CROSS-SECTIONS OF THE GD ISOTOPES

The neutron capture cross sections of Gd-152, Gd-154, Gd-155, Gd-156, Gd-157, and Gd-158 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 by bombarding metallic Li target s with a pulsed proton beam. Capture events were registered with the K arlsruhe 4 pi Barium Fluoride Detector, which was improved by replacin g crystals with high a background and by introducing a pierced crystal at zero degrees with respect to the beam axis. These changes resulted in a significantly increased efficiency for capture events. The main experimental problem was that the samples of the two s isotopes Gd-152 and Gd-154 showed only relatively low enrichment. Nevertheless, the s pectroscopic quality of the BaF2 detector allowed evaluation of the co rresponding corrections for isotopic impurities reliably. The cross se ction ratios could be determined with an overall uncertainty of typica lly 1%, an improvement by factors of five to ten compared to existing data. Severe discrepancies were found with respect to previous results . Maxwellian averaged neutron capture cross sections were calculated f or thermal energies between kT = 10 keV and 100 keV. The new stellar c ross sections were used for an updated analysis of the s-process react ion flow in the mass region between samarium and gadolinium, which is characterized by branchings at Sm-151, Eu-154, and Eu-155. With the cl assical approach, the s-process temperature could be constrained corre sponding to a range of thermal energies between kT = 28 and 33 keV. Th e Gd-152 production in low mass stars was found to depend strongly on the neutron freeze-out at the end of the helium shell burning episodes .