Stellar neutron capture cross sections of the Gd isotopes

The neutron capture cross sections of Gd152, Gd154, Gd155, Gd156, Gd157, and Gd158 were measured in the energy range from 3 to 225 keV at the Karlsruhe 3.75 MV Van de Graaff accelerator. Neutrons were produced via the Li7(p,n)7Be reaction by bombarding metallic Li targets with a pulsed proton beam. Capture events were registered with the Karlsruhe 4 Barium Fluoride Detector, which was improved by replacing crystals with high 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 Gd152 and Gd154 showed only relatively low enrichment. Nevertheless, the spectroscopic quality of the BaF2 detector allowed evaluation of the corresponding corrections for isotopic impurities reliably. The cross section ratios could be determined with an overall uncertainty of typically 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 for thermal energies between kT=10 keV and 100 keV. The new stellar cross sections were used for an updated analysis of the s-process reaction flow in the mass region between samarium and gadolinium, which is characterized by branchings at Sm151, Eu154, and Eu155. With the classical approach, the s-process temperature could be constrained corresponding to a range of thermal energies between kT=28 and 33 keV. The Gd152 production in low mass stars was found to depend strongly on the neutron freeze-out at the end of the helium shell burning episodes.