Stellar neutron capture cross sections of the Yb isotopes - art. no. 065801

The neutron capture cross sections of Yb-170, Yb-171, Yb-172, Yb-173, Yb-17 4, and Yb-176 have been measured in the energy range from 3 to 225 keV rela tive to the gold standard. Neutrons were produced at the Karlsruhe 3.75 MV Van de Graaff Accelerator via the Li-7(p,n)Be-7 reaction by bombarding meta llic Li targets with a pulsed proton beam, and capture events were register ed with the Karlsruhe 4 pi Barium Fluoride Detector. Neutron capture in the even ytterbium isotopes is characterized by a strong population of isomeri c states, lending to unrecognized systematic uncertainties in previous meas urements. For the first time, partial cross sections to ground and isomeric states could be experimentally identified in neutron time-of-flight measur ements for Yb-172, Yb-173, Yb-174, and Yb-176. The present overall uncertai nties of (1-1.5)% correspond to an improvement by factors of 4-10 compared to existing data. Maxwellian averaged neutron capture cross sections were c alculated for thermal energies between kT = 8 keV and 100 keV. In four case s, the results differ by more than 15% from recent evaluations. The s-proce ss analyses based on the present data provide further evidence in favor of stellar models for thermally pulsing low-mass asymptotic giant branch stars .