Stellar neutron capture cross sections of the Lu and Hf isotopes

The neutron capture cross sections of Lu-175,Lu- 176 and Hf-176,Hf- 177,Hf- 178,Hf- 179,Hf- 180 have been measured in the energy range from 3 to 225 keV at the Karlsruhe 3.7 MV Van de Graaff accelerator relative to the gold standard. Neutrons were produced by the Li-7(p,n)Be-7 reaction and capture events were detected by the Karlsruhe 4 pi BaF2 detector. The cross section ratios could be determined with uncertainties between 0.9 and 1.8% about a factor of five more accurate than previous data. A strong population of isomeric states was found in neutron capture of the Hf isotopes, which are only partially explained by CASINO/GEANT simulations based on the known level schemes. Maxwellian averaged neutron capture cross sections were calculated for thermal energies between kT = 8 keV and 100 keV. Severe differences up to 40% were found to the data of a recent evaluation based on existing experimental results. The new data allow for a much more reliable analysis of the important branching in the s-process synthesis path at Lu-176 which can be interpreted as an s-process thermometer.