Stellar neutron capture on 180Tam.: I.: Cross section measurement between 10 keV and 100 keV -: art. no. 055801

The neutron capture cross section of Ta-180(m) has been measured at energies between 10 keV and 100 keV in a time-of-flight experiment at the Karlsruhe 3.7 MV Van de Graaff accelerator. Neutrons were produced via the Li-7(p,n)(7) Be reaction by bombarding metallic Li targets with a pulsed proton beam, and gold was used as a cross section standard. Though the world supply of enriched Ta-180(m) was available for this experiment, the sample consisted of only 150 mg Ta2O5 with a Ta-180(m) content of only 5.5%. The difficult separation of the comparably few capture events in Ta-180(m) from the much larger background due to captures in Ta-181 could be achieved by means of the Karlsruhe 4pi barium fluoride detector, taking advantage of its combination of high efficiency, good energy resolution, and high granularity. The cross section was determined with an overall uncertainty of better than 10% in the energy range from 30 keV to 100 keV and could be used for renormalizing statistical model calculations in the entire energy range of astrophysical interest, which had predicted about two times larger values. Based on these first experimental data, Maxwellian averaged neutron capture cross sections were calculated for thermal energies between kT=8 keV and 100 keV.