The stellar (n, γ) cross section of 180mTa, the rarest stable isotope in nature

The neutron capture cross section of 180mTa has been measured between 10 and 100 keV in a time-of-flight experiment at the Karlsruhe 3.7 MV Van de Graaff accelerator. Neutrons were produced via the 7Li(p, n)7Be 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 180mTa was available for this experiment, the sample consisted of 150 mg Ta2O5 enriched to only 5.5% in 180mTa. The difficult separation of the comparably few capture events in 180mTa from the much larger background due to captures in 181Ta could be achieved by means of the Karlsruhe 4π barium flouride 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 to 100 keV and could be used for renormalizing statistical model calculations outside the investigated energy range, which had predicted about two times larger values. Based on these data, Maxwellian averaged neutron capture cross sections were calculated for thermal energies between kT = 8 and 100 keV. Together with the result of a recent photoactivation experiment, these data were used for a detailed s-process analysis, which indicates that 180mTa is predominantly of s-process origin. (orig.)