Stellar neutron capture onTam180. I. Cross section measurement between10keVand100keV

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The neutron capture cross section of $^{180}\mathrm{Ta}^{m}$ has been measured at energies between $10\phantom{\rule{0.3em}{0ex}}\text{keV}$ and $100\phantom{\rule{0.3em}{0ex}}\text{keV}$ in a time-of-flight experiment at the Karlsruhe $3.7\phantom{\rule{0.3em}{0ex}}\text{MV}$ Van de Graaff accelerator. Neutrons were produced via the $^{7}\mathrm{Li}(p,n)^{7}\mathrm{Be}$ 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 $^{180}\mathrm{Ta}^{m}$ was available for this experiment, the sample consisted of only $150\phantom{\rule{0.3em}{0ex}}\text{mg}$ ${\text{Ta}}_{2}{\mathrm{O}}_{5}$ with a $^{180}\mathrm{Ta}^{m}$ content of only $5.5%$. The difficult separation of the comparably few capture events in $^{180}\mathrm{Ta}^{m}$ from the much larger background due to captures in $^{181}\mathrm{Ta}$ could be achieved by means of the Karlsruhe $4\ensuremath{\pi}$ 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 $30\phantom{\rule{0.3em}{0ex}}\text{keV}$ to $100\phantom{\rule{0.3em}{0ex}}\text{keV}$ 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 $kT=8\phantom{\rule{0.3em}{0ex}}\text{keV}$ and $100\phantom{\rule{0.3em}{0ex}}\text{keV}$.