Fast neutron capture on the Hf isotopes: Cross sections, isomer production, and stellar aspects

Abstract

The ($n,\ensuremath{\gamma}$) cross sections of $^{176}\mathrm{Hf}$, $^{177}\mathrm{Hf}$, $^{178}\mathrm{Hf}$, $^{179}\mathrm{Hf}$, and $^{180}\mathrm{Hf}$ have been measured in the energy range from 3 to 225 keV relative to the gold standard. Neutrons were produced via the $^{7}\mathrm{Li}$($p,n$)$^{7}\mathrm{Be}$ reaction and capture events were registered with the Karlsruhe 4\ensuremath{\pi} barium fluoride detector. The overall uncertainties are between 0.9 and 2.6%, about 5 times smaller than in previous experiments. Partial cross sections to ground and isomeric states could be experimentally identified for neutron capture on $^{176,177,178,179}\mathrm{Hf}$, indicating a strong population of yet-unknown isomeric states in $^{177}\mathrm{Hf}$ and $^{180}\mathrm{Hf}$. This feature was confirmed by extensive GEANT simulations based on calculated capture cascades. The deduced Maxwellian-averaged ($n,\ensuremath{\gamma}$) cross sections for thermal energies between $\mathit{kT}=8$ and 100 keV contribute to the analysis of the s-process branchings at $A=176$ and $A=179/180$ and have significant consequences for the separation of the solar s- and r-process components.