Thermal-Neutron-Capture Gamma-Ray Studies of the Excited States of Odd-AHafnium Isotopes

A lithium-drifted germanium detector was used to investigate the primary high-energy $\gamma$ rays following thermal-neutron capture in the even-$A$ isotopes ${\mathrm{Hf}}^{176}$, ${\mathrm{Hf}}^{178}$, and ${\mathrm{Hf}}^{180}$. The energies, intensities, and isotopic assignments of many of these transitions were determined. The primary transitions from the capture state to the low-lying Nilsson states in ${\mathrm{Hf}}^{179}$ and ${\mathrm{Hf}}^{181}$ were found to exhibit regularities which appear inconsistent with a wholly statistical capture model. These results, together with similar findings of other investigators for the cases of ${\mathrm{W}}^{183}$, ${\mathrm{W}}^{185}$, and ${\mathrm{W}}^{187}$, are discussed in terms of both the statistical model and evidence suggestive of operational selection rules which appear to govern the radiative decay of the capture states. In addition, six previously unreported transitions have been observed and assigned to the reaction ${\mathrm{Hf}}^{177}(n,\gamma ){\mathrm{Hf}}^{178}$.