Rotational and intrinsic states above theKπ=25/2−, T1/2=25day isomer in179Hf

Abstract

Time-correlated, particle-tagged $\ensuremath{\gamma}$ spectroscopy of the stable nucleus ${}^{179}\mathrm{Hf}$ was undertaken with incomplete fusion reactions initiated by beams of ${}^{9}\mathrm{Be}$ and ${}^{7}\mathrm{Li}$ incident on targets of ${}^{176}\mathrm{Yb}.$ Intrinsic and rotational states above the three-quasiparticle ${K}^{\ensuremath{\pi}}{=25/2}^{\ensuremath{-}}, {T}_{1/2}=25 \mathrm{day}$ isomer, ${}^{179}\mathrm{Hf}{}^{m2},$ are reported. The rotational band based on ${}^{179}\mathrm{Hf}{}^{m2}$ has ${g}_{K}\ensuremath{-}{g}_{R}$ values that are consistent with the previously suggested $\ensuremath{\nu}{9/2}^{+}\ensuremath{\bigotimes}{\ensuremath{\pi}}^{2}[{7/2}^{+}{,9/2}^{\ensuremath{-}}]$ configuration assignment. A value of ${g}_{R}=0.34(5)$ was derived for the collective g factor of ${}^{179}\mathrm{Hf}{}^{m2},$ which is considerably higher than that found for the ${9/2}^{+}$ ground state. The difference is consistent with a reduction of the proton pairing strength due to blocking in the ${K}^{\ensuremath{\pi}}{=25/2}^{\ensuremath{-}}\ensuremath{\nu}\ensuremath{\bigotimes}{\ensuremath{\pi}}^{2}$ configuration. A number of ${\ensuremath{\nu}}^{3}{\ensuremath{\pi}}^{2}$ five-quasiparticle configurations were identified, the highest of which is an yrast ${K}^{\ensuremath{\pi}}{=43/2}^{+}, {T}_{1/2}=15(5) \ensuremath{\mu}\mathrm{s}$ isomer. It decays to an yrast ${K}^{\ensuremath{\pi}}{=39/2}^{\ensuremath{-}}$ state, which in turn decays to a rotational band based on a ${K}^{\ensuremath{\pi}}{=33/2}^{\ensuremath{-}}$ state. The ${K}^{\ensuremath{\pi}}{=33/2}^{\ensuremath{-}}$ state decays to the rotational band associated with ${}^{179}\mathrm{Hf}{}^{m2}.$ Semiempirical calculations reproduce the excitation energies of the three- and five-quasiparticle states above ${}^{179}\mathrm{Hf}{}^{m2}$ to within $\ensuremath{\sim}200 \mathrm{keV}.$ The calculations predict that the lowest seven-quasiparticle state will arise from a ${\ensuremath{\nu}}^{5}{\ensuremath{\pi}}^{2}$ configuration with ${K}^{\ensuremath{\pi}}{=47/2}^{\ensuremath{-}},$ which is just beyond the maximum spin accessible with the reactions employed here.