De-excitation of the strongly coupled band in ${}^{177}\mathrm{Au}$ and implications for core intruder configurations in the light Hg isotopes

Excited states in the proton-unbound nuclide ${}^{177}\mathrm{Au}$ were populated in the ${}^{92}\mathrm{Mo}$(${}^{88}\mathrm{Sr}, p2n$) reaction and identified using the Jurogam-II and GREAT spectrometers in conjunction with the RITU gas-filled separator at the University of Jyväskylä Accelerator Laboratory. A strongly coupled band and its decay path to the $11/2^{-}\alpha$-decaying isomer have been identified using recoil-decay tagging. Comparisons with cranked Hartree-Fock-Bogoliubov (HFB) calculations based on Skyrme energy functionals suggest that the band has a prolate deformation and is based upon coupling the odd $1h_{11/2}$ proton hole to the excited $0_2^{+}$ configuration in the ${}^{178}\mathrm{Hg}$ core. Although these configurations might be expected to follow the parabolic trend of core Hg($0_2^{+}$) states as a function of neutron number, the electromagnetic decay paths from the strongly coupled band in ${}^{177}\mathrm{Au}$ are markedly different from those observed in the heavier isotopes above the midshell. This indicates that a significant change in the structure of the underlying ${}^{A+1}\mathrm{Hg}$ core occurs below the neutron midshell.

Figure 1

$\gamma$-ray spectra measured with the JUROGAM-II spectrometer. Spectra showing $\gamma$ rays correlated with ion implantations followed by the characteristic $\alpha$ decay the $11/2^{-}$ isomer in ${}^{177}\mathrm{Au}$ ($E_{\alpha }=6124$ keV) within the same DSSD pixel of the GREAT spectrometer. The recoil-$\alpha$ correlation time was limited to 3 s. (a) $\gamma$-ray singles spectrum, while panels (b) and (c) show $\gamma$ rays in coincidence with 228- and 203-keV transitions. $\gamma$ rays assigned to the strongly coupled band in ${}^{177}\mathrm{Au}$ are labeled by their transition energies. Inband transitions are highlighted with an asterisk.

Figure 2

Level scheme of the states associated with $1h_{11/2}$ proton-hole configuration in ${}^{177}\mathrm{Au}$ deduced in the present work compared with the analogous spherical and deformed configurations in the heavier isotope ${}^{187}\mathrm{Au}$. Level excitation energies are stated relative to the $11/2^{-}$ state. The decay paths from the deformed structures to their respective $11/2^{-}$ states are different in the two isotopes.

Figure 3

(a) $\gamma$-ray energies as a function of initial state angular momentum for $E2$ transitions in the ${}^{177}\mathrm{Au}$ strongly coupled band, ${}^{178}\mathrm{Hg}$ and ${}^{176}\mathrm{Pt}$ intruder bands. The inset gives the rotational alignment calculated as shift of the ${}^{177}\mathrm{Au}$ curve relatively to ${}^{178}\mathrm{Hg}$. (b) Theoretical calculations of $\gamma$-ray energies as a function of initial state angular momentum for $E2$ transitions for the prolate ${}^{178}\mathrm{Hg}$ band and for the ${}^{177}\mathrm{Au}$ strongly coupled band based on $1h_{11/2}, K=9/2$ and $K=11/2$ configurations.