Regularity of core coupled high-spin structure in neutron-rich $^{196-202}$Au isotopes approaching N $=126$

Abstract

Excited states built upon long-lived high-spin isomers in hard-to-reach neutron-rich Au ($Z=79$) isotopes were populated using multi-nucleon transfer reactions between $^{136}$Xe and $^{198}$Pt at 7 MeV/u. These states in $^{196,\,198-202}$Au were identified for the first time using the powerful combination of the VAMOS++ spectrometer, the CATLIFE detection system, and the AGATA $γ$-ray tracking array. Their measured energies exhibit remarkable regularity as a function of neutron number and are seen to be inherited from the energies of yrast-band members in the corresponding Hg ($Z=80$) isotope. Large-scale shell-model calculations reproduce the observed regularity and show that these states arise from the unique-parity orbitals $\pi0h_{11/2}$ and $\nu0i_{13/2}$ coupled to the Hg core. This regularity is due to the dominant proton configurations of the Hg and Au isotopes, where the level energies are almost independent of different neutron-orbital occupancies. The calculated Au wave functions show significant higher-spin components of the corresponding Hg core, unlike what is expected in the conventional interpretation in terms of the weak-coupling/decoupling limits of the particle-core coupling model.