Four-quasiparticle alignments in66Ge

Abstract

The neutron-deficient nucleus ${}^{66}\mathrm{Ge}$ was populated at high spin in two experiments using the reaction ${}^{40}\mathrm{Ca}{(}^{32}\mathrm{S},\ensuremath{\alpha}2p)$ at beam energies of 105 and 95 MeV. In the first experiment, a self-supporting ${}^{40}\mathrm{Ca}$ target was used, while a gold-backed target of similar thickness was used in the second experiment. $\ensuremath{\gamma}$ rays were detected with the EUROBALL array, combined with the charged-particle detector array EUCLIDES and the Neutron Wall. The level scheme of ${}^{66}\mathrm{Ge}$ was extended up to $E\ensuremath{\approx}18\mathrm{MeV}$ and ${I}^{\ensuremath{\pi}}{=(23}^{\ensuremath{-}}).$ Above angular momentum ${10}^{+},$ we found two sequences, connected by energetically staggered $\ensuremath{\Delta}I=1 M1$ transitions. The total Routhian surface calculations describe ${}^{66}\mathrm{Ge}$ at lower spins as a $\ensuremath{\gamma}$-soft nucleus having a moderate deformation of ${\ensuremath{\beta}}_{2}\ensuremath{\approx}0.23,$ while a triaxial deformation is predicted for the band structures above ${I}^{\ensuremath{\pi}}{=10}^{+}.$ To our knowledge, this is the first observation of staggered $M1$ transitions in a deformed four-quasiparticle $\ensuremath{\pi}{(g}_{9/2}^{2})\ensuremath{\nu}{(g}_{9/2}^{2})$ structure.