Non-yrast positive-parity structures in theγ-soft nucleusEr156

Abstract

Weakly populated band structures have been established in $^{156}\mathrm{Er}$ at low to medium spins, following the $^{114}\mathrm{Cd}$($^{48}\mathrm{Ca}$,$6n\ensuremath{\gamma}$) reaction at 215 MeV. High-fold $\ensuremath{\gamma}$-ray coincidence data were recorded in a high-statistics experiment with the Gammasphere spectrometer. Bands built on the second ${0}^{+}$ and ${2}^{+}$ ($\ensuremath{\gamma}$-vibrational) states have been established. A large energy staggering between the even- and odd-spin members of the $\ensuremath{\gamma}$-vibrational band suggests a $\ensuremath{\gamma}$-soft nature of this nucleus. An additional band is discussed as being based on a rotationally aligned $(\ensuremath{\nu}{h}_{9/2},{f}_{7/2}){}^{2}$ structure, coexisting with the systematically observed, more favorable $(\ensuremath{\nu}{i}_{13/2}){}^{2}$ aligned structure seen in this mass region.