Collective oblate band inLa131due to the rotational alignment ofh11/2neutrons

Several rotational bands have been populated to high spin in the γ-soft nucleus ${}_{}{}^{131}{}_{}{}^{}\mathrm{La}$. We present expreimental evidence for a collective oblate rotational band based on a π${\mathrm{h}}_{11/2}$⊗(ν${\mathrm{h}}_{11/2}$ ${)}^2$ three-quasiparticle configuration. It is the rotational alignment of the pair of neutrons from the upper ${\mathrm{h}}_{11/2}$ midshell which drives the nucleus from a prolate (γ=0°) shape to a collectively rotating oblate (γ=-60°) shape. This band coexists with a prolate band that contains a pair of aligned protons from the lower ${\mathrm{h}}_{11/2}$ midshell.