Several rotational bands have been populated to high spin in ${}_{}{}^{127}{}_{}{}^{}\mathrm{Cs}$ following the ${}_{}{}^{120}{}_{}{}^{}\mathrm{Sn}$${(}^{11}$B,4n) reaction. Rotational bands built on low-lying proton ${\mathit{g}}_{7/2}$, ${\mathit{d}}_{5/2}$, and ${\mathit{g}}_{9/2}$ (hole) orbitals, and the unique-parity ${\mathit{h}}_{11/2}$ orbital were observed and identified. For the π${\mathit{g}}_{9/2}$ (hole) case, both signatures were seen in a strongly coupled ΔI=1 band, while for the other cases, decoupled ΔI=2 bands were observed with strong in-band quadrupole transitions. Through comparisons with cranked-shell-model calculations, these band structures are understood to be associated with a prolate (γ∼0°) deformed nuclear shape. At frequencies above ħω=0.3 MeV, the rotational alignment of a pair of ${\mathit{h}}_{11/2}$ neutrons was observed for each of the ${\mathit{h}}_{11/2}$, ${\mathit{g}}_{7/2}$, and ${\mathit{d}}_{5/2}$ bands. This neutron alignment is predicted to drive the nuclear core away from a prolate shape towards the collectively rotating oblate (γ=-60°) shape. Changes in the measured signature splittings for the bands below and above the alignment are consistent with the shape changes.

• Received 5 March 1990

DOI:https://doi.org/10.1103/PhysRevC.42.890