Rotational band structures in 127Cs: Shape changes induced by h11/2 neutron alignment.

Abstract

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 \ensuremath{\pi}${\mathit{g}}_{9/2}$ (hole) case, both signatures were seen in a strongly coupled \ensuremath{\Delta}I=1 band, while for the other cases, decoupled \ensuremath{\Delta}I=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 (\ensuremath{\gamma}\ensuremath{\sim}0\ifmmode^\circ\else\textdegree\fi{}) deformed nuclear shape. At frequencies above \ensuremath{\Elzxh}\ensuremath{\omega}=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 (\ensuremath{\gamma}=-60\ifmmode^\circ\else\textdegree\fi{}) shape. Changes in the measured signature splittings for the bands below and above the alignment are consistent with the shape changes.