First observation of excited states in the Ce96154 nucleus: Rigid rotation at Z=58

A new analysis of the data from measurements of $\ensuremath{\gamma}$ radiation following spontaneous fission of $^{248}\mathrm{Cm}$ and $^{252}\mathrm{Cf}$, performed using the Eurogam2 and Gammashpere arrays, respectively, has revealed, for the first time, excited states in the neutron-rich nucleus $^{154}\mathrm{Ce}_{96}$. Using these fission data we have also improved uncertainties on $\ensuremath{\gamma}$-ray energies in the ground-state bands of several neutron-rich nuclei of the $\mathrm{A}\ensuremath{\approx}150$ mass region. The improved data provided precise systematics of the ${\mathrm{E}}_{\mathrm{exc}}({4}^{+})/{\mathrm{E}}_{\mathrm{exc}}({2}^{+})$ ratio in the region, which indicate that, in the Ce isotopic chain, the rigid-rotation limit is reached at the neutron number $N=96$, two neutrons ``later'' than in the chain of Nd isotopes. The new results suggest the involvement of the proton $9/{2}^{+}[404]$ extruder orbital in generating nuclear deformation in the $\mathrm{A}\ensuremath{\approx}150$ region, in addition to and analogous to the known role played by the neutron $11/{2}^{\ensuremath{-}}[505]$ extruder in this region. The catalytic-type action of both extruders is discussed.