A new analysis of the data from measurements of $\gamma$ 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 $\gamma$-ray energies in the ground-state bands of several neutron-rich nuclei of the $\mathrm{A}\approx 150$ mass region. The improved data provided precise systematics of the ${\mathrm{E}}_{\mathrm{exc}}\left(4^{+}\right)/{\mathrm{E}}_{\mathrm{exc}}\left(2^{+}\right)$ 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 $9/2^{+}\left[404\right]$ extruder orbital in generating nuclear deformation in the $\mathrm{A}\approx 150$ region, in addition to and analogous to the known role played by the neutron $11/2^{-}\left[505\right]$ extruder in this region. The catalytic-type action of both extruders is discussed.

• Received 6 August 2020
• Accepted 8 December 2020

DOI:https://doi.org/10.1103/PhysRevC.102.064321