Lifetime measurements in the neutron-rich 148Ce nuclide at the low-Z boundary of the N=90 shape-phase transition

A quantum shape phase transition (QSPT) in atomic nuclei is characterized by a sudden change of the shape of the nucleus due to changes in the location of the nuclear potential minimum. A first-order QSPT exists in the transition from spherical shapes to the axially symmetric deformed shapes. There are models describing even-even nuclei in terms of their shape and their oscillations and rotations symmetries, such us the U(5) symmetry (for spherical shapes - vibrator) and SU(3) symmetry (for axially symmetric deformed shapes - rotor). Along the QSPT the spherical minimum, corresponding to a spherical shape, starts vanishing and the deformed one, corresponding to a deformed shape, appears. While the nuclear potentials of a vibrator and a symmetric rotor have one minimum, the potential of a nucleus near the critical point (CP) of the first-order QSPT between them presents the two competing minima. The X(5) model provides some signatures of the isotopes at the CP. A first-order QSPT is known in the even-even N=90 isotones with Z=60−66. With Z=58, 148Ce lies in the low-Z boundary of this transition from spherical to axially symmetric deformed shapes. This nucleus is studied in this work. Key observables revealing the nuclear shape, such as the energy ratio R4/2=E(4+1)/E(2+1) and the transition strength ratio B4/2=B(E2;4+1→2+1)/B(E2;2+1→0+1), are compared with the predictions from models describing nuclei along this transition and specially near the CP. The experimental analysis to obtain the lifetimes of the first 2+ and first 4+ states of 148Ce using fast-timing techniques is shown in this work. The data were taken within the EXILL&FATIMA campaign performed at the high-flux reactor of the Institut Laue-Langevin in Grenoble, where fission fragments of 235U and 241Pu were measured by a hybrid spectrometer, consisting of high-resolution germanium and fast-responding lanthanum bromide scintillator detectors. The measurement of the lifetime of the first 4 + state of 148 Ce allows the calculation of the B4/2 observable and contributes in the study of the shape of the nucleus. Beside the comparison with the models, the whole N ≈ 90 region is being investigated within the interacting boson model. The shape evolution of the cerium, the neodymium and the samarium chains is studied and shows the importance of the increasing axially asymmetry