Abstract
The even-even N=90 isotones with Z=60-66 are known to undergo a first order phase transition. Such a phase transition in atomic nuclei is characterized by a sudden change of the shape of the nucleus due to changes in the location of the potential minimum. In these proceedings we report a measurement of the B4/2 ratio of <sup>148</sup>Ce, which will probe the location of the low-Z boundary of the N=90 phase transitional region. The measured B<sub>4/2</sub> value is compared to the prediction from the X(5) symmetry within the interacting boson model at the critical point between the geometrical limits of vibrators and rigid/axial rotors. The EXILL&FATIMA campaign took place at the high-flux reactor of the Institut Laue Langevin, Grenoble, were <sup>235</sup>U and <sup>241</sup>Pu fission fragments were measured by a hybrid spectrometer consisting of high-resolution HPGe and fast LaBr<sub>3</sub>(Ce)-scintillator detectors. The fast LaBr<sub>3</sub>(Ce) detectors in combination with the generalized centroid difference method allowed lifetime measurements in the picosecond region. Furthermore, this kind of analysis can serve as preparation for the FATIMA experiments at FAIR.