Evidence for X(5) critical point symmetry in ¹²⁸Ce

Lifetimes of excited states in ¹²⁸Ce were measured using the recoil distance Doppler-shift (RDDS) and the Doppler-shift attenuation (DSAM) methods. The experiments were performed at the Wright Nuclear Structure Laboratory of Yale University. Excited states of ¹²⁸Ce were populated in the ¹⁰⁰Mo(³²Si,4n) reaction at 120 MeV and the nuclear γ decay was measured with an array of eight Clover detectors positioned at forward and backward angles. The deduced yrast transition strengths together with the energies of the levels within the ground-state (gs) band of ¹²⁸Ce are in agreement with the predicted values for the X(5) critical point symmetry. Thus, we suggest ¹²⁸Ce as a benchmark X(5) nucleus in the mass A ≈ 130 region.