Lifetime measurements in neutron-rich odd-A yttrium isotopes ($^{93-99}$Y): Investigation of shape coexistence and the intertwined quantum phase transition

Abstract

Lifetimes of 16 excited states in the neutron-rich odd-$A$ nuclei $^{93-99}$Y were measured using fast-timing $\gamma$-$\gamma$ coincidence spectroscopy with fast scintillation detectors at the LOHENGRIN recoil separator. Particular attention is given to the region around $N \approx 59$, where rapid changes in nuclear deformation and shape coexistence occur. The lifetimes, determined using the generalized centroid difference method, are compared with interacting boson-fermion model calculations with configuration mixing, in which the odd-$A$ yttrium isotopes are described as a proton coupled to a bosonic core containing normal and intruder configurations. The results provide new constraints on theoretical descriptions of shape coexistence and structural evolution in neutron-rich nuclei near $A \approx 100$, particularly for odd-$A$ systems where experimental information remains limited.