Evidence of oblate-prolate shape coexistence in the strongly-deformed nucleus 119Cs

Abstract Prolate-oblate shape coexistence close to the ground state in the strongly-deformed proton-rich A ≈ 120 nuclei is reported for the first time. One of the four reported bands in 119Cs, built on a 11 / 2 − state at 670 keV, consists of nearly degenerate signature partners, and has properties which unequivocally indicate the strongly-coupled π h 11 / 2 [ 505 ] 11 / 2 − configuration associated with oblate shape. Together with the decoupled π h 11 / 2 [ 541 ] 3 / 2 − band built on the 11 / 2 − prolate state at 110 keV, for which a half-life of T 1 / 2 = 55 ( 5 ) μ s has been measured, the new bands bring evidence of shape coexistence at low spin in the proton-rich strongly deformed A ≈ 120 nuclei, a phenomenon predicted since long time, but not yet observed. Calculations using the particle-number conserving cranked shell model and two dimensional tilted axis cranking covariant density functional theory support and well reproduce the observed oblate and prolate coexisting low-energy states in 119Cs.