Lifetimes in ^121,123Cs and the question of core stiffness

Lifetimes of low-lying states in ^123,123Cs and ^120,122XE are measured using the recoil-distance Doppler-shift method. The investigated nuclei were produced by the ¹⁰⁷Ag+¹⁸O and ¹⁰⁹Ag+¹⁸O reactions. The negative-parity states in ^121,123Cs are described in the framework of the core-quasiparticle coupling model with gamma -soft (the extended Wilets-Jean model) and rigid (the Davydov-Filippov model) cores. The comparison of experimental and theoretical reduced E2 transition probabilities between the yrast negative-parity states in ^121,123Cs seems to indicate that the corresponding cores cannot be fully gamma -soft as in the completely gamma -unstable model of Wilets and Jean (1956). This does not necessarily mean that the cores are as rigid as the Davydov-Filippov model assumes, but rather indicates a localization of the nucleus in the prolate ( gamma <30 degrees ) part of the beta , gamma plane.