Two-phonon γ-vibrational strength in osmium nuclei

Abstract

Collective $E2$ strengths were measured to probe the two-phonon \ensuremath{\gamma}-vibrational character of ${I}_{K}^{\ensuremath{\pi}}{=4}_{4}^{+}$ states for ${}^{188,190}\mathrm{Os}.$ Lifetimes for states with spin up to ${8}^{+}$ of the ground-state band, ${6}^{+}$ of the \ensuremath{\gamma} band, and the ${4}^{+}$ state of the ${K}^{\ensuremath{\pi}}{=4}^{+}$ band were measured by the recoil-distance method following Coulomb excitation by a ${}^{58}\mathrm{Ni}$ projectile at ${E}_{\mathrm{lab}}=275 \mathrm{MeV}.$ These lifetime data are compared with the electromagnetic matrix elements measured by previous Coulomb excitation experiments. The good agreement between these independent measurements supports the conclusion, made by the Coulomb excitation work, that the electromagnetic properties of these osmium nuclei are consistent with the description of \ensuremath{\gamma}-soft collective models. In addition, it supports the evidence that the two-phonon \ensuremath{\gamma}-vibration configuration is the dominant component for the lowest-lying ${I}_{K}^{\ensuremath{\pi}}{=4}_{4}^{+}$ states in osmium nuclei.