2+mixed symmetry state in the O(6)-like nucleus128Xe

Abstract

Excited states in ${}^{128}$Xe have been populated in the fusion reaction ${}^{125}\mathrm{Te}(\ensuremath{\alpha}{,n)}^{128}\mathrm{Xe}$ with the aim of searching for mixed symmetry states at energies around 2 MeV. The ${2}_{4}^{+}$ state at 2.127 MeV decays to the 2${}_{1}^{+}$ state with a transition of 99.4% $M1$ character. The lifetime of this state was measured by the Doppler attenuation technique. The $B(M1)$ value of the \ensuremath{\gamma} transition indicates the mixed symmetry character of the 2${}_{4}^{+}$ state. The experimental results are in good agreement with interacting boson model ($IBM\ensuremath{-}2$) calculations. The decay selection rules of the 2${}^{+}$ mixed-symmetry states are discussed on the basis of the $\ensuremath{\tau}$ quantum numbers and the $d$ parity.