Evolving collectivity at high spins inEr157,Er158, andEr159

Abstract

Lifetimes of high-spin states in ${}_{}{}^{157}{}_{}{}^{}$,158,159Er have been measured following the reaction ${}_{}{}^{128}{}_{}{}^{}\mathrm{Te}$ ${(}^{34}$S,xn) induced by 155-MeV ${}_{}{}^{34}{}_{}{}^{}\mathrm{S}$ ions. A Doppler-shift recoil-distance device was used and γ-γ coincidences were recorded between five germanium detectors at 90° to the beam direction and a Compton-suppressed Ge detector at 0° to record the shifted and unshifted γ-ray peaks. The results for the ground band of ${}_{}{}^{158}{}_{}{}^{}\mathrm{Er}$ clearly show increasing collectivity with spin, an effect probably arising from centrifugal stretching for this N=90 nucleus. In the s band, formed by the alignment of a pair of $i_{13/2}$ neutrons, there is a significant reduction in the collectivity, which appears to have a rotational-frequency dependence similar to that found earlier for ${}_{}{}^{160}{}_{}{}^{}\mathrm{Yb}$ and interpreted as possibly arising from movement toward triaxiality in the nuclear shape. Four states each in the yrast bands of ${}_{}{}^{157}{}_{}{}^{}\mathrm{Er}$ and ${}_{}{}^{159}{}_{}{}^{}\mathrm{Er}$ were also measured, and here, too, a similar reduction of collectivity at higher rotational frequencies is observed.