Evolving collectivity at high spins in 157Er, 158Er, and 159Er.

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 \ensuremath{\gamma}-\ensuremath{\gamma} coincidences were recorded between five germanium detectors at 90\ifmmode^\circ\else\textdegree\fi{} to the beam direction and a Compton-suppressed Ge detector at 0\ifmmode^\circ\else\textdegree\fi{} to record the shifted and unshifted \ensuremath{\gamma}-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.