Quasivibrational bands at high spins in 158Yb.

Abstract

High-spin states in the transitional nucleus $^{158}\mathrm{Yb}$ were populated via several (HI, $4n$) reactions. Two yrast bands were established up to ${I}^{\ensuremath{\pi}}=({40}^{+}) \mathrm{and} ({31}^{\ensuremath{-}})$. In contrast to the negative-parity up to $I=31$ and the positive-parity states up to $I=24$ which show collective rotational patterns, states with ${I}^{\ensuremath{\pi}}={26}^{+}\ensuremath{-}{36}^{+}$ exhibit a nearly vibrational excitation mode. Such a quasivibrational pattern suggests a gradual transition toward oblate shapes and is the first evidence for a band termination in a heavy nucleus.