Quasivibrational bands at high spins in 158Yb.

High-spin states in the transitional nucleus /sup 158/Yb were populated via several (HI,4n) reactions. Two yrast bands were established up to I/sup ..pi../ = (40/sup +/) and (31/sup -/). In contrast to the negative-parity band up to I = 31 and the positive-parity states up to I = 24 which show collective rotational patterns, states with I/sup ..pi../ = 26/sup +/--36/sup +/ 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.