EXOTIC BEHAVIOR AT ULTRAHIGH SPIN VALUES IN LIGHT RARE-EARTH N similar to 90 NUCLEI

The light rare-earth nuclei near N = 90, e.g. Er-158, are textbook examples of the evolution of nuclear structure with excitation energy and angular momentum. They display a variety of different phenomena, such as, multiple back-bends, dramatic shape changes and band termination. However, after several decades of trying, it is only in the last few years that we have been able to observe structures beyond band termination. A spectacular return to collectivity has been found to take place extending discrete gamma-ray spectroscopy into the so-called "ultrahigh-spin regime" (I = 50-70). These sequences, observed in Er-157,Er-158, were initially interpreted as being associated with a particularly stable and energetically favored strongly deformed triaxial shape minimum. However recent quadrupole moment measurements appear to be inconsistent with this early suggestion and have generated a good deal of theoretical discussion. This surprising observation is discussed along with news on similar structures in other neighboring nuclei.