Spin 60 and Beyond: Exotic Behavior at the Limits of Angular Momentum
The light rare-earth nuclei near N = 90, e.g. 158Er, 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 backbends, dramatic shape changes and band termination. However, it has taken several decades before we were 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 latter sequences, observed initially in 157,158Er, were originally 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 now an even more strongly deformed triaxial minimum is preferred. These data have generated a good deal of theoretical discussion and are summarized along with news on similar structures in neighboring nuclei. These results are beginning to reveal a fascinating spectroscopy in this new exotic shape and spin regime but many questions still remain to be answered.