The Fascinating γ -Ray World of the Atomic Nucleus: The Evolution of Nuclear Structure in 158 Er and the Future of γ -Ray Spectroscopy

The rare-earth nucleus 158 Er exhibits a number of beautiful structural changes as it evolves with increasing excitation energy and angular momentum. After undergoing Coriolis induced alignments of high- j neutron and proton pairs, a dramatic prolate collective to oblate non-collective transition takes place via the mechanism of band termination. At the highest spins, a spectacular return to col- lective rotation is observed in the form of triaxial strongly deformed bands. This latter suggestion is based on a comparison of transition quadrupole moments (Qt) between experiment and theory, and long standing predictions that such heavy nuclei will possess nonaxial shapes on their path towards fission. These exciting discoveries in 158 Er have benefited greatly from the progression of γ -ray detector developments through recent decades. The new γ -ray energy tracking technique and the next gen- eration detector arrays utilizing this technique, e.g., GRETINA (1π ) and GRETA (4π ), are briefly discussed.