The return of Collective Rotation in Erbium Nuclei at Ultra High Spin

E. S. Paul, P. J. Twin, A. O. Evans, A. Pipidis, M. A. Riley, J. Simpson et al.,

Physical Review Letters 98, 012501 (2007) and AIP Bulletin of Physics News, Number 807, December 2006.

A new frontier of discrete-line gamma-ray spectroscopy at ultra-high spin has been opened in the rare-earth nuclei Er-157,158 where very weakly populated rotational bands have been established. These structures bypass the well-known "band-terminating" states, marking a return to collectivity that extends discrete gamma-ray spectroscopy to well over 60hbar in these nuclei.

The response of atomic nuclei to increasing angular-momentum values, or rotational stress, continues to be a fundamental and fascinating field of scientific study. Indeed, the quest to observe ever increasing high-spin states in nuclei has driven the field of gamma-ray nuclear spectroscopy for many decades.

The Er-157,158 isotopes have featured prominently as the spectroscopy of nuclei at extreme spin has progressed. Er-158 was one of the initial nuclei in which Coriolis-induced pair-breaking (backbending) was discovered (at spin 12 hbar) and the first nucleus in which the second such alignment was observed at spin 28 hbar. At spin 38 hbar a dramatic change of structure was observed when less-collective band structures become energetically favoured. These bands reach high spin by aligning their valence single-particle angular momenta, outside the Gd-146 doubly magic core, causing the shape of the nucleus to become oblate, and "band termination" is achieved when all the valence-particle spin is exhausted (46 hbar). Indeed, this nucleus provides the textbook example of the phenomenon of band termination in heavy nuclei, which represents a beautiful manifestation of mesoscopic physics since the underlying finite-particle basis of the nuclear angular momentum generation is revealed.

It has been a goal for many years to establish the nature of the states in these nuclei in the spin range from 50 hbar up to the fission limit, well beyond the very favored band-termination states. An experiment was performed by the Nuclear Physics Group at Daresbury in collaboration with Physicists from Liverpool and Florida State University using the highly efficient GAMMASPHERE gamma-ray spectrometer. After an intensive search of the data, four very weakly populated rotational bands were established in the nuclei Er-157,158. The newly identified bands represent a return to collective rotational behaviour and now extend discrete levels in Er-157,158 to well beyond 60 hbar.

The Figure illustrates the spectacular evolution of nuclear structure in Er-158 with increasing angular momentum along with the dramatic changes in nuclear shape that occur, i.e. from prolate collective at low spin, to oblate non-collective at the "band terminating" spins near 50 hbar, and now to collective strongly deformed triaxial shapes up  to I = 65 hbar.