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FSU Nuclear Physics Article Highlighted

Rapidly Rotating Nuclei: "Return of Collective Rotation at Ultra-High Spin in Er-157,158" is selected as an "Editors Suggestion" in Physical Review Letters

Often called the top journal in physics, Physical Review Letters, has this year begun a new experiment to begin selecting/suggesting to its wide readership a small number of papers per issue that were not only of special scientific importance but also sufficiently well written that they merited general interest from the broad physics community. This new "experiment" by the Editors is an attempt to regain some of the original spirit or impact of the journal and "foster unity in physics".

Of the small percentage of papers deemed with such an honor so far, one recently came from the pen of an FSU nuclear physics professor Mark Riley, his students, and his collaborators from Daresbury Laboratory and Liverpool University in the UK, the Lund Institute of Technology in Sweden, and Lawrence Berkeley National Laboratory in the US. A link to the paper can be found at Physical Review Letters 98, 012501 (2007) . In addition this work was selected for mention in the AIP Physics News Update section Cranked Up Nuclear States .

The paper was entitled "Return of Collective Rotation in 157Er and 158Er at Ultrahigh Spin" and reported on the discovery of a dramatic change in behavior of these rare-earth isotopes at extreme angular momentum or spin, see below for more details. Revealing the secrets of these nuclei at such ultrahigh rotational frequencies (roughly 100 million million million revolutions a second!) has taken decades to uncover and was only possible through the use of the world’s most powerful gamma-ray detector "Gammasphere" and the skilled use of precision analysis techniques in order to pull the "golden needle out of the haystack". (As an aside an exact replica of the Gammasphere spectrometer was featured in the 2003 blockbuster movie THE HULK where it turned Bruce Banner into the Hulk!)

This experiment, of which Drs. Mark Riley (FSU), John Simpson (Daresbury) and Eddie Paul (Liverpool), were co-Principal Investigators, had already produced one Physical Review Letter (PRL) in 2004 so to get two such highly significant results from one experiment accepted for publication in PRL is extremely unusual. The two students working on these projects, Akis Pipidis (FSU) and Aled Evans (Liverpool) have now graduated with their Ph.D.s in nuclear physics.

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.

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 60 hbar in these nuclei and thus approaching close to the fission limit.

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 has been featured in textbooks as the outstanding 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. 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. (The quantity plotted as a function of spin or angular momentum is the excitation energy of the various states with respect to a quantum rigid rotor reference.)