A spectroscopic investigation of the γ decays from excited states in 159 Er has been performed in order to study the changing structural properties exhibited as ultrahigh spins (I > 60h) are approached. The nucleus 159 Er was populated by the reaction 116 Cd( 48 Ca,5nγ) at a beam energy of 215 MeV and the resulting γ decays were studied using the Gammasphere spectrometer. New rotational bands and extensions to existing sequences were observed, which are discussed in terms of the cranked shell model, revealing a diverse range of quasiparticle configurations. At spins around 50h there is evidence for a change from dominant prolate collective motion at the yrast line to oblate non-collective structures via the mechanism of band termination. A possible strongly deformed triaxial band occurs at these high spins, which indicates collectivity beyond 50h. The high spin data are interpreted within the framework of cranked Nilsson-Strutinsky calculations.
A classic region of band termination at high spin occurs in rare-earth nuclei with around ten valence nucleons above the 146 Gd closed core. Results are presented here for such non-collective oblate (γ = 60 • ) terminating states in odd-Z 155 Ho, odd-odd 156 Ho, and even-even 156 Er, where they are compared with neighbouring nuclei. In addition to these particularly favoured states, the occurrence of collective triaxial strongly deformed (TSD) bands, bypassing the terminating states and extending to over 65 , is reviewed.
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