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Several rotational bands have been populated to high spin in the /-soft nucleus 13, La. We present experimental evidence for a collective oblate rotational band based on a /T/MI/2® (v/zn/2) 2 threequasiparticle configuration. It is the rotational alignment of the pair of neutrons from the upper h\\/2 midshell which drives the nucleus from a prolate (y -0°) shape to a collectively rotating oblate (y= -60°) shape. This band coexists with a prolate band that contains a pair of aligned protons from the lower /211/2 midshell.

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The exact nature of the lowest K π =2 + rotational bands in all deformed nuclei remains obscure. Traditionally they are assumed to be collective vibrations of the nuclear shape in the γ degree of freedom perpendicular to the nuclear symmetry axis. Very few such γ-bands have been traced past the usual back-bending rotational alignments of high-j nucleons. We have investigated the structure of positive-parity bands in the N=90 nucleus 156 Dy, using the 148 Nd(