Observation of γ vibrations and alignments built on non-ground-state configurations in 156Dy.

The exact nature of the lowest Kμ = 2+ rotational bands in all deformed nuclei remains obscure. Traditionally they are assumed to be collective vibrations of the nuclear shape in the y degree of freedom perpendicular to the nuclear symmetry axis. Very few such γ bands have been traced past the usual backbending rotational alignments of high- j nucléons. We have investigated the structure of positive-parity bands in the N = 90 nucleus 156Dy, using the 148Nd(12C,4n) 156Dy reaction at 65 MeV, observing the resulting y-ray transitions with the Gammasphere array. The even- and odd-spin members of the Kμ = 2+γ band are observed up to 32+ and 31+, respectively. This rotational band faithfully tracks the ground-state configuration to the highest spins. The members of a possible γ vibration built on the aligned yrast S band are observed up to spins 28+ and 27+. An even-spin positive-parity band, observed up to spin 24+, is a candidate for an aligned S band built on the seniority-zero configuration of the 0+2 state at 676 keV. The crossing of this band with the 0+2 band is at ħωc = 0.28(1) MeV and is consistent with the configuration of the 0+2 band not producing any blocking of the monopole pairing. [ABSTRACT FROM AUTHOR]