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

The exact nature of the lowest K-pi = 2(+) rotational bands in all deformed nuclei remains obscure. Traditionally they are assumed to be collective vibrations of the nuclear shape in the. degree of freedom perpendicular to the nuclear symmetry axis. Very few such. bands have been traced past the usual backbending rotational alignments of high-j nucleons. We have investigated the structure of positive-parity bands in the N = 90 nucleus Dy-156, using the Nd-148(C-12,4n) Dy-156 reaction at 65 MeV, observing the resulting. gamma-ray transitions with the Gammasphere array. The even-and odd-spin members of the K-pi = 2(+)gamma 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. 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 h omega(c) = 0.28(1) MeV and is consistent with the configuration of the 0(2)(+) band not producing any blocking of the monopole pairing.