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

The exact nature of the lowest K^π =2_γ ^+ 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 156Dy, using the 148Nd(12C,4n)156Dy reaction at 65 MeV, observing the resulting γ-ray transitions with the Gammasphere array. The even- and odd-spin members of the K^π =2_γ^+ γ-band are observed 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 to spins 28+ and 27+. An even-spin positive-parity band, observed 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 ħω= 0.28(1) MeV and is consistent with the configuration of the 0_2^+ band not producing any blocking of the monopole pairing.