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 γ 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-$j$ nucleons. We have investigated the structure of positive-parity bands in the $N=90$ nucleus ${}^{156}\mathrm{Dy}$, using the ${}^{148}\mathrm{Nd}\left({}^{12}\mathrm{C},4n\right){}^{156}\mathrm{Dy}$ reaction at 65 MeV, observing the resulting γ-ray transitions with the Gammasphere array. The even- and odd-spin members of the $K^{\pi }=2^{+}$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 $\hslash {\omega }_c=0.28\left(1\right)\mathrm{MeV}$ and is consistent with the configuration of the $0_2^{+}$ band not producing any blocking of the monopole pairing.

• Received 11 December 2014
• Revised 16 February 2015

DOI:https://doi.org/10.1103/PhysRevC.91.034330