The model, introduced in a previous paper, for the description of the octupole and quadrupole degrees of freedom in conditions close to the axial symmetry is used to describe the negative-parity band based on the first octupole vibrational state in nuclei close to the critical point of the U(5)-to-SU(3) phase transition. The situation of ${}^{150}\mathrm{Nd}$ and ${}^{152}\mathrm{Sm}$ is discussed in detail. The positive-parity levels of these nuclei, and also the in-band $E2$ transitions, are reasonably accounted for by the X(5) model. With simple assumptions on the nature of the octupole vibrations, it is also possible to describe the negative-parity sector with comparable accuracy without changing the description of the positive-parity part.

• Received 20 November 2009

DOI:https://doi.org/10.1103/PhysRevC.81.034320