Octupole correlations in N=88 Dy 154: Octupole vibration versus stable deformation

We report on low-spin states of Dy154 populated via the reaction Gd155 (He3,4n) with a beam energy of 37.5 MeV from the Separated Sector Cyclotron at iThemba Laboratory. The AFRODITE γ-ray spectrometer was used to establish new E1 transitions between bands of opposite parity. The measurements broaden the N=88 systematics on the relationship between the first excited positive-parity pairing isomer band and the lowest-lying negative-parity band as the nuclear quadrupole deformation decreases with increasing proton number. In a region of strong octupole correlations the data suggest that the spectroscopy of N=88 nuclei is driven by stable octupole deformations and not by vibrations.