Octupole correlations in $N=88{}^{154}\mathrm{Dy}$: Octupole vibration versus stable deformation

We report on low-spin states of ${}^{154}\mathrm{Dy}$ populated via the reaction ${}^{155}\mathrm{Gd}$ $\left({}^3\mathrm{He},4n\right)$ 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.

Figure 1

Partial level scheme of ${}^{154}\mathrm{Dy}$ obtained from the ${}^{155}\mathrm{Gd}$ $\left({}^3\mathrm{He},4n\right)$ reaction at 37.5 MeV. New $E1$ transitions from the octupole band to the $0_2^{+}$ band are shown in blue, and other new transitions are shown in red. Previously published results are shown in black.

Figure 2

Coincidence spectra illustrating the positioning of the $E1$ γ-ray transition depopulation of the octupole band feeding the $0_2^{+}$ band (a) ${}^{154}\mathrm{Dy}\gamma \gamma$ data for the 294-keV $\left(5_1\to 4_2^{+}\right)$ $\gamma$ ray gated by 347 keV. (b) ${}^{154}\mathrm{Dy}\gamma \gamma$ data for the 306-keV $\left(7_1^{-}\to 6_2^{+}\right)$ $\gamma$ ray gated by 407 keV. (c) ${}^{154}\mathrm{Dy}\gamma \gamma$ data for the 259-keV $\left(9_1^{-}\to 8_2^{+}\right)$ $\gamma$ ray gated by 504 keV. (d) ${}^{154}\mathrm{Dy}\gamma \gamma$ data for the 125-keV $\left({11}_1^{-}\to {10}_2^{+}\right)$ $\gamma$ ray gated by 595 keV. The new $E1$ $\gamma$ rays are marked with a blue asterisk.

Figure 3

(a) Frequency ratios ${\omega }^{-}/{\omega }^{+}$ Eq. (2) as a function of spin for the octupole bands divided by the ground-state bands, labeled “gsb” and for the octupole bands divided by $0_2^{+}$ bands in ${}^{154}\mathrm{Dy}$ and octupole-deformed ${}^{224}\mathrm{Ra}$ [1, 2, 14]. For structures with permanent octupole deformation, this ratio would be 1.0. (b) Energy differences $\mathrm{\Delta }E\left(I\right)$ Eq. (3) between negative- and positive-parity states in ${}^{150}\mathrm{Sm},{}^{152}\mathrm{Gd},{}^{154}\mathrm{Dy}$, and octupole-deformed ${}^{224}\mathrm{Ra}$ [1, 3, 15]. For structures with permanent octupole deformation, this ratio would be 0.0.

Figure 4

Ratio of reduced transition probabilities for the out-of-band $E1$ and the in-band $E2$ transitions for ${}^{150}\mathrm{Sm},{}^{152}\mathrm{Gd}$ [19], and ${}^{154}\mathrm{Dy}$ between the $0_2^{+}$ band and the octupole band and between the octupole bands to the ground-state band.