Octupole correlations in the structure of $0_2^{+}$ bands in the $N=88$ nuclei ${}^{150}$Sm and ${}^{152}$Gd

Knowledge of the exact microscopic structure of the 0${}_1$${}^{+}$ ground state and first excited 0${}_2$${}^{+}$ state in ${}^{150}$Sm is required to understand the branching of double β decay to these states from ${}^{150}$Nd. The detailed spectroscopy of ${}^{150}$Sm and ${}^{152}$Gd has been studied using (α,xn) reactions and the γ-ray arrays AFRODITE and JUROGAM II. Consistently strong $E$1 transitions are observed between the excited $K^{\pi }$ $=$ 0${}_2$${}^{+}$ bands and the lowest negative parity bands in both nuclei. These results are discussed in terms of the possible permanent octupole deformation in the first excited $K^{\pi }$ $=$ 0${}_2$${}^{+}$ band and also in terms of the “tidal wave” model of Frauendorf.

Figure 1

Partial level schemes of ${}^{150}$Sm (a) and ${}^{152}$Gd (b) showing the ground state, second vacuum 0${}_2$${}^{+}$, and octupole bands. New transitions and $E$1 transitions from the second vacuum 0${}_2$${}^{+}$ bands to the octupole bands are shown in red.

Figure 2

Coincidence spectra illustrating the positioning of the 0${}_2$${}^{+}$ to octupole band $E$1 transitions. (a) ${}^{150}$Sm γγγ data for the 482 keV 8${}_2$${}^{+}$→7${}_1$${}^{-}$ γ ray. (b) ${}^{150}$Sm γγγ data for the 464 keV 6${}_2$${}^{+}$→5${}_1$${}^{-}$ γ ray. (c) ${}^{152}$Gd γγ data for the 361 keV 10${}_2$${}^{+}$→9${}_1$${}^{-}$ γ ray. (d) ${}^{152}$Gd γγ data for the 159 keV 4${}_2$${}^{+}$→3${}_1$${}^{-}$ γ ray.

Figure 3

Frequency ratios ${\omega }^{-}$/${\omega }^{+}$ as a function of spin I for the ground state band and the octupole band, and for the 0${}_2$${}^{+}$ band and octupole band, for ${}^{152}$Gd and ${}^{150}$Sm. They are compared with the established octupole deformed band in ${}^{220}$Ra [22, 38]. For structures with permanent octupole deformation, this ratio would be 1.0.

Figure 4

(a) Energy difference, Eq. (1), between negative and positive parity states in ${}^{150}$Sm, ${}^{152}$Gd, and ${}^{220}$Ra. (b), (c) Energy in the rotating frame, Routhian, against rotational frequency ħω for the bands in ${}^{152}$Gd and ${}^{150}$Sm.