Backbending in the pear-shaped ${}_{90}{}^{223}\mathrm{Th}$ nucleus: Evidence of a high-spin octupole to quadrupole shape transition in the actinides

Relatively neutron-rich thorium isotopes lie at the heart of a nuclear region of nuclei exhibiting octupole correlation effects. The detailed level structure of ${}^{223}\mathrm{Th}$ has been investigated in measurements of $\gamma$ radiation following the fusion-evaporation channel of the ${}^{208}\mathrm{Pb}({}^{18}\mathrm{O},3n){}^{223}\mathrm{Th}$ reaction at 85 MeV beam energy. The level structure has been extended up to spin $49/2$, and 33 new $\gamma$ rays have been added using triple-$\gamma$ coincidence data. The spins and parities of the newly observed states have been confirmed by angular distribution ratios. In addition to the two known yrast bands based on a $K=5/2$ configuration, a non-yrast band has been established up to spin $35/2$. We interpret this new structure as based on the same configuration as the yrast band in ${}^{221}\mathrm{Th}$ having dominant $K=1/2$ contribution. At the highest spin a backbending occurs around a rotational frequency of $\hslash \omega =0.23$ MeV, very close to the one predicted in ${}^{222}\mathrm{Th}$, where a sharp transition to a reflection-symmetric shape is expected.

Figure 1

Representative triple-gated spectra showing transitions between high-spin levels of ${}^{223}\mathrm{Th}$. Coincidence conditions are indicated. For instance, spectrum (a) shows the $\gamma$ rays appearing in events for which the following set of transitions is also detected: 157 keV and 183 keV and (191 or 213) keV. The transitions of sequences 1a and 1b are indicated in red; sequences 2a and 2b in green; sequences 3a and 3b in blue. (a) Spectrum showing mostly transitions in sequences 1a and 1b. (b) Spectrum showing mostly transitions in sequences 2a and 2b. (c) Spectrum showing mostly transitions in sequences 3a and 3b. The presence of transitions from different bands in each spectrum is linked to the interband communication (see text).

Figure 2

Level scheme of ${}^{223}\mathrm{Th}$ determined by $\gamma$-ray spectroscopy via the ${}^{208}\mathrm{Pb}({}^{18}\mathrm{O},3n){}^{223}\mathrm{Th}$ reaction. The widths of the arrows represents the $\gamma$-ray intensity of the transitions. The newly observed transitions are colored in red. The two main bands of alternating parity are classified in four sequences labeled 1a, 1b for the band of simplex $-i$ and 2a, 2b for the band of simplex $+i$ (each sequence has a fixed parity). A similar notation is used for the non-yrast structure, formed of two sequences labeled 3a and 3b. Spin-parity assignment for the main structure is based on the assumption of a $5/2^{+}$ ground state. Each of the two main bands presents intense stretched $E1$ intersequence transitions as well as stretched $E2$ intrasequence transitions. Strongly converted $M1$ transitions link these two bands: they are not represented here since they do not appear in $\gamma$ spectroscopy. The parities of the non-yrast structure levels are tentative (see text).

Figure 3

Ratio between the rotational frequency of the negative and positive parity bands $\omega (\pi =-)/\omega (\pi =+)$ versus the angular momentum. A value of this ratio close to unity stands for a rigid reflection-asymmetric rotor. The behavior of a perfect aligned octupole phonon is also indicated.

Figure 4

Splitting between states with opposite parity within the same rotational band for various thorium isotopes as a function of spin.

Figure 5

Kinematic moment of inertia for both positive and negative parity states in ${}^{223}\mathrm{Th}$. The full (empty) symbols represent the sequence with simplex number $s=-i$ ($s=+i$). A backbending and an upbending arise from the positive and negative parity bands respectively, in the same simplex structure ($s=+i$) around rotational frequency of $\hslash \omega \simeq 0.23\text{MeV}$.