Parity doublet structures in doubly-odd ${}^{216}\mathrm{Fr}$

Parity doublet structures are established in ${}^{216}\mathrm{Fr}$, which lies at the lower boundary of enhanced octupole collectivity in the trans-lead region. The newly identified levels are established as the simplex partner of a previously reported band leading to parity doublets with small ($\sim 55$ keV) average energy splitting, a feature typical of nuclei with near-static octupole deformation. The observed levels do not follow a regular pattern of rotational bands, indicating low quadrupole collectivity. However, enhanced octupole correlations are evident from the small energy splitting and large B(E1)/B(E2) values. Staggering in E1 transition energies and B(E1)/B(E2) ratios is noted. The enhancement of octupole correlations in ${}^{216}\mathrm{Fr}$ is attributed to the availability of a neutron orbital with a K = 3/2 component.

Figure 1

Smooth Compton-background-subtracted singles $\gamma$-ray spectra illustrating transitions observed at (a) 54 MeV and (b) 62 MeV beam energies. The transitions marked with an asterisk are new.

Figure 2

The partial level scheme of ${}^{216}\mathrm{Fr}$ obtained from the present work. All the transitions are labeled with their energies in keV. Widths of the solid and open area of the arrows are proportional to intensities of $\gamma$ rays and internal conversion electrons, respectively. The newly identified transitions are marked with an asterisk.

Figure 3

Coincidence $\gamma$-ray spectra illustrating transitions in (a) summed double gates of the transitions in sequence “A” of Fig. 2. The transitions used for the gating are marked with the letter “g”. The lower panels show parts of three spectra obtained from gates on the (b) 528, (c) 235, and (d) 206 keV transitions. The transitions marked with “C” are contaminants from ${}^{215}\mathrm{Fr}$. Also, note that not all the transitions assigned to ${}^{216}\mathrm{Fr}$ could be placed in the level scheme of ${}^{216}\mathrm{Fr}$ in Fig. 2.

Figure 4

Coincidence $\gamma$-ray spectra illustrating transitions in (a) the 300 keV gate along with (b) negative and (c) positive parity bands of the sequence “B”. The spectra are obtained by summing double gates on the transitions which are marked with the letter “g” in the corresponding panels. The transitions marked with “C” are contaminants from ${}^{215}\mathrm{Fr}$. Also, note that not all the transitions assigned to ${}^{216}\mathrm{Fr}$ could be placed in the level scheme of ${}^{216}\mathrm{Fr}$ in Fig. 2.

Figure 5

(a) DCO and (b) IPDCO ratios for the transitions in Fig. 2. The filled (open) symbols in the top panel indicate DCO ratios obtained from the quadrupole (dipole) gates. Further, the transitions belonging to the sequences “A” and “B” are distinguished by using square and triangle symbols, respectively. The remaining transitions, viz., 161, 212, 251, and 259 keV, are shown by circles.

Figure 6

Comparison of (a) energy splitting $\left[E\right(s=-1,-i)-E(s=+1,+i\left)\right]$ and (b) spin (I) versus rotational frequency ($\hslash \omega$) in ${}^{223}\mathrm{Th}$ with N = 129 (${}^{216}\mathrm{Fr}$, ${}^{218}\mathrm{Ac}$, ${}^{219}\mathrm{Th}$) and N = 131 (${}^{220}\mathrm{Ac}$) isotones for which the parity doublet structures have been reported. See the text for details. The data are taken from Refs. [7, 8, 29, 31] and the present work.

Figure 7

B(E1)/B(E2) ratios versus initial spin for the transitions in the sequence “A” in Fig. 2 (circles) and corresponding states in ${}^{218}\mathrm{Ac}$ (squares). The data for ${}^{218}\mathrm{Ac}$ are taken from Ref. [7]. The positive and negative parity states are further distinguished by open and filled symbols, respectively.