Exotic collective excitation patterns in triaxially deformed ¹³¹Ba

Abstract

<sec>In the last two decades, several unique phenomena in triaxially deformed nuclei, such as chiral doublet bands and wobbling motion have been revealed. Up to now, there are still many open questions which require further experimental and theoretical studies. To explore the collective motion in ¹³¹Ba, an experiment was performed using the XTU Tandem accelerator in the Legnaro laboratory, Italy. High-spin states of ¹³¹Ba have been populated via the heavy-ion fusion-evaporation ¹²²Sn(¹³C, 4n) reaction. <i>γ</i>-rays, charged particles and neutrons emitted from the residues were detected by the GALILEO array, EUCLIDES silicon ball, and the Neutron Wall, respectively. A total of 1.2<inline-formula><tex-math id="M12">\begin{document}$ \times $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240212_M12.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240212_M12.png"/></alternatives></inline-formula>10⁹ triple- or higher-fold events were collected by the GALILEO data acquisition system. The <i>γ</i>-<i>γ</i>-<i>γ</i> coincidence events were sorted into a three-dimensional histogram (cube) and the analysis was carried out with the RADWARE and GASPWARE software packages.</sec><sec>Through analysis of the coincidences between <i>γ</i>-rays, the most comprehensive level schemes of ¹³¹Ba to date was deduced from the present work. The extended level-scheme consists of 15 rotational bands, and newly observed transitions are marked in red. Three nearly degenerate pairs of doublet bands (Band 3–8) are identified in ¹³¹Ba. Two pairs of chiral doublets (Band 3–6) with configuration <inline-formula><tex-math id="M20">\begin{document}$ {\textit{\pi}}h_{11/2}(g_{7/2},d_{5/2}){\otimes}{\nu}h_{11/2} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240212_M20.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240212_M20.png"/></alternatives></inline-formula> are interpreted as a set of pseudospin-chiral quartet bands. The quartet bands are fed by another pair of chiral doublet bands (Band 7–8) built on a <inline-formula><tex-math id="M21">\begin{document}$ {\textit{\pi}}h^2_{11/2}{\otimes}{\nu}h_{11/2} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240212_M21.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240212_M21.png"/></alternatives></inline-formula> configuration via a series of enhanced E1 transitions. We extracted the energy displacement <i>δ</i>E and the B(E1)/B(E2) branching ratios between the positive-parity band 3 and the negative-parity band 7 in ¹³¹Ba and in comparison with those in ¹²⁴Ba, ²²⁴Th, ¹³³Ce and ¹³⁵Nd. The energy displacement <i>δ</i>E and the B(E1)/B(E2) branching ratios in ¹³¹Ba are comparable with those in ¹²⁴Ba but deviate appreciably from those in ²²⁴Th which has been reported to have stable octupole deformation. The results indicate the existence of octupole correlations in ¹³¹Ba without stable octupole deformation. A new rotational band (Band 10) discovered in the low-spin region exhibits a level structure similar to a wobbling band. Assuming it as a wobbling band, the wobbling frequency was extracted and compared with other reported wobbling bands in the neighboring nuclei. The wobbling frequency of this band decreases with increasing angular momentum, and even exhibits negative value at the highest spin. Considering that the wobbling phonon should contribute a positive amount to the excitation energy, this band is unlikely to be explained by this mechanism. The band may originate from other collective excitation mechanisms such as <i>γ</i> vibration. The newly identified rotational band (Band 9) composed of M1 transitions is tentatively assigned as a magnetic rotational band through a systematic analysis of the level structure. Finally, the configurations of other 4 bands, Band 12-15, are also suggested based on previous researches and the extracted quasiparticle alignments.</sec>