Exotic collective excitation patterns in triaxially deformed <sup>131</sup>Ba
Authors: Wei Rui , Zhou Hou-Bing , Wang Si-Cheng , Ding Bing , Qiang Yun-Hua , Jia Chen-Xu , Chen Hong-Xing , Guo Song , C.M. Petrache , D. Mengoni , A. Astier , E. Dupont , Lü Bing-Feng , D. Bazzacco , A. Boso , A. Goasduff , F. Recchia , D. Testov , F. Galtarossa , G. Jaworski , D.R. Napoli , S. Riccetto , M. Siciliano , J.J. Valiente-Dobon , C. Andreoiu , F.H. Garcia , K. Ortner , K. Whitmore , A. Ataç-Nyberg , T. Bäck , B. Cederwall , E.A. Lawrie , I. Kuti , D. Sohler , T. Marchlewski , J. Srebrny , A. Tucholski ,   College of Physical Science and Technology, Guangxi Normal University, Guilin 541004, China,   Guangxi Key Laboratory of Nuclear Physics and Technology, Guilin 541004, China,   Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China,   University of Chinese Academy of Sciences, Beijing 100049, China,   University Paris-Saclay, CNRS/IN2P3, IJCLab, Orsay 91405, France,   Dipartimento di Fisica e Astronomia, Université degli Studi di Padova, Padova I-35131, Italy,   INFN, Sezione di Padova, I-35131 Padova, Italy,   INFN, Laboratori Nazionali di Legnaro, Legnaro (Padova) I-35020, Italy,   Dipartimento di Fisica e Scienze della Terra, Université di Ferrara, Ferrara, Italy,   Department of Chemistry, Simon Fraser University, Burnaby, BC V5A 1S6, Canada,   KTH Department of Physics, Stockholm S-10691, Sweden,   iThemba LABS, National Research Foundation, PO Box 722, Somerset West 7123, South Africa,   Department of Physics & Astronomy, University of the Western Cape, P/B X17, Bellville ZA-7535, South Africa,   HUN-REN Institute of Nuclear Research, ATOMKI, Debrecen 4001, Hungary,   University of Warsaw, Heavy Ion Laboratory, Pasteura 5a, Warsaw 02-093, Poland
Published: April 2024

<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 <sup>131</sup>Ba, an experiment was performed using the XTU Tandem accelerator in the Legnaro laboratory, Italy. High-spin states of <sup>131</sup>Ba have been populated via the heavy-ion fusion-evaporation <sup>122</sup>Sn(<sup>13</sup>C, 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 $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240212_M12.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240212_M12.png"/></alternatives></inline-formula>10<sup>9</sup> triple- or higher-fold events were collected by the GALILEO data acquisition system. The <i>γ</i>-<i>γ</i>-<i>γ</i> coincidence events were sorted into a three-dimensional histogram (cube) and the analysis was carried out with the RADWARE and GASPWARE software packages.</sec><sec>Through analysis of the coincidences between <i>γ</i>-rays, the most comprehensive level schemes of <sup>131</sup>Ba to date was deduced from the present work. The extended level-scheme consists of 15 rotational bands, and newly observed transitions are marked in red. Three nearly degenerate pairs of doublet bands (Band 3–8) are identified in <sup>131</sup>Ba. Two pairs of chiral doublets (Band 3–6) with configuration <inline-formula><tex-math id="M20">\begin{document}$ {\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 <sup>131</sup>Ba and in comparison with those in <sup>124</sup>Ba, <sup>224</sup>Th, <sup>133</sup>Ce and <sup>135</sup>Nd. The energy displacement <i>δ</i>E and the B(E1)/B(E2) branching ratios in <sup>131</sup>Ba are comparable with those in <sup>124</sup>Ba but deviate appreciably from those in <sup>224</sup>Th which has been reported to have stable octupole deformation. The results indicate the existence of octupole correlations in <sup>131</sup>Ba 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>