Abstract
Exotic nuclei are characterized by having a number of neutrons (or protons) in excess relative to stable nuclei. Their shell structure, which represents single-particle motion in a nucleus1,2, may vary due to nuclear force and excess neutrons3,4,5,6, in a phenomenon called shell evolution7. This effect could be counterbalanced by collective modes causing deformations of the nuclear surface8. Here, we study the interplay between shell evolution and shape deformation by focusing on the magnetic moment of an isomeric state of the neutron-rich nucleus 75Cu. We measure the magnetic moment using highly spin-controlled rare-isotope beams and achieve large spin alignment via a two-step reaction scheme9 that incorporates an angular-momentum-selecting nucleon removal. By combining our experiments with numerical simulations of many-fermion correlations, we find that the low-lying states in 75Cu are, to a large extent, of single-particle nature on top of a correlated 74Ni core. We elucidate the crucial role of shell evolution even in the presence of the collective mode, and within the same framework we consider whether and how the double magicity of the 78Ni nucleus is restored, which is also of keen interest from the perspective of nucleosynthesis in explosive stellar processes.
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Data availability
The data used in the present study are available from the corresponding author upon reasonable request.
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Acknowledgements
The experiment was performed under programme no. NP1412-RIBF124R1 at RIBF, operated by RIKEN Nishina Center for Accelerator-Based Science and CNS, The University of Tokyo. The authors thank the RIKEN accelerator staff for their cooperation during the experiment. This work was supported in part by a JSPS KAKENHI grant (16K05390), and also in part by JSPS and CNRS under the Japan-France Research Cooperative Program. The MCSM calculations were performed on the K computer at RIKEN AICS (hp140210, hp150224, hp160211, hp170230). This work was also supported in part by the HPCI Strategic Program (‘The Origin of Matter and the Universe’) and ‘Priority Issue on Post-K computer’ (Elucidation of the Fundamental Laws and Evolution of the Universe). D.L.B., and A.K. acknowledge support by the Extreme Light Infrastructure Nuclear Physics (ELI-NP) Phase II, a project co-financed by the Romanian Government and the European Union through the European Regional Development Fund – the Competitiveness Operational Programme(1/07.07.2016, COP, ID 1334). D.R. was supported by the P2IO Excellence Center.
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Y.I. designed the experiment, analysed the data and was chiefly responsible for writing the paper. Y.T. and T.O. worked on the theoretical studies, and T.O. helped with writing the paper. The other authors are collaborators on the experiment.
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Ichikawa, Y., Nishibata, H., Tsunoda, Y. et al. Interplay between nuclear shell evolution and shape deformation revealed by the magnetic moment of 75Cu. Nat. Phys. 15, 321–325 (2019). https://doi.org/10.1038/s41567-018-0410-7
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DOI: https://doi.org/10.1038/s41567-018-0410-7
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