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Unexpectedly large charge radii of neutron-rich calcium isotopes

Abstract

Despite being a complex many-body system, the atomic nucleus exhibits simple structures for certain ‘magic’ numbers of protons and neutrons. The calcium chain in particular is both unique and puzzling: evidence of doubly magic features are known in 40,48Ca, and recently suggested in two radioactive isotopes, 52,54Ca. Although many properties of experimentally known calcium isotopes have been successfully described by nuclear theory, it is still a challenge to predict the evolution of their charge radii. Here we present the first measurements of the charge radii of 49,51,52Ca, obtained from laser spectroscopy experiments at ISOLDE, CERN. The experimental results are complemented by state-of-the-art theoretical calculations. The large and unexpected increase of the size of the neutron-rich calcium isotopes beyond N = 28 challenges the doubly magic nature of 52Ca and opens new intriguing questions on the evolution of nuclear sizes away from stability, which are of importance for our understanding of neutron-rich atomic nuclei.

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Figure 1: High-resolution bunched collinear laser spectroscopy at ISOLDE, CERN.
Figure 2: Examples of hyperfine structure spectra measured for the Ca isotopes in the 393-nm 4s 2S1/2 → 4p 2P3/2 ionic transition.
Figure 3: Charge radii of Ca isotopes.

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Acknowledgements

This work was supported by the IAP-project P7/12, the FWO-Vlaanderen, GOA grants 10/010 and 15/010 from KU Leuven, the Max-Planck Society, the ERC Grant No. 307986 STRONGINT, the BMBF contract 05P12RDCIC and 05P15RDCIA, the European Union seventh framework through ENSAR under Contract No. 262010, the US Department of Energy, Office of Science, Office of Nuclear Physics under Award Numbers DEFG02-96ER40963 (University of Tennessee), DE-SC0013365 (Michigan State University), DE-SC0008499 and DE-SC0008511 (NUCLEI SciDAC collaboration), the Field Work Proposal ERKBP57 at Oak Ridge National Laboratory (ORNL), and Contract No. DE-AC05-00OR22725 (ORNL). Computer time was provided by the Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program. This research used resources of the Oak Ridge Leadership Computing Facility at ORNL, and used computational resources of the National Center for Computational Sciences, the National Institute for Computational Sciences. We thank J. Menéndez for very useful discussions. We would like to thank the ISOLDE technical group for their support and assistance.

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R.F.G.R., M.L.B., N.F., M.H., M.K., K.K., R.N., W.Nörtershäuser, J.P. and D.T.Y. performed the experiment. R.F.G.R. performed the data analysis and prepared the figures. G.H. and G.R.J. performed the coupled-cluster calculations. R.F.G.R., K.B., G.H., K.H., G.N., W.Nazarewicz, W.Nörtershäuser, T.P. and A.S. prepared the manuscript. All authors discussed the results and contributed to the manuscript at all stages.

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Correspondence to R. F. Garcia Ruiz.

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Garcia Ruiz, R., Bissell, M., Blaum, K. et al. Unexpectedly large charge radii of neutron-rich calcium isotopes. Nature Phys 12, 594–598 (2016). https://doi.org/10.1038/nphys3645

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