In a recent Physical Review Letters paper (“Surprising Charge-Radius Kink in the Sc Isotopes at N=20”), scientists from Technical University of Darmstadt, University of Jena, Oak Ridge National Laboratory, TRIUMF, McGill University, Augustana University, GSI, Max Planck Institute for Nuclear Physics, the University of Erlangen-Nuremberg, and FRIB discovered that the chain of scandium charge radii extends below the neutron magic number N=20.

Nuclei with a “magic” number of protons or neutrons are more bound than their neighbors. The known magic numbers are 2, 8, 20, 28, 50, 82, and 126.

In this work, charge radii of the neutron-deficient, short-lived isotopes of scandium-40 and scandium-41 were determined using collinear laser spectroscopy at the Beam Cooler and Laser spectroscopy (BECOLA) facility. The scientists discovered the presence of a pronounced kink in charge radii, generally taken as the signature of a shell closure, at N=20. This kink is absent in the neighboring calcium, potassium, and argon isotopic chains.

The paper concluded “a significant puzzle for nuclear theory,” as the theoretical models that explain the trend at N=20 for the calcium isotopes could not reproduce the kink structure. The scientists called for additional measurements of charge radii in the titanium isotopic chain across N=20, as well as further theoretical studies of core polarization effects in that mass region.

This material is based upon work supported by the U.S. Department of Energy Office of Science Office of Nuclear Physics, the U.S. National Science Foundation, the Natural Sciences and Engineering Research Council of Canada, the Arthur B. McDonald Canadian Astroparticle Physics Research Institute, the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), the European Research Council, the National Research Council of Canada, and the European Union’s Horizon 2020 research and innovation program.

Michigan State University (MSU) operates the Facility for Rare Isotope Beams (FRIB) as a user facility for the U.S. Department of Energy Office of Science (DOE-SC), supporting the mission of the DOE-SC Office of Nuclear Physics. User facility operation is supported by the DOE-SC Office of Nuclear Physics as one of 28 DOE-SC user facilities.

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