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Interaction cross sections for 42-51 Ca on a carbon target at 280 MeV/nucleon have been measured for the first time. The neutron number dependence of derived root-mean-square matter radii shows a significant increase beyond the neutron magic number N = 28. Furthermore, this enhancement of matter radii is much larger than that of the previously measured charge radii, indicating a novel growth in neutron skin thickness. A simple examination based on the Fermi-type distribution, and the Mean-Field calculations point out that this anomalous enhancement of the nuclear size beyond N = 28 results from an enlargement of the core by a sudden increase in the surface diffuseness of the neutron density distribution, which implies the swelling of the bare 48 Ca core in Ca isotopes beyond N = 28. PACS numbers: 25.60.Dz Systematic studies of nuclear radii along the isotopic chain have so far elucidated changes in the nuclear structure such as the emergence of a halo as well as the development of neutron skin and nuclear deformation [1][2][3][4][5]. Nuclear charge radii, which represent charge spreads in these nuclei, also give complemental information on the size of the nucleus. It has been revealed that the trend of charge radii along the isotopic chain shows a sudden increase, which is often called a "kink," just after the magic number [6]. In particular, the neutron magic number N = 28 has received considerable attention. Recently, unexpectedly large charge radii were observed in neutron-rich Ca isotopes beyond N = 28 [7]. This sudden growth in charge radii from 48 Ca (N = 28) to 52 Ca represents a challenging problem; it has not been quan-titatively explained by any theoretical calculations other than the Hartree-Fock-Bogolyubov calculation with the Fayans energy density functional [8]. This anomalous phenomenon observed in Ca isotopes is stimulating further studies of nuclear charge radii in a wide mass region [8][9][10][11][12].In contrast, information on the evolution of the size of the neutron density distribution has not been obtained across N = 28. For example, nucleon density distributions ρ m (r) or point-neutron density distributions ρ n (r) for Ca isotopes have been deduced only for stable nuclei, 40,42,44,48 Ca, through the hadron elastic scattering [13][14][15][16][17][18][19][20][21][22][23].The experimental data for root-mean-square (RMS) radii of ρ m (r) or ρ n (r) for Ca isotopes beyond N = 28

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