Interaction cross sections for ${}^{42–51}\mathrm{Ca}$ on a carbon target at $280\mathrm{MeV}/\mathrm{nucleon}$ 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 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 ${}^{48}\mathrm{Ca}$ core in Ca isotopes beyond $N=28$.

• Received 18 September 2019
• Revised 20 December 2019
• Accepted 30 January 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.102501