  Interaction cross sections for $^{42\textrm{-}51}$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 $N=28$.
