The interaction cross sections $\left({\sigma }_I\right)$ of ${}^A\mathrm{Na}$ isotopes $(A=20\mathrm{}–23,25–32)$ on a carbon target have been measured at $950A\mathrm{MeV}$. The effective root-mean-square matter radii of these isotopes were deduced from ${\sigma }_I$ by a Glauber-type calculation. By combining the isotope-shift data with the present data the radii of neutrons have been compared with those of protons for the first time along a chain of stable and unstable isotopes. A monotonic increase in the neutron skin thickness has been observed as the neutron number increases in Na isotopes.

• Received 14 August 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.3241