Matter radii of Na and Mg isotopes

The interaction cross sections (σ1) of ^ANa (A= 20-23, 25-32) and ^AMg (A= 20, 22-25, 27, 29-33) on a carbon target have been measured at a beam energy of around 950 A MeV. The effective root-mean-square matter radii of these isotopes were deduced from σ1 by a Glauber-type calculation. Combining the isotope-shift data with the present Na 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. The Mg data are also consistent with the claim that the formation of a neutron/proton skin is a universal feature in β-unstable nuclei.