Proton Distribution Radii of 12-19C Illuminate Features of Neutron Halos

Proton radii of C12-19 densities derived from first accurate charge changing cross section measurements at 900A MeV with a carbon target are reported. A thick neutron surface evolves from similar to 0.5 fm in C-15 to similar to 1 fm in C-19. The halo radius in C-19 is found to be 6.4 +/- 0.7 fm as large as Li-11. Ab initio calculations based on chiral nucleon-nucleon and three-nucleon forces reproduce the radii well.