Nuclear Charge Radius of 12Be

The nuclear charge radius of Be-12 was precisely determined using the technique of collinear laser spectroscopy on the 2s(1/2) -> 2p(1/2,3/2) transition in the Be+ ion. The mean square charge radius increases from Be-10 to Be-12 by delta < r(c)(2)>(10,12) = 0.69(5) fm(2) compared to delta < r(c)(2)>(10,11) = 0.49(5) fm(2) for the one-neutron halo isotope Be-11. Calculations in the fermionic molecular dynamics approach show a strong sensitivity of the charge radius to the structure of Be-12. The experimental charge radius is consistent with a breakdown of the N = 8 shell closure.