22C:: An s-wave two-neutron halo nucleus

A dripline nucleus C-22 is studied in a Borromean three-body model of C-20+n+n. The valence neutrons, interacting via a realistic potential, are constrained to be orthogonal to the occupied orbits in C-20. We obtain ample results supporting that C-22 is an ideal s-wave two-neutron halo nucleus: The ground state is bound by 390-570 keV, the root mean square neutron and proton radii are 4.0 and 2.4 fm, and the two neutrons are predominantly in (s(1/2))(2) orbits. The binding mechanism of C-22 is discussed. One- and two-body density distributions elucidate the halo character as well as the correlated motion of the neutrons. The reaction cross sections of C-22+C-12 collisions are predicted.