Structure of two-neutron halo in light drip-line nuclei

The formation of two-neutron halo is described using the neutron-neutron (n-n) interaction constrained by the low-energy n-n scattering limit. This method is tested for loosely-bound two neutrons in O-24, where a good agreement with experimental data is found. It is applied to halo neutrons in C-22, where the C-20-core is treated as a correlated-core with mixing of 2s(1/2) orbit. This n-n interaction is shown to be strong enough to produce a two-neutron halo, locating C-22 on the drip line, while C-21 remains unbound. A unique relation between the two neutron separation energy, S-2n, and the radius of neutron halo is presented. The estimated halo radius is found to be consistent with a recent experimental data. The appearance of Efimov states is discussed. The present method is extended to apply to the case of mixed configurations for the two-neutron halos such as B-17 and Li-11.