drip line and proton drip line--and are all unstable with short half-lives, measured in milliseconds; for example, lithium-11 has a half-life of .

Halos in effect represent an excited state with nucleons in an outer quantum shell which has unfilled energy levels "below" it (both in terms of radius and energy). The halo may be made of either neutrons [nn, nnn] or protons [pp, ppp]. Nuclei which have a single neutron halo include ¹¹Be and ¹⁹C. A two-neutron halo is exhibited by