For most nuclides, the density of nucleons in the nucleus is more or less uniform. However near the neutron dripline, some neutrons are only weakly bound to the inner core nucleons. In this recently discovered phenomena, the nuclei consist of a normal nucleus surrounded by a halo of extra neutrons with a diameter very much larger than the core nucleus. The halo neutrons are mostly outside the region of the strong nuclear force between the nucleons. Typically half-lives of these nuclei are around 100 ms.
One such example of a halo nuclide is Li11. It consists of a Li9 core surrounded by a halo of two loosely bound neutrons requiring only 0.3 MeV to remove them. The Li11 nucleus is similar in size to that of Ca48 which has a neutron binding energy of about 8 MeV. Other examples of halo nuclei are He6 (He4 + 2n), He8 (He4 + 4n), Be11 (Be10+1n), Be14, B17 and Ca19.
Proton halo nuclides may also exist for nuclei near the proton dripline. Possible nuclides are B8, N13, and F17.
See also Unbound nuclide
J. Magill and J. Galy, Radioactivity Radionuclides Radiation Springer Verlag, 2005
J. Magill, G. Pfennig, J. Galy, Karlsruhe Nuclide Chart, 7th Edition, 2006.