The density of the nucleus of most nuclides is very nearly identical, except for nuclei near the neutron drip line where some neutrons are only weakly bound to the inner core nucleons. These are the halo nuclei and are a newly discovered and highly unusual phenomenon. They consist of a normal nucleus surrounded by a halo of extra neutrons which extend up to seven times the diameter of the core nucleus. The halflives of these nuclei are typically 100 milliseconds, which is a very long time compared to the characteristic nuclear time scale; they are thus almost 'stable'. The outer halo of neutrons are mostly outside the range of the strong nuclear force between nucleons, their elliptical orbits taking them near the nucleus for only a small percentage of the time. The neutrons may thus be more like the neutron matter within neutron stars.

By Heisenbergs' Uncertainty Principle, the halo neutrons are able to borrow a small extra energy to enable them to remain at long distances from the nucleus for a longish time, thus producing the halo. Lithium-11 is a halo nucleus that consists of a lithium-9 core surrounded by a halo of 2 loosely bound neutrons requiring just 0.3MeV to remove them and is so large as to be similar in size to calcium-48. (This compares to the average 8MeV per nucleon for unconditionally stable nuclei). Other halo nuclei are helium-6 (He-4 + 2n), helium-8 (He-4 + 4n), beryllium-11 (Be-10 + 1n), beryllium-14, boron-17 and the heaviest yet discovered carbon-19. The extra four neutrons in helium-8 are bound fairly closely to the surface forming a neutron skin rather than a neutron halo. See TetraNeutrons. Proton halos may also exist for nuclei near the proton dripline; boron-8, nitrogen-13 and fluorine-17 being strong candidates with possible single-proton halos.

Halo Nucleus He-6, being He-4 + two halo neutrons

N.B. All Borromean nuclei are also neutron halo nuclei.