What is "halo nucleus"

n. (context physics English) An atomic nucleus whose radius is appreciably larger than that predicted by the http://en.wikipedia.org/wiki/liquid%20drop%20model (wherein the nucleus is assumed to be a sphere of constant density).

In nuclear physics, an atomic nucleus is called a halo nucleus or is said to have a nuclear halo when it has a core nucleus surrounded by a halo of orbiting protons or neutrons, which makes the radius of the nucleus appreciably larger than that predicted by the liquid drop model. Halo nuclei form at the extreme edges of the table of nuclides — the neutron drip line and proton drip line — and have short half-lives, measured in milliseconds. These nuclei are studied shortly after their formation in an ion beam.

Typically, an atomic nucleus is a tightly bound group of protons and neutrons. However, in some isotopes, there is an overabundance of one species of nucleon. In some of these cases, a nuclear core and a halo will form.

Often, this property may be detected in scattering experiments, which show the nucleus to be much larger than the otherwise expected value. Normally, the cross-section (corresponding to the classical radius) of the nucleus is proportional to the cube root of its mass, as would be the case for a sphere of constant density. Specifically, for a nucleus of mass number A, the radius r is (approximately)

$$r = r_\circ A^{\frac{1}{3}},$$

where r is 1.2 fm.

One example of a halo nucleus is Li, which has a half-life of 8.6 ms. It contains a core of 3 protons and 6 neutrons, and a halo of two independent and loosely bound neutrons. It decays into Be by the emission of an antineutrino and an electron. Its cross-section of 3.16 fm is close to that of S or, even more impressively, of Pb, both much heavier nuclei.

Nuclei that have a neutron halo include Be and C. A two-neutron halo is exhibited by He, Li, B, B and C. Two-neutron halo nuclei break into three fragments and are called Borromean because of this behavior. He and Be both exhibit a four-neutron halo.

Nuclei that have a proton halo include B and P. A two-proton halo is exhibited by Ne and S. Proton halos are expected to be rarer and more unstable than neutron halos because of the repulsive forces of the excess proton(s).

Experimental confirmation of nuclear halos is recent and ongoing. Additional candidates are suspected. Several nuclides have a halo in the excited state but not in the ground state.