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)
where is 1.2 fm.
One example of a halo nucleus is 11Li, 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 11Be by the emission of an antineutrino and an electron. Its cross-section of 3.16 fm is close to that of 32S or, even more impressively, of 208Pb, both much heavier nuclei.
Experimental confirmation of nuclear halos is recent and ongoing. Additional candidates are suspected. Several nuclides[which?] have a halo in the excited state but not in the ground state.
List of known isotopes with nuclear halo
Nuclei that have a neutron halo include 11Be and 19C. A two-neutron halo is exhibited by 6He, 11Li, 17B, 19B and 22C. Two-neutron halo nuclei break into three fragments and are called Borromean because of this behavior. 8He and 14Be both exhibit a four-neutron halo.
Nuclei that have a proton halo include 8B and 26P. A two-proton halo is exhibited by 17Ne and 27S. Proton halos are expected to be rarer and more unstable than neutron halos because of the repulsive forces of the excess proton(s).
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- Atomic_nucleus#Halo_nuclei_and_strong_force_range_limits (Says "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).")
- "It's Elemental - Isotopes of the Element Lithium". Retrieved 15 April 2015.
- "ISOLDE goes on the trail of superlatives". Retrieved 15 April 2015.
- "Phys. Rev. Lett. 108, 142501 (2012) - Nuclear Charge Radius of 12Be". Physical Review Letters. Retrieved 15 April 2015.
- U.S. National Nuclear Data Center. "NuDat 2.6". Retrieved March 2015. Check date values in:
- http://arxiv.org/abs/0809.2607 Phys.Rev.Lett.102:062503,2009 Nuclear Charge Radii of Be-7,9,10 and the one-neutron halo nucleus Be-11
- http://www.uni-mainz.de/eng/13031.php Atomic Nucleus with Halo: For the First Time, Scientists Measure the Size of a One-Neutron Halo with Lasers (news item about Phys.Rev.Lett.102:062503,2009) Says "The measurements revealed that the average distance between the halo neutrons and the dense core of the nucleus is 7 femtometers. Thus, the halo neutron is about three times as far from the dense core as is the outermost proton, since the core itself has a radius of only 2.5 femtometers."
- http://arxiv.org/abs/nucl-ex/0111001v2 The detection of neutron clusters Discusses 6 events from Be-14 and possible evidence of a tetraneutron bound state.