Halo nucleus

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 $r_\circ$ 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.^[1] Its cross-section of 3.16 fm is close to that of ³²S or, even more impressively, of ²⁰⁸Pb, both much heavier nuclei.^[2]

Nuclei that have a neutron halo include ¹¹Be^[3] 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).

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.

List of known isotopes with nuclear halo

Atomic number	Name	# of nuclear halo isotopes	Nuclear halo isotopes	Halo composition	Half-life (ms)[4]
2	helium	2	helium-6 helium-8	2 neutrons 4 neutrons	801(10) 119.1(12)
3	lithium	1	lithium-11	2 neutrons	8.75(14)
4	beryllium	2	beryllium-11 beryllium-14	1 neutron 4 neutrons	13810(80) 4.35(17)
5	boron	3	boron-8 boron-17 boron-19	1 proton 2 neutrons 2 neutrons	770(3) 5.08(5) 2.92(13)
6	carbon	2	carbon-19 carbon-22	1 neutron 2 neutrons	49(4) 6.1+1.4 -1.2
10	neon	1	neon-17	2 protons	109.2(6)
15	phosphorus	1	phosphorus-26	1 proton	43.7(6)
16	sulfur	1	sulfur-27	2 protons	15.5(15)

References

1. "It's Elemental - Isotopes of the Element Lithium". Retrieved 15 April 2015.
2. "ISOLDE goes on the trail of superlatives". Retrieved 15 April 2015.
3. "Phys. Rev. Lett. 108, 142501 (2012) - Nuclear Charge Radius of $^{12}\mathrm{Be}$". Physical Review Letters. Retrieved 15 April 2015.
4. U.S. National Nuclear Data Center. "NuDat 2.6". Retrieved March 2015.

External links

• http://arxiv.org/abs/0809.2607
• http://www.uni-mainz.de/eng/13031.php
• http://arxiv.org/abs/nucl-ex/0111001v2