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 nuclides, 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 mass radius of 3.16 fm is close to that of ³²S or, even more impressively, of ²⁰⁸Pb, both much heavier nuclei.^[2]

Experimental confirmation of nuclear halos is recent and ongoing. Additional candidates are suspected. Several nuclides including ⁹B, ¹³N, and ¹⁵N are calculated to have a halo in the excited state but not in the ground state.^[3]

List of known nuclides with nuclear halo

Nuclei that have a neutron halo include ¹¹Be^[4] 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, analogously to how all three of the Borromean rings are linked together but no two share a link. For example, the two-neutron halo nucleus ⁶He (which can be taken as a three-body system consisting of an alpha particle and two neutrons) is bound, but neither ⁵He nor the dineutron is. 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).

Atomic number	Name	# of nuclear halo isotopes	Nuclear halo isotopes	Halo composition	Half-life (ms)^[5]
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 4 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

^ "It's Elemental - Isotopes of the Element Lithium". Retrieved 15 April 2015.
^ "ISOLDE goes on the trail of superlatives". 3 May 2004. Retrieved 15 April 2015.
^ Jin-Gen, Chen; Xiang-Zhou, Cai; Hu-Yong, Zhang; Wen-Qing, Shen; Zhong-Zhou, Ren; Wei-Zhou, Jiang; Yu-Gang, Ma; Chen, Zhong; Yi-Bin, Wei; Wei, Guo; Xing-Fei, Zhou; Guo-Liang, Ma; Kun, Wang (2003). "Chinese Phys. Lett. 20 1021 - Proton Halo or Skin in the Excited States of Light Nuclei". Chinese Physics Letters. 20 (7): 1021–1024. doi:10.1088/0256-307X/20/7/314.
^ Krieger, A; Blaum, K; Bissell, M. L; Frömmgen, N; Geppert, Ch; Hammen, M; Kreim, K; Kowalska, M; Krämer, J; Neff, T; Neugart, R; Neyens, G; Nörtershäuser, W; Novotny, Ch; Sánchez, R; Yordanov, D. T (2012). "Phys. Rev. Lett. 108, 142501 (2012) - Nuclear Charge Radius of ¹²Be". Physical Review Letters. 108 (14): 142501. arXiv:1202.4873. doi:10.1103/PhysRevLett.108.142501. PMID 22540787. S2CID 1589595.
^ U.S. National Nuclear Data Center. "NuDat 2.6". Retrieved 13 March 2015.

Nörtershäuser, W.; Tiedemann, D.; Žáková, M.; Andjelkovic, Z.; Blaum, K.; Bissell, M. L.; Cazan, R.; Drake, G. W. F.; Geppert, Ch.; Kowalska, M.; Krämer, J.; Krieger, A.; Neugart, R.; Sánchez, R.; Schmidt-Kaler, F.; Yan, Z.-C.; Yordanov, D. T.; Zimmermann, C. (2009). "Nuclear Charge Radii of Be7,9,10 and the One-Neutron Halo Nucleus Be11". Physical Review Letters. 102 (6): 062503. arXiv:0809.2607. Bibcode:2009PhRvL.102f2503N. doi:10.1103/PhysRevLett.102.062503. PMID 19257582. S2CID 24357745.
"Atomic Nucleus with Halo: For the First Time, Scientists Measure the Size of a One-Neutron Halo with Lasers". 2009. The measurements revealed that the average distance between the halo neutrons and the dense core of the [Be-11] 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. {{cite journal}}: Cite journal requires |journal= (help)
Marqués, F. M.; Labiche, M.; Orr, N. A.; Angélique, J. C.; Axelsson, L.; Benoit, B.; Bergmann, U. C.; Borge, M. J. G.; Catford, W. N.; Chappell, S. P. G.; Clarke, N. M.; Costa, G.; Curtis, N.; d'Arrigo, A.; De Góes Brennand, E.; De Oliveira Santos, F.; Dorvaux, O.; Fazio, G.; Freer, M.; Fulton, B. R.; Giardina, G.; Grévy, S.; Guillemaud-Mueller, D.; Hanappe, F.; Heusch, B.; Jonson, B.; Le Brun, C.; Leenhardt, S.; Lewitowicz, M.; et al. (2002). "Detection of neutron clusters". Physical Review C. 65 (4): 044006. arXiv:nucl-ex/0111001. Bibcode:2002PhRvC..65d4006M. doi:10.1103/PhysRevC.65.044006. S2CID 37431352.