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| 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. The electrons then orbit the core nucleus much further away than the orbit of the halo nucleons. The halo makes the nucleus radius appreciably larger than that predicted by the [[liquid drop model]], wherein the nucleus is assumed to be a sphere of constant density. One example is [[isotopes_of_lithium | <sup>11</sup>Li]] where the nuclear radius is about the size of that of <sup>208</sup>Pb.
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| For a nucleus of [[mass number]] A, the radius r is (approximately)
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| :<math>r = r_\circ A^{\frac{1}{3}},</math>
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| where <math>r_\circ</math> is 1.2 [[femtometer|fm]].
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| 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.
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| 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. This is the same relation as would be seen with a solid sphere.
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| One example of a halo nucleus is <sup>11</sup>[[Lithium#Isotopes|Li]] which has a half life of 8.6 ms. It decays into <sup>11</sup>[[Beryllium#Isotopes|Be]] by the emission of an antineutrino and an electron. <ref>http://education.jlab.org/itselemental/iso003.html</ref> Its cross-section of 3.16 fm is close to that of <sup>32</sup>[[sulfur|S]], or even more impressively that of <sup>208</sup>[[lead|Pb]], both much heavier nuclei.<ref>http://cerncourier.com/cws/article/cern/29077</ref> It contains a core of 3 protons and 6 neutrons, and a halo of two independent and loosely bound neutrons.
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| Nuclei which have a '''neutron halo''' include <sup>11</sup>[[Beryllium#Isotopes|Be]]<ref name="Krieger2012">{{cite journal|last=Krieger|first=A.|coauthors=K. Blaum, M. L. Bissell, N. Frömmgen, Ch. Geppert, M. Hammen, K. Kreim, M. Kowalska, J. Krämer, T. Neff, R. Neugart, G. Neyens, W. Nörtershäuser, Ch. Novotny, R. Sánchez, D. T. Yordanov|year=2012|title=Nuclear Charge Radius of 12Be|journal=Physical Review Letters|volume=108|issue=14|issn=0031-9007|doi=10.1103/PhysRevLett.108.142501}}</ref> and <sup>19</sup>[[Carbon#Isotopes|C]]. A two-neutron halo is exhibited by <sup>6</sup>[[Helium#Isotopes|He]], <sup>11</sup>[[Lithium#Isotopes|Li]], <sup>17</sup>[[Boron#Isotopes|B]], <sup>19</sup>[[Boron#Isotopes|B]] and <sup>22</sup>[[Carbon#Isotopes|C]]. Two-neutron halo nuclei break into three fragments and are called [[Borromean nucleus|Borromean]] because of this behavior. <sup>8</sup>[[Helium#Isotopes|He]] and <sup>14</sup>[[Beryllium#Isotopes|Be]] both exhibit a four-neutron halo.
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| Nuclei which have a '''proton halo''' include <sup>8</sup>[[Boron#Isotopes|B]] and <sup>26</sup>[[Phosphorus#Isotopes|P]]. A two-proton halo is exhibited by <sup>17</sup>[[Neon#Isotopes|Ne]] and <sup>27</sup>[[Sulfur#Isotopes|S]]. Proton halos are expected to be more rare and unstable than the neutron examples, because of the repulsive forces of the excess proton(s).
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| Halo nuclei form at the extreme edges of the [[chart of the 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.
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| 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]].
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| ==List of known isotopes with nuclear halo==
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| {|class="sortable wikitable" border="1" align="center"
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| ! Atomic<br />number || Name || # of nuclear<br />halo isotopes || Nuclear halo<br />isotopes || Halo<br />composition
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| |- style="background:{{element color|Noble gases}}" align="center"
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| | 2 ||[[helium]] || 2 || [[helium-6]]<br />[[helium-8]] || 2 neutrons <br />4 neutrons
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| |- style="background:{{element color|Alkali metals}}" align="center"
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| | 3 ||[[lithium]] || 1 || [[lithium-11]] || 2 neutrons
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| |- style="background:{{element color|Alkaline earth metals}}" align="center"
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| | 4 ||[[beryllium]] || 2 || [[beryllium-11]]<br />[[beryllium-14]] || 1 neutron<br />4 neutrons
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| |- style="background:{{element color|Metalloids}}" align="center"
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| | 5 ||[[boron]] || 3 || [[boron-8]]<br />[[boron-17]]<br />[[boron-19]] || 1 proton<br />2 neutrons<br />2 neutrons
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| |- style="background:{{element color|Other nonmetals}}" align="center"
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| | 6 ||[[carbon]] || 2 || [[carbon-19]]<br />[[carbon-22]] || 1 neutron<br />2 neutrons
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| |- style="background:{{element color|Noble gases}}" align="center"
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| | 10 ||[[neon]] || 1 || [[neon-17]] || 2 protons
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| |- style="background:{{element color|Other nonmetals}}" align="center"
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| | 15 ||[[phosphorus]] || 1 || [[phosphorus-26]] || 1 proton
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| |- style="background:{{element color|Other nonmetals}}" align="center"
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| | 16 ||[[sulfur]] || 1 || [[sulfur-27]] || 2 protons
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| |}
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| ==References==
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| {{reflist|1}}
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| ==External links==
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| *http://arxiv.org/abs/0809.2607
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| *http://www.uni-mainz.de/eng/13031.php
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| *http://arxiv.org/abs/nucl-ex/0111001v2
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| [[Category:Nuclear physics]]
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The child of any , is aware of patience and dedication are important elements with regards to an effective job- . His first album, Stay Me, created the most notable hits “All My Girlfriends “Country and Say” Person,” while his hard work, Doin’ Point, identified the performer-3 directly No. 4 single people: Else Contacting Is a Fantastic Thing.”
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