Editing Magic number (physics) (section)

==Doubly magic==
Nuclei which have neutron numbers and proton ([[atomic number|atomic]]) numbers both equal to one of the magic numbers are called "doubly magic", and are generally very stable against decay.<ref>{{Cite web|url=https://www.periodic-table.org/what-is-stable-nuclei-unstable-nuclei-definition/|title=What is Stable Nuclei - Unstable Nuclei - Definition|date=2019-05-22|website=Periodic Table|language=en-GB|access-date=2019-12-22}}</ref> The known doubly magic isotopes are [[helium-4]], [[helium]]-10, [[oxygen-16]], [[calcium-40]], [[calcium-48]], [[nickel]]-48, nickel-56, nickel-78, [[tin]]-100, tin-132, and [[lead]]-208. While only helium-4, oxygen-16, calcium-40, and lead-208 are completely stable, calcium-48 is extremely long-lived and therefore found naturally, disintegrating only by a very inefficient [[double beta decay|double beta minus decay]] process. Double beta decay in general is so rare that several nuclides exist which are predicted to decay by this mechanism but in which no such decay has yet been observed. Even in nuclides whose double beta decay has been confirmed through observations, half lives usually exceed the [[age of the universe]] by orders of magnitude, and emitted beta or gamma radiation is for virtually all practical purposes irrelevant. On the other hand, helium-10 is extremely unstable, and has a [[half-life]] of just {{val|260|(40)|u=[[yoctoseconds]]}} ({{val|2.6e-22|(4)|u=s}}).

Doubly magic effects may allow the existence of stable isotopes which otherwise would not have been expected. An example is [[calcium-40]], with 20 neutrons and 20 protons, which is the heaviest stable isotope made of the same number of protons and neutrons. Both [[calcium-48]] and [[Isotopes of nickel#Notable isotopes|nickel]]-48 are doubly magic because calcium-48 has 20 protons and 28 neutrons while nickel-48 has 28 protons and 20 neutrons. Calcium-48 is very neutron-rich for such a relatively light element, but like calcium-40, it is stabilized by being doubly magic. As an exception, although [[oxygen-28]] has 8 protons and 20 neutrons, it is unbound with respect to four-neutron decay and appears to lack closed neutron shells, so it is not regarded as doubly magic.<ref>{{Cite journal |last1=Kondo |first1=Y. |last2=Achouri |first2=N. L. |last3=Falou |first3=H. Al |last4=Atar |first4=L. |last5=Aumann |first5=T. |last6=Baba |first6=H. |last7=Boretzky |first7=K. |last8=Caesar |first8=C. |last9=Calvet |first9=D. |last10=Chae |first10=H. |last11=Chiga |first11=N. |last12=Corsi |first12=A. |last13=Delaunay |first13=F. |last14=Delbart |first14=A. |last15=Deshayes |first15=Q. |date=2023-08-31 |title=First observation of 28O |journal=Nature |language=en |volume=620 |issue=7976 |pages=965–970 |doi=10.1038/s41586-023-06352-6 |issn=0028-0836 |pmc=10630140 |pmid=37648757}}</ref>

Magic number shell effects are seen in ordinary abundances of elements: helium-4 is among the most abundant (and stable) nuclei in the universe<ref>{{Cite web |url= http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin2.html#c1 |title= The Most Tightly Bound Nuclei |website=HyperPhysics|first=C. R. |last=Nave}}</ref> and lead-208 is the heaviest [[stable nuclide|stable]] [[nuclide]] ([[Observationally stable|at least]] by known experimental observations). [[Alpha decay]] (the emission of a <sup>4</sup>He nucleus – also known as an alpha particle – by a heavy element undergoing radioactive decay) is common in part due to the extraordinary stability of helium-4, which makes this type of decay energetically favored in most heavy nuclei over [[neutron emission]], [[proton emission]] or any other type of [[cluster decay]]. The stability of <sup>4</sup>He also leads to the absence of stable [[Isobar (nuclide)|isobars]] of mass number 5 and 8; indeed, all nuclides of those mass numbers decay within fractions of a second to produce alpha particles.

Magic effects can keep unstable nuclides from decaying as rapidly as would otherwise be expected. For example, the nuclides tin-100 and tin-132 are examples of doubly magic [[isotopes of tin]] that are unstable, and represent endpoints beyond which stability drops off rapidly. Nickel-48, discovered in 1999, is the most proton-rich doubly magic nuclide known.<ref>{{cite web|last=W. |first=P. |title=Twice-magic metal makes its debut - isotope of nickel |publisher=[[Science News]] |date=October 23, 1999 |url=http://www.findarticles.com/p/articles/mi_m1200/is_17_156/ai_57799535 |archive-url=https://archive.today/20120524134125/http://www.findarticles.com/p/articles/mi_m1200/is_17_156/ai_57799535 |url-status=dead |archive-date=May 24, 2012 |access-date=2006-09-29 }}</ref> At the other extreme, nickel-78 is also doubly magic, with 28 protons and 50 neutrons, a ratio observed only in much heavier elements, apart from [[tritium]] with one proton and two neutrons (<sup>78</sup>Ni: 28/50&nbsp;=&nbsp;0.56; <sup>238</sup>U: 92/146&nbsp;=&nbsp;0.63).<ref>{{cite web | title = Tests confirm nickel-78 is a 'doubly magic' isotope |  publisher = [[Phys.org]]  | date = September 5, 2014 | url = http://phys.org/news/2014-09-nickel-doubly-magic-isotope.html | access-date = 2014-09-09 }}</ref>

In December 2006, [[hassium]]-270, with 108 protons and 162 neutrons, was discovered by an international team of scientists led by the [[Technical University of Munich]], having a [[half-life]] of 9&nbsp;seconds.{{NUBASE2016|ref|page=030001–134}} Hassium-270 evidently forms part of an [[island of stability]], and may even be doubly magic due to the deformed ([[American football (ball)|American football]]- or [[rugby ball]]-like) shape of this nucleus.<ref name="Focus">{{Cite magazine|url=http://focus.aps.org/story/v18/st19|title=A Nuclear Magic Trick|volume=18|access-date=2006-12-25|author=Mason Inman|date=2006-12-14|magazine= Physical Review Focus}}</ref><ref>{{cite journal|last1=Dvorak|first1=J.|last2=Brüchle|first2=W.|last3=Chelnokov|first3=M.|last4=Dressler|first4=R.|last5=Düllmann|first5=Ch. E.|last6=Eberhardt|first6=K.|last7=Gorshkov|first7=V.|last8=Jäger|first8=E.|last9=Krücken|first9=R.|last10=Kuznetsov|first10=A.|last11=Nagame|first11=Y.|last12=Nebel|first12=F.|last13=Novackova|first13=Z.|last14=Qin|first14=Z.|last15=Schädel|first15=M.|last16=Schausten|first16=B.|last17=Schimpf|first17=E.|last18=Semchenkov|first18=A.|last19=Thörle|first19=P.|last20=Türler|first20=A.|last21=Wegrzecki|first21=M.|last22=Wierczinski|first22=B.|last23=Yakushev|first23=A.|last24=Yeremin|first24=A.|title=Doubly Magic Nucleus <sub>108</sub><sup>270</sup>Hs<sub>162</sub> |journal=Physical Review Letters|volume=97|issue=24|pages=242501|year=2006|doi=10.1103/PhysRevLett.97.242501|pmid=17280272|bibcode=2006PhRvL..97x2501D|url=https://www.dora.lib4ri.ch/psi/islandora/object/psi%3A16351}}</ref>

Although ''Z''&nbsp;=&nbsp;92 and ''N''&nbsp;=&nbsp;164 are not magic numbers, the undiscovered neutron-rich nucleus [[uranium]]-256 may be doubly magic and spherical due to the difference in size between low- and high-[[angular momentum]] orbitals, which alters the shape of the [[nuclear potential]].<ref name=magickoura>{{cite journal|last1=Koura|first1=H.|last2=Chiba|first2=S.|date=2013|title=Single-Particle Levels of Spherical Nuclei in the Superheavy and Extremely Superheavy Mass Region|journal=Journal of the Physical Society of Japan|volume=82|issue=1|pages=014201|url=https://www.researchgate.net/publication/258799250 |doi=10.7566/JPSJ.82.014201|bibcode=2013JPSJ...82a4201K}}</ref>