Editing Helium (section)

===Isotopes===
{{Main|Isotopes of helium}}
There are nine known [[isotope]]s of helium of which two, [[helium-3]] and [[helium-4]], are [[stable isotope|stable]]. In the Earth's atmosphere, one atom is {{chem|3|He}} for every million that are {{chem|4|He}}.<ref name="nbb">{{Cite book| author = Emsley, John| title = Nature's Building Blocks| publisher = Oxford University Press| date = 2001| location = Oxford| pages = 175–179| isbn = 978-0-19-850341-5}}</ref> Unlike most elements, helium's isotopic abundance varies greatly by origin, due to the different formation processes. The most common isotope, helium-4, is produced on Earth by [[alpha decay]] of heavier radioactive elements; the alpha particles that emerge are fully ionized helium-4 nuclei. Helium-4 is an unusually stable nucleus because its [[nucleon]]s are arranged into [[Nuclear shell model|complete shells]]. It was also formed in enormous quantities during [[Big Bang nucleosynthesis]].<ref name="bigbang" />

Helium-3 is present on Earth only in trace amounts. Most of it has been present since Earth's formation, though some falls to Earth trapped in [[cosmic dust]].<ref name="heliumfundamentals">{{cite web |url = http://www.mantleplumes.org/HeliumFundamentals.html |title = Helium Fundamentals |author = Anderson, Don L. |author2 = Foulger, G. R. |author3 = Meibom, A. |date = 2006-09-02 |access-date = 2008-07-20 |publisher = MantlePlumes.org |archive-url = https://web.archive.org/web/20070208194933/http://www.mantleplumes.org/HeliumFundamentals.html |archive-date = 2007-02-08 |url-status = live }}</ref> Trace amounts are also produced by the [[beta decay]] of [[tritium]].<ref>{{Cite journal|title= Half-Life of Tritium| journal=Physical Review|volume= 72|issue= 10|date= 1947| pages= 972|last= Novick|first=Aaron| doi=10.1103/PhysRev.72.972.2|bibcode = 1947PhRv...72..972N }}</ref> Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten, and these ratios can be used to investigate the origin of rocks and the composition of the Earth's [[Mantle (geology)|mantle]].<ref name="heliumfundamentals" /> {{chem|3|He}} is much more abundant in stars as a product of nuclear fusion. Thus in the [[interstellar medium]], the proportion of {{chem|3|He}} to {{chem|4|He}} is about 100 times higher than on Earth.<ref>{{Cite journal|title=Isotopic Composition and Abundance of Interstellar Neutral Helium Based on Direct Measurements| journal=Astrophysics| volume=45| issue=2|date=2002| pages=131–142| last1=Zastenker | first1=G. N. | doi=10.1023/A:1016057812964|bibcode = 2002Ap.....45..131Z|last2=Salerno | first2=E. | last3=Buehler |first3=F.|last4=Bochsler | first4=P.|last5=Bassi | first5=M. |last6=Agafonov | first6=Yu. N. |last7=Eisomont| first7=N. A. |last8=Khrapchenkov | first8=V. V. | last9=Busemann | first9=H.| s2cid=116957905| display-authors = 8 }}</ref> Extraplanetary material, such as [[Moon|lunar]] and [[asteroid]] [[regolith]], have trace amounts of helium-3 from being bombarded by [[solar wind]]s. The [[Moon]]'s surface contains helium-3 at concentrations on the order of 10 [[Parts per billion|ppb]], much higher than the approximately 5 [[Parts per trillion|ppt]] found in the Earth's atmosphere.<ref>{{cite web|url = http://fti.neep.wisc.edu/research/he3|title = Lunar Mining of Helium-3|date = 2007-10-19|access-date = 2008-07-09|publisher = Fusion Technology Institute of the University of Wisconsin-Madison|archive-url = https://web.archive.org/web/20100609234057/http://fti.neep.wisc.edu/research/he3|archive-date = 2010-06-09|url-status = live}}</ref><ref>{{cite journal|url= http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2175.pdf|title= The estimation of helium-3 probable reserves in lunar regolith|author= Slyuta, E. N.|author2= Abdrakhimov, A. M.|author3= Galimov, E. M.|journal= Lunar and Planetary Science Conference|issue= 1338|pages= 2175|date= 2007|access-date= 2008-07-20|bibcode= 2007LPI....38.2175S|archive-url= https://web.archive.org/web/20080705122316/http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2175.pdf|archive-date= 2008-07-05|url-status= live}}</ref> A number of people, starting with Gerald Kulcinski in 1986,<ref>{{Cite news|url = http://www.thespacereview.com/article/536/1|title = A fascinating hour with Gerald Kulcinski|author = Hedman, Eric R.|date = 2006-01-16|work = The Space Review|access-date = 2008-07-20|archive-url = https://web.archive.org/web/20110109082500/http://thespacereview.com/article/536/1|archive-date = 2011-01-09|url-status = dead}}</ref> have proposed to explore the Moon, mine lunar regolith, and use the helium-3 for [[Nuclear fusion|fusion]].

Liquid helium-4 can be cooled to about {{convert|1|K|C F}} using [[evaporative cooling]] in a [[1-K pot]]. Similar cooling of helium-3, which has a lower boiling point, can achieve about {{val|0.2|u=kelvin}} in a [[Helium-3#Cryogenics|helium-3 refrigerator]]. Equal mixtures of liquid {{chem|3|He}} and {{chem|4|He}} below {{val|0.8|u=K}} separate into two immiscible phases due to their dissimilarity (they follow different [[quantum statistics]]: helium-4 atoms are [[boson]]s while helium-3 atoms are [[fermion]]s).<ref name = enc/> [[Dilution refrigerator]]s use this immiscibility to achieve temperatures of a few millikelvins.<ref>{{Cite journal | doi = 10.1016/j.cryogenics.2021.103390|issn=0011-2275| title = Development of Dilution refrigerators – A review | journal = Cryogenics| volume = 121| year = 2022| last1 = Zu | first1 = H.| last2 = Dai | first2 = W.| last3 = de Waele | first3 = A.T.A.M.|s2cid=244005391}}</ref>

It is possible to produce [[exotic helium isotopes]], which rapidly decay into other substances. The shortest-lived heavy helium isotope is the [[nuclear drip line|unbound]] helium-10 with a [[half-life]] of {{val|2.6|(4)|e=-22|u=s}}.{{NUBASE2020|ref}} Helium-6 decays by emitting a [[beta particle]] and has a half-life of 0.8 second. Helium-7 and helium-8 are created in certain [[nuclear reaction]]s.<ref name="enc" /> Helium-6 and helium-8 are known to exhibit a [[nuclear halo]].<ref name = enc/>