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The deuterium isotope's name is formed from the Greek ''deuteros'', meaning "second", to denote the two particles composing the nucleus.<ref name="diplogen">{{cite journal |doi=10.1038/nchem.1273 |pmid=22354440 |author=O'Leary, Dan |title=The deeds to deuterium |journal=Nature Chemistry |volume=4 |issue=3 |page=236 |year=2012 |bibcode=2012NatCh...4..236O}}</ref> Deuterium was discovered and named in 1931 by [[Harold Urey]]. When the neutron was discovered in 1932, this made the nuclear structure of deuterium obvious, and Urey won the [[List of Nobel laureates in Chemistry|Nobel Prize]] in 1934 “for his discovery of heavy hydrogen”. Soon after deuterium's discovery, Urey and others produced samples of "[[heavy water]]" in which the deuterium content had been highly concentrated.
Deuterium is destroyed in the interiors of stars faster than it is produced. Other natural processes are thought to produce only an insignificant amount of deuterium. Nearly all deuterium found in nature was produced in the [[Big Bang]] 13.8 billion years ago, as the basic or primordial ratio of hydrogen-1 to deuterium (about 26 atoms of deuterium per million hydrogen atoms) has its origin from that time. This is the ratio found in the gas giant planets, such as Jupiter.<ref name="diplogen" /><ref name="nature2">{{cite journal |doi=10.1038/nature10519 |journal=Nature |volume=478 |pages=218–220 |year=2011 |title=Ocean-like water in the Jupiter-family comet 103P/Hartley 2 |last1=Hartogh |first1=Paul |last2=Lis |first2=Dariusz C. |last3=Bockelée-Morvan |first3=Dominique |last4=De Val-Borro |first4=Miguel |last5=Biver |first5=Nicolas |last6=Küppers |first6=Michael |last7=Emprechtinger |first7=Martin |last8=Bergin |first8=Edwin A. |last9=Crovisier |first9=Jacques |displayauthors=8 |issue=7368 |pmid=21976024 |bibcode=2011Natur.478..218H}}</ref><ref name="Hersant">{{cite journal |url= |quote=see fig. 7. for a review of D/H ratios in various astronomical objects |doi=10.1086/321355 |title=A Two‐dimensional Model for the Primordial Nebula Constrained by D/H Measurements in the Solar System: Implications for the Formation of Giant Planets |year=2001 |last1=Hersant |first1=Franck |last2=Gautier |first2=Daniel |last3=Hure |first3=Jean‐Marc |journal=The Astrophysical Journal |volume=554 |pages=391–407 |bibcode=2001ApJ...554..391H |issue=1}}</ref> However, other astronomical bodies are found to have different ratios of deuterium to hydrogen-1. This is thought to be a result of natural isotope separation processes that occur from solar heating of ices in comets. Like the [[water cycle]] in Earth's weather, such heating processes may enrich deuterium with respect to protium. The analysis of deuterium/protium ratios in comets found results very similar to the mean ratio in Earth's oceans (156 atoms of deuterium per million hydrogens). This reinforces theories that much of Earth's ocean water is of cometary origin.<ref name="nature2" /><ref name="Hersant" /> <!--News reports of Hubble measurements of "6 atoms of D per 10,000" in Jupiter are wrong; the correct figure is 6 parts D per 100,000 by weight, which is 30 parts per million atom-fraction, close to the Galileo result of 26 parts per million, atom-fraction--> The deuterium/protium ratio of the comet 67P/Churyumov-Gerasimenko, as measured by the [[Rosetta (spacecraft)|Rosetta space probe]], is about three times that of earth water. This figure is the highest yet measured in a comet.<ref name="">{{cite journal |doi=10.1126/science.1261952 |journal=Science |year=2014 |title=67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio |last1=Altwegg |first1=K. |last2=Balsiger |first2=H. |last3=Bar-Nun |first3=A. |last4=Berthelier |first4=J. J. |display-authors=etal |volume=347 |issue=6220 |pages=1261952 |bibcode=2015Sci...347A.387A |pmid=25501976|url= }} retrieved Dec 12, 2014</ref>
Deuterium/protium ratios thus continue to be an active topic of research in both astronomy and climatology.
The proton and neutron making up deuterium can be [[Dissociation (chemistry)|dissociated]] through [[neutral current]] interactions with [[neutrino]]s. The [[Cross section (physics)|cross section]] for this interaction is comparatively large, and deuterium was successfully used as a neutrino target in the [[Sudbury Neutrino Observatory]] experiment.
Diatomic deuterium (D<sub>2</sub>) has ortho and para [[Spin isomers of hydrogen|nuclear spin isomers]] like diatomic hydrogen, but with [[Spin isomers of hydrogen#Deuterium|differences in the number and population of spin states and rotational levels]], which occur because the deuteron is a [[boson]] with nuclear spin equal to one.<ref name=Hollas>{{cite book |last1=Hollas |first1=J. Michael |title=Modern Spectroscopy |date=1996 |publisher=John Wiley and Sons |isbn=0 -471 -96523 -5 |page=115 |edition=3rd}}</ref>
====Isospin singlet state of the deuteron====
Due to the similarity in mass and nuclear properties between the proton and neutron, they are sometimes considered as two symmetric types of the same object, a [[nucleon]]. While only the proton has an electric charge, this is often negligible due to the weakness of the [[electromagnetic interaction]] relative to the [[strong nuclear interaction]]. The symmetry relating the proton and neutron is known as [[isospin]] and denoted ''I'' (or sometimes ''T'').
Isospin is an [[SU(2)]] symmetry, like ordinary [[Spin (physics)|spin]], so is completely analogous to it. The proton and neutron form an [[spin doublet|isospin doublet]], with a [[Spin-½|"down" state]] (↓) being a neutron, and an [[Spin-½|"up" state]] (↑) being a proton.<ref>{{cite journal |last1=Kalmbach |first1=Gudrun H.E. |title=Deuteron States |journal=Nessa Journal of Physics |date=2017 |volume=1 |issue=2 |pages=1-171–17 |url= |accessdate=3 May 2019 |publisher=Nessa Publishers}}</ref>
A pair of nucleons can either be in an antisymmetric state of isospin called [[Singlet state|singlet]], or in a symmetric state called [[Spin triplet|triplet]]. In terms of the "down" state and "up" state, the singlet is
:<math>\frac{1}{\sqrt{2}}\Big( |\uparrow \downarrow \rangle - |\downarrow \uparrow \rangle\Big).</math>, which can also be written :<math>\frac{1}{\sqrt{2}}\Big( |p n \rangle - |n p \rangle\Big).</math>