Editing Uranium (section)

===Origin===
Like all elements with [[atomic weight]]s higher than that of [[iron]], uranium is only naturally formed by the [[r-process]] (rapid neutron capture) in [[supernova]]e and [[neutron star merger]]s.<ref>{{cite web |url=http://herschel.jpl.nasa.gov/chemicalOrigins.shtml |title=History/Origin of Chemicals |publisher=NASA |access-date=1 January 2013}}</ref> Primordial thorium and uranium are only produced in the r-process, because the [[s-process]] (slow neutron capture) is too slow and cannot pass the gap of instability after bismuth.<ref name="B2FH">{{cite journal | author1=Burbidge, E. M. | author2=Burbidge, G. R. | author3=Fowler, W. A. | author4=Hoyle, F. | year=1957 | title=Synthesis of the Elements in Stars | journal=[[Reviews of Modern Physics]] | volume=29 | issue=4 | page=547 | bibcode=1957RvMP...29..547B | doi=10.1103/RevModPhys.29.547 | doi-access=free }}</ref><ref>{{cite book|last=Clayton|first=Donald D.|author-link=Donald D. Clayton|title=Principles of Stellar Evolution and Nucleosynthesis|publisher=Mc-Graw-Hill|location=New York |date=1968|pages=577–91|isbn=978-0226109534}}</ref> Besides the two extant primordial uranium isotopes, {{sup|235}}U and {{sup|238}}U, the r-process also produced significant quantities of [[uranium-236|{{sup|236}}U]], which has a shorter half-life and so is an [[extinct radionuclide]], having long since decayed completely to {{sup|232}}Th. Further uranium-236 was produced by the decay of [[plutonium-244|{{sup|244}}Pu]], accounting for the observed higher-than-expected abundance of thorium and lower-than-expected abundance of uranium.<ref name="thoruranium">{{cite journal |last1=Trenn |first1=Thaddeus J. |date=1978 |title=Thoruranium (U-236) as the extinct natural parent of thorium: The premature falsification of an essentially correct theory |journal=Annals of Science |volume=35 |issue=6 |pages=581–97 |doi=10.1080/00033797800200441}}</ref> While the natural abundance of uranium has been supplemented by the decay of extinct [[plutonium-242|{{sup|242}}Pu]] (half-life 375,000&nbsp;years) and {{sup|247}}Cm (half-life 16&nbsp;million years), producing {{sup|238}}U and {{sup|235}}U respectively, this occurred to an almost negligible extent due to the shorter half-lives of these parents and their lower production than {{sup|236}}U and {{sup|244}}Pu, the parents of thorium: the {{sup|247}}Cm/{{sup|235}}U ratio at the formation of the Solar System was {{val|7.0e-5|1.6}}.<ref>
{{cite journal |last1=Tissot |first1=François L. H. |last2=Dauphas |first2=Nicolas |last3=Grossmann |first3=Lawrence |date=4 March 2016 |title=Origin of uranium isotope variations in early solar nebula condensates |journal=Science Advances |volume=2 |issue=3 |doi=10.1126/sciadv.1501400|pmid=26973874 |pmc=4783122 |arxiv=1603.01780 |bibcode=2016SciA....2E1400T |page=e1501400}}</ref>