Editing Thorium (section)

===Radiometric dating===
Two radiometric dating methods involve thorium isotopes: [[uranium–thorium dating]], based on the decay of [[uranium-234|<sup>234</sup>U]] to <sup>230</sup>Th, and [[ionium–thorium dating]], which measures the ratio of <sup>232</sup>Th to <sup>230</sup>Th.{{efn|The name ''ionium'' for <sup>230</sup>Th is a remnant from a period when different isotopes were not recognised to be the same element and were given different names.}} These rely on the fact that <sup>232</sup>Th is a primordial radioisotope, but <sup>230</sup>Th only occurs as an intermediate decay product in the decay chain of <sup>238</sup>U.<ref name="uth" /> Uranium–thorium dating is a relatively short-range process because of the short half-lives of <sup>234</sup>U and <sup>230</sup>Th relative to the age of the Earth: it is also accompanied by a sister process involving the alpha decay of <sup>235</sup>U into <sup>231</sup>Th, which very quickly becomes the longer-lived <sup>231</sup>Pa, and this process is often used to check the results of uranium–thorium dating. Uranium–thorium dating is commonly used to determine the age of [[calcium carbonate]] materials such as [[speleothem]] or [[coral]], because uranium is more soluble in water than thorium and protactinium, which are selectively precipitated into ocean-floor [[sediment]]s, where their ratios are measured. The scheme has a range of several hundred thousand years.<ref name="uth">{{cite web |url=http://www3.nd.edu/~nsl/Lectures/phys178/pdf/chap3_6.pdf |title=3–6: Uranium Thorium Dating |publisher=Institute for Structure and Nuclear Astrophysics, [[University of Notre Dame]] |access-date=7 October 2017 |archive-date=21 April 2021 |archive-url=https://web.archive.org/web/20210421193422/https://www3.nd.edu/~nsl/Lectures/phys178/pdf/chap3_6.pdf }}</ref><ref>{{cite web|last=Davis |first=O.|url=http://www.geo.arizona.edu/Antevs/ecol438/uthdating.html |title=Uranium-Thorium Dating |publisher=Department of Geosciences, [[University of Arizona]] |archive-date=28 March 2017 |archive-url=https://web.archive.org/web/20170328095352/http://www.geo.arizona.edu/Antevs/ecol438/uthdating.html |access-date=7 October 2017}}</ref> Ionium–thorium dating is a related process, which exploits the insolubility of thorium (both <sup>232</sup>Th and <sup>230</sup>Th) and thus its presence in ocean sediments to date these sediments by measuring the ratio of <sup>232</sup>Th to <sup>230</sup>Th.<ref name="rafferty2010">{{citation|title=Geochronology, Dating, and Precambrian Time: The Beginning of the World As We Know It|date=2010|last1=Rafferty|first1=J. P.|series=The Geologic History of Earth|page=150|publisher=[[Rosen Publishing]]|isbn=978-1-61530-125-6}}</ref><ref name="vertes2010">{{citation|title=Handbook of Nuclear Chemistry|date=2010|last1=Vértes|first1=A.|volume=5|page=800|edition=2nd|publisher=Springer Science+Business Media|editor1-last=Nagy|editor2-last=Klencsár|editor3-last=Lovas|editor4-last=Rösch|editor1-first=S.|editor2-first=Z.|editor3-first=R. G.|editor4-first=F.|display-editors=3|isbn=978-1-4419-0719-6}}</ref> Both of these dating methods assume that the proportion of <sup>230</sup>Th to <sup>232</sup>Th is a constant during the period when the sediment layer was formed, that the sediment did not already contain thorium before contributions from the decay of uranium, and that the thorium cannot migrate within the sediment layer.<ref name="rafferty2010" /><ref name="vertes2010" />