Editing Protactinium (section)

==Applications==
Although protactinium is situated in the periodic table between uranium and thorium, both of which have numerous applications, there are currently no uses for protactinium outside scientific research owing to its scarcity, high radioactivity, and high toxicity.<ref name="ANL" />

<sup>231</sup>Pa arises naturally from the decay of natural <sup>235</sup>U, and artificially in nuclear reactors by the reaction <sup>232</sup>Th&nbsp;+&nbsp;n&nbsp;→&nbsp;<sup>231</sup>Th&nbsp;+&nbsp;2n and the subsequent [[beta decay]] of <sup>231</sup>Th. It was once thought to be able to support a nuclear chain reaction, which could in principle be used to build [[nuclear weapon]]s; the [[physicist]] {{Interlanguage link multi|Walter Seifritz|de|3=Walter_Seifritz}} once estimated the associated [[critical mass]] as {{val|750|180|u=kg}}.<ref>Seifritz, Walter (1984) ''Nukleare Sprengkörper – Bedrohung oder Energieversorgung für die Menschheit'', Thiemig-Verlag, {{ISBN|3-521-06143-4}}.</ref> However, the possibility of criticality of <sup>231</sup>Pa has since been ruled out.<ref name=AVG231 /><ref>{{cite journal|author=Ganesan, S.|url=http://www.iisc.ernet.in/currsci/sept10/researcharticle.pdf|title=A Re-calculation of Criticality Property of <sup>231</sup>Pa Using New Nuclear Data|journal=[[Current Science]]|year=1999|volume=77|issue=5|pages=667–677|access-date=21 March 2013|archive-date=3 March 2016|archive-url=https://web.archive.org/web/20160303231501/http://www.iisc.ernet.in/currsci/sept10/researcharticle.pdf|url-status=dead}}</ref>

With the advent of highly sensitive [[Mass spectrometry|mass spectrometers]], an application of <sup>231</sup>Pa as a tracer in geology and [[paleoceanography]] has become possible. In this application, the ratio of <sup>231</sup>Pa to <sup>230</sup>Th is used for [[radiometric dating]] of sediments which are up to 175,000 years old, and in modeling of the formation of minerals.<ref name="brit" /> In particular, its evaluation in oceanic sediments helped to reconstruct the movements of [[North Atlantic]] water bodies during the last melting of [[Ice age|Ice Age]] [[glacier]]s.<ref>{{cite journal|doi = 10.1038/nature02494|display-authors = 4|author = McManus, J. F.|author2 = Francois, R.|author3 = Gherardi, J.-M.|author4 = Keigwin, L. D.|author5 = Brown-Leger, S.|name-list-style = amp|title = Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes|journal = [[Nature (journal)|Nature]]|date = 2004|volume = 428|issue = 6985|pages = 834–837|pmid = 15103371|url = http://www.seas.harvard.edu/climate/pdf/mcmanus-2004.pdf|bibcode = 2004Natur.428..834M|s2cid = 205210064|access-date = 29 November 2010|archive-date = 10 April 2013|archive-url = https://web.archive.org/web/20130410051109/http://www.seas.harvard.edu/climate/pdf/mcmanus-2004.pdf|url-status = dead}}</ref><!--10.1016/S0016-7037(98)00255-5--> Some of the protactinium-related dating variations rely on analysis of the relative concentrations of several long-living members of the uranium decay chain – uranium, protactinium, and thorium, for example. These elements have 6, 5, and 4 valence electrons, thus favoring +6, +5, and +4 oxidation states respectively, and display different physical and chemical properties. Thorium and protactinium, but not uranium compounds, are poorly soluble in aqueous solutions and precipitate into sediments; the precipitation rate is faster for thorium than for protactinium. The concentration analysis for both protactinium-231 (half-life 32,760 years) and <sup>230</sup>Th (half-life 75,380 years) improves measurement accuracy compared to when only one isotope is measured; this double-isotope method is also weakly sensitive to inhomogeneities in the spatial distribution of the isotopes and to variations in their precipitation rate.<ref name="brit">Articles "Protactinium" and "Protactinium-231 – thorium-230 dating" in Encyclopædia Britannica, 15th edition, 1995, p. 737</ref><ref>{{cite journal|last1=Cheng|first1=H.|title=Uranium-thorium-protactinium dating systematics|journal=[[Geochimica et Cosmochimica Acta]]|volume=62|pages=3437|date=1998|doi=10.1016/S0016-7037(98)00255-5|bibcode=1998GeCoA..62.3437C|issue=21–22|last2=Edwards|first2=R.Lawrence|last3=Murrell|first3=M. T.|last4=Benjamin|first4=T. M.}}</ref>