Editing Thorium (section)

==Isotopes==
{{Main|Isotopes of thorium}}
There are seven naturally occurring isotopes of Thorium but none are stable. <sup>232</sup>Th is one of the two nuclides beyond bismuth (the other being [[uranium-238|<sup>238</sup>U]]) that have half-lives measured in billions of years; its half-life is 14.05&nbsp;billion&nbsp;years, about three times the [[age of the Earth]], and slightly longer than the [[age of the universe]]. Four-fifths of the thorium present at Earth's formation has survived to the present.<ref name="NUBASE">{{cite journal |last1=Audi |first1=G. |last2=Bersillon |first2=O. |last3=Blachot |first3=J. |last4=Wapstra |first4=A.H. |title=The Nubase evaluation of nuclear and decay properties |journal=Nuclear Physics A |date=December 2003 |volume=729 |issue=1 |pages=3–128 |doi=10.1016/j.nuclphysa.2003.11.001 |bibcode=2003NuPhA.729....3A |url=http://hal.in2p3.fr/in2p3-00014184/file/democrite-00014184.pdf |archive-date=16 April 2023 |access-date=10 October 2021 |archive-url=https://web.archive.org/web/20230416024455/http://hal.in2p3.fr/in2p3-00014184/file/democrite-00014184.pdf |url-status=live }}</ref><ref>{{CIAAW2003}}</ref><ref>{{cite journal |last1=Wieser |first1=M. E. |title=Atomic weights of the elements 2005 (IUPAC Technical Report) |journal=Pure and Applied Chemistry |date=1 January 2006 |volume=78 |issue=11 |pages=2051–2066 |doi=10.1351/pac200678112051 |s2cid=94552853 |doi-access=free }}</ref> <sup>232</sup>Th is the only isotope of thorium occurring in quantity in nature.<ref name="NUBASE" /> Its stability is attributed to its closed [[nuclear shell|nuclear subshell]] with 142 neutrons.<ref>{{cite book |last=Nagy |first=S. |date=2009 |title=Radiochemistry and Nuclear Chemistry |volume=2 |publisher=EOLSS Publications |page=374 |isbn=978-1-84826-127-3}}</ref><ref>{{cite book |last=Griffin |first=H. C. |date=2010 |title=Handbook of Nuclear Chemistry |publisher=[[Springer Science+Business Media]] |page=668 |isbn=978-1-4419-0719-6 |editor1-last=Vértes |editor1-first=A. |editor2-last=Nagy |editor2-first=S. |editor3-last=Klencsár |editor3-first=Z. |editor4-last=Lovas |editor4-first=R. G. |editor5-last=Rösch |editor5-first=F. |display-editors=3 |chapter=Natural Radioactive Decay Chains}}</ref> Thorium has a characteristic terrestrial isotopic composition, with [[standard atomic weight|atomic weight]] {{val|232.0377|0.0004}}.{{CIAAW2021}} It is one of only four radioactive elements (along with bismuth, protactinium and uranium) that occur in large enough quantities on Earth for a standard atomic weight to be determined.{{CIAAW2021}}

Thorium nuclei are susceptible to [[alpha decay]] because the strong nuclear force cannot overcome the electromagnetic repulsion between their protons.<ref name="beiser">{{cite book|title=Concepts of Modern Physics|chapter-url=http://phy240.ahepl.org/Concepts_of_Modern_Physics_by_Beiser.pdf|year=2003|publisher=[[McGraw-Hill Education]]|isbn=978-0-07-244848-1|chapter=Nuclear Transformations|pages=432–434|edition=6|first=A.|last=Beiser|access-date=4 July 2016|archive-date=4 October 2016|archive-url=https://web.archive.org/web/20161004204701/http://phy240.ahepl.org/Concepts_of_Modern_Physics_by_Beiser.pdf}}</ref> The alpha decay of <sup>232</sup>Th initiates the 4''n'' [[decay chain]] which includes isotopes with a [[mass number]] divisible by 4 (hence the name; it is also called the thorium series after its progenitor). This chain of consecutive alpha and [[beta decay]]s begins with the decay of <sup>232</sup>Th to <sup>228</sup>Ra and terminates at <sup>208</sup>Pb.<ref name="NUBASE" /> Any sample of thorium or its compounds contains traces of these daughters, which are isotopes of [[thallium]], [[lead]], bismuth, polonium, [[radon]], [[radium]], and actinium.<ref name="NUBASE" /> Natural thorium samples can be chemically purified to extract useful daughter nuclides, such as <sup>212</sup>Pb, which is used in [[nuclear medicine]] for [[cancer therapy]].<ref>{{cite press release |url=http://us.areva.com/EN/home-2564/areva-inc-areva-med-launches-production-of-lead212-at-new-facility.html |title=AREVA Med launches production of lead-212 at new facility |publisher=[[Areva]] |year=2013 |access-date=1 January 2017 |archive-date=19 December 2020 |archive-url=https://web.archive.org/web/20201219225628/http://us.areva.com/EN/home-2564/areva-inc-areva-med-launches-production-of-lead212-at-new-facility.html }}</ref><ref>{{cite web|url=http://minerals.usgs.gov/minerals/pubs/commodity/thorium/myb1-2011-thori.pdf|title=Mineral Yearbook 2012|publisher=[[United States Geological Survey]]|access-date=30 September 2017|archive-date=11 April 2013|archive-url=https://web.archive.org/web/20130411121452/http://minerals.usgs.gov/minerals/pubs/commodity/thorium/myb1-2011-thori.pdf|url-status=live}}</ref> <sup>227</sup>Th (alpha emitter with an 18.68 days half-life) can also be used in cancer treatments such as [[Targeted alpha-particle therapy|targeted alpha therapies]].<ref>{{cite journal |last1=Ramdahl |first1=Thomas |last2=Bonge-Hansen |first2=Hanne T. |last3=Ryan |first3=Olav B. |last4=Larsen |first4=Åsmund |last5=Herstad |first5=Gunnar |last6=Sandberg |first6=Marcel |last7=Bjerke |first7=Roger M. |last8=Grant |first8=Derek |last9=Brevik |first9=Ellen M. |last10=Cuthbertson |first10=Alan S. |title=An efficient chelator for complexation of thorium-227 |journal=Bioorganic & Medicinal Chemistry Letters |date=September 2016 |volume=26 |issue=17 |pages=4318–4321 |doi=10.1016/j.bmcl.2016.07.034 |pmid=27476138 |doi-access=free }}</ref><ref>{{cite journal |last1=Deblonde |first1=Gauthier J.-P. |last2=Lohrey |first2=Trevor D. |last3=Booth |first3=Corwin H. |last4=Carter |first4=Korey P. |last5=Parker |first5=Bernard F. |last6=Larsen |first6=Åsmund |last7=Smeets |first7=Roger |last8=Ryan |first8=Olav B. |last9=Cuthbertson |first9=Alan S. |last10=Abergel |first10=Rebecca J. |title=Solution Thermodynamics and Kinetics of Metal Complexation with a Hydroxypyridinone Chelator Designed for Thorium-227 Targeted Alpha Therapy |journal=Inorganic Chemistry |date=19 November 2018 |volume=57 |issue=22 |pages=14337–14346 |doi=10.1021/acs.inorgchem.8b02430 |pmid=30372069 |osti=1510758 |s2cid=53115264 |url=https://escholarship.org/uc/item/7nz4j81s |archive-date=11 May 2021 |access-date=3 February 2019 |archive-url=https://web.archive.org/web/20210511140331/https://escholarship.org/uc/item/7nz4j81s |url-status=live }}</ref><ref>{{cite journal |last1=Captain |first1=Ilya |last2=Deblonde |first2=Gauthier J.-P. |last3=Rupert |first3=Peter B. |last4=An |first4=Dahlia D. |last5=Illy |first5=Marie-Claire |last6=Rostan |first6=Emeline |last7=Ralston |first7=Corie Y. |last8=Strong |first8=Roland K. |last9=Abergel |first9=Rebecca J. |title=Engineered Recognition of Tetravalent Zirconium and Thorium by Chelator–Protein Systems: Toward Flexible Radiotherapy and Imaging Platforms |journal=Inorganic Chemistry |date=21 November 2016 |volume=55 |issue=22 |pages=11930–11936 |doi=10.1021/acs.inorgchem.6b02041 |pmid=27802058 |osti=1458481 |url=http://www.escholarship.org/uc/item/2nx8r6pz |archive-date=29 April 2021 |access-date=3 February 2019 |archive-url=https://web.archive.org/web/20210429025503/https://escholarship.org/uc/item/2nx8r6pz |url-status=live }}</ref> <sup>232</sup>Th also very occasionally undergoes [[spontaneous fission]] rather than alpha decay, and has left evidence of doing so in its minerals (as trapped [[xenon]] gas formed as a fission product), but the [[partial half-life]] of this process is very large at over 10<sup>21</sup>&nbsp;years and alpha decay predominates.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=53–55}}<ref>{{cite journal |last1=Bonetti |first1=R. |last2=Chiesa |first2=C. |first3=A. |last3=Guglielmetti |first4=R. |last4=Matheoud |first5=G. |last5=Poli |first6=V. L. |last6=Mikheev |first7=S. P. |last7=Tretyakova |display-authors=3 |year=1995 |title=First observation of spontaneous fission and search for cluster decay of <sup>232</sup>Th |journal=Physical Review C |volume=51 |issue=5 |pages=2530–2533 |doi=10.1103/PhysRevC.51.2530|pmid=9970335 |bibcode=1995PhRvC..51.2530B}}</ref>

[[File:Decay Chain Thorium.svg|thumb|upright=1.25|alt=Ball-and-arrow presentation of the thorium decay series|The 4''n'' [[decay chain]] of <sup>232</sup>Th, commonly called the "thorium series"]]
In total, 32 [[radioisotope]]s have been characterised, which range in mass number from 207<ref name="Yang207">{{cite journal |title=New isotope <sup>207</sup>Th and odd-even staggering in α-decay energies for nuclei with ''Z''&nbsp;>&nbsp;82 and ''N''&nbsp;<&nbsp;126 |last=Yang |first=H. B. |display-authors=et al. |journal=Physical Review C |year=2022 |volume=105 |number=L051302 |doi=10.1103/PhysRevC.105.L051302|bibcode=2022PhRvC.105e1302Y |s2cid=248935764 }}</ref> to 238.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=53–55}} After <sup>232</sup>Th, the most stable of them (with respective half-lives) are <sup>230</sup>Th (75,380&nbsp;years), <sup>229</sup>Th (7,917&nbsp;years), <sup>228</sup>Th (1.92&nbsp;years), <sup>234</sup>Th (24.10&nbsp;days), and <sup>227</sup>Th (18.68&nbsp;days). All of these isotopes occur in nature as [[trace radioisotope]]s due to their presence in the decay chains of <sup>232</sup>Th, <sup>235</sup>U, <sup>238</sup>U, and <sup>237</sup>[[neptunium|Np]]: the last of these is long [[extinct radionuclide|extinct]] in nature due to its short half-life (2.14&nbsp;million&nbsp;years), but is continually produced in minute traces from [[neutron capture]] in uranium ores. All of the remaining thorium isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes.<ref name="NUBASE" /> <sup>233</sup>Th (half-life 22&nbsp;minutes) occurs naturally as the result of [[neutron activation]] of natural <sup>232</sup>Th.<ref name=4n1>{{cite journal |last1=Peppard |first1=D. F. |last2=Mason |first2=G. W. |last3=Gray |first3=P. R. |last4=Mech |first4=J. F. |title=Occurrence of the (4n + 1) series in nature |journal=Journal of the American Chemical Society |date=1952 |volume=74 |issue=23 |pages=6081–6084 |doi=10.1021/ja01143a074 |bibcode=1952JAChS..74.6081P |url=https://digital.library.unt.edu/ark:/67531/metadc172698/m2/1/high_res_d/metadc172698.pdf |archive-url=https://web.archive.org/web/20190429182951/https://digital.library.unt.edu/ark:/67531/metadc172698/m2/1/high_res_d/metadc172698.pdf |archive-date=29 April 2019 |url-status=live }}</ref> <sup>226</sup>Th (half-life 31&nbsp;minutes) has not yet been observed in nature, but would be produced by the still-unobserved [[double beta decay]] of natural <sup>226</sup>Ra.<ref name="Tretyak2002">{{Cite journal
 |last1=Tretyak |first1=V.I. 
 |last2=Zdesenko |first2=Yu.G. 
 |year=2002
 |title=Tables of Double Beta Decay Data — An Update
 |journal=[[At. Data Nucl. Data Tables]] |volume=80 |issue=1 |pages=83–116
 |doi=10.1006/adnd.2001.0873
|bibcode=2002ADNDT..80...83T }}</ref>

In deep [[seawater]]s the isotope <sup>230</sup>Th makes up to {{val|0.02|u=%}} of natural thorium.{{NUBASE2020|ref}} This is because its parent <sup>238</sup>U is soluble in water, but <sup>230</sup>Th is insoluble and precipitates into the sediment. Uranium ores with low thorium concentrations can be purified to produce gram-sized thorium samples of which over a quarter is the <sup>230</sup>Th isotope, since <sup>230</sup>Th is one of the daughters of <sup>238</sup>U.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=53–55}} The [[International Union of Pure and Applied Chemistry]] (IUPAC) reclassified thorium as a binuclidic element in 2013; it had formerly been considered a [[mononuclidic element]].<ref name="CIAAWthorium"/>

Thorium has three known [[nuclear isomer]]s (or metastable states), <sup>216m1</sup>Th, <sup>216m2</sup>Th, and <sup>229m</sup>Th. <sup>229m</sup>Th has the lowest known excitation energy of any isomer,<ref name="Ruchowska">{{cite journal|last1=Ruchowska |first1=E. |title=Nuclear structure of <sup>229</sup>Th |journal=Physical Review C|volume= 73|page=044326|date=2006 |doi=10.1103/PhysRevC.73.044326|issue=4 |bibcode= 2006PhRvC..73d4326R |display-authors=3 |last2=Płóciennik |first2=W. A. |last3=Żylicz |first3=J. |last4=Mach |last5=Kvasil |last6=Algora |last7=Amzal |last8=Bäck |last9=Borge |last10=Boutami |last11=Butler |last12=Cederkäll |last13=Cederwall |last14=Fogelberg |last15=Fraile |last16=Fynbo |last17=Hagebø |last18=Hoff |last19=Gausemel |last20=Jungclaus |last21=Kaczarowski |last22=Kerek |last23=Kurcewicz |last24=Lagergren |last25=Nacher |last26=Rubio |last27=Syntfeld |last28=Tengblad |last29=Wasilewski |last30=Weissman|url=https://cds.cern.ch/record/974608 |hdl=10261/12130 |hdl-access=free }}</ref> measured to be {{val|7.6|0.5|u=eV}}. This is so low that when it undergoes [[isomeric transition]], the emitted gamma radiation is in the [[ultraviolet]] range.<ref name="Beck">{{cite journal |last1=Beck |first1=B. R. |title=Energy splitting in the ground state doublet in the nucleus <sup>229</sup>Th |journal=[[Physical Review Letters]] |volume=98 |page=142501 |date=2007 |doi=10.1103/PhysRevLett.98.142501 |pmid=17501268 |bibcode=2007PhRvL..98n2501B |issue=14 |display-authors=3 |last2=Becker |first2=J. A. |last3=Beiersdorfer |first3=P. |last4=Brown |last5=Moody |last6=Wilhelmy |last7=Porter |last8=Kilbourne |last9=Kelley |url=https://zenodo.org/record/1233955 |archive-date=13 April 2023 |access-date=24 August 2019 |archive-url=https://web.archive.org/web/20230413111140/https://zenodo.org/record/1233955 |url-status=live }}</ref><ref name="nuclear_clock">{{cite journal |journal=[[Nature (journal)|Nature]]
| volume=533 |issue=7601 |pages=47–51| year=2016| title= Direct detection of the <sup>229</sup>Th nuclear clock transition| first1=L. |last1=von der Wense| first2=B. |last2=Seiferle| first3=M. |last3=Laatiaoui| first4=Jürgen B. |last4=Neumayr| first5=Hans-Jörg |last5=Maier| first6=Hans-Friedrich |last6=Wirth| first7=Christoph |last7=Mokry| first8=Jörg |last8=Runke| first9=Klaus |last9=Eberhardt| first10=Christoph E. |last10=Düllmann| first11=Norbert G. |last11=Trautmann| first12=Peter G. |last12=Thirolf| display-authors=3| doi=10.1038/nature17669| pmid=27147026 | bibcode=2016Natur.533...47V|arxiv=1710.11398| s2cid=205248786 }}</ref>{{efn|Gamma rays are distinguished by their origin in the nucleus, not their wavelength; hence there is no lower limit to gamma energy derived from radioactive decay.<ref>{{cite book|last1=Feynman|first1=R.|author-link=Richard Feynman|last2=Leighton|first2=R.|last3=Sands|first3=M.|title=The Feynman Lectures on Physics|volume=1|publisher=[[Addison-Wesley]]|year=1963|pages=2–5|isbn=978-0-201-02116-5|url=https://feynmanlectures.caltech.edu/I_02.html|access-date=13 January 2018|archive-date=17 February 2021|archive-url=https://web.archive.org/web/20210217134956/https://www.feynmanlectures.caltech.edu/I_02.html|url-status=live}}</ref>}} The nuclear transition from <sup>229</sup>Th to <sup>229m</sup>Th is being investigated for a [[nuclear clock]].<ref name="nuclear_clock" />

Different isotopes of thorium are chemically identical, but have slightly differing physical properties: for example, the densities of pure <sup>228</sup>Th, <sup>229</sup>Th, <sup>230</sup>Th, and <sup>232</sup>Th are respectively expected to be 11.5, 11.6, 11.6, and 11.7&nbsp;g/cm<sup>3</sup>.<ref name="critical">{{cite web|publisher= [[Institut de radioprotection et de sûreté nucléaire]]|title= Evaluation of nuclear criticality safety data and limits for actinides in transport|page= 15|url= http://ec.europa.eu/energy/nuclear/transport/doc/irsn_sect03_146.pdf|access-date=20 December 2010 |archive-url=https://web.archive.org/web/20070710105629/http://ec.europa.eu/energy/nuclear/transport/doc/irsn_sect03_146.pdf |archive-date=10 July 2007}}</ref> The isotope <sup>229</sup>Th is expected to be [[fissionable]] with a bare [[critical mass]] of 2839&nbsp;kg, although with steel [[neutron reflector|reflectors]] this value could drop to 994&nbsp;kg.<ref name="critical" />{{efn|name="fissionable"|A ''fissionable'' nuclide is capable of undergoing fission (even with a low probability) after capturing a high-energy neutron. Some of these nuclides can be induced to fission with low-energy thermal neutrons with a high probability; they are referred to as ''fissile''. A ''fertile'' nuclide is one that could be bombarded with neutrons to produce a fissile nuclide. [[Critical mass]] is the mass of a ball of a material which could undergo a sustained [[nuclear chain reaction]].}} <sup>232</sup>Th is not fissionable, but it is [[fertile material|fertile]] as it can be converted to fissile [[uranium-233|<sup>233</sup>U]] by neutron capture and subsequent beta decay.<ref name="critical" />{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}}

===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" />