View source for Isotopes of thorium

[[File:Th-228 nitrate (12.5 mCi) - Photo by Dr Andrew R. Burgoyne - Oak Ridge National Laboratory.jpg|thumb|[[:File:Th-228 nitrate (12.5 mCi) - Photo by Dr Andrew R. Burgoyne - Oak Ridge National Laboratory.jpg|Th-228 nitrate (12.5 mCi) - Photo by Dr Andrew R. Burgoyne - Oak Ridge National Laboratory]]]]
{{Short description|none}}
{{Infobox thorium isotopes}}

[[Thorium]] (<sub>90</sub>Th) has seven naturally occurring [[isotope]]s but none are stable. One isotope, [[thorium-232|<sup>232</sup>Th]], is ''relatively'' stable, with a half-life of 1.405×10<sup>10</sup> years, considerably longer than the [[age of the Earth]], and even slightly longer than the generally accepted [[age of the universe]]. This isotope makes up nearly all natural thorium, so thorium was considered to be [[mononuclidic elements|mononuclidic]]. However, in 2013, [[International Union of Pure and Applied Chemistry|IUPAC]] reclassified thorium as binuclidic, due to large amounts of <sup>230</sup>Th in deep seawater. Thorium has a characteristic terrestrial isotopic composition and thus a standard atomic weight can be given.

Thirty-one [[radioisotope]]s have been characterized, with the most stable being <sup>232</sup>Th, <sup>230</sup>Th with a half-life of 75,380 years, <sup>229</sup>Th with a half-life of 7,917 years,<ref name="Varga14" /> and <sup>228</sup>Th with a half-life of 1.92 years. All of the remaining [[radioactive]] isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes. One isotope, <sup>229</sup>Th, has a [[nuclear isomer]] (or metastable state) with a remarkably low excitation energy,<ref name="Ruchowska">{{cite journal |author=E. Ruchowska|title=Nuclear structure of <sup>229</sup>Th |journal=[[Physical Review C]]|volume=73 |issue=4 |pages=044326 |year=2006 |doi=10.1103/PhysRevC.73.044326 |bibcode = 2006PhRvC..73d4326R |hdl=10261/12130 |hdl-access=free |url=https://cds.cern.ch/record/974608/files/PhysRevC.73.044326.pdf }}</ref> recently measured to be {{val|8.355,733,554,021|(8)|u=eV}}{{^|Uncertainty omitted in lead for conciseness}}<ref name=Tiedau2024/><ref name="Zhang2024" /> It has been proposed to perform laser spectroscopy of the <sup>229</sup>Th nucleus and use the low-energy transition for the development of a [[nuclear clock]] of extremely high accuracy.<ref name=PR-Tiedau2024/><ref name="Peik2003">{{cite journal
| first1 = E.
| last1 = Peik
| first2 = Chr.
| last2 = Tamm
| title = Nuclear laser spectroscopy of the 3.5&nbsp;eV transition in <sup>229</sup>Th
| url = https://www.few.vu.nl/~wimu/Varying-Constants-Papers/Th-229-Peik-Tamm-EuLett-2003.pdf
| journal = Europhysics Letters
| volume = 61
| issue = 2
| pages = 181–186
| date = 2003-01-15
| doi = 10.1209/epl/i2003-00210-x
| bibcode = 2003EL.....61..181P
| s2cid = 250818523
| access-date = 2024-04-30
| archive-url = https://web.archive.org/web/20240414032917/http://www.few.vu.nl/~wimu/Varying-Constants-Papers/Th-229-Peik-Tamm-EuLett-2003.pdf
| archive-date = 2024-04-14
| url-status = live
}}</ref><ref name="Campbell2012">{{cite journal
| first1=C.J. |last1=Campbell
| first2=A.G. |last2=Radnaev
| first3=A.  |last3=Kuzmich
| first4=V.A. |last4=Dzuba
| first5=V.V. |last5=Flambaum
| first6=A. |last6=Derevianko
| title = A single ion nuclear clock for metrology at the 19th decimal place
| journal = [[Physical Review Letters]]
| volume = 108 | issue = 12 | article-number = 120802
| date = 2012
|page=120802
| doi = 10.1103/PhysRevLett.108.120802
| pmid = 22540568
| arxiv = 1110.2490
| bibcode = 2012PhRvL.108l0802C
| s2cid = 40863227
| url = https://sites.lsa.umich.edu/kuzmich-lab/wp-content/uploads/sites/90/2014/05/229ThClock.pdf
| access-date=2024-04-30
}}</ref>

The known isotopes of thorium range in [[mass number]] from 207<ref name="Yang207"/> to 238.

== List of isotopes ==
{{Anchor|Thorium-209m|Thorium-233m|Thorium-239}}

<!--Please delete anchor(s) from the list above or table below if adding a dedicated isotope section(s).-->

{{Isotopes table
| symbol = Th
| refs = NUBASE2020, AME2020 II<!-- updated 2024-10-05 -->
| notes = m, histname, unc(), mass#, hl-nst, spin(), spin#, EC, CD, IT, daughter-st
}}
|-id=Thorium-207
| <sup>207</sup>Th<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>
|
| style="text-align:right" | 90
| style="text-align:right" | 117
| 
| {{val|9.7|46.6|4.4|u=ms}}
| [[alpha decay|α]]
| <sup>203</sup>Ra
| 
|
|
|-id=Thorium-208
| <sup>208</sup>Th
|
| style="text-align:right" | 90
| style="text-align:right" | 118
| 208.017915(34)
| 2.4(12)&nbsp;ms
| α
| <sup>204</sup>Ra
| 0+
|
|
|-id=Thorium-209
| <sup>209</sup>Th
|
| style="text-align:right" | 90
| style="text-align:right" | 119
| 209.017998(27)
| 3.1(12)&nbsp;ms
| α
| <sup>205</sup>Ra
| 13/2+
|
|
|-id=Thorium-210
| <sup>210</sup>Th
| 
| style="text-align:right" | 90
| style="text-align:right" | 120
| 210.015094(20)
| 16.0(36)&nbsp;ms
| α
| <sup>206</sup>Ra
| 0+
| 
| 
|-id=Thorium-211
| <sup>211</sup>Th
| 
| style="text-align:right" | 90
| style="text-align:right" | 121
| 211.014897(92)
| 48(20)&nbsp;ms
| α
| <sup>207</sup>Ra
| 5/2−#
| 
| 
|-id=Thorium-212
| <sup>212</sup>Th
| 
| style="text-align:right" | 90
| style="text-align:right" | 122
| 212.013002(11)
| 31.7(13)&nbsp;ms
| α
| <sup>208</sup>Ra
| 0+
| 
| 
|-id=Thorium-213
| <sup>213</sup>Th
| 
| style="text-align:right" | 90
| style="text-align:right" | 123
| 213.0130115(99)
| 144(21)&nbsp;ms
| α
| <sup>209</sup>Ra
| 5/2−
| 
| 
|-id=Thorium-213m
| style="text-indent:1em" | <sup>213m</sup>Th
|
| colspan="3" style="text-indent:2em" | 1180.0(14)&nbsp;keV
| 1.4(4)&nbsp;μs
| [[Isomeric transition|IT]]
| <sup>213</sup>Th
| (13/2)+
|
|
|-id=Thorium-214
| <sup>214</sup>Th
|
| style="text-align:right" | 90
| style="text-align:right" | 124
| 214.011481(11)
| 87(10)&nbsp;ms
| α
| <sup>210</sup>Ra
| 0+
|
|
|-id=Thorium-214m
| style="text-indent:1em" | <sup>214m</sup>Th
|
| colspan="3" style="text-indent:2em" | 2181.0(27)&nbsp;keV
| 1.24(12)&nbsp;μs
| IT
| <sup>214</sup>Th
| 8+#
|
|
|-id=Thorium-215
| <sup>215</sup>Th
|
| style="text-align:right" | 90
| style="text-align:right" | 125
| 215.0117246(68)
| 1.35(14)&nbsp;s
| α
| <sup>211</sup>Ra
| (1/2−)
|
|
|-id=Thorium-215m
| style="text-indent:1em" | <sup>215m</sup>Th
|
| colspan="3" style="text-indent:2em" | 1471(50)#&nbsp;keV
| 770(60)&nbsp;ns
| IT
| <sup>215</sup>Th
| 9/2+#
|
|
|-id=Thorium-216
| <sup>216</sup>Th
| 
| style="text-align:right" | 90
| style="text-align:right" | 126
| 216.011056(12)
| 26.28(16)&nbsp;ms
| α
| <sup>212</sup>Ra
| 0+
| 
| 
|-id=Thorium-216m1
| rowspan=2 style="text-indent:1em" | <sup>216m1</sup>Th
| rowspan=2|
| rowspan=2 colspan="3" style="text-indent:2em" | 2041(8)&nbsp;keV
| rowspan=2|135.4(29)&nbsp;μs
| IT (97.2%)
| <sup>216</sup>Th
| rowspan=2|8+
| rowspan=2|
| rowspan=2|
|-
| α (2.8%)
| <sup>212</sup>Ra
|-id=Thorium-216m2
| style="text-indent:1em" | <sup>216m2</sup>Th
|
| colspan="3" style="text-indent:2em" | 2648(8)&nbsp;keV
| 580(26)&nbsp;ns
| IT
| <sup>216</sup>Th
| (11−)
|
|
|-id=Thorium-216m3
| style="text-indent:1em" | <sup>216m3</sup>Th
|
| colspan="3" style="text-indent:2em" | 3682(8)&nbsp;keV
| 740(70)&nbsp;ns
| IT
| <sup>216</sup>Th
| (14+)
|
|
|-id=Thorium-217
| <sup>217</sup>Th
|
| style="text-align:right" | 90
| style="text-align:right" | 127
| 217.013103(11)
| 248(4)&nbsp;μs
| α
| <sup>213</sup>Ra
| 9/2+#
|
|
|-id=Thorium-217m1
| style="text-indent:1em" | <sup>217m1</sup>Th
|
| colspan="3" style="text-indent:2em" | 673.3(1)&nbsp;keV
| 141(50)&nbsp;ns
| IT
| <sup>217</sup>Th
| (15/2−)
|
|
|-id=Thorium-217m2
| style="text-indent:1em" | <sup>217m2</sup>Th
|
| colspan="3" style="text-indent:2em" | 2307(32)&nbsp;keV
| 71(14)&nbsp;μs
| IT
| <sup>217</sup>Th
| (25/2+)
|
|
|-id=Thorium-218
| <sup>218</sup>Th
|
| style="text-align:right" | 90
| style="text-align:right" | 128
| 218.013276(11)
| 122(5)&nbsp;ns
| α
| <sup>214</sup>Ra
| 0+
|
|
|-id=Thorium-219
| <sup>219</sup>Th
| 
| style="text-align:right" | 90
| style="text-align:right" | 129
| 219.015526(61)
| 1.023(18)&nbsp;μs
| α
| <sup>215</sup>Ra
| 9/2+#
| 
| 
|-id=Thorium-220
| <sup>220</sup>Th
| 
| style="text-align:right" | 90
| style="text-align:right" | 130
| 220.015770(15)
| 10.2(3)&nbsp;μs
| α
| <sup>216</sup>Ra
| 0+
| 
| 
|-id=Thorium-221
| <sup>221</sup>Th
|
| style="text-align:right" | 90
| style="text-align:right" | 131
| 221.0181858(86)
| 1.75(2)&nbsp;ms
| α
| <sup>217</sup>Ra
| 7/2+#
|
|
|-id=Thorium-222
| <sup>222</sup>Th
| 
| style="text-align:right" | 90
| style="text-align:right" | 132
| 222.018468(11)
| 2.24(3)&nbsp;ms
| α
| <sup>218</sup>Ra
| 0+
| 
| 
|-id=Thorium-223
| <sup>223</sup>Th
|
| style="text-align:right" | 90
| style="text-align:right" | 133
| 223.0208111(85)
| 0.60(2)&nbsp;s
| α
| <sup>219</sup>Ra
| (5/2)+
|
|
|-id=Thorium-224
| <sup>224</sup>Th
| 
| style="text-align:right" | 90
| style="text-align:right" | 134
| 224.021466(10)
| 1.04(2)&nbsp;s
| α<ref group="n">Theorized to also undergo β<sup>+</sup>β<sup>+</sup> decay to <sup>224</sup>Ra</ref>
| <sup>220</sup>Ra
| 0+
| 
| 
|-id=Thorium-225
| rowspan=2|<sup>225</sup>Th
| rowspan=2|
| rowspan=2 style="text-align:right" | 90
| rowspan=2 style="text-align:right" | 135
| rowspan=2|225.0239510(55)
| rowspan=2|8.75(4)&nbsp;min
| α (~90%)
| <sup>221</sup>Ra
| rowspan=2|3/2+
| rowspan=2|
| rowspan=2|
|-
| [[Electron capture|EC]] (~10%)
| <sup>225</sup>Ac
|-id=Thorium-226
| rowspan=2|<sup>226</sup>Th
| rowspan=2|
| rowspan=2 style="text-align:right" | 90
| rowspan=2 style="text-align:right" | 136
| rowspan=2|226.0249037(48)
| rowspan=2|30.70(3)&nbsp;min
| α
| <sup>222</sup>Ra
| rowspan=2|0+
| rowspan=2|
| rowspan=2|
|-
| [[cluster decay|CD]] (<{{val|3.2e-12}}%)
| '''<sup>208</sup>Pb'''<br>'''<sup>18</sup>O'''
|-id=Thorium-227
| <sup>227</sup>Th
| Radioactinium
| style="text-align:right" | 90
| style="text-align:right" | 137
| 227.0277025(22)
| 18.693(4)&nbsp;d
| α
| <sup>223</sup>Ra
| (1/2+)
| Trace<ref group="n" name="as">Intermediate [[decay product]] of [[Uranium-235|<sup>235</sup>U]]</ref>
|
|-
| rowspan=2|<sup>228</sup>Th
| rowspan=2|Radiothorium
| rowspan=2 style="text-align:right" | 90
| rowspan=2 style="text-align:right" | 138
| rowspan=2|228.0287397(19)
| rowspan=2|1.9125(7)&nbsp;y
| α
| <sup>224</sup>Ra
| rowspan=2|0+
| rowspan=2|Trace<ref group="n">Intermediate decay product of [[thorium-232|<sup>232</sup>Th]]</ref>
| rowspan=2|
|-
| [[Cluster decay|CD]] (1.13×10<sup>−11</sup>%)
| '''<sup>208</sup>Pb'''<br><sup>20</sup>O
|-
| <sup>229</sup>Th
|
| style="text-align:right" | 90
| style="text-align:right" | 139
| 229.0317614(26)
| 7916(17)&nbsp;y
| α
| <sup>225</sup>Ra
| 5/2+
| Trace<ref group=n>Intermediate decay product of [[neptunium-237|<sup>237</sup>Np]]</ref>
|
|-
| style="text-indent:1em" | <sup>229m</sup>Th
|
| colspan="3" style="text-indent:2em" | 8.355733554021(8) eV<ref name=Zhang2024 />
| 7(1)&nbsp;μs<ref name="Seiferle2017" />
| IT<ref group="n" name=229m>Neutral <sup>229m</sup>Th decays rapidly by [[internal conversion]], ejecting an electron.  There is not enough energy to eject a second electron, so <sup>229m</sup>Th<sup>+</sup> ions live much longer, decaying by gamma emission.  See {{alink|Thorium-229m}}.</ref>
| <sup>229</sup>Th<sup>+</sup>
| 3/2+
|
|
|-
| style="text-indent:1em" | <sup>229m</sup>Th<sup>+</sup>
|
| colspan="3" style="text-indent:2em" | 8.355733554021(8) eV<ref name=Zhang2024 />
| 29(1)&nbsp;min<ref name="Elwell2024" />
| [[Gamma decay|γ]]<ref group="n" name=229m/>
| <sup>229</sup>Th<sup>+</sup>
| 3/2+
|
|
|-
| rowspan=3|<sup>230</sup>Th<ref group="n">Used in [[Uranium–thorium dating]]</ref>
| rowspan=3|Ionium
| rowspan=3 style="text-align:right" | 90
| rowspan=3 style="text-align:right" | 140
| rowspan=3|230.0331323(13)
| rowspan=3|7.54(3)×10<sup>4</sup>&nbsp;y
| α
| <sup>226</sup>Ra
| rowspan=3|0+
| rowspan=3|0.0002(2)<ref group="n" name="us">Intermediate decay product of [[Uranium-238|<sup>238</sup>U]]</ref>
| rowspan=3|
|-
| CD (5.8×10<sup>−11</sup>%)
| <sup>206</sup>Hg<br><sup>24</sup>Ne
|-
| [[Spontaneous fission|SF]] (<4×10<sup>−12</sup>%)
| (various)
|-
| <sup>231</sup>Th
| Uranium Y
| style="text-align:right" | 90
| style="text-align:right" | 141
| 231.0363028(13)
| 25.52(1)&nbsp;h
| β<sup>−</sup>
| <sup>231</sup>Pa
| 5/2+
| Trace<ref group="n" name="as" />
| 
|-
| rowspan=4|[[Thorium-232|<sup>232</sup>Th]]<ref group="n">[[Primordial nuclide|Primordial]] [[radionuclide]]</ref>
| rowspan=4|Thorium
| rowspan=4 style="text-align:right" | 90
| rowspan=4 style="text-align:right" | 142
| rowspan=4|232.0380536(15)
| rowspan=4|'''1.40(1)×10<sup>10</sup>&nbsp;y'''
| α<ref group="n">Theorized to also undergo β<sup>−</sup>β<sup>−</sup> decay to <sup>232</sup>U</ref>
| <sup>228</sup>Ra
| rowspan=4|0+
| rowspan=4|0.9998(2)
| rowspan=4|
|-
| SF (1.1×10<sup>−9</sup>%)
| (various)
|-
| CD (<2.78×10<sup>−10</sup>%)
| <sup>208</sup>Hg<br><sup>24</sup>Ne
|-
| CD (<2.78×10<sup>−10</sup>%)
| <sup>206</sup>Hg<br><sup>26</sup>Ne
|-
| <sup>233</sup>Th
|
| style="text-align:right" | 90
| style="text-align:right" | 143
| 233.0415801(15)
| 21.83(4)&nbsp;min
| β<sup>−</sup>
| <sup>233</sup>Pa
| 1/2+
| Trace<ref group="n">Produced in neutron capture by <sup>232</sup>Th</ref>
|
|-
| <sup>234</sup>Th
| Uranium X<sub>1</sub>
| style="text-align:right" | 90
| style="text-align:right" | 144
| 234.0435998(28)
| 24.107(24)&nbsp;d
| β<sup>−</sup>
| <sup>234m</sup>Pa
| 0+
| Trace<ref group="n" name="us" />
|
|-id=Thorium-235
| <sup>235</sup>Th
|
| style="text-align:right" | 90
| style="text-align:right" | 145
| 235.047255(14)
| 7.2(1)&nbsp;min
| β<sup>−</sup>
| <sup>235</sup>Pa
| 1/2+#
|
|
|-id=Thorium-236
| <sup>236</sup>Th
|
| style="text-align:right" | 90
| style="text-align:right" | 146
| 236.049657(15)
| 37.3(15)&nbsp;min
| β<sup>−</sup>
| <sup>236</sup>Pa
| 0+
|
|
|-id=Thorium-237
| <sup>237</sup>Th
|
| style="text-align:right" | 90
| style="text-align:right" | 147
| 237.053629(17)
| 4.8(5)&nbsp;min
| β<sup>−</sup>
| <sup>237</sup>Pa
| 5/2+#
|
|
|-id=Thorium-238
| <sup>238</sup>Th
|
| style="text-align:right" | 90
| style="text-align:right" | 148
| 238.05639(30)#
| 9.4(20)&nbsp;min
| β<sup>−</sup>
| <sup>238</sup>Pa
| 0+
|
|
{{Isotopes table/footer}}

== Uses ==
Thorium has been suggested for use in [[thorium-based nuclear power]].

In many countries the use of thorium in consumer products is banned or discouraged because it is radioactive.

It is currently used in cathodes of vacuum tubes, for a combination of physical stability at high temperature and a low work energy required to remove an electron from its surface.

It has, for about a century, been used in [[Gas mantle|mantle]]s of gas and vapor lamps such as [[Gas lighting|gas lights]] and camping lanterns.

===Low dispersion lenses===
Thorium was also used in certain glass elements of '''Aero-Ektar''' lenses made by [[Kodak]] during World War II. Thus they are mildly radioactive.<ref>[http://new55project.blogspot.co.uk/2012/02/f25-aero-ektar-lenses.html f2.5 Aero Ektar Lenses ]{{Dead link|date=February 2023 |bot=InternetArchiveBot |fix-attempted=yes }} Some images.</ref> Two of the glass elements in the [[f number|f/2.5]] Aero-Ektar lenses are 11% and 13% thorium by weight. The thorium-containing glasses were used because they have a high refractive index with a low dispersion (variation of index with wavelength), a highly desirable property. Many surviving Aero-Ektar lenses have a tea colored tint, possibly due to radiation damage to the glass.

These lenses were used for aerial reconnaissance because the radiation level is not high enough to fog film over a short period. This would indicate the radiation level is reasonably safe. However, when not in use, it would be prudent to store these lenses as far as possible from normally inhabited areas; allowing the inverse square relationship to attenuate the radiation.<ref>{{Cite web|title = Aero-Ektar Lenses|url = http://home.earthlink.net/~michaelbriggs/aeroektar/aeroektar.html|author = Michael S. Briggs|date = January 16, 2002|access-date = 2015-08-28|archive-url = https://web.archive.org/web/20150812205035/http://home.earthlink.net/~michaelbriggs/aeroektar/aeroektar.html|archive-date = August 12, 2015|url-status = dead}}</ref>

==Actinides vs. fission products==
{{Actinidesvsfissionproducts}}
{{Clear}}

== Notable isotopes ==

=== Thorium-228 ===
'''<sup>228</sup>Th''' is an [[isotope]] of [[thorium]] with 138 [[neutrons]]. It was once named Radiothorium, due to its occurrence in the [[disintegration chain]] of thorium-232. It has a [[half-life]] of 1.9116 years. It undergoes [[alpha decay]] to [[isotopes of radium|<sup>224</sup>Ra]]. Occasionally it decays by the unusual route of [[cluster decay]], emitting a nucleus of [[isotopes of oxygen|<sup>20</sup>O]] and producing stable [[isotopes of lead|<sup>208</sup>Pb]]. It is a daughter isotope of [[Uranium-232|<sup>232</sup>U]] in the thorium decay series.

<sup>228</sup>Th has an atomic weight of 228.0287411&nbsp;grams/mole.

Together with its decay product <sup>224</sup>Ra it is used for alpha particle radiation therapy.<ref>{{cite web | url=https://www.scatecinnovation.no/artikler/thor-medical-production-of-alpha-emitters-for-cancer-treatment | title=Thor Medical – production of alpha emitters for cancer treatment  | date=May 2023}}</ref>

===Thorium-229===
'''<sup>229</sup>Th''' is a [[radioactive]] [[isotope]] of [[thorium]] that decays by [[alpha particle|alpha]] emission with a [[half-life]] of 7917 years.<ref name="Varga14" />
<sup>229</sup>Th is produced by the decay of [[uranium-233]], and its principal use is for the production of the [[nuclear medicine|medical isotopes]] [[actinium-225]] and [[bismuth-213]].<ref>[http://www.ne.doe.gov/pdfFiles/U233RptConMarch2001.pdf Report to Congress on the extraction of medical isotopes from U-233] {{webarchive|url=https://web.archive.org/web/20110927225711/http://www.ne.doe.gov/pdfFiles/U233RptConMarch2001.pdf |date=2011-09-27 }}. U.S. Department of Energy. March 2001</ref>

====Thorium-229m====
<sup>229</sup>Th has a [[nuclear isomer]], {{SimpleNuclide|Th|229|m}}, with a remarkably low excitation energy of {{val|{{#expr:(2020407384335*6.62607015/1.602176634e12) round 12}}|(8)|u=eV}}.<ref name=Zhang2024/>

Due to this low energy, the lifetime of <sup>229m</sup>Th very much depends on the electronic environment of the nucleus. In neutral <sup>229</sup>Th, the isomer decays by [[internal conversion]] within a few microseconds.<ref>{{cite journal
| last1=Karpeshin |first1=F.F. |last2=Trzhaskovskaya |first2=M.B.
| title = Impact of the electron environment on the lifetime of the <sup>229</sup>Th<sup>m</sup> low-lying isomer  
| journal = [[Physical Review C]]
| volume = 76 | issue=5 | date = November 2007 | article-number = 054313
| page=<!--Fuck off Citation Bot-->
| doi = 10.1103/PhysRevC.76.054313
|bibcode=2007PhRvC..76e4313K
}}</ref><ref name="Tkalya" /><ref name="Seiferle2017">{{cite journal
| last1=Seiferle |first1=B. |last2=von der Wense |first2=L. |last3=Thirolf |first3=P.G.
| title = Lifetime measurement of the <sup>229</sup>Th nuclear isomer  
| journal = [[Physical Review Letters]]
| volume = 118 | issue=4 | date = January 2017 | article-number = 042501
| page=<!--Fuck off Citation Bot-->
| doi = 10.1103/PhysRevLett.118.042501
| pmid=28186791 | arxiv=1801.05205
| bibcode=2017PhRvL.118d2501S |s2cid=37518294
| quote=A half-life of {{val|7|1|u=us}} has been measured
}}</ref>  However, the isomeric energy is not enough to remove a second electron (thorium's second ionization energy is {{val|11.5|u=eV}}), so internal conversion is impossible in Th<sup>+</sup> ions. Radiative decay occurs with a half-life {{#expr:(ln(1740/7e-6)/ln10) round 1}} orders of magnitude longer, in excess of 1000&nbsp;seconds.<ref name="Tkalya">{{cite journal
| last1=Tkalya |first1=Eugene V. |last2=Schneider |first2=Christian
| last3=Jeet |first3=Justin |last4=Hudson |first4=Eric R.
| title = Radiative lifetime and energy of the low-energy isomeric level in <sup>229</sup>Th
| journal = [[Physical Review C]]
| volume = 92 | issue=5 | date = 25 November 2015 | article-number = 054324
| page=<!--Fuck off Citation Bot-->
| doi = 10.1103/PhysRevC.92.054324
| arxiv=1509.09101
| bibcode=2015PhRvC..92e4324T |s2cid=118374372
}}</ref><ref>{{cite journal
| last1=Minkov |first1=Nikolay |last2=Pálffy |first2=Adriana
| title = Reduced transition probabilities for the gamma decay of the 7.8 eV isomer in <sup>229m</sup>Th 
| journal = [[Phys. Rev. Lett.]]
| volume = 118 | issue=21 | article-number = 212501
| date = 23 May 2017
| page=<!--Fuck off Citation Bot-->
| doi = 10.1103/PhysRevLett.118.212501
| arxiv = 1704.07919
| pmid = 28598657
| bibcode=2017PhRvL.118u2501M |s2cid=40694257
}}</ref>  Embedded in [[ionic crystals]], ionization is not quite 100%, so a small amount of internal conversion occurs, leading to a recently measured lifetime of ≈{{val|600|u=s}},{{r|Tiedau2024|Elwell2024}}<!--Slightly different in different crystals--> which can be extrapolated to a lifetime for isolated ions of {{val|1740|50|u=s}}.{{r|Tiedau2024}}

This excitation energy corresponds to a photon frequency of {{val|2020407384335|2|u=kHz}} (wavelength {{val|{{#expr:(299792458e6/2020407384335) round 10}}|(15)|u=nm}}).<ref name=Zhang2024>{{cite journal
 |title=Frequency ratio of the <sup>229m</sup>Th nuclear isomeric transition and the <sup>87</sup>Sr atomic clock
 |first1=Chuankun |last1=Zhang  |first2=Tian |last2=Ooi
 |first3=Jacob S. |last3=Higgins  |first4=Jack F. |last4=Doyle
 |first5=Lars |last5=von der Wense  |first6=Kjeld |last6=Beeks
 |first7=Adrian |last7=Leitner  |first8=Georgy |last8=Kazakov
 |first9=Peng |last9=Li  |first10=Peter G. |last10=Thirolf
 |first11=Thorsten |last11=Schumm  |first12=Jun |last12=Ye |author-link12=Jun Ye
 |journal=[[Nature (journal)|Nature]] |volume=633 |issue=8028 |pages=63–70
 |date=4 September 2024
 |doi=10.1038/s41586-024-07839-6
 |arxiv=2406.18719
 |quote=The transition frequency between the {{math|1=''I'' = 5/2}} ground state and the {{math|1=''I'' = 3/2}} excited state is determined as: {{math|1=
''𝜈''<sub>Th</sub> = {{sfrac|1|6}} (''𝜈''<sub>a</sub> + 2''𝜈''<sub>b</sub> + 2''𝜈''<sub>c</sub> + ''𝜈''<sub>d</sub>) = {{val|2020407384335|(2)|u=kHz}}}}.
}}</ref>{{r|Thirolf2024|Tiedau2024|Elwell2024}}  Although in the very high frequency [[Ultraviolet#Subtypes|vacuum ultraviolet]] frequency range, it is possible to build a [[Ultraviolet#Ultraviolet lasers|laser operating at this frequency]], giving the only known opportunity for direct laser excitation of a nuclear state,<ref>{{cite journal
| first1=E.V. |last1=Tkalya
| first2=V.O. |last2=Varlamov
| first3=V.V. |last3=Lomonosov
| first4=S.A. |last4=Nikulin
| title=Processes of the nuclear isomer <sup>229m</sup>Th(3/2<sup>+</sup>, 3.5±1.0 eV) Resonant excitation by optical photons
| journal=Physica Scripta
| volume=53 | issue=3
| pages=296–299
| date=1996
| doi=10.1088/0031-8949/53/3/003
| bibcode=1996PhyS...53..296T
| s2cid=250744766
}}</ref> which could have applications like a [[nuclear clock]] of very high accuracy<ref name="Peik2003" /><ref name="Campbell2012" /><ref>{{cite journal
| title=Towards a <sup>229</sup>Th-based nuclear clock
| first1=Lars |last1=von der Wense
| first2=Benedict |last2=Seiferle
| first3=Peter G. |last3=Thirolf
| journal=Measurement Techniques |volume=60 |issue=12 |pages=1178–1192 |date=March 2018
| doi=10.1007/s11018-018-1337-1
| arxiv=1811.03889
| bibcode=2018MeasT..60.1178V | s2cid=119359298
}}</ref><ref name=Thirolf2020>{{cite conference
 |title='Phase Transition' in the 'Thorium-Isomer Story'
 |first=Peter G. |last=Thirolf  |display-authors=etal
 |journal=[[Acta Physica Polonica B]]
 |volume=51 |issue=3 |pages=561–570
 |date=March 2020
 |url=https://www.actaphys.uj.edu.pl/R/51/3/561/pdf
 |conference=XXXVI Mazurian Lakes Conference on Physics (1–7 November 2019)
 |conference-url=https://mazurian.fuw.edu.pl/wp-content/uploads/2019/09/2019Program.pdf
 |location=[[Piaski, Pisz County]], Poland<!--Damn, there are a lot of places names "Piaski" in Poland!-->
 |doi=10.5506/APhysPolB.51.561 |doi-access=free
 |arxiv=2108.13388
}} Originally presented as ''Characterization of the elusive <sup>229m</sup>Th isomer – milestones towards a nuclear clock''.</ref> or as a [[qubit]] for [[quantum computing]].<ref>{{cite journal
| first1=S. |last1=Raeder
| first2=V. |last2=Sonnenschein
| first3=T. |last3=Gottwald
| first4=I.D. |last4=Moore
| first5=M. |last5=Reponen
| first6=S. |last6=Rothe
| first7=N. |last7=Trautmann
| first8=K. |last8=Wendt
| title = Resonance ionization spectroscopy of thorium isotopes - towards a laser spectroscopic identification of the low-lying 7.6 eV isomer of <sup>229</sup>Th
| journal = [[J. Phys. B: At. Mol. Opt. Phys.]]
| volume = 44 | issue = 16 | article-number = 165005
| page=<!--Fuck off Citation Bot-->
| date = July 2011
| doi = 10.1088/0953-4075/44/16/165005
| arxiv=1105.4646
|bibcode=2011JPhB...44p5005R
|s2cid=118379032
}}</ref>

These applications were for a long time impeded by imprecise measurements of the isomeric energy, as laser excitation's exquisite precision makes it difficult to use to search a wide frequency range.  There were many investigations, both theoretical and experimental, trying to determine the transition energy precisely and to specify other properties of the isomeric state of <sup>229</sup>Th (such as the lifetime and the magnetic moment) before the frequency was accurately measured in 2024.{{r|Tiedau2024|Thirolf2024|Elwell2024}}

=====History=====
Early measurements were performed via [[gamma ray spectroscopy]], producing the {{val|29.5855|u=keV}} excited state of <sup>229</sup>Th, and measuring the difference in emitted gamma ray energies as it decays to either the <sup>229m</sup>Th (90%) or <sup>229</sup>Th (10%) isomeric states.  In 1976, Kroger and Reich sought to understand [[coriolis force]] effects in [[Nilsson model|deformed nuclei]], and attempted to match thorium's gamma-ray spectrum to theoretical nuclear shape models.  To their surprise, the known nuclear states could not be reasonably classified into different [[total angular momentum]] quantization levels.  They concluded that some states previously identified as <sup>229</sup>Th actually arose from a spin-{{sfrac|3|2}} nuclear isomer, <sup>229m</sup>Th, with a remarkably low excitation energy.<ref>{{cite journal
| first1=L.A. |last1=Kroger
| first2=C.W. |last2=Reich
| title = Features of the low energy level scheme of <sup>229</sup>Th as observed in the α-decay of <sup>233</sup>U
| journal = [[Nuclear Physics A]]
| volume = 259
| issue=1
| pages = 29–60
| date = 1976
| doi = 10.1016/0375-9474(76)90494-2
|bibcode=1976NuPhA.259...29K
}}</ref> 

At that time the energy was inferred to be below 100 eV, purely based on the non-observation of the isomer's direct decay. However, in 1990, further measurements led to the conclusion that the energy is almost certainly below 10 eV,<ref name="Helmer1990">{{cite journal
|  title = Energy separation of the doublet of intrinsic states at the ground state of <sup>229</sup>Th
| last1=Reich |first1=C. W.  |author2=Helmer, R. G.
|  journal = [[Physical Review Letters]]
|  volume = 64
|  issue = 3
|  pages = 271–273
|date=Jan 1990
|  doi = 10.1103/PhysRevLett.64.271
|pmid=10041937  |  publisher = American Physical Society
|bibcode = 1990PhRvL..64..271R |url=https://zenodo.org/record/1233878 }}
</ref> making it one of the lowest known isomeric excitation energies. In the following years, the energy was further constrained to {{val|3.5|1.0|u=eV}}, which was for a long time the accepted energy value.<ref name="Helmer1994">{{cite journal 
| journal=[[Physical Review C]]
| volume=49 | issue=4| pages=1845–1858 
| date=April 1994 
| last1=Helmer |first1=R. G. |last2=Reich |first2=C. W.
| title = An Excited State of <sup>229</sup>Th at 3.5&nbsp;eV 
| doi=10.1103/PhysRevC.49.1845 
| pmid=9969412 | bibcode = 1994PhRvC..49.1845H
| url=https://zenodo.org/record/1233767}}</ref> 

Improved gamma ray spectroscopy measurements using an advanced high-resolution X-ray microcalorimeter were carried out in 2007, yielding a new value for the transition energy of {{val|7.6|0.5|u=eV}},<ref name="Beck">{{cite journal |author=B. R. Beck |title=Energy splitting in the ground state doublet in the nucleus <sup>229</sup>Th |journal=[[Physical Review Letters]] |volume=98 |issue=14 |pages=142501|date=2007-04-06 |doi=10.1103/PhysRevLett.98.142501 |pmid=17501268 |bibcode=2007PhRvL..98n2501B |s2cid=12092700 |url=https://zenodo.org/record/1233955 |display-authors=etal}}</ref> corrected to {{val|7.8|0.5|u=eV}} in 2009.<ref name="BeckReview">{{cite conference
| title = Improved value for the energy splitting of the ground-state doublet in the nucleus <sup>229</sup>Th
| vauthors = Beck BR, Wu CY, Beiersdorfer P, Brown GV, Becker JA, Moody KJ, Wilhelmy JB, Porter FS, Kilbourne CA, Kelley RL
| date = 2009-07-30
| conference = 12th Int. Conf. on Nuclear Reaction Mechanisms
| location = Varenna, Italy
| id = LLNL-PROC-415170
| url = https://e-reports-ext.llnl.gov/pdf/375773.pdf
| access-date = 2014-05-14
| archive-url = https://web.archive.org/web/20170127104504/https://e-reports-ext.llnl.gov/pdf/375773.pdf
| archive-date = 2017-01-27
| url-status = dead
}}</ref> This higher energy has two consequences which had not been considered by earlier attempts to observe emitted photons:
* Because it is above thorium's {{val|6.08|u=eV}} first ionization energy, neutral <sup>229m</sup>Th will decay radiatively with an extremely low likelihood, and
* Because it is above the {{val|6.2|u=eV}} vacuum ultraviolet cutoff, the produced photons cannot travel through air.

But even knowing the higher energy, most of the searches in the 2010s for light emitted by the isomeric decay failed to observe any signal,<ref>{{Cite journal
 |last1=Jeet |first1=Justin  |first2=Christian |last2=Schneider  |first3=Scott T. |last3=Sullivan
 |first4=Wade G. |last4=Rellergert  |first5=Saed |last5=Mirzadeh  |first6=A. |last6=Cassanho
 |first7=H. P. |last7=Jenssen  |first8=Eugene V. |last8=Tkalya  |first9=Eric R. |last9=Hudson
 |display-authors=6
 |title=Results of a Direct Search Using Synchrotron Radiation for the Low-Energy
 |journal=[[Physical Review Letters]]
 |volume=114 |issue=25 |pages=253001 |date=23 June 2015
 |doi=10.1103/physrevlett.114.253001 |pmid=26197124 |arxiv=1502.02189 |bibcode=2015PhRvL.114y3001J
|s2cid=1322253 }}</ref><ref>{{Cite journal
 |last1=Yamaguchi |first1=A.  |last2=Kolbe |first2=M.  |last3=Kaser |first3=H.
 |last4=Reichel |first4=T.  |last5=Gottwald |first5=A.  |last6=Peik |first6=E.
 |title=Experimental search for the low-energy nuclear transition in <sup>229</sup>Th with undulator radiation
 |journal=New Journal of Physics |language=en
 |volume=17 |issue=5 |pages=053053 |date=May 2015
 |doi=10.1088/1367-2630/17/5/053053 |bibcode=2015NJPh...17e3053Y |doi-access=free
}}</ref><ref>{{cite thesis
| first = Lars | last = von der Wense
| degree = PhD
| title = On the direct detection of <sup>229m</sup>Th
| publisher = [[Ludwig Maximilian University of Munich]]
| date = 2016
| isbn=978-3-319-70461-6
| url = https://edoc.ub.uni-muenchen.de/20492/7/Wense_Lars_von_der.pdf
}}</ref><ref>{{cite journal
| title=On an attempt to optically excite the nuclear isomer in Th-229
| first1=S. |last1=Stellmer
| first2=G. |last2=Kazakov
| first3=M. |last3=Schreitl
| first4=H. |last4=Kaser
| first5=M. |last5=Kolbe
| first6=T. |last6=Schumm
| journal=[[Physical Review A]] |volume=97 |pages=062506 |date=2018
| issue=6 | doi= 10.1103/PhysRevA.97.062506
| arxiv=1803.09294 | bibcode=2018PhRvA..97f2506S| s2cid=4946329 }}</ref> pointing towards a potentially strong non-radiative decay channel. A direct detection of photons emitted in the isomeric decay was claimed in 2012<ref name="Zhao2012">{{cite journal
 |last1=Zhao |first1=Xinxin
 |date=18 October 2012
 |title=Observation of the Deexcitation of the <sup>229m</sup>Th Nuclear Isomer
 |journal=[[Physical Review Letters]] |volume=109 |issue=16 |article-number=160801
 |doi=10.1103/PhysRevLett.109.160801
 |first2=Yenny Natali |last2=Martinez de Escobar
 |first3=Robert |last3=Rundberg
 |first4=Evelyn M. |last4=Bond
 |first5=Allen |last5=Moody
 |first6=David J. |last6=Vieira
 |page=<!--Fuck off Citation Bot-->
 |bibcode=2012PhRvL.109p0801Z
 |pmid=23215066
|doi-access=free
}}</ref> and again in 2018.<ref name=":1">{{Cite arXiv
 |last1=Borisyuk|first1=P. V.  |last2=Chubunova|first2=E. V.  |last3=Kolachevsky|first3=N. N.
 |last4=Lebedinskii|first4=Yu Yu  |last5=Vasiliev|first5=O. S.  |last6=Tkalya|first6=E. V.
 |date=2018-04-01
 |title=Excitation of <sup>229</sup>Th nuclei in laser plasma: the energy and half-life of the low-lying isomeric state
 |eprint=1804.00299 |class=nucl-th
}}</ref> However, both reports were subject to controversial discussions within the community.<ref>{{Cite journal
 |last1=Peik|first1=Ekkehard
 |last2=Zimmermann|first2=Kai
 |date=2013-07-03
 |title=Comment on "Observation of the Deexcitation of the <sup>229m</sup>Th Nuclear Isomer"
 |journal=[[Physical Review Letters]] |volume=111 |issue=1
 |article-number=018901
 |doi=10.1103/PhysRevLett.111.018901
 |pmid=23863029 |bibcode=2013PhRvL.111a8901P
 |quote=While we do not exclude that the decay of the <sup>229m</sup>Th isomer has contributed to the photon emission observed in [1], we conclude that the sought-after signal would be heavily masked by background from other nuclear decays and radioluminescence induced in the MgF<sub>2</sub> plates.
}}</ref><ref>{{Cite journal
 |last1=Thirolf|first1=Peter G. |last2=Seiferle|first2=Benedict  |last3=von der Wense |first3=Lars
 |date=2019-10-28
 |title=The 229-thorium isomer: doorway to the road from the atomic clock to the nuclear clock
 |journal=[[Journal of Physics B: Atomic, Molecular and Optical Physics]]
 |volume=52 |issue=20 |article-number=203001
 |doi=10.1088/1361-6455/ab29b8 |bibcode=2019JPhB...52t3001T |doi-access=free
 |url=https://s3.cern.ch/inspire-prod-files-6/6da1f99f16760317d4532b9394b15842
}}</ref>

A direct detection of electrons being emitted in the [[internal conversion]] decay channel of <sup>229m</sup>Th was achieved in 2016.<ref name=":0">{{cite journal 
| journal=[[Nature (journal)|Nature]]
| volume=533 | issue=7601 | pages=47–51
| date=5 May 2016
| title = Direct detection of the <sup>229</sup>Th nuclear clock transition
| first1=Lars | last1=von der Wense
| first2=Benedict | last2=Seiferle
| first3=Mustapha | 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=6
| doi=10.1038/nature17669
| pmid=27147026 | bibcode=2016Natur.533...47V| arxiv=1710.11398| s2cid=205248786 }}</ref> However, at the time the isomer's transition energy could only be weakly constrained to between 6.3 and 18.3&nbsp;eV. Finally, in 2019, non-optical electron spectroscopy of the internal conversion electrons emitted in the isomeric decay allowed for a determination of the isomer's excitation energy to {{val|8.28|0.17|u=eV}}.<ref name="SeiferleEnergy">{{cite journal
| title = Energy of the <sup>229</sup>Th nuclear clock transition
| journal=[[Nature (journal)|Nature]]
| volume=573  | issue=7773 | pages=243–246
| date=12 September 2019
| first1=B. |last1=Seiferle
| first2=L. |last2=von der Wense
| first3=P.V.  |last3=Bilous
| first4=I. |last4=Amersdorffer
| first5=C. |last5=Lemell
| first6=F. |last6=Libisch
| first7=S. |last7=Stellmer
| first8=T. |last8=Schumm
| first9=C.E. |last9=Düllmann
| first10=A. |last10=Pálffy
| first11=P.G. |last11=Thirolf
| doi=10.1038/s41586-019-1533-4
| pmid=31511684
| arxiv=1905.06308
| bibcode=2019Natur.573..243S
| s2cid=155090121
}}</ref> However, this value appeared at odds with the 2018 preprint showing that a similar signal as an {{val|8.4|u=eV}} xenon VUV photon can be shown, but with about {{val|1.3|0.2|0.1|u=eV}} less energy and a (retrospectively correct) {{val|1880|170|u=s}} lifetime.<ref name=":1" /> In that paper, <sup>229</sup>Th was embedded in [[silicon dioxide|SiO<sub>2</sub>]], possibly resulting in an energy shift and altered lifetime, although the states involved are primarily nuclear, shielding them from electronic interactions.

In another 2018 experiment, it was possible to perform a first laser-spectroscopic characterization of the nuclear properties of <sup>229m</sup>Th.<ref>{{cite journal
| last1=Thielking |first1=J. |last2=Okhapkin |first2=M.V. |last3=Przemyslaw |first3=G.
| last4=Meier |first4=D.M. |last5=von der Wense |first5=L.  |last6=Seiferle |first6=B.
| last7=Düllmann |first7=C.E.  |last8=Thirolf |first8=P.G.  |last9=Peik |first9=E.
| title = Laser spectroscopic characterization of the nuclear-clock isomer <sup>229m</sup>Th
| journal = Nature
| volume = 556
| issue=7701 | year = 2018
| pages = 321–325
| doi = 10.1038/s41586-018-0011-8
|pmid=29670266 | arxiv=1709.05325
|bibcode=2018Natur.556..321T |s2cid=4990345 }}</ref> In this experiment, [[laser spectroscopy]] of the <sup>229</sup>Th atomic shell was conducted using a <sup>229</sup>Th<sup>2+</sup> ion cloud with 2% of the ions in the nuclear excited state. This allowed probing for the hyperfine shift induced by the different nuclear spin states of the ground and the isomeric state. In this way, a first experimental value for the magnetic dipole and the electric quadrupole moment of <sup>229m</sup>Th could be inferred.

In 2019, the isomer's excitation energy was constrained to {{val|8.28|0.17|u=eV}} based on the direct detection of internal conversion electrons<ref name="SeiferleEnergy" /> and a secure population of <sup>229m</sup>Th from the nuclear ground state was achieved by excitation of the {{val|29|u=keV}} nuclear excited state via synchrotron radiation.<ref name="Masuda2019">{{cite journal
| title = X-ray pumping of the <sup>229</sup>Th nuclear clock isomer
| journal=[[Nature (journal)|Nature]]
| volume=573  | issue=7773 | pages=238–242
| date=12 September 2019
| first1=T. |last1=Masuda
| first2=A. |last2=Yoshimi
| first3=A. |last3=Fujieda
| first4=H. |last4=Fujimoto
| first5=H. |last5=Haba
| first6=H. |last6=Hara
| first7=T. |last7=Hiraki
| first8=H. |last8=Kaino
| first9=Y. |last9=Kasamatsu
| first10=S. |last10=Kitao
| first11=K. |last11=Konashi
| first12=Y. |last12= Miyamoto
| first13=K. |last13=Okai
| first14=S. |last14=Okubo
| first15=N. |last15=Sasao
| first16=M. |last16=Seto
| first17=T. |last17=Schumm
| first18=Y. |last18=Shigekawa
| first19=K. |last19=Suzuki
| first20=S. |last20=Stellmer
| first21=K. |last21=Tamasaku
| first22=S. |last22=Uetake
| first23=M. |last23=Watanabe
| first24=T. |last24=Watanabe
| first25=Y. |last25=Yasuda
| first26=A. |last26=Yamaguchi
| first27=Y. |last27=Yoda
| first28=T. |last28=Yokokita
| first29=M. |last29=Yoshimura
| first30=K. |last30=Yoshimura
| display-authors=6
| doi=10.1038/s41586-019-1542-3
| pmid=31511686
| arxiv=1902.04823
| bibcode=2019Natur.573..238M
| s2cid=119083861
}}</ref>  Additional measurements by a different group in 2020 produced a figure of {{val|8.10|0.17|u=eV}} ({{val|153.1|3.2|u=nm}} wavelength).<ref>{{cite journal
 |first1=Tomas |last1=Sikorsky  |first2=Jeschua |last2=Geist  |first3=Daniel |last3=Hengstler
 |first4=Sebastian |last4=Kempf  |first5=Loredana |last5=Gastaldo  |first6=Christian |last6=Enss
 |first7=Christoph |last7=Mokry  |first8=Jörg |last8=Runke  |first9=Christoph E. |last9=Düllmann
 |first10=Peter |last10=Wobrauschek  |first11=Kjeld |last11=Beeks  |first12=Veronika |last12=Rosecker
 |first13=Johannes H. |last13=Sterba  |first14=Georgy |last14=Kazakov  |first15=Thorsten |last15=Schumm
 |first16=Andreas |last16=Fleischmann
 |display-authors=6
 |title=Measurement of the <sup>229</sup>Th Isomer Energy with a Magnetic Microcalorimeter
 |journal=[[Physical Review Letters]]
 |volume=125 |issue=14 |article-number=142503 |date=2 October 2020
 |page=<!--Fuck off Citation Bot-->
 |doi=10.1103/PhysRevLett.125.142503 |pmid=33064540 |arxiv=2005.13340
 |bibcode=2020PhRvL.125n2503S |s2cid=218900580
}}</ref>  Combining these measurements, the expected transition energy is {{val|8.12|0.11|u=eV}}.<ref>{{cite news
 |first=Lars |last=von der Wense
 |title=Ticking Toward a Nuclear Clock
 |journal=[[Physics (magazine)|Physics]] |volume=13 |article-number=152 |date=28 September 2020
 |url=https://physics.aps.org/articles/v13/152
}}</ref>

In September 2022, spectroscopy on decaying samples determined the excitation energy to be {{val|8.338|0.024|u=eV}}.<ref>{{Cite journal |last1=Kraemer |first1=Sandro |last2=Moens |first2=Janni |last3=Athanasakis-Kaklamanakis |first3=Michail |last4=Bara |first4=Silvia |last5=Beeks |first5=Kjeld |last6=Chhetri |first6=Premaditya |last7=Chrysalidis |first7=Katerina |last8=Claessens |first8=Arno |last9=Cocolios |first9=Thomas E. |last10=Correia |first10=João G. M. |last11=Witte |first11=Hilde De |last12=Ferrer |first12=Rafael |last13=Geldhof |first13=Sarina |last14=Heinke |first14=Reinhard |last15=Hosseini |first15=Niyusha |date=May 2023 |title=Observation of the radiative decay of the 229Th nuclear clock isomer |url=https://www.nature.com/articles/s41586-023-05894-z |journal=Nature |language=en |volume=617 |issue=7962 |pages=706–710 |doi=10.1038/s41586-023-05894-z |pmid=37225880 |bibcode=2023Natur.617..706K |issn=1476-4687|arxiv=2209.10276 }}</ref>

In April 2024, two separate groups finally reported precision laser excitation Th<sup>4+</sup> cations doped into [[ionic crystals]] (of [[Calcium fluoride|CaF<sub>2</sub>]] and LiSrAlF<sub>6</sub> with additional interstitial [[Fluoride|F<sup>−</sup> anions]] for charge compensation), giving a precise (~1 [[part per million]]) measurement of the transition energy.<ref name=Thirolf2024>{{cite journal
 |title=Shedding Light on the Thorium-229 Nuclear Clock Isomer
 |first=Peter |last=Thirolf
 |date=April 29, 2024
 |journal=[[Physics (magazine)|Physics]] |volume=17 |article-number=71
 |page=<!--Fuck off Citation Bot-->
 |doi=10.1103/Physics.17.71
 |url=https://physics.aps.org/articles/v17/71
}}</ref><ref name=PR-Tiedau2024>{{cite press release
 |title=Atomic Nucleus Excited with Laser: A Breakthrough after Decades
 |date=29 April 2024
 |publisher=[[TU Wien]]
 |url=https://www.tuwien.at/en/tu-wien/news/news-articles/news/lange-erhoffter-durchbruch-erstmals-atomkern-mit-laser-angeregt
 |access-date=29 April 2024
}}</ref><ref name=Tiedau2024>{{cite journal
 |title=Laser Excitation of the Th-229 Nucleus
 |first1=J. |last1=Tiedau  |first2=M. V. |last2=Okhapkin  |first3=K. |last3=Zhang
 |first4=J. |last4=Thielking  |first5=G. |last5=Zitzer  |first6=E. |last6=Peik
 |first7=F. |last7=Schaden  |first8=T. |last8=Pronebner  |first9=I. |last9=Morawetz
 |first10=L. |last10=Toscani De Col |first11=F. |last11=Schneider  |first12=A. |last12=Leitner
 |first13=M. |last13=Pressler  |first14=G.A. |last14=Kazakov  |first15=K. |last15=Beeks
 |first16=T. |last16=Sikorsky  |first17=T. |last17=Schumm
 |display-authors=6
 |journal=[[Physical Review Letters]] |volume=132 |issue=18 |article-number=182501
 |date=29 April 2024
 |page=<!--Fuck off Citation Bot-->
 |doi=10.1103/PhysRevLett.132.182501
 |bibcode=2024PhRvL.132r2501T |url=https://www.tuwien.at/fileadmin/Assets/tu-wien/News/2024/Thorium_Preprint.pdf
 |quote=The nuclear resonance for the Th<sup>4+</sup> ions in Th:CaF<sub>2</sub> is measured at the wavelength {{val|148.3821|(5)|u=nm}}, frequency {{val|2020.409|(7)|u=THz}}, and the fluorescence lifetime in the crystal is {{val|630|(15)|u=s}}, corresponding to an isomer half-life of {{val|1740|(50)|u=s}} for a nucleus isolated in vacuum.
}}</ref><ref name=Elwell2024>{{cite arXiv
 |title=Laser excitation of the <sup>229</sup>Th nuclear isomeric transition in a solid-state host
 |first1=R. |last1=Elwell  |first2=Christian |last2=Schneider  |first3=Justin |last3=Jeet
 |first4=J. E. S. |last4=Terhune  |first5=H. W. T. |last5=Morgan  |first6=A. N. |last6=Alexandrova
 |first7=Hoang Bao |last7=Tran Tan  |first8=Andrei |last8=Derevianko  |first9=Eric R. |last9=Hudson
 |eprint=2404.12311 |class=physics.atom-ph
 |date=18 April 2024
 |quote=a narrow, laser-linewidth-limited spectral feature at {{val|148.38219|(4)|errend=<sub>stat</sub>(20)<sub>sys</sub>|u=nm}} ({{val|2020407.3|(5)|errend=<sub>stat</sub>(30)<sub>sys</sub>|u=GHz}}) that decays with a lifetime of {{val|568|(13)|errend=<sub>stat</sub>(20)<sub>sys</sub>|u=s}}. This feature is assigned to the excitation of the <sup>229</sup>Th nuclear isomeric state, whose energy is found to be {{val|8.355733|(2)|errend=<sub>stat</sub>(10)<sub>sys</sub>|u=eV}} in <sup>229</sup>Th:LiSrAlF<sub>6</sub>. 
}}</ref>  A one-[[part-per-trillion]]  ({{val|e=-12}}) measurement soon followed in June 2024,<ref name=Zhang2024/><ref name=Howlett2024>{{cite news
| first=Joseph |last=Howlett
| title=The First Nuclear Clock Will Test if Fundamental Constants Change
| date=4 September 2024
| journal=[[Quanta Magazine]]
| url=https://www.quantamagazine.org/the-first-nuclear-clock-will-test-if-fundamental-constants-change-20240904/
}}</ref> and future high-precision lasers will measure the frequency up to the {{val|e=-18}} accuracy of the best [[atomic clock]]s.{{r|Zhang2024|Campbell2012|Thirolf2020}}

===Thorium-230===
'''<sup>230</sup>Th''' is a [[radioactive]] [[isotope]] of [[thorium]] that can be used to date [[coral]]s and determine [[ocean current]] flux. '''Ionium''' was a name given early in the study of radioactive elements to the <sup>230</sup>Th isotope produced in the [[decay chain]] of [[Uranium-238|<sup>238</sup>U]] before it was realized that ionium and thorium are chemically identical. The symbol '''Io''' was used for this supposed element. (The name is still used in [[ionium–thorium dating]].)

===Thorium-231===
'''<sup>231</sup>Th''' has 141 [[neutrons]]. It is the decay product of [[uranium-235]]. It is found in very small amounts on the [[earth]] and has a [[half-life]] of 25.5 hours.<ref>{{cite journal|first1=G. B.|last1=Knight|first2=R. L.|last2=Macklin|title=Radiations of Uranium Y|journal=Physical Review|date=1 January 1949|pages=34–38|volume=75|issue=1|doi=10.1103/PhysRev.75.34|bibcode=1949PhRv...75...34K}}</ref> When it decays, it emits a [[beta ray]] and forms [[protactinium-231]]. It has a decay energy of 0.39&nbsp;MeV. It has a mass of 231.0363043 [[Atomic mass unit|u]].

===Thorium-232===
{{main|Thorium-232}}
'''<sup>232</sup>Th''' is the only [[primordial nuclide]] of [[thorium]] and makes up effectively all of natural thorium, with other isotopes of thorium appearing only in trace amounts as relatively short-lived [[decay product]]s of [[uranium]] and thorium.<ref>{{cite web |url=http://ie.lbl.gov/education/parent/Th_iso.htm |title=Isotopes of Thorium (Z=90) |work=Isotopes Project |publisher=[[Lawrence Berkeley National Laboratory]] |access-date=2010-01-18 |url-status=dead |archive-url=https://web.archive.org/web/20100203162843/http://ie.lbl.gov/education/parent/Th_iso.htm |archive-date=2010-02-03 }}</ref> 
The isotope decays by [[alpha decay]] with a [[half-life]] of 1.405{{E|10}} years, over three times the [[age of the Earth]] and approximately the [[age of the universe]].
Its [[decay chain]] is the [[thorium series]], eventually ending in [[lead-208]]. The remainder of the chain is quick; the longest half-lives in it are 5.75 years for [[radium-228]] and 1.91 years for [[thorium-228]], with all other half-lives totaling less than 15 days.<ref>{{cite web |url=http://hepwww.rl.ac.uk/ukdmc/Radioactivity/Th_chain/Th_chain.html |title=Th-232 Decay Chain |author=Rutherford Appleton Laboratory |access-date=2010-01-25 |author-link=Rutherford Appleton Laboratory |archive-url=https://web.archive.org/web/20120319190834/http://hepwww.rl.ac.uk/ukdmc/Radioactivity/Th_chain/Th_chain.html |archive-date=2012-03-19 |url-status=dead }}</ref>

<sup>232</sup>Th is a [[fertile material]] able to [[neutron capture|absorb]] a [[neutron]] and undergo [[Nuclear transmutation|transmutation]] into the [[fissile]] [[nuclide]] [[uranium-233]], which is the basis of the [[thorium fuel cycle]].<ref>{{cite web |url=http://www.world-nuclear.org/info/inf62.html |title=Thorium |author=World Nuclear Association |access-date=2010-01-25 |author-link=World Nuclear Association |archive-date=2013-02-16 |archive-url=https://web.archive.org/web/20130216102005/http://www.world-nuclear.org/info/inf62.html |url-status=dead }}</ref>
In the form of [[Thorotrast]], a [[thorium dioxide]] [[suspension (chemistry)|suspension]], it was used as a [[contrast medium]] in early [[X-ray]] diagnostics. Thorium-232 is now classified as [[carcinogen]]ic.<ref>{{cite journal |last1= Krasinskas |first1= Alyssa M |last2= Minda |first2= Justina |last3= Saul |first3= Scott H |last4= Shaked |first4= Abraham |last5= Furth |first5= Emma E |title=Redistribution of thorotrast into a liver allograft several years following transplantation: a case report |journal= Mod. Pathol. |volume= 17 |pages= 117–120 |year= 2004 |doi= 10.1038/modpathol.3800008 |pmid=14631374 |issue=1|doi-access= free }}</ref>

===Thorium-233===
'''<sup>233</sup>Th''' is an isotope of [[thorium]] that decays into [[protactinium-233]] through beta decay. It has a half-life of 21.83 minutes.{{NUBASE2020|ref}} Traces occur in nature as the result of natural [[neutron activation]] of <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 |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=2019-04-29 |url-status=live }}</ref>

===Thorium-234===
'''<sup>234</sup>Th''' is an [[isotope]] of [[thorium]] whose [[Atomic nucleus|nuclei]] contain 144 [[neutron]]s. <sup>234</sup>Th has a [[half-life]] of 24.1 days, and when it decays, it emits a [[beta particle]], and in doing so, it [[Nuclear transmutation|transmutes]] into [[protactinium]]-234. <sup>234</sup>Th has a mass of 234.0436 [[atomic mass unit]]s, and it has a decay energy of about 270&nbsp;keV. [[Uranium]]-238 usually decays into this isotope of thorium (although in rare cases it can undergo [[spontaneous fission]] instead).

==References==
{{Reflist}}
* Isotope masses from:
**{{NUBASE 2003}}
* Isotopic compositions and standard atomic masses from:
**{{CIAAW2003}}
**{{CIAAW 2005}}
* Half-life, spin, and isomer data selected from the following sources.
**{{cite journal |author=G. Audi |author2=A. H. Wapstra |author3=C. Thibault |author4=J. Blachot |author5=O. Bersillon |year=2003 |title=The NUBASE evaluation of nuclear and decay properties |url=http://amdc.in2p3.fr/nubase/Nubase2003.pdf |journal=[[Nuclear Physics A]] |volume=729 |issue=1 |pages=3–128 |doi=10.1016/j.nuclphysa.2003.11.001 |bibcode=2003NuPhA.729....3A |url-status=dead |archive-url=https://web.archive.org/web/20110720233206/http://amdc.in2p3.fr/nubase/Nubase2003.pdf |archive-date=2011-07-20 }}
**{{NNDC}}
**{{CRC85|chapter=11}}

{{Navbox element isotopes}}
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[[Category:Isotopes of thorium| ]]
[[Category:Thorium]]
[[Category:Lists of isotopes by element|Thorium]]