Isotopes of thorium

{{Short description|none}} {{Infobox thorium isotopes}}

Thorium (₉₀Th) has seven naturally occurring isotopes but none are stable. One isotope, ²³²Th, is ''relatively'' stable, with a half-life of 1.40×10¹⁰ 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. However, in 2013, IUPAC reclassified thorium as binuclidic, due to large amounts of ²³⁰Th in deep seawater. Thorium has a characteristic terrestrial isotopic composition and thus a standard atomic weight can be given.

Thirty-one radioisotopes have been characterized, with the most stable being ²³²Th, ²³⁰Th with a half-life of 75,400 years, ²²⁹Th with a half-life of 7,916 years, and ²²⁸Th with a half-life of 1.91 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, ²²⁹Th, 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 ²²⁹Th |journal=Physical Review C|volume=73 |issue=4 |article-number=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 ²²⁹Th 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 ²²⁹Th | 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 --> IsotopeFRIB, IsomerFRIB | notes = m, histname, unc(), mass#, hl-nst, spin(), spin#, EC, CD, IT, daughter-st, discoveryname }} |-id=Thorium-207 | ²⁰⁷Th<ref name="Yang207">{{cite journal |title=New isotope ²⁰⁷Th 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 | | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/207.pdf 2022] | {{val|9.7|46.6|4.4|u=ms}} | α | ²⁰³Ra | | | |-id=Thorium-208 | ²⁰⁸Th | | style="text-align:right" | 90 | style="text-align:right" | 118 | 208.017915(34) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/208.pdf 2010] | 2.4(12)&nbsp;ms | α | ²⁰⁴Ra | 0+ | | |-id=Thorium-209 | ²⁰⁹Th | | style="text-align:right" | 90 | style="text-align:right" | 119 | 209.017998(27) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/209.pdf 1996] | 3.1(12)&nbsp;ms | α | ²⁰⁵Ra | 13/2+ | | |-id=Thorium-210 | ²¹⁰Th | | style="text-align:right" | 90 | style="text-align:right" | 120 | 210.015094(20) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/210.pdf 1995] | 16.0(36)&nbsp;ms | α | ²⁰⁶Ra | 0+ | | |-id=Thorium-211 | ²¹¹Th | | style="text-align:right" | 90 | style="text-align:right" | 121 | 211.014897(92) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/211.pdf 1995] | 48(20)&nbsp;ms | α | ²⁰⁷Ra | 5/2−# | | |-id=Thorium-212 | ²¹²Th | | style="text-align:right" | 90 | style="text-align:right" | 122 | 212.013002(11) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/212.pdf 1980] | 31.7(13)&nbsp;ms | α | ²⁰⁸Ra | 0+ | | |-id=Thorium-213 | ²¹³Th | | style="text-align:right" | 90 | style="text-align:right" | 123 | 213.0130115(99) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/213.pdf 1968] | 144(21)&nbsp;ms | α | ²⁰⁹Ra | 5/2− | | |-id=Thorium-213m | style="text-indent:1em" | ^213mTh | | colspan="3" style="text-indent:2em" | 1180.0(14)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/90/213Th-1.pdf 2007] | 1.4(4)&nbsp;μs | IT | ²¹³Th | (13/2)+ | | |-id=Thorium-214 | ²¹⁴Th | | style="text-align:right" | 90 | style="text-align:right" | 124 | 214.011481(11) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/214.pdf 1968] | 87(10)&nbsp;ms | α | ²¹⁰Ra | 0+ | | |-id=Thorium-214m | style="text-indent:1em" | ^214mTh | | colspan="3" style="text-indent:2em" | 2181.0(27)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/90/214Th-1.pdf 2007] | 1.24(12)&nbsp;μs | IT | ²¹⁴Th | 8+# | | |-id=Thorium-215 | ²¹⁵Th | | style="text-align:right" | 90 | style="text-align:right" | 125 | 215.0117246(68) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/215.pdf 1968] | 1.35(14)&nbsp;s | α | ²¹¹Ra | (1/2−) | | |-id=Thorium-215m | style="text-indent:1em" | ^215mTh | | colspan="3" style="text-indent:2em" | 1471(50)#&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/90/215Th-1.pdf 2005] | 770(60)&nbsp;ns | IT | ²¹⁵Th | 9/2+# | | |-id=Thorium-216 | ²¹⁶Th | | style="text-align:right" | 90 | style="text-align:right" | 126 | 216.011056(12) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/216.pdf 1968] | 26.28(16)&nbsp;ms | α | ²¹²Ra | 0+ | | |-id=Thorium-216m1 | rowspan=2 style="text-indent:1em" | ^216m1Th | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 2041(8)&nbsp;keV | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/90/216Th-1.pdf 1983] | rowspan=2|135.4(29)&nbsp;μs | IT (97.2%) | ²¹⁶Th | rowspan=2|8+ | rowspan=2| | rowspan=2| |- | α (2.8%) | ²¹²Ra |-id=Thorium-216m2 | style="text-indent:1em" | ^216m2Th | | colspan="3" style="text-indent:2em" | 2648(8)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/90/216Th-3.pdf 2001] | 580(26)&nbsp;ns | IT | ²¹⁶Th | (11−) | | |-id=Thorium-216m3 | style="text-indent:1em" | ^216m3Th | | colspan="3" style="text-indent:2em" | 3682(8)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/90/216Th-2.pdf 2001] | 740(70)&nbsp;ns | IT | ²¹⁶Th | (14+) | | |-id=Thorium-217 | ²¹⁷Th | | style="text-align:right" | 90 | style="text-align:right" | 127 | 217.013103(11) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/217.pdf 1968] | 248(4)&nbsp;μs | α | ²¹³Ra | 9/2+# | | |-id=Thorium-217m1 | style="text-indent:1em" | ^217m1Th | | colspan="3" style="text-indent:2em" | 673.3(1)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/90/217Th-1.pdf 1989] | 141(50)&nbsp;ns | IT | ²¹⁷Th | (15/2−) | | |-id=Thorium-217m2 | style="text-indent:1em" | ^217m2Th | | colspan="3" style="text-indent:2em" | 2307(32)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/90/217Th-2.pdf 2005] | 71(14)&nbsp;μs | IT | ²¹⁷Th | (25/2+) | | |-id=Thorium-218 | ²¹⁸Th | | style="text-align:right" | 90 | style="text-align:right" | 128 | 218.013276(11) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/218.pdf 1973] | 122(5)&nbsp;ns | α | ²¹⁴Ra | 0+ | | |-id=Thorium-219 | ²¹⁹Th | | style="text-align:right" | 90 | style="text-align:right" | 129 | 219.015526(61) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/219.pdf 1973] | 1.023(18)&nbsp;μs | α | ²¹⁵Ra | 9/2+# | | |-id=Thorium-220 | ²²⁰Th | | style="text-align:right" | 90 | style="text-align:right" | 130 | 220.015770(15) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/220.pdf 1973] | 10.2(3)&nbsp;μs | α | ²¹⁶Ra | 0+ | | |-id=Thorium-221 | ²²¹Th | | style="text-align:right" | 90 | style="text-align:right" | 131 | 221.0181858(86) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/221.pdf 1970] | 1.75(2)&nbsp;ms | α | ²¹⁷Ra | 7/2+# | | |-id=Thorium-222 | ²²²Th | | style="text-align:right" | 90 | style="text-align:right" | 132 | 222.018468(11) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/222.pdf 1970] | 2.24(3)&nbsp;ms | α | ²¹⁸Ra | 0+ | | |-id=Thorium-223 | ²²³Th | | style="text-align:right" | 90 | style="text-align:right" | 133 | 223.0208111(85) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/223.pdf 1952] | 0.60(2)&nbsp;s | α | ²¹⁹Ra | (5/2)+ | | |-id=Thorium-224 | ²²⁴Th | | style="text-align:right" | 90 | style="text-align:right" | 134 | 224.021466(10) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/224.pdf 1949] | 1.04(2)&nbsp;s | α<ref group="n">Theorized to also undergo β⁺β⁺ decay to ²²⁴Ra</ref> | ²²⁰Ra | 0+ | | |-id=Thorium-225 | rowspan=2|²²⁵Th | rowspan=2| | rowspan=2 style="text-align:right" | 90 | rowspan=2 style="text-align:right" | 135 | rowspan=2|225.0239510(55) | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/225.pdf 1949] | rowspan=2|8.75(4)&nbsp;min | α (~90%) | ²²¹Ra | rowspan=2|3/2+ | rowspan=2| | rowspan=2| |- | EC (~10%) | ²²⁵Ac |-id=Thorium-226 | rowspan=2|²²⁶Th | rowspan=2| | rowspan=2 style="text-align:right" | 90 | rowspan=2 style="text-align:right" | 136 | rowspan=2|226.0249037(48) | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/226.pdf 1948] | rowspan=2|30.70(3)&nbsp;min | α | ²²²Ra | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | CD (<{{val|3.2e-12}}%) | '''²⁰⁸Pb'''{{br}}'''¹⁸O''' |-id=Thorium-227 | ²²⁷Th | Radioactinium | style="text-align:right" | 90 | style="text-align:right" | 137 | 227.0277025(22) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/227.pdf 1906] | 18.693(4)&nbsp;d | α | ²²³Ra | (1/2+) | Trace<ref group="n" name="as">Intermediate decay product of ²³⁵U</ref> | |-id=Thorium-228 | rowspan=2|²²⁸Th | rowspan=2|Radiothorium | rowspan=2 style="text-align:right" | 90 | rowspan=2 style="text-align:right" | 138 | rowspan=2|228.0287397(19) | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/228.pdf 1905] | rowspan=2|1.9125(7)&nbsp;y | α | ²²⁴Ra | rowspan=2|0+ | rowspan=2|Trace<ref group="n">Intermediate decay product of ²³²Th</ref> | rowspan=2| |- | CD (1.13×10⁻¹¹%) | '''²⁰⁸Pb'''{{br}}²⁰O |-id=Thorium-229 | ²²⁹Th | | style="text-align:right" | 90 | style="text-align:right" | 139 | 229.0317614(26) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/229.pdf 1947] | 7916(17)&nbsp;y | α | ²²⁵Ra | 5/2+ | Trace<ref group=n>Intermediate decay product of ²³⁷Np</ref> | |-id=Thorium-229m | style="text-indent:1em" | ^229mTh | | colspan="3" style="text-indent:2em" | 8.355733554021(8)&nbsp;eV<ref name="Zhang">{{cite journal |last1=Zhang|first1=Ch. |last2=von der Wense|first2=L. |last3=Doyle |first3=J.F. |display-authors=et al. |title=²²⁹ThF₄ thin films for solid-state nuclear clocks |journal=Nature |volume=636 |year=2024 |number=603 |doi=10.1038/s41586-024-08256-5}}</ref> | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/90/229Th-1.pdf 1990] | 7(1)&nbsp;μs<ref name="Seiferle">{{cite journal |last1=Seiferle |first1=B |last2=v.d.Wense |first2=L. |last3=Thirolf |first3=P.G. |title=Lifetime Measurement of the ²²⁹Th Nuclear Isomer |journal=Phys. Rev. Lett. |volume=118 |year=2017 |number=042501 |doi=10.1103/PhysRevLett.118.042501|arxiv=1801.05205 }}</ref> | IT<ref group="n" name=229m>Neutral ^229mTh decays rapidly by internal conversion, ejecting an electron. There is not enough energy to eject a second electron, so ^229mTh⁺ ions live much longer, decaying by gamma emission. See {{alink|Thorium-229m}}.</ref> | ²²⁹Th⁺ | 3/2+ | | |-id=Thorium-229m | style="text-indent:1em" | ^229mTh⁺ | | colspan="3" style="text-indent:2em" | 8.355733554021(8)&nbsp;eV<ref name=Zhang /> | style="text-align:center" | (1990)<ref group="n">Same isomeric state as above</ref> | 29(1)&nbsp;min<ref name="Elwell">{{cite journal |last1=Elwell |first1=R |last2=Schneider |first2=Ch. |last3=Jeet |first3=J |display-authors=et al. |title=Laser Excitation of the ²²⁹Th Nuclear Isomeric Transition in a Solid-State Host |journal=Phys. Rev. Lett. |volume=133 |year=2024 |number=013201 |doi=10.1103/PhysRevLett.133.013201}}</ref> | γ<ref group="n" name=229m/> | ²²⁹Th⁺ | 3/2+ | | |-id=Thorium-230 | rowspan=3|²³⁰Th<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 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/89/230.pdf 1973] | rowspan=3|7.54(3)×10⁴&nbsp;y | α | ²²⁶Ra | rowspan=3|0+ | rowspan=3|0.0002(2)<ref group="n" name="us">Intermediate decay product of ²³⁸U</ref> | rowspan=3| |- | CD (5.8×10⁻¹¹%) | ²⁰⁶Hg{{br}}²⁴Ne |- | SF (<4×10⁻¹²%) | (various) |-id=Thorium-231 | ²³¹Th | Uranium Y | style="text-align:right" | 90 | style="text-align:right" | 141 | 231.0363028(13) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/231.pdf 1911] | 25.52(1)&nbsp;h | β⁻ | ²³¹Pa | 5/2+ | Trace<ref group="n" name="as" /> | |-id=Thorium-232 | rowspan=3|²³²Th<ref group="n">Primordial radionuclide</ref> | rowspan=3|Thorium | rowspan=3 style="text-align:right" | 90 | rowspan=3 style="text-align:right" | 142 | rowspan=3|232.0380536(15) | rowspan=3 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/232.pdf 1898] | rowspan=3|'''1.40(1)×10¹⁰&nbsp;y''' | α<ref group="n">Theorized to also undergo β⁻β⁻ decay to ²³²U</ref> | ²²⁸Ra | rowspan=3|0+ | rowspan=3|0.9998(2) | rowspan=3| |- | SF (1.1 × 10⁻⁹%) | (various) |- | CD (<2.78×10⁻¹⁰%) | ^208,206Hg{{br}}^24,26Ne |-id=Thorium-233 | ²³³Th | | style="text-align:right" | 90 | style="text-align:right" | 143 | 233.0415801(15) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/233.pdf 1935] | 21.83(4)&nbsp;min | β⁻ | ²³³Pa | 1/2+ | Trace<ref group="n">Produced in neutron capture by ²³²Th</ref> | |-id=Thorium-234 | ²³⁴Th | Uranium X₁ | style="text-align:right" | 90 | style="text-align:right" | 144 | 234.0435998(28) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/234.pdf 1900] | 24.107(24)&nbsp;d | β⁻ | ^234mPa<ref>ENSDF analysis available at {{NNDC}}</ref> | 0+ | Trace<ref group="n" name="us" /> | |-id=Thorium-235 | ²³⁵Th | | style="text-align:right" | 90 | style="text-align:right" | 145 | 235.047255(14) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/235.pdf 1969] | 7.2(1)&nbsp;min | β⁻ | ²³⁵Pa | 1/2+# | | |-id=Thorium-236 | ²³⁶Th | | style="text-align:right" | 90 | style="text-align:right" | 146 | 236.049657(15) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/236.pdf 1973] | 37.3(15)&nbsp;min | β⁻ | ²³⁶Pa | 0+ | | |-id=Thorium-237 | ²³⁷Th | | style="text-align:right" | 90 | style="text-align:right" | 147 | 237.053629(17) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/237.pdf 1993] | 4.8(5)&nbsp;min | β⁻ | ²³⁷Pa | 5/2+# | | |-id=Thorium-238 | ²³⁸Th | | style="text-align:right" | 90 | style="text-align:right" | 148 | 238.05639(30)# | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/90/238.pdf 1999] | 9.4(20)&nbsp;min | β⁻ | ²³⁸Pa | 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 mantles of gas and vapor lamps such as 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/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}}</ref>

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

== Notable isotopes ==

=== Thorium-228 === '''²²⁸Th''' 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 1.9125 years. It undergoes alpha decay to ²²⁴Ra. Occasionally it decays by the unusual route of cluster decay, emitting a nucleus of ²⁰O and producing stable ²⁰⁸Pb. It is a daughter isotope of ²³²U and responsible for its radiological hazard.

Together with its decay product ²²⁴Ra 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 === '''²²⁹Th''' is a radioactive isotope of thorium that decays by alpha emission with a half-life of 7916 years. ²²⁹Th is produced by the decay of uranium-233, and its principal use is for the production of the 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 ==== ²²⁹Th 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 ^229mTh very much depends on the electronic environment of the nucleus. In neutral ²²⁹Th, 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 ²²⁹Th^m 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 ²²⁹Th 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⁺ 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 ²²⁹Th | 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 ^229mTh | 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 ^229mTh nuclear isomeric transition and the ⁸⁷Sr 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 |volume=633 |issue=8028 |pages=63–70 |date=4 September 2024 |doi=10.1038/s41586-024-07839-6 |pmid=39232152 |arxiv=2406.18719 |bibcode=2024Natur.633...63Z |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''_Th = {{sfrac|1|6}} (''𝜈''_a + 2''𝜈''_b + 2''𝜈''_c + ''𝜈''_d) = {{val|2020407384335|(2)|u=kHz}}}}. }}</ref>{{r|Thirolf2024|Tiedau2024|Elwell2024}} Although in the very high frequency vacuum ultraviolet frequency range, it is possible to build a 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 ^229mTh(3/2⁺, 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 ²²⁹Th-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 ^229mTh 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 ²²⁹Th | 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 ²²⁹Th (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 ²²⁹Th, and measuring the difference in emitted gamma ray energies as it decays to either the ^229mTh (90%) or ²²⁹Th (10%) isomeric states. In 1976, Kroger and Reich sought to understand coriolis force effects in 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 ²²⁹Th actually arose from a spin-{{sfrac|3|2}} nuclear isomer, ^229mTh, 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 ²²⁹Th as observed in the α-decay of ²³³U | 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&nbsp;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&nbsp;eV,<ref name="Helmer1990">{{cite journal | title = Energy separation of the doublet of intrinsic states at the ground state of ²²⁹Th | 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 ²²⁹Th 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 ²²⁹Th |journal=Physical Review Letters |volume=98 |issue=14 |article-number=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 ²²⁹Th | 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 }}</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 ^229mTh 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 |article-number=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 ²²⁹Th with undulator radiation |journal=New Journal of Physics |language=en |volume=17 |issue=5 |article-number=053053 |date=May 2015 |doi=10.1088/1367-2630/17/5/053053 |bibcode=2015NJPh...17e3053Y |doi-access=free }}</ref><ref>{{cite thesis |last=von der Wense |first=Lars |title=On the direct detection of ^229mTh |date=2016 |degree=PhD |publisher=Ludwig-Maximilians-Universität München |url=https://edoc.ub.uni-muenchen.de/20492/7/Wense_Lars_von_der.pdf |isbn=978-3-319-70461-6}}</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 |article-number=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 ^229mTh 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 ²²⁹Th 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 ^229mTh 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 ^229mTh 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 MgF₂ 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 ^229mTh was achieved in 2016.<ref name=":0">{{cite journal | journal=Nature | volume=533 | issue=7601 | pages=47–51 | date=5 May 2016 | title = Direct detection of the ²²⁹Th 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 ²²⁹Th nuclear clock transition | 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, ²²⁹Th was embedded in SiO₂, 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 ^229mTh.<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 ^229mTh | 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 ²²⁹Th atomic shell was conducted using a ²²⁹Th²⁺ 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 ^229mTh 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 ^229mTh 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 ²²⁹Th nuclear clock isomer | 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 ²²⁹Th 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 |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 Th⁴⁺ cations doped into ionic crystals (of CaF₂ and LiSrAlF₆ with additional interstitial F⁻ 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 |volume=17 |article-number=71 |page=<!--Fuck off Citation Bot--> |doi=10.1103/Physics.17.71 |bibcode=2024PhyOJ..17...71T |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 |pmid=38759160 |bibcode=2024PhRvL.132r2501T |url=https://www.tuwien.at/fileadmin/Assets/tu-wien/News/2024/Thorium_Preprint.pdf |quote=The nuclear resonance for the Th⁴⁺ ions in Th:CaF₂ 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. |archive-date=29 April 2024 |access-date=29 April 2024 |archive-url=https://web.archive.org/web/20240429092810/https://www.tuwien.at/fileadmin/Assets/tu-wien/News/2024/Thorium_Preprint.pdf |url-status=dead }}</ref><ref name=Elwell2024>{{cite arXiv |title=Laser excitation of the ²²⁹Th 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=_stat(20)_sys|u=nm}} ({{val|2020407.3|(5)|errend=_stat(30)_sys|u=GHz}}) that decays with a lifetime of {{val|568|(13)|errend=_stat(20)_sys|u=s}}. This feature is assigned to the excitation of the ²²⁹Th nuclear isomeric state, whose energy is found to be {{val|8.355733|(2)|errend=_stat(10)_sys|u=eV}} in ²²⁹Th:LiSrAlF₆. }}</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 clocks.{{r|Zhang2024|Campbell2012|Thirolf2020}}

=== Thorium-230 === '''²³⁰Th''' is a radioactive isotope of thorium that can be used to date corals (uranium-thorium dating) and determine ocean current flux. '''Ionium''' (symbol '''Io''') was the name given early in the study of radioactive elements to the ²³⁰Th isotope produced in the decay chain of ²³⁸U before the nature of isotopes was fully realized. The name is still used in ionium–thorium dating, another dating method using this isotope.

=== Thorium-231 === '''²³¹Th''' 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 25.52 hours.<ref>{{cite journal|first1=G. B.|last1=Knight|first2=R. L.|last2=Macklin|title=Radiations of Uranium Y|url=https://archive.org/details/sim_physical-review_1949-01-01_75_1/page/34|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, with a decay energy of 0.39&nbsp;MeV.

=== Thorium-232 === {{main|Thorium-232}} '''²³²Th''' 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 products 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 |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 1.40{{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 a week.

²³²Th is a fertile material able to absorb a neutron and undergo 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 }}</ref> In the form of Thorotrast, a thorium dioxide suspension, it was used as a contrast medium in early X-ray diagnostics. Thorium-232 is now classified as carcinogenic.<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 === '''²³³Th''' is an isotope of thorium that decays into protactinium-233 through beta decay, then into uranium-233 to join the neptunium series decay chain. It has a half-life of 21.83 minutes. Traces occur in nature as the result of natural neutron activation of ²³²Th.<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 (4''n'' + 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=2019-04-29 |url-status=live }}</ref>

=== Thorium-234 === '''²³⁴Th''' is an isotope of thorium whose nuclei contain 144 neutrons. ²³⁴Th has a half-life of 24.11 days; it emits a beta particle, transmuting into protactinium-234 with a decay energy around 0.27&nbsp;MeV. Uranium-238 almost always produces isotope of thorium on decay (although in rare cases it undergoes spontaneous fission, and even more rarely double beta decay).

== 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 |archive-url=https://web.archive.org/web/20110720233206/http://amdc.in2p3.fr/nubase/Nubase2003.pdf |archive-date=2011-07-20 }} ** {{NNDC}} ** {{CRC85|chapter=11}}

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Category:Isotopes of thorium Category:Thorium Thorium