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Thorium (₉₀Th) has seven naturally occurring isotopes but none are stable. One isotope, Th, is relatively stable, with a half-life of 1.405×10 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 Th 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 Th, Th with a half-life of 75,380 years, Th with a half-life of 7,917 years, and 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, Th, has a nuclear isomer (or metastable state) with a remarkably low excitation energy, recently measured to be 8.355733554021(8) eV It has been proposed to perform laser spectroscopy of the Th nucleus and use the low-energy transition for the development of a nuclear clock of extremely high accuracy.

The known isotopes of thorium range in mass number from 207 to 238.

List of isotopes

1. Th – Excited nuclear isomer.
2. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
3. # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
4. Bold half-life – nearly stable, half-life longer than age of universe.
5. Modes of decay:

   CD:	Cluster decay
   EC:	Electron capture
   IT:	Isomeric transition

6. Bold symbol as daughter – Daughter product is stable.
7. ( ) spin value – Indicates spin with weak assignment arguments.
8. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
9. Theorized to also undergo ββ decay to Ra
10. ^ Intermediate decay product of U
11. Intermediate decay product of Th
12. Intermediate decay product of Np
13. ^ Neutral Th decays rapidly by internal conversion, ejecting an electron. There is not enough energy to eject a second electron, so Th ions live much longer, decaying by gamma emission. See § Thorium-229m.
14. Used in Uranium–thorium dating
15. ^ Intermediate decay product of U
16. Primordial radionuclide
17. Theorized to also undergo ββ decay to U
18. Produced in neutron capture by Th

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. 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.

Actinides vs. fission products

Notable isotopes

Thorium-228

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 Ra. Occasionally it decays by the unusual route of cluster decay, emitting a nucleus of O and producing stable Pb. It is a daughter isotope of U in the thorium decay series.

Th has an atomic weight of 228.0287411 grams/mole.

Together with its decay product Ra it is used for alpha particle radiation therapy.

Thorium-229

Th is a radioactive isotope of thorium that decays by alpha emission with a half-life of 7917 years. Th 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.

Thorium-229m

Th has a nuclear isomer,
Th
, with a remarkably low excitation energy of 8.355733554021(8) eV.

Due to this low energy, the lifetime of Th very much depends on the electronic environment of the nucleus. In neutral Th, the isomer decays by internal conversion within a few microseconds. However, the isomeric energy is not enough to remove a second electron (thorium's second ionization energy is 11.5 eV), so internal conversion is impossible in Th ions. Radiative decay occurs with a half-life 8.4 orders of magnitude longer, in excess of 1000 seconds. Embedded in ionic crystals, ionization is not quite 100%, so a small amount of internal conversion occurs, leading to a recently measured lifetime of ≈600 s, which can be extrapolated to a lifetime for isolated ions of 1740±50 s.

This excitation energy corresponds to a photon frequency of 2020407384335±2 kHz (wavelength 148.3821828827(15) nm). 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, which could have applications like a nuclear clock of very high accuracy or as a qubit for quantum computing.

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 Th (such as the lifetime and the magnetic moment) before the frequency was accurately measured in 2024.

History

Early measurements were performed via gamma ray spectroscopy, producing the 29.5855 keV excited state of Th, and measuring the difference in emitted gamma ray energies as it decays to either the Th (90%) or Th (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 Th actually arose from a spin-⁠3/2⁠ nuclear isomer, Th, with a remarkably low excitation energy.

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, making it one of the lowest known isomeric excitation energies. In the following years, the energy was further constrained to 3.5±1.0 eV, which was for a long time the accepted energy value.

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 7.6±0.5 eV, corrected to 7.8±0.5 eV in 2009. This higher energy has two consequences which had not been considered by earlier attempts to observe emitted photons:

• Because it is above thorium's 6.08 eV first ionization energy, neutral Th will decay radiatively with an extremely low likelihood, and
• Because it is above the 6.2 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, pointing towards a potentially strong non-radiative decay channel. A direct detection of photons emitted in the isomeric decay was claimed in 2012 and again in 2018. However, both reports were subject to controversial discussions within the community.

A direct detection of electrons being emitted in the internal conversion decay channel of Th was achieved in 2016. However, at the time the isomer's transition energy could only be weakly constrained to between 6.3 and 18.3 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 8.28±0.17 eV. However, this value appeared at odds with the 2018 preprint showing that a similar signal as an 8.4 eV xenon VUV photon can be shown, but with about 1.3+0.2
−0.1 eV less energy and a (retrospectively correct) 1880±170 s lifetime. In that paper, Th 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 Th. In this experiment, laser spectroscopy of the Th atomic shell was conducted using a Th 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 Th could be inferred.

In 2019, the isomer's excitation energy was constrained to 8.28±0.17 eV based on the direct detection of internal conversion electrons and a secure population of Th from the nuclear ground state was achieved by excitation of the 29 keV nuclear excited state via synchrotron radiation. Additional measurements by a different group in 2020 produced a figure of 8.10±0.17 eV (153.1±3.2 nm wavelength). Combining these measurements, the expected transition energy is 8.12±0.11 eV.

In September 2022, spectroscopy on decaying samples determined the excitation energy to be 8.338±0.024 eV.

In April 2024, two separate groups finally reported precision laser excitation Th 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. A one-part-per-trillion (10) measurement soon followed in June 2024, and future high-precision lasers will measure the frequency up to the 10 accuracy of the best atomic clocks.

Thorium-230

Th is a radioactive isotope of thorium that can be used to date corals and determine ocean current flux. Ionium was a name given early in the study of radioactive elements to the Th isotope produced in the decay chain of 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

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. When it decays, it emits a beta ray and forms protactinium-231. It has a decay energy of 0.39 MeV. It has a mass of 231.0363043 u.

Thorium-232

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 products of uranium and thorium. The isotope decays by alpha decay with a half-life of 1.405×10 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.

Th 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. 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.

Thorium-233

Th is an isotope of thorium that decays into protactinium-233 through beta decay. It has a half-life of 21.83 minutes. Traces occur in nature as the result of natural neutron activation of Th.

Thorium-234

Th is an isotope of thorium whose nuclei contain 144 neutrons. Th has a half-life of 24.1 days, and when it decays, it emits a beta particle, and in doing so, it transmutes into protactinium-234. Th has a mass of 234.0436 atomic mass units, and it has a decay energy of about 270 keV. Uranium-238 usually decays into this isotope of thorium (although in rare cases it can undergo spontaneous fission instead).

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• Isotope masses from:
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• "News & Notices: Standard Atomic Weights Revised". International Union of Pure and Applied Chemistry. 19 October 2005.
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  • G. Audi; A. H. Wapstra; C. Thibault; J. Blachot; O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A. 729 (1): 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001. Archived from the original (PDF) on 2011-07-20.
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v t e Isotopes of the chemical elements
Group 1 2   3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Period Hydrogen and alkali metals Alkaline earth metals Pnicto­gens Chal­co­gens Halo­gens Noble gases ① Isotopes § ListH1 Isotopes § ListHe2 ② Isotopes § ListLi3 Isotopes § ListBe4 Isotopes § ListB5 Isotopes § ListC6 Isotopes § ListN7 Isotopes § ListO8 Isotopes § ListF9 Isotopes § ListNe10 ③ Isotopes § ListNa11 Isotopes § ListMg12 Isotopes § ListAl13 Isotopes § ListSi14 Isotopes § ListP15 Isotopes § ListS16 Isotopes § ListCl17 Isotopes § ListAr18 ④ Isotopes § ListK19 Isotopes § ListCa20 Isotopes § ListSc21 Isotopes § ListTi22 Isotopes § ListV23 Isotopes § ListCr24 Isotopes § ListMn25 Isotopes § ListFe26 Isotopes § ListCo27 Isotopes § ListNi28 Isotopes § ListCu29 Isotopes § ListZn30 Isotopes § ListGa31 Isotopes § ListGe32 Isotopes § ListAs33 Isotopes § ListSe34 Isotopes § ListBr35 Isotopes § ListKr36 ⑤ Isotopes § ListRb37 Isotopes § ListSr38 Isotopes § ListY39 Isotopes § ListZr40 Isotopes § ListNb41 Isotopes § ListMo42 Isotopes § ListTc43 Isotopes § ListRu44 Isotopes § ListRh45 Isotopes § ListPd46 Isotopes § ListAg47 Isotopes § ListCd48 Isotopes § ListIn49 Isotopes § ListSn50 Isotopes § ListSb51 Isotopes § ListTe52 Isotopes § ListI53 Isotopes § ListXe54 ⑥ Isotopes § ListCs55 Isotopes § ListBa56 Isotopes § ListLu71 Isotopes § ListHf72 Isotopes § ListTa73 Isotopes § ListW74 Isotopes § ListRe75 Isotopes § ListOs76 Isotopes § ListIr77 Isotopes § ListPt78 Isotopes § ListAu79 Isotopes § ListHg80 Isotopes § ListTl81 Isotopes § ListPb82 Isotopes § ListBi83 Isotopes § ListPo84 Isotopes § ListAt85 Isotopes § ListRn86 ⑦ Isotopes § ListFr87 Isotopes § ListRa88 Isotopes § ListLr103 Isotopes § ListRf104 Isotopes § ListDb105 Isotopes § ListSg106 Isotopes § ListBh107 Isotopes § ListHs108 Isotopes § ListMt109 Isotopes § ListDs110 Isotopes § ListRg111 Isotopes § ListCn112 Isotopes § ListNh113 Isotopes § ListFl114 Isotopes § ListMc115 Isotopes § ListLv116 Isotopes § ListTs117 Isotopes § ListOg118 ⑧ Isotopes § ListUue119 Isotopes § ListUbn120 Isotopes § ListLa57 Isotopes § ListCe58 Isotopes § ListPr59 Isotopes § ListNd60 Isotopes § ListPm61 Isotopes § ListSm62 Isotopes § ListEu63 Isotopes § ListGd64 Isotopes § ListTb65 Isotopes § ListDy66 Isotopes § ListHo67 Isotopes § ListEr68 Isotopes § ListTm69 Isotopes § ListYb70   Isotopes § ListAc89 Isotopes § ListTh90 Isotopes § ListPa91 Isotopes § ListU92 Isotopes § ListNp93 Isotopes § ListPu94 Isotopes § ListAm95 Isotopes § ListCm96 Isotopes § ListBk97 Isotopes § ListCf98 Isotopes § ListEs99 Isotopes § ListFm100 Isotopes § ListMd101 Isotopes § ListNo102
Table of nuclides Categories: Isotopes Tables of nuclides Metastable isotopes Isotopes by element

• Isotopes of thorium
• Thorium
• Lists of isotopes by element