Isotopes of thorium

Main isotopes of Chemistry:thorium (90Th) | Iso­tope | Decay

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| abun­dance | half-life (t1/2) | mode | pro­duct
227Th  | trace | 18.68 d  | α | 223Ra
228Th  | trace  | 1.9116 y  | α  | 224Ra
229Th  | trace  | 7917 y[1] | α  | 225Ra
230Th  | 0.02%  | 75400 y  | α  | 226Ra
231Th  | trace  | 25.5 h  | β− | 231Pa
232Th  | 99.98%  | 1.405×1010 y  | α  | 228Ra
234Th  | trace  | 24.1 d  | β− | 234Pa
Standard atomic weight Ar, standard(Th)|

* 232.0377(4)[2]

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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,[3] recently measured to be 8.355733554021(8) eV[4][5] 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.[6][7][8]

The known isotopes of thorium range in mass number from 207[9] to 238.

## Contents

* 1 List of isotopes
* 2 Uses
* 2.1 Low dispersion lenses
* 3 Actinides vs. fission products
* 4 Notable isotopes
* 4.1 Thorium-228
* 4.2 Thorium-229
* 4.2.1 Thorium-229m
* 4.2.2 History
* 4.3 Thorium-230
* 4.4 Thorium-231
* 4.5 Thorium-232
* 4.6 Thorium-233
* 4.7 Thorium-234
* 5 References

## List of isotopes

Nuclide
[n 1] | Historic
name  | Z | N | Isotopic mass (u)
[n 2][n 3] | Half-life
[n 4] | Decay
mode
[n 5] | Daughter
isotope
[n 6] | Spin and
parity
[n 7][n 8] | Physics:Natural abundance (mole fraction)

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Excitation energy  | Normal proportion  | Range of variation
207Th[9] |  | 90  | 117  |  | 2022 | 9.7+46.6
−4.4 ms | α | 203Ra  |  |  |
208Th  |  | 90  | 118  | 208.017915(34)  | 2010 | 2.4(12) ms  | α  | 204Ra  | 0+  |  |
209Th  |  | 90  | 119  | 209.017998(27)  | 1996 | 3.1(12) ms  | α  | 205Ra  | 13/2+  |  |
210Th  |  | 90  | 120  | 210.015094(20)  | 1995 | 16.0(36) ms  | α  | 206Ra  | 0+  |  |
211Th  |  | 90  | 121  | 211.014897(92)  | 1995 | 48(20) ms  | α  | 207Ra  | 5/2−#  |  |
212Th  |  | 90  | 122  | 212.013002(11)  | 1980 | 31.7(13) ms  | α  | 208Ra  | 0+  |  |
213Th  |  | 90  | 123  | 213.0130115(99)  | 1968 | 144(21) ms  | α  | 209Ra  | 5/2−  |  |
213mTh  |  | 1180.0(14) keV  | 2007 | 1.4(4) μs  | IT  | 213Th  | (13/2)+  |  |
214Th  |  | 90  | 124  | 214.011481(11)  | 1968 | 87(10) ms  | α  | 210Ra  | 0+  |  |
214mTh  |  | 2181.0(27) keV  | 2007 | 1.24(12) μs  | IT  | 214Th  | 8+#  |  |
215Th  |  | 90  | 125  | 215.0117246(68)  | 1968 | 1.35(14) s  | α  | 211Ra  | (1/2−)  |  |
215mTh  |  | 1471(50)# keV  | 2005 | 770(60) ns  | IT  | 215Th  | 9/2+#  |  |
216Th  |  | 90  | 126  | 216.011056(12)  | 1968 | 26.28(16) ms  | α  | 212Ra  | 0+  |  |
216m1Th  |  | 2041(8) keV  | 1983 | 135.4(29) μs  | IT (97.2%)  | 216Th  | 8+  |  |
α (2.8%)  | 212Ra
216m2Th  |  | 2648(8) keV  | 2001 | 580(26) ns  | IT  | 216Th  | (11−)  |  |
216m3Th  |  | 3682(8) keV  | 2001 | 740(70) ns  | IT  | 216Th  | (14+)  |  |
217Th  |  | 90  | 127  | 217.013103(11)  | 1968 | 248(4) μs  | α  | 213Ra  | 9/2+#  |  |
217m1Th  |  | 673.3(1) keV  | 1989 | 141(50) ns  | IT  | 217Th  | (15/2−)  |  |
217m2Th  |  | 2307(32) keV  | 2005 | 71(14) μs  | IT  | 217Th  | (25/2+)  |  |
218Th  |  | 90  | 128  | 218.013276(11)  | 1973 | 122(5) ns  | α  | 214Ra  | 0+  |  |
219Th  |  | 90  | 129  | 219.015526(61)  | 1973 | 1.023(18) μs  | α  | 215Ra  | 9/2+#  |  |
220Th  |  | 90  | 130  | 220.015770(15)  | 1973 | 10.2(3) μs  | α  | 216Ra  | 0+  |  |
221Th  |  | 90  | 131  | 221.0181858(86)  | 1970 | 1.75(2) ms  | α  | 217Ra  | 7/2+#  |  |
222Th  |  | 90  | 132  | 222.018468(11)  | 1970 | 2.24(3) ms  | α  | 218Ra  | 0+  |  |
223Th  |  | 90  | 133  | 223.0208111(85)  | 1952 | 0.60(2) s  | α  | 219Ra  | (5/2)+  |  |
224Th  |  | 90  | 134  | 224.021466(10)  | 1949 | 1.04(2) s  | α[n 9] | 220Ra  | 0+  |  |
225Th  |  | 90  | 135  | 225.0239510(55)  | 1949 | 8.75(4) min  | α (~90%)  | 221Ra  | 3/2+  |  |
EC (~10%)  | 225Ac
226Th  |  | 90  | 136  | 226.0249037(48)  | 1948 | 30.70(3) min  | α  | 222Ra  | 0+  |  |
CD (<3.2×10−12%)  | 208Pb
18O
227Th  | Radioactinium  | 90  | 137  | 227.0277025(22)  | 1906 | 18.693(4) d  | α  | 223Ra  | (1/2+)  | Trace[n 10] |
228Th  | Radiothorium  | 90  | 138  | 228.0287397(19)  | 1905 | 1.9125(7) y  | α  | 224Ra  | 0+  | Trace[n 11] |
CD (1.13×10−11%)  | 208Pb
20O
229Th  |  | 90  | 139  | 229.0317614(26)  | 1947 | 7916(17) y  | α  | 225Ra  | 5/2+  | Trace[n 12] |
229mTh  |  | 8.355733554021(8) eV[10] | 1990 | 7(1) μs[11] | IT[n 13] | 229Th+ | 3/2+  |  |
229mTh+ |  | 8.355733554021(8) eV[10] | (1990)[n 14] | 29(1) min[12] | γ[n 13] | 229Th+ | 3/2+  |  |
230Th[n 15] | Ionium  | 90  | 140  | 230.0331323(13)  | 1907 | 7.54(3)×104 y  | α  | 226Ra  | 0+  | 0.0002(2)[n 16] |
CD (5.8×10−11%)  | 206Hg
24Ne
SF (<4×10−12%)  | (various)
231Th  | Uranium Y  | 90  | 141  | 231.0363028(13)  | 1911 | 25.52(1) h  | β− | 231Pa  | 5/2+  | Trace[n 10] |
232Th[n 17] | Thorium  | 90  | 142  | 232.0380536(15)  | 1898 | 1.40(1)×1010 y | α[n 18] | 228Ra  | 0+  | 0.9998(2)  |
SF (1.1 × 10−9%)  | (various)
CD (<2.78×10−10%)  | 208,206Hg
24,26Ne
233Th  |  | 90  | 143  | 233.0415801(15)  | 1935 | 21.83(4) min  | β− | 233Pa  | 1/2+  | Trace[n 19] |
234Th  | Uranium X1 | 90  | 144  | 234.0435998(28)  | 1900 | 24.107(24) d  | β− | 234mPa[13] | 0+  | Trace[n 16] |
235Th  |  | 90  | 145  | 235.047255(14)  | 1969 | 7.2(1) min  | β− | 235Pa  | 1/2+#  |  |
236Th  |  | 90  | 146  | 236.049657(15)  | 1973 | 37.3(15) min  | β− | 236Pa  | 0+  |  |
237Th  |  | 90  | 147  | 237.053629(17)  | 1993 | 4.8(5) min  | β− | 237Pa  | 5/2+#  |  |
238Th  |  | 90  | 148  | 238.05639(30)#  | 1999 | 9.4(20) min  | β− | 238Pa  | 0+  |  |

1. ↑ mTh - 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 β+β+ decay to 224Ra
10. ↑ 10.0 10.1 Intermediate decay product of 235U
11. ↑ Intermediate decay product of 232Th
12. ↑ Intermediate decay product of 237Np
13. ↑ 13.0 13.1 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 Template:Alink.
14. ↑ Same isomeric state as above
15. ↑ Used in uranium–thorium dating
16. ↑ 16.0 16.1 Intermediate decay product of 238U
17. ↑ Primordial radionuclide
18. ↑ Theorized to also undergo β−β− decay to 232U
19. ↑ Produced in neutron capture by 232Th

## 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.[14] 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.[15]

## Actinides vs. fission products

Actinides and fission products by half-life v * d * e

Actinides[16] by decay chain | Half-life
range (y)  | Fission products of 235U by yield<ref>Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.</ref>
4n | 4n+1 | 4n+2 | 4n+3
| 4.5–7%  | 0.04–1.25%  | <0.001%
228Ra№ |  |  |  | 4–6  | †  |  | 155Euþ |
244Cmƒ | 241Puƒ | 250Cf  | 227Ac№ | 10–29  | 90Sr  | 85Kr  | 113mCdþ
232Uƒ |  | 238Puƒ | 243Cmƒ | 29–97  | 137Cs  | 151Smþ | 121mSn
248Bk[17] | 249Cfƒ | 242mAmƒ |  | 141–351  |

No fission products
have a half-life
in the range of
100–210 k years ...

| 241Amƒ |  | 251Cfƒ[18] | 430–900
|  | 226Ra№ | 247Bk  | 1.3 k – 1.6 k
240Pu  | 229Th  | 246Cmƒ | 243Amƒ | 4.7 k – 7.4 k
| 245Cmƒ | 250Cm  |  | 8.3 k – 8.5 k
|  |  | 239Puƒ | 24.1 k
|  | 230Th№ | 231Pa№ | 32 k – 76 k
236Npƒ | 233Uƒ | 234U№ |  | 150 k – 250 k  | ‡  | 99Tc₡ | 126Sn
248Cm  |  | 242Pu  |  | 327 k – 375 k  |  | 79Se₡
|  |  |  | 1.53 M  | 93Zr
| 237Npƒ |  |  | 2.1 M – 6.5 M  | 135Cs₡ | 107Pd
236U  |  |  | 247Cmƒ | 15 M – 24 M  |  | 129I₡
244Pu  |  |  |  | 80 M  |

... nor beyond 15.7 M years[19]

232Th№ |  | 238U№ | 235Uƒ№ | 0.7 G – 14.1 G

Legend for superscript symbols
₡ has thermal neutron capture cross section in the range of 8–50 barns
ƒ fissile
m metastable isomer
№ primarily a naturally occurring radioactive material (NORM)
þ neutron poison (thermal neutron capture cross section greater than 3k barns)
† range 4–97 y: Medium-lived fission product
‡ over 200,000 y: Long-lived fission product

## 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.[20]

### 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.[21]

#### Thorium-229m

229Th has a nuclear isomer, The element Chemistry:Th does not exist., with a remarkably low excitation energy of 8.355733554021(8) eV.[5] This gives it several remarkable properties.

Because this energy is on the same order of magnitude as electron ionization energy (for reference, the ionization energy of hydrogen is 13.6 eV), the decay rate of 229mTh is sensitive to the electronic environment of the nucleus. In neutral 229Th, the isomer decays by internal conversion within a few microseconds.[22][23][24] 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.[23][25] 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,[4][26] which can be extrapolated to a lifetime for isolated ions of 1740±50 s.[4]

Normal nuclear excitation energy corresponds to gamma-ray photons, but this excitation energy corresponds to a photon frequency of 2020407384335±2 kHz (wavelength 148.3821828827(15) nm),[5][27][4][26] which is just a far ultraviolet. This means it is possible to build a laser operating at this frequency, giving the only known opportunity for direct laser excitation of a nuclear state.[28] This could have applications like a nuclear clock of very high accuracy[7][8][29][30] or as a qubit for quantum computing.[31]

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), until the frequency was accurately measured in 2024.[4][27][26]

#### History

Early measurements of thorium isomers were performed via gamma ray spectroscopy, producing the 29.5855 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-⁠3/2⁠ nuclear isomer, 229mTh, with a remarkably low excitation energy.[32]

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,[33] 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.[34]

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,[35] corrected to 7.8±0.5 eV in 2009.[36] 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 229mTh 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,[37][38][39][40] pointing towards a potentially strong non-radiative decay channel. A direct detection of photons emitted in the isomeric decay was claimed in 2012[41] and again in 2018.[42] However, both reports were controversial within the community.[43][44]

A direct detection of electrons being emitted in the internal conversion decay channel of 229mTh was achieved in 2016.[45] 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.[46] 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.[42] 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.[47] 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 8.28±0.17 eV based on the direct detection of internal conversion electrons[46] and a secure population of 229mTh from the nuclear ground state was achieved by excitation of the 29 keV nuclear excited state via synchrotron radiation.[48] Additional measurements by a different group in 2020 produced a figure of 8.10±0.17 eV (153.1±3.2 nm wavelength).[49] Combining these measurements, the expected transition energy is 8.12±0.11 eV.[50]

In September 2022, spectroscopy on decaying samples determined the excitation energy to be 8.338±0.024 eV.[51]

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.[27][6][4][26] A one-part-per-trillion (10−12) measurement soon followed in June 2024,[5][52] and future high-precision lasers will measure the frequency up to the 10−18 accuracy of the best atomic clocks.[5][8][30]

### 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.[53] When it decays, it emits a beta ray and forms protactinium-231, with a decay energy of 0.39 MeV.

### Thorium-232

Main article: Physics: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.[54] The isotope decays by alpha decay with a half-life of 1.40×1010 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.[55] 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.[56]

### 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.[57]

### 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 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

1. ↑ Varga, Z.; Nicholl, A.; Mayer, K. (2014). "Determination of the 229Th half-life". Physical Review C 89 (6): 064310. doi:10.1103/PhysRevC.89.064310.
2. ↑ Meija, Juris; Coplen, Tyler B.; Berglund, Michael; Brand, Willi A.; De Bièvre, Paul; Gröning, Manfred; Holden, Norman E.; Irrgeher, Johanna et al. (2016). "Atomic weights of the elements 2013 (IUPAC Technical Report)". Pure and Applied Chemistry 88 (3): 265–91. doi:10.1515/pac-2015-0305.
3. ↑ E. Ruchowska (2006). "Nuclear structure of 229Th". Physical Review C 73 (4). doi:10.1103/PhysRevC.73.044326. Bibcode: 2006PhRvC..73d4326R. https://cds.cern.ch/record/974608/files/PhysRevC.73.044326.pdf.
4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Tiedau, J.; Okhapkin, M. V.; Zhang, K.; Thielking, J.; Zitzer, G.; Peik, E. et al. (29 April 2024). "Laser Excitation of the Th-229 Nucleus". Physical Review Letters 132 (18). doi:10.1103/PhysRevLett.132.182501. PMID 38759160. Bibcode: 2024PhRvL.132r2501T. https://www.tuwien.at/fileadmin/Assets/tu-wien/News/2024/Thorium_Preprint.pdf. Retrieved 29 April 2024. "The nuclear resonance for the Th4+ ions in Th:CaF2 is measured at the wavelength 148.3821(5) nm, frequency 2020.409(7) THz, and the fluorescence lifetime in the crystal is 630(15) s, corresponding to an isomer half-life of 1740(50) s for a nucleus isolated in vacuum.".
5. ↑ 5.0 5.1 5.2 5.3 5.4 Zhang, Chuankun; Ooi, Tian; Higgins, Jacob S.; Doyle, Jack F.; von der Wense, Lars; Beeks, Kjeld; Leitner, Adrian; Kazakov, Georgy et al. (4 September 2024). "Frequency ratio of the 229mTh nuclear isomeric transition and the 87Sr atomic clock". Nature 633 (8028): 63–70. doi:10.1038/s41586-024-07839-6. PMID 39232152. Bibcode: 2024Natur.633...63Z. "The transition frequency between the I = 5/2 ground state and the I = 3/2 excited state is determined as: 𝜈Th = ⁠1/6⁠ (𝜈a \+ 2𝜈b \+ 2𝜈c \+ 𝜈d) = 2020407384335(2) kHz.".
6. ↑ 6.0 6.1 "Atomic Nucleus Excited with Laser: A Breakthrough after Decades" (Press release). TU Wien. 29 April 2024. Retrieved 29 April 2024.
7. ↑ 7.0 7.1 Peik, E.; Tamm, Chr. (2003-01-15). "Nuclear laser spectroscopy of the 3.5 eV transition in 229Th". Europhysics Letters 61 (2): 181–186. doi:10.1209/epl/i2003-00210-x. Bibcode: 2003EL.....61..181P. https://www.few.vu.nl/~wimu/Varying-Constants-Papers/Th-229-Peik-Tamm-EuLett-2003.pdf. Retrieved 2024-04-30.
8. ↑ 8.0 8.1 8.2 Campbell, C.J.; Radnaev, A.G.; Kuzmich, A.; Dzuba, V.A.; Flambaum, V.V.; Derevianko, A. (2012). "A single ion nuclear clock for metrology at the 19th decimal place". Physical Review Letters 108 (12). doi:10.1103/PhysRevLett.108.120802. PMID 22540568. Bibcode: 2012PhRvL.108l0802C. https://sites.lsa.umich.edu/kuzmich-lab/wp-content/uploads/sites/90/2014/05/229ThClock.pdf. Retrieved 2024-04-30.
9. ↑ 9.0 9.1 Yang, H. B. (2022). "New isotope 207Th and odd-even staggering in α-decay energies for nuclei with Z > 82 and N < 126". Physical Review C 105 (L051302). doi:10.1103/PhysRevC.105.L051302. Bibcode: 2022PhRvC.105e1302Y.
10. ↑ 10.0 10.1 Zhang, Ch.Expression error: Unrecognized word "et". (2024). "229ThF4 thin films for solid-state nuclear clocks". Nature 636 (603). doi:10.1038/s41586-024-08256-5.
11. ↑ Seiferle, B; v.d.Wense, L.; Thirolf, P.G. (2017). "Lifetime Measurement of the 229Th Nuclear Isomer". Phys. Rev. Lett. 118 (042501). doi:10.1103/PhysRevLett.118.042501.
12. ↑ Elwell, RExpression error: Unrecognized word "et". (2024). "Laser Excitation of the 229Th Nuclear Isomeric Transition in a Solid-State Host". Phys. Rev. Lett. 133 (013201). doi:10.1103/PhysRevLett.133.013201.
13. ↑ ENSDF analysis available at National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory. http://www.nndc.bnl.gov/nudat2/.
14. ↑ f2.5 Aero Ektar Lenses  Some images.
15. ↑ Michael S. Briggs (January 16, 2002). "Aero-Ektar Lenses". http://home.earthlink.net/~michaelbriggs/aeroektar/aeroektar.html.
16. ↑ Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
17. ↑ Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics 71 (2): 299. doi:10.1016/0029-5582(65)90719-4. Bibcode: 1965NucPh..71..299M.
"The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk248 with a half-life greater than 9 y. No growth of Cf248 was detected, and a lower limit for the β− half-life can be set at about 104 y. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 y."
18. ↑ This is the heaviest nuclide with a half-life of at least four years before the "Sea of Instability".
19. ↑ Excluding those "classically stable" nuclides with half-lives significantly in excess of 232Th; e.g., while 113mCd has a half-life of only fourteen years, that of 113Cd is nearly eight quadrillion years.
20. ↑ "Thor Medical – production of alpha emitters for cancer treatment". May 2023. https://www.scatecinnovation.no/artikler/thor-medical-production-of-alpha-emitters-for-cancer-treatment.
21. ↑ Report to Congress on the extraction of medical isotopes from U-233 . U.S. Department of Energy. March 2001
22. ↑ Karpeshin, F.F.; Trzhaskovskaya, M.B. (November 2007). "Impact of the electron environment on the lifetime of the 229Thm low-lying isomer". Physical Review C 76 (5). doi:10.1103/PhysRevC.76.054313. Bibcode: 2007PhRvC..76e4313K.
23. ↑ 23.0 23.1 Tkalya, Eugene V.; Schneider, Christian; Jeet, Justin; Hudson, Eric R. (25 November 2015). "Radiative lifetime and energy of the low-energy isomeric level in 229Th". Physical Review C 92 (5). doi:10.1103/PhysRevC.92.054324. Bibcode: 2015PhRvC..92e4324T.
24. ↑ Seiferle, B.; von der Wense, L.; Thirolf, P.G. (January 2017). "Lifetime measurement of the 229Th nuclear isomer". Physical Review Letters 118 (4). doi:10.1103/PhysRevLett.118.042501. PMID 28186791. Bibcode: 2017PhRvL.118d2501S. "A half-life of 7±1 μs has been measured".
25. ↑ Minkov, Nikolay; Pálffy, Adriana (23 May 2017). "Reduced transition probabilities for the gamma decay of the 7.8 eV isomer in 229mTh". Phys. Rev. Lett. 118 (21). doi:10.1103/PhysRevLett.118.212501. PMID 28598657. Bibcode: 2017PhRvL.118u2501M.
26. ↑ 26.0 26.1 26.2 26.3 Elwell, R.; Schneider, Christian; Jeet, Justin; Terhune, J. E. S.; Morgan, H. W. T.; Alexandrova, A. N.; Tran Tan, Hoang Bao; Derevianko, Andrei; Hudson, Eric R. (18 April 2024). "Laser excitation of the 229Th nuclear isomeric transition in a solid-state host". arXiv:2404.12311 [physics.atom-ph]. "a narrow, laser-linewidth-limited spectral feature at 148.38219(4)stat(20)sys nm (2020407.3(5)stat(30)sys GHz) that decays with a lifetime of 568(13)stat(20)sys s. This feature is assigned to the excitation of the 229Th nuclear isomeric state, whose energy is found to be 8.355733(2)stat(10)sys eV in 229Th:LiSrAlF6."
27. ↑ 27.0 27.1 27.2 Thirolf, Peter (April 29, 2024). "Shedding Light on the Thorium-229 Nuclear Clock Isomer". Physics 17. doi:10.1103/Physics.17.71. Bibcode: 2024PhyOJ..17...71T. https://physics.aps.org/articles/v17/71.
28. ↑ Tkalya, E.V.; Varlamov, V.O.; Lomonosov, V.V.; Nikulin, S.A. (1996). "Processes of the nuclear isomer 229mTh(3/2+, 3.5±1.0 eV) Resonant excitation by optical photons". Physica Scripta 53 (3): 296–299. doi:10.1088/0031-8949/53/3/003. Bibcode: 1996PhyS...53..296T.
29. ↑ von der Wense, Lars; Seiferle, Benedict; Thirolf, Peter G. (March 2018). "Towards a 229Th-based nuclear clock". Measurement Techniques 60 (12): 1178–1192. doi:10.1007/s11018-018-1337-1. Bibcode: 2018MeasT..60.1178V.
30. ↑ 30.0 30.1 Thirolf, Peter G. (March 2020). "'Phase Transition' in the 'Thorium-Isomer Story'". XXXVI Mazurian Lakes Conference on Physics (1–7 November 2019). 51. Piaski, Pisz County, Poland. 561–570. doi:10.5506/APhysPolB.51.561. https://www.actaphys.uj.edu.pl/R/51/3/561/pdf. Originally presented as Characterization of the elusive 229mTh isomer – milestones towards a nuclear clock.
31. ↑ Raeder, S.; Sonnenschein, V.; Gottwald, T.; Moore, I.D.; Reponen, M.; Rothe, S.; Trautmann, N.; Wendt, K. (July 2011). "Resonance ionization spectroscopy of thorium isotopes - towards a laser spectroscopic identification of the low-lying 7.6 eV isomer of 229Th". J. Phys. B: At. Mol. Opt. Phys. 44 (16). doi:10.1088/0953-4075/44/16/165005. Bibcode: 2011JPhB...44p5005R.
32. ↑ Kroger, L.A.; Reich, C.W. (1976). "Features of the low energy level scheme of 229Th as observed in the α-decay of 233U". Nuclear Physics A 259 (1): 29–60. doi:10.1016/0375-9474(76)90494-2. Bibcode: 1976NuPhA.259...29K.
33. ↑ Reich, C. W.; Helmer, R. G. (Jan 1990). "Energy separation of the doublet of intrinsic states at the ground state of 229Th". Physical Review Letters (American Physical Society) 64 (3): 271–273. doi:10.1103/PhysRevLett.64.271. PMID 10041937. Bibcode: 1990PhRvL..64..271R. https://zenodo.org/record/1233878.
34. ↑ Helmer, R. G.; Reich, C. W. (April 1994). "An Excited State of 229Th at 3.5 eV". Physical Review C 49 (4): 1845–1858. doi:10.1103/PhysRevC.49.1845. PMID 9969412. Bibcode: 1994PhRvC..49.1845H. https://zenodo.org/record/1233767.
35. ↑ B. R. Beck (2007-04-06). "Energy splitting in the ground state doublet in the nucleus 229Th". Physical Review Letters 98 (14). doi:10.1103/PhysRevLett.98.142501. PMID 17501268. Bibcode: 2007PhRvL..98n2501B. https://zenodo.org/record/1233955.
36. ↑ "Improved value for the energy splitting of the ground-state doublet in the nucleus 229Th". 12th Int. Conf. on Nuclear Reaction Mechanisms. Varenna, Italy. 2009-07-30. LLNL-PROC-415170. https://e-reports-ext.llnl.gov/pdf/375773.pdf. Retrieved 2014-05-14.
37. ↑ Jeet, Justin; Schneider, Christian; Sullivan, Scott T.; Rellergert, Wade G.; Mirzadeh, Saed; Cassanho, A. et al. (23 June 2015). "Results of a Direct Search Using Synchrotron Radiation for the Low-Energy". Physical Review Letters 114 (25). doi:10.1103/physrevlett.114.253001. PMID 26197124. Bibcode: 2015PhRvL.114y3001J.
38. ↑ Yamaguchi, A.; Kolbe, M.; Kaser, H.; Reichel, T.; Gottwald, A.; Peik, E. (May 2015). "Experimental search for the low-energy nuclear transition in 229Th with undulator radiation" (in en). New Journal of Physics 17 (5). doi:10.1088/1367-2630/17/5/053053. Bibcode: 2015NJPh...17e3053Y.
39. ↑ von der Wense, Lars (2016). On the direct detection of 229mTh (PDF) (PhD thesis). Ludwig-Maximilians-Universität München. ISBN 978-3-319-70461-6.
40. ↑ Stellmer, S.; Kazakov, G.; Schreitl, M.; Kaser, H.; Kolbe, M.; Schumm, T. (2018). "On an attempt to optically excite the nuclear isomer in Th-229". Physical Review A 97 (6). doi:10.1103/PhysRevA.97.062506. Bibcode: 2018PhRvA..97f2506S.
41. ↑ Zhao, Xinxin; Martinez de Escobar, Yenny Natali; Rundberg, Robert; Bond, Evelyn M.; Moody, Allen; Vieira, David J. (18 October 2012). "Observation of the Deexcitation of the 229mTh Nuclear Isomer". Physical Review Letters 109 (16). doi:10.1103/PhysRevLett.109.160801. PMID 23215066. Bibcode: 2012PhRvL.109p0801Z.
42. ↑ 42.0 42.1 Borisyuk, P. V.; Chubunova, E. V.; Kolachevsky, N. N.; Lebedinskii, Yu Yu; Vasiliev, O. S.; Tkalya, E. V. (2018-04-01). "Excitation of 229Th nuclei in laser plasma: the energy and half-life of the low-lying isomeric state". arXiv:1804.00299 [nucl-th].
43. ↑ Peik, Ekkehard; Zimmermann, Kai (2013-07-03). "Comment on "Observation of the Deexcitation of the 229mTh Nuclear Isomer"". Physical Review Letters 111 (1). doi:10.1103/PhysRevLett.111.018901. PMID 23863029. Bibcode: 2013PhRvL.111a8901P. "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.".229mTh+Nuclear+Isomer"&rft.jtitle=[[Physics:Physical+Review+Letters|Physical+Review+Letters]]&rft.aulast=Peik&rft.aufirst=Ekkehard&rft.au=Peik, Ekkehard&rft.au=Zimmermann, Kai&rft.date=2013-07-03&rft.volume=111&rft.issue=1&rft_id=info:doi/10.1103/PhysRevLett.111.018901&rft_id=info:pmid/23863029&rft_id=info:bibcode/2013PhRvL.111a8901P&rfr_id=info:sid/en.wikibooks.org:Physics:Isotopes_of_thorium">
44. ↑ Thirolf, Peter G.; Seiferle, Benedict; von der Wense, Lars (2019-10-28). "The 229-thorium isomer: doorway to the road from the atomic clock to the nuclear clock". Journal of Physics B: Atomic, Molecular and Optical Physics 52 (20). doi:10.1088/1361-6455/ab29b8. Bibcode: 2019JPhB...52t3001T. https://s3.cern.ch/inspire-prod-files-6/6da1f99f16760317d4532b9394b15842.
45. ↑ von der Wense, Lars; Seiferle, Benedict; Laatiaoui, Mustapha; Neumayr, Jürgen B.; Maier, Hans-Jörg; Wirth, Hans-Friedrich et al. (5 May 2016). "Direct detection of the 229Th nuclear clock transition". Nature 533 (7601): 47–51. doi:10.1038/nature17669. PMID 27147026. Bibcode: 2016Natur.533...47V.
46. ↑ 46.0 46.1 Seiferle, B.; von der Wense, L.; Bilous, P.V.; Amersdorffer, I.; Lemell, C.; Libisch, F.; Stellmer, S.; Schumm, T. et al. (12 September 2019). "Energy of the 229Th nuclear clock transition". Nature 573 (7773): 243–246. doi:10.1038/s41586-019-1533-4. PMID 31511684. Bibcode: 2019Natur.573..243S.
47. ↑ Thielking, J.; Okhapkin, M.V.; Przemyslaw, G.; Meier, D.M.; von der Wense, L.; Seiferle, B.; Düllmann, C.E.; Thirolf, P.G. et al. (2018). "Laser spectroscopic characterization of the nuclear-clock isomer 229mTh". Nature 556 (7701): 321–325. doi:10.1038/s41586-018-0011-8. PMID 29670266. Bibcode: 2018Natur.556..321T.
48. ↑ Masuda, T.; Yoshimi, A.; Fujieda, A.; Fujimoto, H.; Haba, H.; Hara, H. et al. (12 September 2019). "X-ray pumping of the 229Th nuclear clock isomer". Nature 573 (7773): 238–242. doi:10.1038/s41586-019-1542-3. PMID 31511686. Bibcode: 2019Natur.573..238M.
49. ↑ Sikorsky, Tomas; Geist, Jeschua; Hengstler, Daniel; Kempf, Sebastian; Gastaldo, Loredana; Enss, Christian et al. (2 October 2020). "Measurement of the 229Th Isomer Energy with a Magnetic Microcalorimeter". Physical Review Letters 125 (14). doi:10.1103/PhysRevLett.125.142503. PMID 33064540. Bibcode: 2020PhRvL.125n2503S.
50. ↑ von der Wense, Lars (28 September 2020). "Ticking Toward a Nuclear Clock". Physics 13. https://physics.aps.org/articles/v13/152.
51. ↑ Kraemer, Sandro; Moens, Janni; Athanasakis-Kaklamanakis, Michail; Bara, Silvia; Beeks, Kjeld; Chhetri, Premaditya; Chrysalidis, Katerina; Claessens, Arno et al. (May 2023). "Observation of the radiative decay of the 229Th nuclear clock isomer" (in en). Nature 617 (7962): 706–710. doi:10.1038/s41586-023-05894-z. ISSN 1476-4687. PMID 37225880. Bibcode: 2023Natur.617..706K. https://www.nature.com/articles/s41586-023-05894-z.
52. ↑ Howlett, Joseph (4 September 2024). "The First Nuclear Clock Will Test if Fundamental Constants Change". Quanta Magazine. https://www.quantamagazine.org/the-first-nuclear-clock-will-test-if-fundamental-constants-change-20240904/.
53. ↑ Knight, G. B.; Macklin, R. L. (1 January 1949). "Radiations of Uranium Y". Physical Review 75 (1): 34–38. doi:10.1103/PhysRev.75.34. Bibcode: 1949PhRv...75...34K. https://archive.org/details/sim_physical-review_1949-01-01_75_1/page/34.
54. ↑ "Isotopes of Thorium (Z=90)". Isotopes Project. Lawrence Berkeley National Laboratory. http://ie.lbl.gov/education/parent/Th_iso.htm.
55. ↑ World Nuclear Association. "Thorium". http://www.world-nuclear.org/info/inf62.html.
56. ↑ Krasinskas, Alyssa M; Minda, Justina; Saul, Scott H; Shaked, Abraham; Furth, Emma E (2004). "Redistribution of thorotrast into a liver allograft several years following transplantation: a case report". Mod. Pathol. 17 (1): 117–120. doi:10.1038/modpathol.3800008. PMID 14631374.
57. ↑ Peppard, D. F.; Mason, G. W.; Gray, P. R.; Mech, J. F. (1952). "Occurrence of the (4n \+ 1) series in nature". Journal of the American Chemical Society 74 (23): 6081–6084. doi:10.1021/ja01143a074. Bibcode: 1952JAChS..74.6081P. https://digital.library.unt.edu/ark:/67531/metadc172698/m2/1/high_res_d/metadc172698.pdf.

* Isotope masses from:
* Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A 729: 3–128, doi:10.1016/j.nuclphysa.2003.11.001, Bibcode: 2003NuPhA.729....3A, https://hal.archives-ouvertes.fr/in2p3-00020241/document
* Isotopic compositions and standard atomic masses from:
* Template:CIAAW2003
* Wieser, Michael E. (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry 78 (11): 2051–2066. doi:10.1351/pac200678112051.
* Half-life, spin, and isomer data selected from the following sources.
* G. Audi; A. H. Wapstra; C. Thibault; J. Blachot; O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties". Nuclear Physics A 729 (1): 3–128. doi:10.1016/j.nuclphysa.2003.11.001. Bibcode: 2003NuPhA.729....3A. http://amdc.in2p3.fr/nubase/Nubase2003.pdf.
* National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory. http://www.nndc.bnl.gov/nudat2/.
* Lide, David R., ed (2004). "11. Table of the Isotopes". CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9.

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* Table of nuclides
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