Isotopes of thorium

Thorium (₉₀Th) has seven naturally occurring isotopes but none are stable. One isotope, ²³²Th, is relatively stable, with a half-life of 1.405×10¹⁰ 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.

Isotopes of thorium (₉₀Th)

Main isotopes[1] Decay 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[2] α 225Ra 230Th 0.02% 75400 y α 226Ra 231Th trace 25.5 h β− 231Pa 232Th 100.0% 1.405×1010 y α 228Ra 233Th trace 21.83 min β− 233Pa 234Th trace 24.1 d β− 234Pa
Standard atomic weight Ar°(Th)
	232.0377±0.0004[3] 232.04±0.01 (abridged)[4]
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Thirty-one radioisotopes have been characterized, with the most stable being ²³²Th, ²³⁰Th with a half-life of 75,380 years, ²²⁹Th with a half-life of 7,917 years,^[2] and ²²⁸Th 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, ²²⁹Th, has a nuclear isomer (or metastable state) with a remarkably low excitation energy,^[5] recently measured to be 8.355733554021(8) eV^[6][7] 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.^[8][9][10]

The known isotopes of thorium range in mass number from 207^[11] to 238.

List of isotopes

Nuclide [n 1]	Historic name	Z	N	Isotopic mass (Da)[12] [n 2][n 3]	Half-life[1] [n 4]	Decay mode[1] [n 5]	Daughter isotope [n 6]	Spin and parity[1] [n 7][n 8]	Natural abundance (mole fraction)
		Excitation energy							Normal proportion[1]	Range of variation
207Th[11]		90	117		9.7+46.6 −4.4 ms	α	203Ra
208Th		90	118	208.017915(34)	2.4(12) ms	α	204Ra	0+
209Th		90	119	209.017998(27)	3.1(12) ms	α	205Ra	13/2+
210Th		90	120	210.015094(20)	16.0(36) ms	α	206Ra	0+
211Th		90	121	211.014897(92)	48(20) ms	α	207Ra	5/2−#
212Th		90	122	212.013002(11)	31.7(13) ms	α	208Ra	0+
213Th		90	123	213.0130115(99)	144(21) ms	α	209Ra	5/2−
213mTh		1180.0(14) keV			1.4(4) μs	IT	213Th	(13/2)+
214Th		90	124	214.011481(11)	87(10) ms	α	210Ra	0+
214mTh		2181.0(27) keV			1.24(12) μs	IT	214Th	8+#
215Th		90	125	215.0117246(68)	1.35(14) s	α	211Ra	(1/2−)
215mTh		1471(50)# keV			770(60) ns	IT	215Th	9/2+#
216Th		90	126	216.011056(12)	26.28(16) ms	α	212Ra	0+
216m1Th		2041(8) keV			135.4(29) μs	IT (97.2%)	216Th	8+
						α (2.8%)	212Ra
216m2Th		2648(8) keV			580(26) ns	IT	216Th	(11−)
216m3Th		3682(8) keV			740(70) ns	IT	216Th	(14+)
217Th		90	127	217.013103(11)	248(4) μs	α	213Ra	9/2+#
217m1Th		673.3(1) keV			141(50) ns	IT	217Th	(15/2−)
217m2Th		2307(32) keV			71(14) μs	IT	217Th	(25/2+)
218Th		90	128	218.013276(11)	122(5) ns	α	214Ra	0+
219Th		90	129	219.015526(61)	1.023(18) μs	α	215Ra	9/2+#
220Th		90	130	220.015770(15)	10.2(3) μs	α	216Ra	0+
221Th		90	131	221.0181858(86)	1.75(2) ms	α	217Ra	7/2+#
222Th		90	132	222.018468(11)	2.24(3) ms	α	218Ra	0+
223Th		90	133	223.0208111(85)	0.60(2) s	α	219Ra	(5/2)+
224Th		90	134	224.021466(10)	1.04(2) s	α[n 9]	220Ra	0+
225Th		90	135	225.0239510(55)	8.75(4) min	α (~90%)	221Ra	3/2+
						EC (~10%)	225Ac
226Th		90	136	226.0249037(48)	30.70(3) min	α	222Ra	0+
						CD (<3.2×10−12%)	208Pb 18O
227Th	Radioactinium	90	137	227.0277025(22)	18.693(4) d	α	223Ra	(1/2+)	Trace[n 10]
228Th	Radiothorium	90	138	228.0287397(19)	1.9125(7) y	α	224Ra	0+	Trace[n 11]
						CD (1.13×10−11%)	208Pb 20O
229Th		90	139	229.0317614(26)	7916(17) y	α	225Ra	5/2+	Trace[n 12]
229mTh		8.355733554021(8) eV[7]			7(1) μs[13]	IT[n 13]	229Th+	3/2+
229mTh+		8.355733554021(8) eV[7]			29(1) min[14]	γ[n 13]	229Th+	3/2+
230Th[n 14]	Ionium	90	140	230.0331323(13)	7.54(3)×104 y	α	226Ra	0+	0.0002(2)[n 15]
						CD (5.8×10−11%)	206Hg 24Ne
						SF (<4×10−12%)	(Various)
231Th	Uranium Y	90	141	231.0363028(13)	25.52(1) h	β−	231Pa	5/2+	Trace[n 10]
232Th[n 16]	Thorium	90	142	232.0380536(15)	1.40(1)×1010 y	α[n 17]	228Ra	0+	0.9998(2)
						SF (1.1×10−9%)	(various)
						CD (<2.78×10−10%)	208Hg 24Ne
						CD (<2.78×10−10%)	206Hg 26Ne
233Th		90	143	233.0415801(15)	21.83(4) min	β−	233Pa	1/2+	Trace[n 18]
234Th	Uranium X1	90	144	234.0435998(28)	24.107(24) d	β−	234mPa	0+	Trace[n 15]
235Th		90	145	235.047255(14)	7.2(1) min	β−	235Pa	1/2+#
236Th		90	146	236.049657(15)	37.3(15) min	β−	236Pa	0+
237Th		90	147	237.053629(17)	4.8(5) min	β−	237Pa	5/2+#
238Th		90	148	238.05639(30)#	9.4(20) min	β−	238Pa	0+
This table header & footer: view

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. Intermediate decay product of ²³⁵U
11. Intermediate decay product of ²³²Th
12. Intermediate decay product of ²³⁷Np
13. 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 § Thorium-229m.
14. Used in Uranium–thorium dating
15. Intermediate decay product of ²³⁸U
16. Primordial radionuclide
17. Theorized to also undergo β⁻β⁻ decay to ²³²U
18. 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.^[15] 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.^[16]

Actinides vs. fission products

Actinides and fission products by half-life v t e
Actinides[17] by decay chain				Half-life range (a)	Fission products of 235U by yield[18]
4n	4n + 1	4n + 2	4n + 3		4.5–7%	0.04–1.25%	<0.001%
228Ra№				4–6 a		155Euþ
248Bk[19]				> 9 a
244Cmƒ	241Puƒ	250Cf	227Ac№	10–29 a	90Sr	85Kr	113mCdþ
232Uƒ		238Puƒ	243Cmƒ	29–97 a	137Cs	151Smþ	121mSn
	249Cfƒ	242mAmƒ		141–351 a	No fission products have a half-life in the range of 100 a–210 ka ...
	241Amƒ		251Cfƒ[20]	430–900 a
		226Ra№	247Bk	1.3–1.6 ka
240Pu	229Th	246Cmƒ	243Amƒ	4.7–7.4 ka
	245Cmƒ	250Cm		8.3–8.5 ka
			239Puƒ	24.1 ka
		230Th№	231Pa№	32–76 ka
236Npƒ	233Uƒ	234U№		150–250 ka	99Tc₡	126Sn
248Cm		242Pu		327–375 ka		79Se₡
				1.33 Ma	135Cs₡
	237Npƒ			1.61–6.5 Ma	93Zr	107Pd
236U			247Cmƒ	15–24 Ma		129I₡
244Pu				80 Ma	... nor beyond 15.7 Ma[21]
232Th№		238U№	235Uƒ№	0.7–14.1 Ga
₡,  has thermal neutron capture cross section in the range of 8–50 barns ƒ,  fissile №,  primarily a naturally occurring radioactive material (NORM) þ,  neutron poison (thermal neutron capture cross section greater than 3k barns)

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 1.9116 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 in the thorium decay series.

²²⁸Th has an atomic weight of 228.0287411 grams/mole.

Together with its decay product ²²⁴Ra it is used for alpha particle radiation therapy.^[22]

Thorium-229

²²⁹Th is a radioactive isotope of thorium that decays by alpha emission with a half-life of 7917 years.^{[2] 229}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.^[23]

Thorium-229m

²²⁹Th has a nuclear isomer, ^229m
Th
, with a remarkably low excitation energy of 8.355733554021(8) eV.^[7]

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.^[24][25][13] 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.^[25][26] 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,^[6][14] which can be extrapolated to a lifetime for isolated ions of 1740±50 s.^[6]

This excitation energy corresponds to a photon frequency of 2020407384335±2 kHz (wavelength 148.3821828827(15) nm).^{[7][27][6][14]} 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,^[28] which could have applications like a nuclear clock of very high accuracy^{[9][10][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) before the frequency was accurately measured in 2024.^[6][27][14]

History

Early measurements 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, this technique first indicated that ²²⁹Th has a 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 likelyhood, 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 subject to controversial discussions 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][8][6][14]} A one-part-per-trillion (10⁻¹²) measurement soon followed in June 2024,^[7][52] and future high-precision lasers will measure the frequency up to the 10⁻¹⁸ accuracy of the best atomic clocks.^[7][10][30]

Thorium-230

²³⁰Th 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 ²³⁰Th isotope produced in the decay chain of ²³⁸U 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

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 25.5 hours.^[53] 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

Main article: 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 1.405×10¹⁰ 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 15 days.^[55]

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.^[56] 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.^[57]

Thorium-233

²³³Th is an isotope of thorium that decays into protactinium-233 through beta decay. It has a half-life of 21.83 minutes.^[1] Traces occur in nature as the result of natural neutron activation of ²³²Th.^[58]

Thorium-234

²³⁴Th is an isotope of thorium whose nuclei contain 144 neutrons. ²³⁴Th 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. ²³⁴Th 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).

References

1. Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
2. 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.
3. "Standard Atomic Weights: Thorium". CIAAW. 2013.
4. Prohaska, Thomas; Irrgeher, Johanna; Benefield, Jacqueline; Böhlke, John K.; Chesson, Lesley A.; Coplen, Tyler B.; Ding, Tiping; Dunn, Philip J. H.; Gröning, Manfred; Holden, Norman E.; Meijer, Harro A. J. (2022-05-04). "Standard atomic weights of the elements 2021 (IUPAC Technical Report)". Pure and Applied Chemistry. doi:10.1515/pac-2019-0603. ISSN 1365-3075.
5. E. Ruchowska (2006). "Nuclear structure of ²²⁹Th" (PDF). Physical Review C. 73 (4): 044326. Bibcode:2006PhRvC..73d4326R. doi:10.1103/PhysRevC.73.044326. hdl:10261/12130.
6. 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" (PDF). Physical Review Letters. 132 (18) 182501. Bibcode:2024PhRvL.132r2501T. doi:10.1103/PhysRevLett.132.182501. 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.
7. Zhang, Chuankun; Ooi, Tian; Higgins, Jacob S.; Doyle, Jack F.; von der Wense, Lars; Beeks, Kjeld; Leitner, Adrian; Kazakov, Georgy; Li, Peng; Thirolf, Peter G.; Schumm, Thorsten; Ye, Jun (4 September 2024). "Frequency ratio of the ^229mTh nuclear isomeric transition and the ⁸⁷Sr atomic clock". Nature. 633 (8028): 63–70. arXiv:2406.18719. doi:10.1038/s41586-024-07839-6. The transition frequency between the I = 5/2 ground state and the I = 3/2 excited state is determined as: 𝜈_Th = ⁠1/6⁠ (𝜈_a + 2𝜈_b + 2𝜈_c + 𝜈_d) = 2020407384335(2) kHz.
8. "Atomic Nucleus Excited with Laser: A Breakthrough after Decades" (Press release). TU Wien. 29 April 2024. Retrieved 29 April 2024.
9. Peik, E.; Tamm, Chr. (2003-01-15). "Nuclear laser spectroscopy of the 3.5 eV transition in ²²⁹Th" (PDF). Europhysics Letters. 61 (2): 181–186. Bibcode:2003EL.....61..181P. doi:10.1209/epl/i2003-00210-x. S2CID 250818523. Archived (PDF) from the original on 2024-04-14. Retrieved 2024-04-30.
10. 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" (PDF). Physical Review Letters. 108 (12) 120802: 120802. arXiv:1110.2490. Bibcode:2012PhRvL.108l0802C. doi:10.1103/PhysRevLett.108.120802. PMID 22540568. S2CID 40863227. Retrieved 2024-04-30.
11. Yang, H. B.; et al. (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). Bibcode:2022PhRvC.105e1302Y. doi:10.1103/PhysRevC.105.L051302. S2CID 248935764.
12. Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*". Chinese Physics C. 45 (3): 030003. doi:10.1088/1674-1137/abddaf.
13. Seiferle, B.; von der Wense, L.; Thirolf, P.G. (January 2017). "Lifetime measurement of the ²²⁹Th nuclear isomer". Physical Review Letters. 118 (4) 042501. arXiv:1801.05205. Bibcode:2017PhRvL.118d2501S. doi:10.1103/PhysRevLett.118.042501. PMID 28186791. S2CID 37518294. A half-life of 7±1 μs has been measured
14. 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.
15. f2.5 Aero Ektar Lenses ^{[permanent dead link]} Some images.
16. Michael S. Briggs (January 16, 2002). "Aero-Ektar Lenses". Archived from the original on August 12, 2015. Retrieved 2015-08-28.
17. 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.
18. Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.
19. 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. Bibcode:1965NucPh..71..299M. doi:10.1016/0029-5582(65)90719-4.
    "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 [years]. No growth of Cf²⁴⁸ was detected, and a lower limit for the β⁻ half-life can be set at about 10⁴ [years]. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 [years]."
20. This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
21. 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 eight quadrillion years.
22. "Thor Medical – production of alpha emitters for cancer treatment". May 2023.
23. Report to Congress on the extraction of medical isotopes from U-233 Archived 2011-09-27 at the Wayback Machine. U.S. Department of Energy. March 2001
24. Karpeshin, F.F.; Trzhaskovskaya, M.B. (November 2007). "Impact of the electron environment on the lifetime of the ²²⁹Th^m low-lying isomer". Physical Review C. 76 (5) 054313. Bibcode:2007PhRvC..76e4313K. doi:10.1103/PhysRevC.76.054313.
25. 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) 054324. arXiv:1509.09101. Bibcode:2015PhRvC..92e4324T. doi:10.1103/PhysRevC.92.054324. S2CID 118374372.
26. 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) 212501. arXiv:1704.07919. Bibcode:2017PhRvL.118u2501M. doi:10.1103/PhysRevLett.118.212501. PMID 28598657. S2CID 40694257.
27. Thirolf, Peter (April 29, 2024). "Shedding Light on the Thorium-229 Nuclear Clock Isomer". Physics. 17 71. doi:10.1103/Physics.17.71.
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. Bibcode:1996PhyS...53..296T. doi:10.1088/0031-8949/53/3/003. S2CID 250744766.
29. von der Wense, Lars; Seiferle, Benedict; Thirolf, Peter G. (March 2018). "Towards a ²²⁹Th-based nuclear clock". Measurement Techniques. 60 (12): 1178–1192. arXiv:1811.03889. Bibcode:2018MeasT..60.1178V. doi:10.1007/s11018-018-1337-1. S2CID 119359298.
30. Thirolf, Peter G.; et al. (March 2020). 'Phase Transition' in the 'Thorium-Isomer Story'. XXXVI Mazurian Lakes Conference on Physics (1–7 November 2019) (PDF). Acta Physica Polonica B. Vol. 51, no. 3. Piaski, Pisz County, Poland. pp. 561–570. arXiv:2108.13388. doi:10.5506/APhysPolB.51.561. 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) 165005. arXiv:1105.4646. Bibcode:2011JPhB...44p5005R. doi:10.1088/0953-4075/44/16/165005. S2CID 118379032.
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. Bibcode:1976NuPhA.259...29K. doi:10.1016/0375-9474(76)90494-2.
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. 64 (3). American Physical Society: 271–273. Bibcode:1990PhRvL..64..271R. doi:10.1103/PhysRevLett.64.271. PMID 10041937.
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. Bibcode:1994PhRvC..49.1845H. doi:10.1103/PhysRevC.49.1845. PMID 9969412.
35. B. R. Beck; et al. (2007-04-06). "Energy splitting in the ground state doublet in the nucleus ²²⁹Th". Physical Review Letters. 98 (14): 142501. Bibcode:2007PhRvL..98n2501B. doi:10.1103/PhysRevLett.98.142501. PMID 17501268. S2CID 12092700.
36. Beck BR, Wu CY, Beiersdorfer P, Brown GV, Becker JA, Moody KJ, Wilhelmy JB, Porter FS, Kilbourne CA, Kelley RL (2009-07-30). Improved value for the energy splitting of the ground-state doublet in the nucleus ²²⁹Th (PDF). 12th Int. Conf. on Nuclear Reaction Mechanisms. Varenna, Italy. LLNL-PROC-415170. Archived from the original (PDF) on 2017-01-27. 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): 253001. arXiv:1502.02189. Bibcode:2015PhRvL.114y3001J. doi:10.1103/physrevlett.114.253001. PMID 26197124. S2CID 1322253.
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". New Journal of Physics. 17 (5): 053053. Bibcode:2015NJPh...17e3053Y. doi:10.1088/1367-2630/17/5/053053.
39. von der Wense, Lars (2016). On the direct detection of ^229mTh (PDF) (PhD thesis). Ludwig Maximilian University of Munich. 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): 062506. arXiv:1803.09294. Bibcode:2018PhRvA..97f2506S. doi:10.1103/PhysRevA.97.062506. S2CID 4946329.
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) 160801. Bibcode:2012PhRvL.109p0801Z. doi:10.1103/PhysRevLett.109.160801. PMID 23215066.
42. 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) 018901. Bibcode:2013PhRvL.111a8901P. doi:10.1103/PhysRevLett.111.018901. PMID 23863029. 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.
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) 203001. Bibcode:2019JPhB...52t3001T. doi:10.1088/1361-6455/ab29b8.
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. arXiv:1710.11398. Bibcode:2016Natur.533...47V. doi:10.1038/nature17669. PMID 27147026. S2CID 205248786.
46. Seiferle, B.; von der Wense, L.; Bilous, P.V.; Amersdorffer, I.; Lemell, C.; Libisch, F.; Stellmer, S.; Schumm, T.; Düllmann, C.E.; Pálffy, A.; Thirolf, P.G. (12 September 2019). "Energy of the ²²⁹Th nuclear clock transition". Nature. 573 (7773): 243–246. arXiv:1905.06308. Bibcode:2019Natur.573..243S. doi:10.1038/s41586-019-1533-4. PMID 31511684. S2CID 155090121.
47. Thielking, J.; Okhapkin, M.V.; Przemyslaw, G.; Meier, D.M.; von der Wense, L.; Seiferle, B.; Düllmann, C.E.; Thirolf, P.G.; Peik, E. (2018). "Laser spectroscopic characterization of the nuclear-clock isomer ^229mTh". Nature. 556 (7701): 321–325. arXiv:1709.05325. Bibcode:2018Natur.556..321T. doi:10.1038/s41586-018-0011-8. PMID 29670266. S2CID 4990345.
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. arXiv:1902.04823. Bibcode:2019Natur.573..238M. doi:10.1038/s41586-019-1542-3. PMID 31511686. S2CID 119083861.
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) 142503. arXiv:2005.13340. Bibcode:2020PhRvL.125n2503S. doi:10.1103/PhysRevLett.125.142503. PMID 33064540. S2CID 218900580.
50. von der Wense, Lars (28 September 2020). "Ticking Toward a Nuclear Clock". Physics. Vol. 13.
51. Kraemer, Sandro; Moens, Janni; Athanasakis-Kaklamanakis, Michail; Bara, Silvia; Beeks, Kjeld; Chhetri, Premaditya; Chrysalidis, Katerina; Claessens, Arno; Cocolios, Thomas E.; Correia, João G. M.; Witte, Hilde De; Ferrer, Rafael; Geldhof, Sarina; Heinke, Reinhard; Hosseini, Niyusha (May 2023). "Observation of the radiative decay of the 229Th nuclear clock isomer". Nature. 617 (7962): 706–710. Bibcode:2023Natur.617..706K. doi:10.1038/s41586-023-05894-z. ISSN 1476-4687. PMID 37225880.
52. Howlett, Joseph (4 September 2024). "The First Nuclear Clock Will Test if Fundamental Constants Change". Quanta Magazine.
53. Knight, G. B.; Macklin, R. L. (1 January 1949). "Radiations of Uranium Y". Physical Review. 75 (1): 34–38. Bibcode:1949PhRv...75...34K. doi:10.1103/PhysRev.75.34.
54. "Isotopes of Thorium (Z=90)". Isotopes Project. Lawrence Berkeley National Laboratory. Archived from the original on 2010-02-03. Retrieved 2010-01-18.
55. Rutherford Appleton Laboratory. "Th-232 Decay Chain". Archived from the original on 2012-03-19. Retrieved 2010-01-25.
56. World Nuclear Association. "Thorium". Archived from the original on 2013-02-16. Retrieved 2010-01-25.
57. 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.
58. Peppard, D. F.; Mason, G. W.; Gray, P. R.; Mech, J. F. (1952). "Occurrence of the (4n + 1) series in nature" (PDF). Journal of the American Chemical Society. 74 (23): 6081–6084. doi:10.1021/ja01143a074. Archived (PDF) from the original on 2019-04-29.

• 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, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
• Isotopic compositions and standard atomic masses from:
  • de Laeter, John Robert; Böhlke, John Karl; De Bièvre, Paul; Hidaka, Hiroshi; Peiser, H. Steffen; Rosman, Kevin J. R.; Taylor, Philip D. P. (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry. 75 (6): 683–800. doi:10.1351/pac200375060683.
  • 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.
• "News & Notices: Standard Atomic Weights Revised". International Union of Pure and Applied Chemistry. 19 October 2005.
• 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" (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.
  • National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory.
  • Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.). CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9.