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 1.40×10¹⁰ 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.

Isotopes of thorium (₉₀Th)

Main isotopes Decay Isotope abun­dance half-life (t1/2) mode pro­duct ^227Th trace 18.693 d α ^223Ra ^228Th trace 1.9125 y α ^224Ra ^229Th trace 7916 y α ^225Ra ^230Th 0.02% 75400 y α ^226Ra ^231Th trace 25.52 h β^− ^231Pa ^232Th 100.0% 1.40×10^10 y α ^228Ra ^233Th trace 21.83 min β^− ^233Pa ^234Th trace 24.11 d β^− ^234Pa
Standard atomic weight Ar°(Th)
	232.0377±0.0004 232.04±0.01 (abridged)
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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, recently measured to be 8.355733554021(8) eV 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.

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

List of isotopes

Nuclide	Historic name	Z	N	Isotopic mass (Da)	Half-life	Decay mode	Daughter isotope	Spin and parity	Natural abundance (mole fraction)
		Excitation energy							Normal proportion	Range of variation
^207Th		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	^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
^228Th	Radiothorium	90	138	228.0287397(19)	1.9125(7) y	α	^224Ra	0+	Trace
						CD (1.13×10^−11%)	^208Pb ^20O
^229Th		90	139	229.0317614(26)	7916(17) y	α	^225Ra	5/2+	Trace
^229mTh		8.355733554021(8) eV			7(1) μs	IT	^229Th^+	3/2+
^229mTh^+		8.355733554021(8) eV			29(1) min	γ	^229Th^+	3/2+
^230Th	Ionium	90	140	230.0331323(13)	7.54(3)×10^4 y	α	^226Ra	0+	0.0002(2)
						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
^232Th	Thorium	90	142	232.0380536(15)	1.40(1)×10^10 y	α	^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)	21.83(4) min	β^−	^233Pa	1/2+	Trace
^234Th	Uranium X1	90	144	234.0435998(28)	24.107(24) d	β^−	^234mPa	0+	Trace
^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:

   EC:	Electron capture
   CD:	Cluster decay
   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. 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

Actinides and fission products by half-life v t e
Actinides by decay chain				Half-life range (a)	Fission products of ^235U by yield
4n (Thorium)	4n + 1 (Neptunium)	4n + 2 (Radium)	4n + 3 (Actinium)		4.5–7%	0.04–1.25%	<0.001%
^228Ra^№				4–6 a		^155Eu^þ
^248Bk				> 9 a
^244Cm^ƒ	^241Pu^ƒ	^250Cf	^227Ac^№	10–29 a	^90Sr	^85Kr	^113mCd^þ
^232U^ƒ		^238Pu^ƒ	^243Cm^ƒ	29–97 a	^137Cs	^151Sm^þ	^121mSn
	^249Cf^ƒ	^242mAm^ƒ		141–351 a	No fission products have a half-life in the range of 100 a–210 ka ...
	^241Am^ƒ		^251Cf^ƒ	430–900 a
		^226Ra^№	^247Bk	1.3–1.6 ka
^240Pu	^229Th	^246Cm^ƒ	^243Am^ƒ	4.7–7.4 ka
	^245Cm^ƒ	^250Cm		8.3–8.5 ka
			^239Pu^ƒ	24.1 ka
		^230Th^№	^231Pa^№	32–76 ka
^236Np^ƒ	^233U^ƒ	^234U^№		150–250 ka	^99Tc^₡	^126Sn
^248Cm		^242Pu		327–375 ka		^79Se^₡
				1.33 Ma	^135Cs^₡
	^237Np^ƒ			1.61–6.5 Ma	^93Zr	^107Pd
^236U			^247Cm^ƒ	15–24 Ma		^129I^₡
^244Pu				80 Ma	... nor beyond 15.7 Ma
^232Th^№		^238U^№	^235U^ƒ№	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 1.9125 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.

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.

Thorium-229m

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

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. 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 ²²⁹Th (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 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.

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 ^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, 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 ^229mTh 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, 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. 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 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. 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 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. A one-part-per-trillion (10⁻¹²) measurement soon followed in June 2024, and future high-precision lasers will measure the frequency up to the 10⁻¹⁸ accuracy of the best atomic clocks.

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

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. 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. 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

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.

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).