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Actinides and fission products by half-life v t e
Actinides^[1] by decay chain				Half-life range (a)	Fission products of ²³⁵U by yield^[2]
4n	4n + 1	4n + 2	4n + 3	Half-life range (a)	4.5–7%	0.04–1.25%	<0.001%
²²⁸Ra^№				4–6 a		¹⁵⁵Eu^þ
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
²⁴⁸Bk^[3]	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^[4]	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^₡
				1.53 Ma	⁹³Zr
	²³⁷Np^ƒ			2.1–6.5 Ma	¹³⁵Cs^₡	¹⁰⁷Pd
²³⁶U			²⁴⁷Cm^ƒ	15–24 Ma		¹²⁹I^₡
²⁴⁴Pu				80 Ma	... nor beyond 15.7 Ma^[5]
²³²Th^№		²³⁸U^№	²³⁵U^ƒ№	0.7–14.1 Ga	... nor beyond 15.7 Ma^[5]
₡, 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)

Although thorium (Th), with atomic number 90, has 6 naturally occurring isotopes, none of these isotopes are stable; however, one isotope, ²³²Th, is relatively stable, with a half-life of 14.05 billion 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. As such, thorium is considered to be mononuclidic. It has a characteristic terrestrial isotopic composition and thus an atomic mass can be given. Standard atomic mass: 232.03806(2) u.

Thirty radioisotopes have been characterized, with the most stable (after ²³²Th) being ²³⁰Th with a half-life of 75,380 years, ²²⁹Th with a half-life of 7,340 years, 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,^[6] recently measured to be 7.6 ± 0.5 eV.^[7]

The known isotopes of thorium range in mass number from 209^[8] to 238.

Some 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.

Th-228 has an atomic weight of 228.0287411 grams/mole. Uranium-232 decays to this nuclide by alpha emission.

Thorium-229

²²⁹Th is a radioactive isotope of thorium that decays by alpha emission with a half-life of 7340 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.^[9]

Thorium-229m

Gamma ray spectroscopy has indicated that ²²⁹Th has a nuclear isomer ^229mTh with a remarkably low excitation energy. This would make it the lowest-energy nuclear isomer known, and it might be possible to excite this nuclear state using lasers with wavelengths in the vacuum ultraviolet. The isomer might have application for high density energy storage,^[10] an accurate clock,^[11] as a qubit for quantum computing, or to test the effect of the chemical environment on nuclear decay rates.^[12]

The lifetime of the isomer has been measured to be 6±1 hours. The measurement was done by collecting recoiled ^229mTh atoms in a MgF_s crystal and measuring the light emission variation over time.^[13] If this isomer were to decay it would produce a gamma ray (defined by its origin, not its wavelength) in the ultraviolet range.

The isomer transition energy of ²²⁹Th is currently derived from indirect measurements of the gamma-ray spectrum resulting from the decay of ²³³U. In 1989–1993 first measurements were performed using high-quality germanium detectors, resulting in an estimate of E = 3.5±1.0 eV for the ²²⁹Th isomer transition energy.^[14]^[15] This unnaturally low value triggered a multitude of investigations, both theoretical and experimental, trying to determine the transition energy precisely and to specify other properties of the isomer state of ²²⁹Th (such as the lifetime and the magnetic moment). However, searches for direct photon emission from the low-lying excited state have failed to report an unambiguous signal. New indirect measurements with an advanced high-resolution x-ray microcalorimeter were carried out in 2007^[7] yielding a new value for the transition energy of E = 7.6±0.5 eV, corrected to E = 7.8±0.5 eV in 2009.^[16] This value is currently the most accepted one in the community but cannot be considered definite until a direct measurement is made successfully. The shift into the VUV domain probably explains why previous attempts to directly observe the transition were unsuccessful.

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. 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 grams/mole.

Thorium-232

As Thorium is mononuclidic, the main article on thorium effectively discusses this isotope.

²³²Th is the only primordial isotope 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.^[17]

²³²Th decays by alpha decay with a half-life of 1.405×10¹⁰ years, over three times the age of the earth and more than 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 5 days.^[18]

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.^[19]

In the form of Thorotrast, a thorium dioxide suspension, it was used as contrast medium in early X-ray diagnostics. Thorium-232 is now classified as carcinogenic.^[20]

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.^[21]

Thorium-234

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

Table

nuclide symbol	historic name	Z(p)	N(n)	isotopic mass (u)	half-life^{[n 1]}	decay mode(s)^[22]^{[n 2]}	daughter isotope(s)^{[n 3]}	nuclear spin	representative isotopic composition (mole fraction)	range of natural variation (mole fraction)
nuclide symbol	historic name	excitation energy			half-life^{[n 1]}	decay mode(s)^[22]^{[n 2]}	daughter isotope(s)^{[n 3]}	nuclear spin	representative isotopic composition (mole fraction)	range of natural variation (mole fraction)
²⁰⁹Th		90	119	209.01772(11)	7(5) ms [3.8(+69−15)]			5/2−#
²¹⁰Th		90	120	210.015075(27)	17(11) ms [9(+17−4) ms]	α	²⁰⁶Ra	0+
²¹⁰Th		90	120	210.015075(27)	17(11) ms [9(+17−4) ms]	β⁺ (rare)	²¹⁰Ac	0+
²¹¹Th		90	121	211.01493(8)	48(20) ms [0.04(+3−1) s]	α	²⁰⁷Ra	5/2−#
²¹¹Th		90	121	211.01493(8)	48(20) ms [0.04(+3−1) s]	β⁺ (rare)	²¹¹Ac	5/2−#
²¹²Th		90	122	212.01298(2)	36(15) ms [30(+20-10) ms]	α (99.7%)	²⁰⁸Ra	0+
²¹²Th		90	122	212.01298(2)	36(15) ms [30(+20-10) ms]	β⁺ (.3%)	²¹²Ac	0+
²¹³Th		90	123	213.01301(8)	140(25) ms	α	²⁰⁹Ra	5/2−#
²¹³Th		90	123	213.01301(8)	140(25) ms	β⁺ (rare)	²¹³Ac	5/2−#
²¹⁴Th		90	124	214.011500(18)	100(25) ms	α	²¹⁰Ra	0+
²¹⁵Th		90	125	215.011730(29)	1.2(2) s	α	²¹¹Ra	(1/2−)
²¹⁶Th		90	126	216.011062(14)	26.8(3) ms	α (99.99%)	²¹²Ra	0+
²¹⁶Th		90	126	216.011062(14)	26.8(3) ms	β⁺ (.006%)	²¹⁶Ac	0+
^216m1Th		2042(13) keV			137(4) µs			(8+)
^216m2Th		2637(20) keV			615(55) ns			(11−)
²¹⁷Th		90	127	217.013114(22)	240(5) µs	α	²¹³Ra	(9/2+)
²¹⁸Th		90	128	218.013284(14)	109(13) ns	α	²¹⁴Ra	0+
²¹⁹Th		90	129	219.01554(5)	1.05(3) µs	α	²¹⁵Ra	9/2+#
²¹⁹Th		90	129	219.01554(5)	1.05(3) µs	β⁺ (10⁻⁷%)	²¹⁹Ac	9/2+#
²²⁰Th		90	130	220.015748(24)	9.7(6) µs	α	²¹⁶Ra	0+
²²⁰Th		90	130	220.015748(24)	9.7(6) µs	EC (2×10⁻⁷%)	²²⁰Ac	0+
²²¹Th		90	131	221.018184(10)	1.73(3) ms	α	²¹⁷Ra	(7/2+)
²²²Th		90	132	222.018468(13)	2.237(13) ms	α	²¹⁸Ra	0+
²²²Th		90	132	222.018468(13)	2.237(13) ms	EC (1.3×10⁻⁸%)	²²²Ac	0+
²²³Th		90	133	223.020811(10)	0.60(2) s	α	²¹⁹Ra	(5/2)+
²²⁴Th		90	134	224.021467(12)	1.05(2) s	α	²²⁰Ra	0+
²²⁴Th		90	134	224.021467(12)	1.05(2) s	β⁺β⁺ (rare)	²²⁴Ra	0+
²²⁵Th		90	135	225.023951(5)	8.72(4) min	α (90%)	²²¹Ra	(3/2)+
²²⁵Th		90	135	225.023951(5)	8.72(4) min	EC (10%)	²²⁵Ac	(3/2)+
²²⁶Th		90	136	226.024903(5)	30.57(10) min	α	²²²Ra	0+
²²⁷Th	Radioactinium	90	137	227.0277041(27)	18.68(9) d	α	²²³Ra	1/2+	Trace^{[n 4]}
²²⁸Th	Radiothorium	90	138	228.0287411(24)	1.9116(16) y	α	²²⁴Ra	0+	Trace^{[n 5]}
²²⁸Th	Radiothorium	90	138	228.0287411(24)	1.9116(16) y	CD (1.3×10⁻¹¹%)	²⁰⁸Pb ²⁰O	0+	Trace^{[n 5]}
²²⁹Th		90	139	229.031762(3)	7.34(16)×10³ y	α	²²⁵Ra	5/2+
^229mTh		0.0076(5) keV			70(50) h	IT	²²⁹Th	3/2+
²³⁰Th^{[n 6]}	Ionium	90	140	230.0331338(19)	7.538(30)×10⁴ y	α	²²⁶Ra	0+	Trace ^{[n 7]}
						CD (5.6×10⁻¹¹%)	²⁰⁶Hg ²⁴Ne
						SF (5×10⁻¹¹%)	(Various)
²³¹Th	Uranium Y	90	141	231.0363043(19)	25.52(1) h	β⁻	²³¹Pa	5/2+	Trace^{[n 4]}
²³¹Th	Uranium Y	90	141	231.0363043(19)	25.52(1) h	α (10⁻⁸%)	²²⁷Ra	5/2+	Trace^{[n 4]}
²³²Th^{[n 8]}	Thorium	90	142	232.0380553(21)	1.405(6)×10¹⁰ y	α	²²⁸Ra	0+	1.0000
						β⁻β⁻ (rare)	²³²U
						SF (1.1×10⁻⁹%)	(various)
						CD (2.78×10⁻¹⁰%)	¹⁸²Yb ²⁶Ne ²⁴Ne
²³³Th		90	143	233.0415818(21)	21.83(4) min	β⁻	²³³Pa	1/2+
²³⁴Th	Uranium X1	90	144	234.043601(4)	24.10(3) d	β⁻	^234mPa	0+	Trace^{[n 7]}
²³⁵Th		90	145	235.04751(5)	7.2(1) min	β⁻	²³⁵Pa	(1/2+)#
²³⁶Th		90	146	236.04987(21)#	37.5(2) min	β⁻	²³⁶Pa	0+
²³⁷Th		90	147	237.05389(39)#	4.8(5) min	β⁻	²³⁷Pa	5/2+#
²³⁸Th		90	148	238.0565(3)#	9.4(20) min	β⁻	²³⁸Pa	0+

^ Bold for nuclides with half-lives longer than the age of the universe (nearly stable)
^ Abbreviations:
CD: Cluster decay
EC: Electron capture
IT: Isomeric transition
SF: Spontaneous fission
^ Bold for stable isotopes
^ ^a ^b Intermediate decay product of ²³⁵U
^ Intermediate decay product of ²³²Th
^ Used in Uranium-thorium dating
^ ^a ^b Intermediate decay product of ²³⁸U
^ Primordial radionuclide

Notes

Geologically exceptional samples are known in which the isotopic composition lies outside the reported range. The uncertainty in the atomic mass may exceed the stated value for such specimens.
Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC, which use expanded uncertainties.

Uses

Thorium has been suggested for use as a source of nuclear energy. Presumably, it would need to be exposed to neutrons in a nuclear reactor, to convert the common isotope to some species that is fissionable.

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. Its radioactivity is a consideration for its non-nuclear uses but is too small to rule it out.

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.

As these lenses were used for aerial reconnaissance, 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.^[23]

References

^ 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.
^ Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.
^ 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]."
^ This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
^ 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.
^ E. Ruchowska (2006). "Nuclear structure of ²²⁹Th". Phys. Rev. C. 73 (4): 044326. Bibcode:2006PhRvC..73d4326R. doi:10.1103/PhysRevC.73.044326.
^ ^a ^b 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.
^ H. Ikezoe; et al. (1996). "alpha decay of a new isotope of ²⁰⁹Th". Physical Review C. 54 (4): 2043. Bibcode:1996PhRvC..54.2043I. doi:10.1103/PhysRevC.54.2043.
^ Report to Congress on the extraction of medical isotopes from U-233. U.S. Department of Energy. March 2001
^ Poppe, C. H.; Weiss, M. S.; Anderson, J. D. (1992). "Nuclear isomers as ultra-high-energy-density materials". Air Force Meeting on High Energy Density Materials, Lancaster, CA. Bibcode:1992hedm.meet...23P. {{cite conference}}: Unknown parameter |booktitle= ignored (|book-title= suggested) (help)
^ 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. Retrieved 2013-12-14.
^ Tkalya, Eugene V.; Zherikhin, Alexander N.; Zhudov, Valerii I. (2000). "Decay of the low-energy nuclear isomer ²²⁹Th^m (3/2⁺, 3.5±1.0 eV) in solids (dielectrics and metals): A new scheme of experimental research". Physical Review C. 61 (6): 064308. Bibcode:2000PhRvC..61f4308T. doi:10.1103/PhysRevC.61.064308.
^ Zhao, Xinxin (2012). "Observation of the Deexcitation of the ^{229m}Th Nuclear Isomer". Physical Review Letters. 109 (16). doi:10.1103/PhysRevLett.109.160801. ISSN 0031-9007. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ Reich, C. W. and Helmer, R. G. (Jan 1990). "Energy separation of the doublet of intrinsic states at the ground state of ²²⁹Th". Phys. Rev. Lett. 64 (3). American Physical Society: 271–273. Bibcode:1990PhRvL..64..271R. doi:10.1103/PhysRevLett.64.271.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ Helmer, R. G.; Reich, C. W. (1994). "An Excited State of Th-229 at 3.5 eV". Physical Review C. 49 (4): 1845–1858. Bibcode:1994PhRvC..49.1845H. doi:10.1103/PhysRevC.49.1845.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ Beck B R, Wu C Y, Beiersdorfer P, Brown G V, Becker J A, Moody K J, Wilhelmy J B, Porter F S, Kilbourne C A and Kelley R L (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.{{cite conference}}: CS1 maint: multiple names: authors list (link)
^ Isotopes Project Home Page, Lawrence Berkeley National Laboratory. "Isotopes of Thorium (Z=90)". Retrieved 2010-01-18. {{cite web}}: |author= has generic name (help)
^ Rutherford Appleton Laboratory. "Th-232 Decay Chain". Retrieved 2010-01-25.
^ World Nuclear Association. "Thorium". Retrieved 2010-01-25.
^ Krasinskas, Alyssa M; Minda, Justina; Saul, Scott H; Shaked, Abraham; Furth, Emma E; et al. (2004). "Redistribution of thorotrast into a liver allograft several years following transplantation: a case report". Nature. 17 (1): 117–120. doi:10.1038/modpathol.3800008. PMID 14631374.
^ Georges, Audi (2003). "The NUBASE Evaluation of Nuclear and Decay Properties". Nuclear Physics A. 729. Atomic Mass Data Center: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.
^ "Universal Nuclide Chart". nucleonica. {{cite web}}: Unknown parameter |registration= ignored (|url-access= suggested) (help)
^ Michael S. Briggs (January 16, 2002). "Aero-Ektar Lenses". Retrieved 2015-08-28.

Isotope masses from:
- 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: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.
Isotopic compositions and standard atomic masses from:
- J. R. de Laeter; J. K. Böhlke; P. De Bièvre; H. Hidaka; H. S. Peiser; K. J. R. Rosman; P. D. P. Taylor (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry. 75 (6): 683–800. doi:10.1351/pac200375060683.
- M. E. Wieser (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry. 78 (11): 2051–2066. doi:10.1351/pac200678112051. {{cite journal}}: Unknown parameter |laysummary= ignored (help)
Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.
- 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: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.
- National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. Retrieved September 2005. {{cite web}}: Check date values in: |accessdate= (help)
- N. E. Holden (2004). "Table of the Isotopes". In D. R. Lide (ed.). CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. Section 11. ISBN 978-0-8493-0485-9. {{cite book}}: Unknown parameter |nopp= ignored (|no-pp= suggested) (help)

[22] Bold for nuclides with half-lives longer than the age of the universe (nearly stable)

[24] Abbreviations:
CD: Cluster decay
EC: Electron capture
IT: Isomeric transition
SF: Spontaneous fission

[25] Bold for stable isotopes

[as-26] Intermediate decay product of ²³⁵U

[27] Intermediate decay product of ²³²Th

[28] Used in Uranium-thorium dating

[us-29] Intermediate decay product of ²³⁸U

[30] Primordial radionuclide

[1] 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.

[2] Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.

[3] 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]."

[4] This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".

[5] 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.

[Ruchowska-6] E. Ruchowska (2006). "Nuclear structure of ²²⁹Th". Phys. Rev. C. 73 (4): 044326. Bibcode:2006PhRvC..73d4326R. doi:10.1103/PhysRevC.73.044326.

[Beck-7] 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.

[Ikezoe-8] H. Ikezoe; et al. (1996). "alpha decay of a new isotope of ²⁰⁹Th". Physical Review C. 54 (4): 2043. Bibcode:1996PhRvC..54.2043I. doi:10.1103/PhysRevC.54.2043.

[9] Report to Congress on the extraction of medical isotopes from U-233. U.S. Department of Energy. March 2001

[10] Poppe, C. H.; Weiss, M. S.; Anderson, J. D. (1992). "Nuclear isomers as ultra-high-energy-density materials". Air Force Meeting on High Energy Density Materials, Lancaster, CA. Bibcode:1992hedm.meet...23P. {{cite conference}}: Unknown parameter |booktitle= ignored (|book-title= suggested) (help)

[11] 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. Retrieved 2013-12-14.

[Tkalya-12] Tkalya, Eugene V.; Zherikhin, Alexander N.; Zhudov, Valerii I. (2000). "Decay of the low-energy nuclear isomer ²²⁹Th^m (3/2⁺, 3.5±1.0 eV) in solids (dielectrics and metals): A new scheme of experimental research". Physical Review C. 61 (6): 064308. Bibcode:2000PhRvC..61f4308T. doi:10.1103/PhysRevC.61.064308.

[Zhao2012-13] Zhao, Xinxin (2012). "Observation of the Deexcitation of the ^{229m}Th Nuclear Isomer". Physical Review Letters. 109 (16). doi:10.1103/PhysRevLett.109.160801. ISSN 0031-9007. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[Helmer1990-14] Reich, C. W. and Helmer, R. G. (Jan 1990). "Energy separation of the doublet of intrinsic states at the ground state of ²²⁹Th". Phys. Rev. Lett. 64 (3). American Physical Society: 271–273. Bibcode:1990PhRvL..64..271R. doi:10.1103/PhysRevLett.64.271.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[Helmer1994-15] Helmer, R. G.; Reich, C. W. (1994). "An Excited State of Th-229 at 3.5 eV". Physical Review C. 49 (4): 1845–1858. Bibcode:1994PhRvC..49.1845H. doi:10.1103/PhysRevC.49.1845.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[BeckReview-16] Beck B R, Wu C Y, Beiersdorfer P, Brown G V, Becker J A, Moody K J, Wilhelmy J B, Porter F S, Kilbourne C A and Kelley R L (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.{{cite conference}}: CS1 maint: multiple names: authors list (link)

[17] Isotopes Project Home Page, Lawrence Berkeley National Laboratory. "Isotopes of Thorium (Z=90)". Retrieved 2010-01-18. {{cite web}}: |author= has generic name (help)

[18] Rutherford Appleton Laboratory. "Th-232 Decay Chain". Retrieved 2010-01-25.

[19] World Nuclear Association. "Thorium". Retrieved 2010-01-25.

[20] Krasinskas, Alyssa M; Minda, Justina; Saul, Scott H; Shaked, Abraham; Furth, Emma E; et al. (2004). "Redistribution of thorotrast into a liver allograft several years following transplantation: a case report". Nature. 17 (1): 117–120. doi:10.1038/modpathol.3800008. PMID 14631374.

[NUBASE-21] Georges, Audi (2003). "The NUBASE Evaluation of Nuclear and Decay Properties". Nuclear Physics A. 729. Atomic Mass Data Center: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.

[23] "Universal Nuclide Chart". nucleonica. {{cite web}}: Unknown parameter |registration= ignored (|url-access= suggested) (help)

[31] Michael S. Briggs (January 16, 2002). "Aero-Ektar Lenses". Retrieved 2015-08-28.

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