Isotopes of thorium

Actinides and fission products by half-life
Actinides by decay chain				Half-life range (y)	Fission products of 235U by yield
4n	4n+1	4n+2	4n+3
						4.5–7%	0.04–1.25%	<0.001%
228Ra№				4–6	†		155Euþ
244Cm	241Puƒ	250Cf	227Ac№	10–29		90Sr	85Kr	113mCdþ
232Uƒ		238Pu№	243Cmƒ	29–97		137Cs	151Smþ	121mSn
248Bk	249Cfƒ	242mAmƒ		141–351	No fission products have a half-life in the range of 100–210 k years ...
	241Amƒ		251Cfƒ	430–900
		226Ra№	247Bk	1.3 k – 1.6 k
240Pu	229Th№	246Cm	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
	237Np№			2.1 M – 6.5 M		135Cs₡	107Pd
236U№			247Cmƒ	15 M – 24 M			129I₡
244Pu№				80 M	... nor beyond 15.7 M years
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 №  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

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.
Relative atomic mass: 232.03806(2).

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, recently measured to be 7.6 ± 0.5 eV.

The known isotopes of thorium range in mass number from 209 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.

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, an accurate clock, as a qubit for quantum computing, or to test the effect of the chemical environment on nuclear decay rates.

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. 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. 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 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. 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.

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.

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 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. It has a half-life of 21.83 minutes.

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	decay mode(s)	daughter isotope(s)	nuclear spin	representative isotopic composition (mole fraction)	range of natural variation (mole fraction)
		excitation energy
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+
						β+ (rare)	210Ac
211Th		90	121	211.01493(8)	48(20) ms [0.04(+3−1) s]	α	207Ra	5/2−#
						β+ (rare)	211Ac
212Th		90	122	212.01298(2)	36(15) ms [30(+20-10) ms]	α (99.7%)	208Ra	0+
						β+ (.3%)	212Ac
213Th		90	123	213.01301(8)	140(25) ms	α	209Ra	5/2−#
						β+ (rare)	213Ac
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+
						β+ (.006%)	216Ac
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+#
						β+ (10−7%)	219Ac
220Th		90	130	220.015748(24)	9.7(6) µs	α	216Ra	0+
						EC (2×10−7%)	220Ac
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+
						EC (1.3×10−8%)	222Ac
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+
						β+β+ (rare)	224Ra
225Th		90	135	225.023951(5)	8.72(4) min	α (90%)	221Ra	(3/2)+
						EC (10%)	225Ac
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
228Th	Radiothorium	90	138	228.0287411(24)	1.9116(16) y	α	224Ra	0+	Trace
						CD (1.3×10−11%)	208Pb 20O
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	Ionium	90	140	230.0331338(19)	7.538(30)×104 y	α	226Ra	0+	Trace
						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
						α (10−8%)	227Ra
232Th	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
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+

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.

References

• Isotope masses from:
• Isotopic compositions and standard atomic masses from:
• Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.

Isotopes of actinium	Isotopes of thorium	Isotopes of protactinium
Table of nuclides