Actinides and fission products by half-life
Actinides[1] by decay chain Half-life
range (a)
Fission products by yield[2]
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
249Cfƒ 242mAmƒ 141–351

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

241Am 251Cfƒ[3] 430–900
226Ra 247Bk 1.3k–1.6k
240Pu 229Th 246Cm 243Am 4.7k–7.4k
245Cmƒ 250Cm 8.3k–8.5k
239Puƒ 24.1k
230Th 231Pa 32k–76k
236Npƒ 233Uƒ 234U 150k–250k 99Tc 126Sn
248Cm 242Pu 327k–375k 79Se
1.53M 93Zr
237Np 2.1M–6.5M 135Cs 107Pd
236U 247Cmƒ 15M–24M 129I
244Pu 80M

...nor beyond 15.7M[4]

232Th 238U 235Uƒ№ 0.7G–14G

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

Although thorium (Th) 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,[5] recently measured to be 7.6 ± 0.5 eV.[6]

The known isotopes of thorium range in mass number from 209[7] to 238.

Some notable isotopes


228Th is an isotope of thorium which has 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.


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.[8]


Gamma ray spectroscopy has indicated that 229Th has a nuclear isomer 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,[9] an accurate clock,[10] as a qubit for quantum computing, or to test the effect of the chemical environment on nuclear decay rates.[11]

The half-life of this excited state is not known, though it is estimated at 5 hours. 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 [12] .[13] 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 [6] 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.[14] 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.


230Th is a radioactive isotope of thorium which 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.)


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.


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.[15]

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.[16]

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.[17]

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.[18]


233Th is an isotope of thorium that decays into protactinium-233 through beta decay. It has a half-life of 21.83 minutes.[19]


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.)


Z(p) N(n)  
isotopic mass (u)
half-life[n 1] decay
mode(s)[20][n 2]
isotopes(s)[n 3]
(mole fraction)
range of natural
(mole fraction)
excitation energy
209Th 90 119 209.01772(11) 7(5) ms
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[n 4]
228Th Radiothorium 90 138 228.0287411(24) 1.9116(16) a α 224Ra 0+ Trace[n 5]
CD (1.3×10−11%) 208Pb
229Th 90 139 229.031762(3) 7.34(16)×103 a 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 a α 226Ra 0+ Trace[n 7]
CD (5.6×10−11%) 206Hg
SF (5×10−11%) (Various)
231Th Uranium Y 90 141 231.0363043(19) 25.52(1) h β- 231Pa 5/2+ Trace[n 4]
α (10−8%) 227Ra
232Th[n 8] Thorium 90 142 232.0380553(21) 1.405(6)×1010 a α 228Ra 0+ 1.0000
β-β- (rare) 232U
SF (1.1×10−9%) (various)
CD (2.78×10−10%) 182Yb
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+


  • 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.


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.


  • 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