Isotopes of thorium
Actinides and fission products by half-life
|Actinides by decay chain||Half-life
|Fission products by yield|
No fission products
...nor beyond 15.7M
Legend for superscript symbols
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, recently measured to be 7.6 ± 0.5 eV.
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.
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, an accurate clock, as a qubit for quantum computing, or to test the effect of the chemical environment on nuclear decay rates.
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  . 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.
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.
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.
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.)
isotopic mass (u)
|range of natural
|216Th||90||126||216.011062(14)||26.8(3) ms||α (99.99%)||212Ra||0+|
|216m1Th||2042(13) keV||137(4) µs||(8+)|
|216m2Th||2637(20) keV||615(55) ns||(11-)|
|225Th||90||135||225.023951(5)||8.72(4) min||α (90%)||221Ra||(3/2)+|
|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]|
|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]|
|231Th||Uranium Y||90||141||231.0363043(19)||25.52(1) h||β-||231Pa||5/2+||Trace[n 4]|
|232Th[n 8]||Thorium||90||142||232.0380553(21)||1.405(6)×1010 a||α||228Ra||0+||1.0000|
|234Th||Uranium X1||90||144||234.043601(4)||24.10(3) d||β-||234mPa||0+||Trace[n 7]|
- Bold for nuclides with half-lives longer than the age of the universe (nearly stable)
CD: Cluster decay
EC: Electron capture
IT: Isomeric transition
SF: Spontaneous fission
- Bold for stable isotopes
- Intermediate decay product of 235U
- Intermediate decay product of 232Th
- Used in Uranium-thorium dating
- Intermediate decay product of 238U
- Primordial radionuclide
- 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.
- 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 isotopes have half-lives of at least four years (the longest-lived isotope in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at a notable 1600 years, thus merits the element's inclusion here.
- Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
- This is the heaviest isotope with a half-life of at least four years before the "Sea of Instability".
- Excluding those "classically stable" isotopes with half-lives significantly in excess of 232Th, e.g. while 113mCd has a half-life of only fourteen years, that of 113Cd is nearly eight quadrillion.
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- 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
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- 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.
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- Isotopes Project Home Page, Lawrence Berkeley National Laboratory. "Isotopes of Thorium (Z=90)". Retrieved 2010-01-18.
- Rutherford Appleton Laboratory. "Th-232 Decay Chain". Retrieved 2010-01-25.
- World Nuclear Association. "Thorium". Retrieved 2010-01-25.
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- Isotope masses from:
- 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 and 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.
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- Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.
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- National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. Retrieved September 2005.
- N. E. Holden (2004). "Table of the Isotopes". In D. R. Lide. CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. Section 11. ISBN 978-0-8493-0485-9.
|Isotopes of actinium||Isotopes of thorium||Isotopes of protactinium|
|Table of nuclides|
|Isotopes of the chemical elements|