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. 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
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
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, 229mTh, with an excitation energy of 8.355733554021(8) eV. This is by far the lowest of all nuclear isomers. Because this energy lies between thorium's first and second ionization energies of 6.3 and 11.5 eV, the decay rate of 229mTh is sensitive to the electronic environment of the nucleus. In neutral 229mTh, the isomer decays by internal conversion to 229Th+ within a few microseconds. However, the isomeric energy is not enough to remove a second electron, so internal conversion is impossible in 229mTh+ ions; they are forced to decay radiatively 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. Any photon emitted by nuclear decay is called a gamma ray, but this "gamma ray" has a frequency of 2020407384335±2 kHz (wavelength 148.3821828827(15) nm), in te far ultraviolet. This means it is possible to build a laser operating at this frequency, giving the only known opportunity for direct laser excitation of a nuclear state. This 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 its lifetime and magnetic moment), until the frequency was accurately measured in 2024. The first prototype nuclear clocks were announced in June 2026.
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