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).
Contents
- Thorium 228
- Thorium 229
- Thorium 229m
- Thorium 230
- Thorium 231
- Thorium 232
- Thorium 233
- Thorium 234
- Notes
- Uses
- Low dispersion lenses
- References
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.
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 MgF2 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.
In 2016, the transition was directly detected, however the energy was only weakly constrained, to between 6.3 and 18.3 eV (200–70 nm), because the experiment was optimized for detection rather than precision measurement.
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 (kiloelectronvolts). Uranium-238 usually decays into this isotope of thorium. (It can undergo spontaneous fission.)
Notes
Uses
Thorium has been suggested for use in Thorium-based nuclear power.
Its radioactivity is a consideration for its non-nuclear uses but is too small to rule it out.
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.
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.