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.^[13] 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.^[14]

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 in the thorium decay series.

²²⁸Th has an atomic weight of 228.0287411 grams/mole.

Together with its decay product ²²⁴Ra it is used for alpha particle radiation therapy.^[20]

Thorium-229

²²⁹Th is a radioactive isotope of thorium that decays by alpha emission with a half-life of 7917 years.^{[2] 229}Th 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.^[21]

Thorium-229m

It has been suggested that this section be split out into another article titled Thorium-229m. (Discuss) (April 2023)

In 1976, gamma ray spectroscopy first indicated that ²²⁹Th has a nuclear isomer, ^229mTh, with a remarkably low excitation energy.^[22] At that time the energy was inferred to be below 100 eV, purely based on the non-observation of the isomer's direct decay. However, in 1990, further measurements led to the conclusion that the energy is almost certainly below 10 eV,^[23] making the isomer to be the one of lowest known excitation energy. In the following years, the energy was further constrained to 3.5 ± 1.0 eV, which was for a long time the accepted energy value.^[24] Such low energy soon raised some interest as it conceptually allows for direct laser excitation of the nuclear state,^[25] which could have applications like a nuclear clock of very high accuracy^[7][8] or as a qubit for quantum computing.^[26]

The direct observation of photons emitted in the isomeric decay would significantly help to pin down the isomeric energy value. Unfortunately, until today, there has been no fully conclusive report on the detection of photons emitted in the decay of ^229mTh. Instead, improved gamma ray spectroscopy measurements using 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,^[27] corrected to E = 7.8 ± 0.5 eV in 2009.^[28] This shift in isomeric energy from 3.5 eV to 7.8 eV possibly explains why several early attempts to directly observe the transition were unsuccessful. Still, most of the recent searches for light emitted in the isomeric decay failed to observe any signal,^{[29][30][31][32]} pointing towards a potentially strong non-radiative decay channel. A direct detection of photons emitted in the isomeric decay was claimed in 2012^[33] and again in 2018.^[34] However, both reports are currently subject to controversial discussions within the community.^[35][36]

A direct detection of electrons being emitted in the internal conversion decay channel of ^229mTh was achieved in 2016.^[37] However, at the time the isomer's transition energy could only be weakly constrained to between 6.3 and 18.3 eV. Finally, in 2019, non-optical electron spectroscopy of the internal conversion electrons emitted in the isomeric decay allowed for a determination of the isomer's excitation energy to 8.28±0.17 eV, which poses today's most precise energy value.^[6] However, this value appears at odds with the 2018 preprint showing that a similar signal as an 8.4 eV xenon VUV photon can be shown, but with about 1.3+0.2
−0.1 eV less energy and an 1880 s lifetime.^[34] In that paper, ²²⁹Th was embedded in SiO₂, possibly resulting in an energy shift and altered lifetime, although the states involved are primarily nuclear, shielding them from electronic interactions.

As a peculiarity of the extremely low excitation energy, the lifetime of ^229mTh very much depends on the electronic environment of the nucleus. In ²²⁹Th ions, the internal conversion decay channel is energetically forbidden, as the isomeric energy is below the energy that is required for further ionization of Th⁺. This leads to a lifetime that may approach the radiative lifetime of ^229mTh, for which no measurement exists, but which has been theoretically predicted to be in the range between 10³ and 10⁴ seconds.^[38][39] Experimentally, for ^229mTh²⁺ and ^229mTh³⁺ ions, an isomeric lifetime of longer than 1 minute was found.^[37] Opposed to that, in neutral ²²⁹Th atoms the internal conversion decay channel is allowed, leading to an isomeric lifetime which is reduced by 9 orders of magnitude to about 10 microseconds.^[40][38] A lifetime in the range of a few microseconds was indeed confirmed in 2017 for neutral, surface bound ^229mTh atoms, based on the detection of the internal conversion decay signal.^[12]

In a 2018 experiment, it was possible to perform a first laser-spectroscopic characterization of the nuclear properties of ^229mTh.^[41] In this experiment, laser spectroscopy of the ²²⁹Th atomic shell was conducted using a ²²⁹Th²⁺ ion cloud with 2% of the ions in the nuclear excited state. This allowed to probe for the hyperfine shift induced by the different nuclear spin states of the ground and the isomeric state. In this way, a first experimental value for the magnetic dipole and the electric quadrupole moment of ^229mTh could be inferred.

In 2019, the isomer's excitation energy was constrained to 8.28±0.17 eV based on the direct detection of internal conversion electrons^[6] and a secure population of ^229mTh from the nuclear ground state was achieved by excitation of the 29 keV nuclear excited state via synchrotron radiation.^[42] Additional measurements by a different group in 2020 produced a figure of 8.10±0.17 eV (153.1±3.2 nm wavelength).^[43] Combining these measurements, the expected transition energy is 8.12±0.11 eV.^[44]

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.^[51] 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

²³²Th is the only primordial nuclide 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.^[52] The isotope decays by alpha decay with a half-life of 1.405×10¹⁰ years, over three times the age of the Earth and approximately 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 15 days.^[53]

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.^[54] In the form of Thorotrast, a thorium dioxide suspension, it was used as a contrast medium in early X-ray diagnostics. Thorium-232 is now classified as carcinogenic.^[55]

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.^[1] Traces occur in nature as the result of natural neutron activation of ²³²Th.^[56]

Thorium-234

²³⁴Th is an isotope of thorium whose nuclei contain 144 neutrons. ²³⁴Th has a half-life of 24.1 days, and when it decays, it emits a beta particle, and in doing so, it transmutes into protactinium-234. ²³⁴Th 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 (although in rare cases it can undergo spontaneous fission instead).