One isotope, 229Th, has a nuclear isomer (or metastable state) with a remarkably low excitation energy,[5] recently measured to be 8.355733554021(8) eV[6][7] 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.
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.
Thorium was also used in certain glass elements of Aero-Ektar lenses made by Kodak during World War II.
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.
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.
[25][26] 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,[6][14] which can be extrapolated to a lifetime for isolated ions of 1740±50 s.[6] This excitation energy corresponds to a photon frequency of 2020407384335±2 kHz (wavelength 148.3821828827(15) nm).
[31] 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 the lifetime and the magnetic moment) before the frequency was accurately measured in 2024.
In 1976, Kroger and Reich sought to understand coriolis force effects in deformed nuclei, and attempted to match thorium's gamma-ray spectrum to theoretical nuclear shape models.
To their surprise, the known nuclear states could not be reasonably classified into different total angular momentum quantization levels.
They concluded that some states previously identified as 229Th actually arose from a spin-3/2 nuclear isomer, 229mTh, with a remarkably low excitation energy.
[32] At that time the energy was inferred to be below 100 eV, purely based on the non-observation of the isomer's direct decay.
[36] This higher energy has two consequences which had not been considered by earlier attempts to observe emitted photons: But even knowing the higher energy, most of the searches in the 2010s for light emitted by the isomeric decay failed to observe any signal,[37][38][39][40] pointing towards a potentially strong non-radiative decay channel.
[42] 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.
[51] In April 2024, two separate groups finally reported precision laser excitation Th4+ cations doped into ionic crystals (of CaF2 and LiSrAlF6 with additional interstitial F− anions for charge compensation), giving a precise (~1 part per million) measurement of the transition energy.
[55] 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.
[56] In the form of Thorotrast, a thorium dioxide suspension, it was used as a contrast medium in early X-ray diagnostics.