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, , with a remarkably low excitation energy of . However, the isomeric energy is not enough to remove a second electron (thorium's second ionization energy is ), so internal conversion is impossible in Th+ ions. Radiative decay occurs with a half-life 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 ≈, which can be extrapolated to a lifetime for isolated ions of . This excitation energy corresponds to a photon frequency of (wavelength ). Although in the very high frequency
vacuum ultraviolet frequency range, it is possible to build a
laser operating at this frequency, giving the only known opportunity for direct laser excitation of a nuclear state, which 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 the lifetime and the magnetic moment) before the frequency was accurately measured in 2024.
History Early measurements were performed via
gamma ray spectroscopy, producing the excited state of 229Th, and measuring the difference in emitted gamma ray energies as it decays to either the 229mTh (90%) or 229Th (10%) isomeric states. 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- nuclear isomer, 229mTh, with a remarkably low excitation energy. 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, making it one of the lowest known isomeric excitation energies. In the following years, the energy was further constrained to , which was for a long time the accepted energy value. corrected to in 2009. This higher energy has two consequences which had not been considered by earlier attempts to observe emitted photons: • Because it is above thorium's first ionization energy, neutral 229mTh will decay radiatively with an extremely low likelihood, and • Because it is above the vacuum ultraviolet cutoff, the produced photons cannot travel through air. 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, and again in 2018. However, both reports were subject to controversial discussions within the community. A direct detection of electrons being emitted in the
internal conversion decay channel of 229mTh was achieved in 2016. However, this value appeared at odds with the 2018 preprint showing that a similar signal as an xenon VUV photon can be shown, but with about less energy and a (retrospectively correct) lifetime. Additional measurements by a different group in 2020 produced a figure of ( wavelength). Combining these measurements, the expected transition energy is . In September 2022, spectroscopy on decaying samples determined the excitation energy to be . 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. A one-
part-per-trillion () measurement soon followed in June 2024, and future high-precision lasers will measure the frequency up to the accuracy of the best
atomic clocks.
Thorium-230 230Th is a
radioactive isotope of
thorium that can be used to date
corals (
uranium-thorium dating) and determine
ocean current flux.
Ionium (symbol
Io) was the name given early in the study of radioactive elements to the 230Th isotope produced in the
decay chain of
238U before the nature of isotopes was fully realized. The name is still used in
ionium–thorium dating, another dating method using this isotope.
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 25.52 hours. When it decays, it emits a
beta ray and forms
protactinium-231, with a decay energy of 0.39 MeV.
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. The isotope decays by
alpha decay with a
half-life of 1.40 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 a week. 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 a
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, then into
uranium-233 to join the
neptunium series decay chain. It has a half-life of 21.83 minutes. Traces occur in nature as the result of natural
neutron activation of 232Th.
Thorium-234 234Th is an
isotope of
thorium whose
nuclei contain 144
neutrons. 234Th has a
half-life of 24.11 days; it emits a
beta particle,
transmuting into
protactinium-234 with a decay energy around 0.27 MeV.
Uranium-238 almost always produces isotope of thorium on decay (although in rare cases it undergoes
spontaneous fission, and even more rarely
double beta decay). == References ==