Thorium Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high me ...

(₉₀Th) has seven naturally occurring

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

s but none are stable. One isotope, ²³²Th, is ''relatively'' stable, with a half-life of 1.405×10¹⁰ years, considerably longer than the

age of the Earth The age of Earth is estimated to be 4.54 ± 0.05 billion years This age may represent the age of Earth's accretion, or core formation, or of the material from which Earth formed. This dating is based on evidence from radiometric age-dating of ...

, and even slightly longer than the generally accepted

age of the universe In physical cosmology, the age of the universe is the time elapsed since the Big Bang. Astronomers have derived two different measurements of the age of the universe: a measurement based on direct observations of an early state of the universe, ...

. This isotope makes up nearly all natural thorium, so thorium was considered to be mononuclidic. However, in 2013,

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

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

radioisotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...

s have been characterized, with the most stable being ²³²Th, ²³⁰Th with a half-life of 75,380 years, ²²⁹Th with a half-life of 7,917 years, and ²²⁸Th with a half-life of 1.92 years. All of the remaining

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

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 A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy excited state, higher energy levels than in the ground state of the same nucleus. "Metastable" describes nuclei whose excited ...

(or metastable state) with a remarkably low excitation energy, recently measured to be 8.28 ± 0.17 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 A nuclear clock or nuclear optical clock is a notional clock that would use the frequency of a Atomic nucleus, nuclear transition as its reference frequency, in the same manner as an atomic clock uses the frequency of an Atomic electron transition ...

of extremely high accuracy. The known isotopes of thorium range in

mass number The mass number (symbol ''A'', from the German word ''Atomgewicht'' tomic weight, also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It is approxima ...

from 207 to 238.

List of isotopes

, - , ²⁰⁷Th , , style="text-align:right" , 90 , style="text-align:right" , 117 , , 9.7(+46.6−4.4) ms , α , ²⁰³Ra , , , , - , ²⁰⁸Th , , style="text-align:right" , 90 , style="text-align:right" , 118 , 208.01791(4) , 1.7(+1.7-0.6) ms , α , ²⁰⁴Ra , 0+ , , , - , ²⁰⁹Th , , style="text-align:right" , 90 , style="text-align:right" , 119 , 209.01772(11) , 7(5) ms
.8(+69−15), α , ²⁰⁵Ra , 5/2−# , , , - , rowspan=2, ²¹⁰Th , rowspan=2, , rowspan=2 style="text-align:right" , 90 , rowspan=2 style="text-align:right" , 120 , rowspan=2, 210.015075(27) , rowspan=2, 17(11) ms
(+17−4) ms, α , ²⁰⁶Ra , rowspan=2, 0+ , rowspan=2, , rowspan=2, , - , β⁺ (rare) , ²¹⁰Ac , - , rowspan=2, ²¹¹Th , rowspan=2, , rowspan=2 style="text-align:right" , 90 , rowspan=2 style="text-align:right" , 121 , rowspan=2, 211.01493(8) , rowspan=2, 48(20) ms
.04(+3−1) s, α , ²⁰⁷Ra , rowspan=2, 5/2−# , rowspan=2, , rowspan=2, , - , β⁺ (rare) , ²¹¹Ac , - , rowspan=2, ²¹²Th , rowspan=2, , rowspan=2 style="text-align:right" , 90 , rowspan=2 style="text-align:right" , 122 , rowspan=2, 212.01298(2) , rowspan=2, 36(15) ms
0(+20-10) ms, α (99.7%) , ²⁰⁸Ra , rowspan=2, 0+ , rowspan=2, , rowspan=2, , - , β⁺ (.3%) , ²¹²Ac , - , rowspan=2, ²¹³Th , rowspan=2, , rowspan=2 style="text-align:right" , 90 , rowspan=2 style="text-align:right" , 123 , rowspan=2, 213.01301(8) , rowspan=2, 140(25) ms , α , ²⁰⁹Ra , rowspan=2, 5/2−# , rowspan=2, , rowspan=2, , - , β⁺ (rare) , ²¹³Ac , - , ²¹⁴Th , , style="text-align:right" , 90 , style="text-align:right" , 124 , 214.011500(18) , 100(25) ms , α , ²¹⁰Ra , 0+ , , , - , ²¹⁵Th , , style="text-align:right" , 90 , style="text-align:right" , 125 , 215.011730(29) , 1.2(2) s , α , ²¹¹Ra , (1/2−) , , , - , rowspan=2, ²¹⁶Th , rowspan=2, , rowspan=2 style="text-align:right" , 90 , rowspan=2 style="text-align:right" , 126 , rowspan=2, 216.011062(14) , rowspan=2, 26.8(3) ms , α (99.99%) , ²¹²Ra , rowspan=2, 0+ , rowspan=2, , rowspan=2, , - , β⁺ (.006%) , ²¹⁶Ac , - , style="text-indent:1em" , ^216m1Th , , colspan="3" style="text-indent:2em" , 2042(13) keV , 137(4) μs , , , (8+) , , , - , style="text-indent:1em" , ^216m2Th , , colspan="3" style="text-indent:2em" , 2637(20) keV , 615(55) ns , , , (11−) , , , - , ²¹⁷Th , , style="text-align:right" , 90 , style="text-align:right" , 127 , 217.013114(22) , 240(5) μs , α , ²¹³Ra , (9/2+) , , , - , ²¹⁸Th , , style="text-align:right" , 90 , style="text-align:right" , 128 , 218.013284(14) , 109(13) ns , α , ²¹⁴Ra , 0+ , , , - , rowspan=2, ²¹⁹Th , rowspan=2, , rowspan=2 style="text-align:right" , 90 , rowspan=2 style="text-align:right" , 129 , rowspan=2, 219.01554(5) , rowspan=2, 1.05(3) μs , α , ²¹⁵Ra , rowspan=2, 9/2+# , rowspan=2, , rowspan=2, , - , β⁺ (10⁻⁷%) , ²¹⁹Ac , - , rowspan=2, ²²⁰Th , rowspan=2, , rowspan=2 style="text-align:right" , 90 , rowspan=2 style="text-align:right" , 130 , rowspan=2, 220.015748(24) , rowspan=2, 9.7(6) μs , α , ²¹⁶Ra , rowspan=2, 0+ , rowspan=2, , rowspan=2, , - , EC (2×10⁻⁷%) , ²²⁰Ac , - , ²²¹Th , , style="text-align:right" , 90 , style="text-align:right" , 131 , 221.018184(10) , 1.73(3) ms , α , ²¹⁷Ra , (7/2+) , , , - , rowspan=2, ²²²Th , rowspan=2, , rowspan=2 style="text-align:right" , 90 , rowspan=2 style="text-align:right" , 132 , rowspan=2, 222.018468(13) , rowspan=2, 2.237(13) ms , α , ²¹⁸Ra , rowspan=2, 0+ , rowspan=2, , rowspan=2, , - , EC (1.3×10⁻⁸%) , ²²²Ac , - , ²²³Th , , style="text-align:right" , 90 , style="text-align:right" , 133 , 223.020811(10) , 0.60(2) s , α , ²¹⁹Ra , (5/2)+ , , , - , rowspan=3, ²²⁴Th , rowspan=3, , rowspan=3 style="text-align:right" , 90 , rowspan=3 style="text-align:right" , 134 , rowspan=3, 224.021467(12) , rowspan=3, 1.05(2) s , α , ²²⁰Ra , rowspan=3, 0+ , rowspan=3, , rowspan=3, , - , β⁺β⁺ (rare) , ²²⁴Ra , - , CD (rare) , ²⁰⁸Pb
¹⁶O , - , rowspan=2, ²²⁵Th , rowspan=2, , rowspan=2 style="text-align:right" , 90 , rowspan=2 style="text-align:right" , 135 , rowspan=2, 225.023951(5) , rowspan=2, 8.72(4) min , α (90%) , ²²¹Ra , rowspan=2, (3/2)+ , rowspan=2, , rowspan=2, , - , EC (10%) , ²²⁵Ac , - , ²²⁶Th , , style="text-align:right" , 90 , style="text-align:right" , 136 , 226.024903(5) , 30.57(10) min , α , ²²²Ra , 0+ , , , - , ²²⁷Th , Radioactinium , style="text-align:right" , 90 , style="text-align:right" , 137 , 227.0277041(27) , 18.68(9) d , α , ²²³Ra , 1/2+ , TraceIntermediate

decay product In nuclear physics, a decay product (also known as a daughter product, daughter isotope, radio-daughter, or daughter nuclide) is the remaining nuclide left over from radioactive decay. Radioactive decay often proceeds via a sequence of steps ( ...

of ²³⁵U , , - , rowspan=2, ²²⁸Th , rowspan=2, Radiothorium , rowspan=2 style="text-align:right" , 90 , rowspan=2 style="text-align:right" , 138 , rowspan=2, 228.0287411(24) , rowspan=2, 1.9116(16) y , α , ²²⁴Ra , rowspan=2, 0+ , rowspan=2, TraceIntermediate decay product of ²³²Th , rowspan=2, , - , CD (1.3×10⁻¹¹%) , ²⁰⁸Pb
²⁰O , - , ²²⁹Th , , style="text-align:right" , 90 , style="text-align:right" , 139 , 229.031762(3) , 7.34(16)×10³ y , α , ²²⁵Ra , 5/2+ , TraceIntermediate decay product of ²³⁷Np , , - , style="text-indent:1em" , ^229mTh , , colspan="3" style="text-indent:2em" , 8.3(2) eV , 7(1) μs , IT , ²²⁹Th , 3/2+ , , , - , rowspan=3, ²³⁰ThUsed in

Uranium–thorium dating Uranium–thorium dating, also called thorium-230 dating, uranium-series disequilibrium dating or uranium-series dating, is a radiometric dating technique established in the 1960s which has been used since the 1970s to determine the age of calciu ...

, rowspan=3, Ionium , rowspan=3 style="text-align:right" , 90 , rowspan=3 style="text-align:right" , 140 , rowspan=3, 230.0331338(19) , rowspan=3, 7.538(30)×10⁴ y , α , ²²⁶Ra , rowspan=3, 0+ , rowspan=3, 0.0002(2)Intermediate decay product of ²³⁸U , rowspan=3, , - , CD (5.6×10⁻¹¹%) , ²⁰⁶Hg
²⁴Ne , - , SF (5×10⁻¹¹%) , (Various) , - , rowspan=2, ²³¹Th , rowspan=2, Uranium Y , rowspan=2 style="text-align:right" , 90 , rowspan=2 style="text-align:right" , 141 , rowspan=2, 231.0363043(19) , rowspan=2, 25.52(1) h , β⁻ , ²³¹Pa , rowspan=2, 5/2+ , rowspan=2, Trace , rowspan=2, , - , α (10⁻⁸%) , ²²⁷Ra , - , rowspan=4, ²³²Th Primordial

radionuclide A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transfer ...

, rowspan=4, Thorium , rowspan=4 style="text-align:right" , 90 , rowspan=4 style="text-align:right" , 142 , rowspan=4, 232.0380553(21) , rowspan=4, 1.405(6)×10¹⁰ y , α , ²²⁸Ra , rowspan=4, 0+ , rowspan=4, 0.9998(2) , rowspan=4, , - , β⁻β⁻ (rare) , ²³²U , - , SF (1.1×10⁻⁹%) , (various) , - , CD (2.78×10⁻¹⁰%) , ¹⁸²Yb
²⁶Ne
²⁴Ne , - , ²³³Th , , style="text-align:right" , 90 , style="text-align:right" , 143 , 233.0415818(21) , 21.83(4) min , β⁻ , ²³³Pa , 1/2+ , , , - , ²³⁴Th , Uranium X₁ , style="text-align:right" , 90 , style="text-align:right" , 144 , 234.043601(4) , 24.10(3) d , β⁻ , ^234mPa , 0+ , Trace , , - , ²³⁵Th , , style="text-align:right" , 90 , style="text-align:right" , 145 , 235.04751(5) , 7.2(1) min , β⁻ , ²³⁵Pa , (1/2+)# , , , - , ²³⁶Th , , style="text-align:right" , 90 , style="text-align:right" , 146 , 236.04987(21)# , 37.5(2) min , β⁻ , ²³⁶Pa , 0+ , , , - , ²³⁷Th , , style="text-align:right" , 90 , style="text-align:right" , 147 , 237.05389(39)# , 4.8(5) min , β⁻ , ²³⁷Pa , 5/2+# , , , - , ²³⁸Th , , style="text-align:right" , 90 , style="text-align:right" , 148 , 238.0565(3)# , 9.4(20) min , β⁻ , ²³⁸Pa , 0+ , ,

Uses

Thorium has been suggested for use in

thorium-based nuclear power Thorium-based nuclear power generation is fueled primarily by the nuclear fission of the isotope uranium-233 produced from the fertile element thorium. A thorium fuel cycle can offer several potential advantages over a uranium fuel cycleA nuclea ...

. 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

thorium Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high me ...

with 138

neutrons The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave ...

. It was once named Radiothorium, due to its occurrence in the

disintegration chain In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay direc ...

of thorium-232. It has a

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...

of 1.9116 years. It undergoes

alpha decay Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an atom ...

to ²²⁴Ra. Occasionally it decays by the unusual route of

cluster decay Cluster decay, also named heavy particle radioactivity or heavy ion radioactivity, is a rare type of nuclear decay in which an atomic nucleus emits a small "cluster" of neutrons and protons, more than in an alpha particle, but less than a typic ...

, 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.

Thorium-229

²²⁹Th is a

that decays by

alpha Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', or ell, άλφα, álfa) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter aleph , whic ...

emission with a

of 7917 years. ²²⁹Th is produced by the decay of

uranium-233 Uranium-233 (233U or U-233) is a fissile Isotopes of uranium, isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a Nuclear fuel, reactor fuel. It ha ...

, and its principal use is for the production of the

medical isotopes A medical isotope is an isotope used in medicine. The first uses of isotopes in medicine were in radiopharmaceuticals, and this is still the most common use. However more recently, separated stable isotopes have also come into use. Examples of ...

actinium-225 Actinium-225 (225Ac, Ac-225) is an isotope of actinium. It undergoes alpha decay to francium-221 with a half-life of 10 days, and is an intermediate decay product in the neptunium series (the decay chain starting at 237Np). Except for minuscu ...

and

bismuth-213 Bismuth (83Bi) has 41 known isotopes, ranging from 184Bi to 224Bi. Bismuth has no stable isotopes, but does have one very long-lived isotope; thus, the standard atomic weight can be given as . Although bismuth-209 is now known to be unstable, it ...

Thorium-229m

In 1976,

gamma ray spectroscopy Gamma-ray spectroscopy is the quantitative study of the energy spectra of gamma-ray sources, such as in the nuclear industry, geochemical investigation, and astrophysics. Most radioactive sources produce gamma rays, which are of various energi ...

first indicated that ²²⁹Th has a

, ^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 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. Such low energy soon raised some interest as it conceptually allows for direct laser excitation of the nuclear state, which leads to some interesting potential applications, e.g. the development of a

of very high accuracy or as a

qubit In quantum computing, a qubit () or quantum bit is a basic unit of quantum information—the quantum version of the classic binary bit physically realized with a two-state device. A qubit is a two-state (or two-level) quantum-mechanical system, ...

for

quantum computing Quantum computing is a type of computation whose operations can harness the phenomena of quantum mechanics, such as superposition, interference, and entanglement. Devices that perform quantum computations are known as quantum computers. Though ...

. Nuclear laser excitation of ^229mTh and therefore also the development of a

has so far been impeded by an insufficient knowledge about the isomeric properties. A precise knowledge of the isomeric energy is of particular importance in this context, as it determines the required laser technology and shortens the scanning times when searching for the direct excitation. This triggered a multitude of 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). 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, corrected to E = 7.8 ± 0.5 eV in 2009. 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, pointing towards a potentially strong non-radiative decay channel. A direct detection of photons emitted in the isomeric decay was claimed in 2012 and again in 2018. However, both reports are currently subject to controversial discussions within the community. A direct detection of electrons being emitted in the

internal conversion Internal conversion is a non-radioactive, atomic decay process where an excited nucleus interacts electromagnetically with one of the orbital electrons of an atom. This causes the electron to be emitted (ejected) from the atom. Thus, in internal ...

decay channel of ^229mTh was achieved in 2016. 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 , which poses today's most precise energy value. 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 less energy and an 1880 s lifetime. 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 to 10⁴ seconds. Experimentally, for ^229mTh²⁺ and ^229mTh³⁺ ions, an isomeric lifetime of longer than 1 minute was found. 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. 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. In a 2018 experiment, it was possible to perform a first laser-spectroscopic characterization of the nuclear properties of ^229mTh. In this experiment,

laser spectroscopy Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...

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 based on the direct detection of internal conversion electrons 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. Additional measurements by a different group in 2020 produced a figure of ( wavelength). Combining these measurements, we have an expected transition energy of . The 29189.93 eV excited state of ²²⁹Th decays to the isomeric state with a probability of 90%. Both measurements are further important steps towards the development of a

. Also gamma spectroscopy experiments confirmed the 8.3 eV energy splitting from the distance to the 29189.93 eV level. 8.28 eV (150 nm) is reachable as a 7th harmonic of an ytterbium fiber laser by VUV frequency comb. Continuous wave phase matching for harmonic generation may be available.

Thorium-230

²³⁰Th is a

that can be used to date

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...

s and determine

ocean current An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, s ...

flux. Ionium was a name given early in the study of radioactive elements to the ²³⁰Th isotope produced in the

decay chain In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay direct ...

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

. It is the decay product of

uranium-235 Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exis ...

. It is found in very small amounts on the

earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

and has a

of 25.5 hours. When it decays, it emits a

beta ray A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β� ...

and forms

protactinium-231 Protactinium (91Pa) has no stable isotopes. The three naturally occurring isotopes allow a standard atomic weight to be given. Thirty radioisotopes of protactinium have been characterized, with the most stable being 231Pa with a half-life of 32, ...

. 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 In geochemistry, geophysics and nuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on Earth that have existed in their current form since before Earth was formed. Primordial nuclides were present in the ...

and makes up effectively all of natural thorium, with other isotopes of thorium appearing only in trace amounts as relatively short-lived

s of

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

and thorium. The isotope decays by

with a

of 1.405 years, over three times the

and approximately the

. Its

is the

thorium series In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay directly ...

, eventually ending in

lead-208 Lead (82Pb) has four stable isotopes: 204Pb, 206Pb, 207Pb, 208Pb. Lead-204 is entirely a primordial nuclide and is not a radiogenic nuclide. The three isotopes lead-206, lead-207, and lead-208 represent the ends of three decay chains: the uraniu ...

. The remainder of the chain is quick; the longest half-lives in it are 5.75 years for

radium-228 Radium (88Ra) has no stable or nearly stable isotopes, and thus a standard atomic weight cannot be given. The longest lived, and most common, isotope of radium is 226Ra with a half-life of . 226Ra occurs in the decay chain of 238U (often referre ...

and 1.91 years for

thorium-228 Thorium (90Th) has seven naturally occurring isotopes but none are stable. One isotope, thorium-232, 232Th, is ''relatively'' stable, with a half-life of 1.405×1010 years, considerably longer than the age of the Earth, and even slightly longer t ...

, with all other half-lives totaling less than 15 days. ²³²Th is a

fertile material Fertile material is a material that, although not itself fissionable by thermal neutrons, can be converted into a fissile material by neutron absorption and subsequent nuclei conversions. Naturally occurring fertile materials Naturally occurring ...

able to absorb a

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...

and undergo transmutation into the

fissile In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of thermal energy. The predominant neutron energy may be typ ...

nuclide A nuclide (or nucleide, from nucleus, also known as nuclear species) is a class of atoms characterized by their number of protons, ''Z'', their number of neutrons, ''N'', and their nuclear energy state. The word ''nuclide'' was coined by Truman ...

, which is the basis of the

thorium fuel cycle The thorium fuel cycle is a nuclear fuel cycle that uses an isotope of thorium, , as the fertile material. In the reactor, is transmuted into the fissile artificial uranium isotope which is the nuclear fuel. Unlike natural uranium, natural tho ...

. In the form of

Thorotrast Thorotrast is a suspension containing particles of the radioactive compound thorium dioxide, ThO2; it was used as a radiocontrast agent in clinical radiography in the 1930s to 1950s. It is no longer used clinically. Thorium compounds produce ...

, a

thorium dioxide Thorium dioxide (ThO2), also called thorium(IV) oxide, is a crystalline solid, often white or yellow in colour. Also known as thoria, it is produced mainly as a by-product of lanthanide and uranium production. Thorianite is the name of the minera ...

suspension Suspension or suspended may refer to: Science and engineering * Suspension (topology), in mathematics * Suspension (dynamical systems), in mathematics * Suspension of a ring, in mathematics * Suspension (chemistry), small solid particles suspend ...

, it was used as a

contrast medium A contrast agent (or contrast medium) is a substance used to increase the contrast of structures or fluids within the body in medical imaging. Contrast agents absorb or alter external electromagnetism or ultrasound, which is different from radiop ...

in early

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

diagnostics. Thorium-232 is now classified as

carcinogen A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive substan ...

ic.

Thorium-233

²³³Th is an isotope of

that decays into

protactinium-233 Protactinium (91Pa) has no stable isotopes. The three naturally occurring isotopes allow a standard atomic weight to be given. Thirty radioisotopes of protactinium have been characterized, with the most stable being 231Pa with a half-life of 32, ...

through beta decay. It has a half-life of 21.83 minutes.

Thorium-234

²³⁴Th is an

whose nuclei contain 144

s. ²³⁴Th has a

of 24.1 days, and when it decays, it emits a

beta particle A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β� ...

, and in doing so, it transmutes into

protactinium Protactinium (formerly protoactinium) is a chemical element with the symbol Pa and atomic number 91. It is a dense, silvery-gray actinide metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds ...

-234. ²³⁴Th has a mass of 234.0436

atomic mass unit The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at ...

s (amu), and it has a decay energy of about 270 keV ( kiloelectronvolts).

Uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

-238 usually decays into this isotope of thorium (although in rare cases it can undergo

spontaneous fission Spontaneous fission (SF) is a form of radioactive decay that is found only in very heavy chemical elements. The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 56 (e.g., iron-56); spontaneous breakdo ...

instead).

References

* Isotope masses from: ** * Isotopic compositions and standard atomic masses from: ** ** * Half-life, spin, and isomer data selected from the following sources. ** ** ** {{Authority control Thorium