Thorium Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and ha ...

(₉₀Th) has seven naturally occurring

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

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 (astrophysics), accretion, or Internal structure of Earth, core formation, or of the material from which Earth formed. This dating ...

, and even slightly longer than the generally accepted age 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 numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable. This excess energy can be used in one of three ...

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 conside ...

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 levels (higher energy levels). "Metastable" describes nuclei whose excited states have Half-life, half-lives of ...

(or metastable state) with a remarkably low excitation energy, recently measured to be 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. The known isotopes of thorium range in

mass number The mass number (symbol ''A'', from the German word: ''Atomgewicht'', "atomic 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 appro ...

from 207 to 238.

List of isotopes

, -id=Thorium-207 , ²⁰⁷Th , , style="text-align:right" , 90 , style="text-align:right" , 117 , , , α , ²⁰³Ra , , , , -id=Thorium-208 , ²⁰⁸Th , , style="text-align:right" , 90 , style="text-align:right" , 118 , 208.017915(34) , 2.4(12) ms , α , ²⁰⁴Ra , 0+ , , , -id=Thorium-209 , ²⁰⁹Th , , style="text-align:right" , 90 , style="text-align:right" , 119 , 209.017998(27) , 3.1(12) ms , α , ²⁰⁵Ra , 13/2+ , , , -id=Thorium-210 , ²¹⁰Th , , style="text-align:right" , 90 , style="text-align:right" , 120 , 210.015094(20) , 16.0(36) ms , α , ²⁰⁶Ra , 0+ , , , -id=Thorium-211 , ²¹¹Th , , style="text-align:right" , 90 , style="text-align:right" , 121 , 211.014897(92) , 48(20) ms , α , ²⁰⁷Ra , 5/2−# , , , -id=Thorium-212 , ²¹²Th , , style="text-align:right" , 90 , style="text-align:right" , 122 , 212.013002(11) , 31.7(13) ms , α , ²⁰⁸Ra , 0+ , , , -id=Thorium-213 , ²¹³Th , , style="text-align:right" , 90 , style="text-align:right" , 123 , 213.0130115(99) , 144(21) ms , α , ²⁰⁹Ra , 5/2− , , , -id=Thorium-213m , style="text-indent:1em" , ^213mTh , , colspan="3" style="text-indent:2em" , 1180.0(14) keV , 1.4(4) μs , IT , ²¹³Th , (13/2)+ , , , -id=Thorium-214 , ²¹⁴Th , , style="text-align:right" , 90 , style="text-align:right" , 124 , 214.011481(11) , 87(10) ms , α , ²¹⁰Ra , 0+ , , , -id=Thorium-214m , style="text-indent:1em" , ^214mTh , , colspan="3" style="text-indent:2em" , 2181.0(27) keV , 1.24(12) μs , IT , ²¹⁴Th , 8+# , , , -id=Thorium-215 , ²¹⁵Th , , style="text-align:right" , 90 , style="text-align:right" , 125 , 215.0117246(68) , 1.35(14) s , α , ²¹¹Ra , (1/2−) , , , -id=Thorium-215m , style="text-indent:1em" , ^215mTh , , colspan="3" style="text-indent:2em" , 1471(50)# keV , 770(60) ns , IT , ²¹⁵Th , 9/2+# , , , -id=Thorium-216 , ²¹⁶Th , , style="text-align:right" , 90 , style="text-align:right" , 126 , 216.011056(12) , 26.28(16) ms , α , ²¹²Ra , 0+ , , , -id=Thorium-216m1 , rowspan=2 style="text-indent:1em" , ^216m1Th , rowspan=2, , rowspan=2 colspan="3" style="text-indent:2em" , 2041(8) keV , rowspan=2, 135.4(29) μs , IT (97.2%) , ²¹⁶Th , rowspan=2, 8+ , rowspan=2, , rowspan=2, , - , α (2.8%) , ²¹²Ra , -id=Thorium-216m2 , style="text-indent:1em" , ^216m2Th , , colspan="3" style="text-indent:2em" , 2648(8) keV , 580(26) ns , IT , ²¹⁶Th , (11−) , , , -id=Thorium-216m3 , style="text-indent:1em" , ^216m3Th , , colspan="3" style="text-indent:2em" , 3682(8) keV , 740(70) ns , IT , ²¹⁶Th , (14+) , , , -id=Thorium-217 , ²¹⁷Th , , style="text-align:right" , 90 , style="text-align:right" , 127 , 217.013103(11) , 248(4) μs , α , ²¹³Ra , 9/2+# , , , -id=Thorium-217m1 , style="text-indent:1em" , ^217m1Th , , colspan="3" style="text-indent:2em" , 673.3(1) keV , 141(50) ns , IT , ²¹⁷Th , (15/2−) , , , -id=Thorium-217m2 , style="text-indent:1em" , ^217m2Th , , colspan="3" style="text-indent:2em" , 2307(32) keV , 71(14) μs , IT , ²¹⁷Th , (25/2+) , , , -id=Thorium-218 , ²¹⁸Th , , style="text-align:right" , 90 , style="text-align:right" , 128 , 218.013276(11) , 122(5) ns , α , ²¹⁴Ra , 0+ , , , -id=Thorium-219 , ²¹⁹Th , , style="text-align:right" , 90 , style="text-align:right" , 129 , 219.015526(61) , 1.023(18) μs , α , ²¹⁵Ra , 9/2+# , , , -id=Thorium-220 , ²²⁰Th , , style="text-align:right" , 90 , style="text-align:right" , 130 , 220.015770(15) , 10.2(3) μs , α , ²¹⁶Ra , 0+ , , , -id=Thorium-221 , ²²¹Th , , style="text-align:right" , 90 , style="text-align:right" , 131 , 221.0181858(86) , 1.75(2) ms , α , ²¹⁷Ra , 7/2+# , , , -id=Thorium-222 , ²²²Th , , style="text-align:right" , 90 , style="text-align:right" , 132 , 222.018468(11) , 2.24(3) ms , α , ²¹⁸Ra , 0+ , , , -id=Thorium-223 , ²²³Th , , style="text-align:right" , 90 , style="text-align:right" , 133 , 223.0208111(85) , 0.60(2) s , α , ²¹⁹Ra , (5/2)+ , , , -id=Thorium-224 , ²²⁴Th , , style="text-align:right" , 90 , style="text-align:right" , 134 , 224.021466(10) , 1.04(2) s , αTheorized to also undergo β⁺β⁺ decay to ²²⁴Ra , ²²⁰Ra , 0+ , , , -id=Thorium-225 , rowspan=2, ²²⁵Th , rowspan=2, , rowspan=2 style="text-align:right" , 90 , rowspan=2 style="text-align:right" , 135 , rowspan=2, 225.0239510(55) , rowspan=2, 8.75(4) min , α (~90%) , ²²¹Ra , rowspan=2, 3/2+ , rowspan=2, , rowspan=2, , - , EC (~10%) , ²²⁵Ac , -id=Thorium-226 , rowspan=2, ²²⁶Th , rowspan=2, , rowspan=2 style="text-align:right" , 90 , rowspan=2 style="text-align:right" , 136 , rowspan=2, 226.0249037(48) , rowspan=2, 30.70(3) min , α , ²²²Ra , rowspan=2, 0+ , rowspan=2, , rowspan=2, , - , CD (<%) , ²⁰⁸Pb
¹⁸O , -id=Thorium-227 , ²²⁷Th , Radioactinium , style="text-align:right" , 90 , style="text-align:right" , 137 , 227.0277025(22) , 18.693(4) 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 ( d ...

of ²³⁵U , , - , rowspan=2, ²²⁸Th , rowspan=2, Radiothorium , rowspan=2 style="text-align:right" , 90 , rowspan=2 style="text-align:right" , 138 , rowspan=2, 228.0287397(19) , rowspan=2, 1.9125(7) y , α , ²²⁴Ra , rowspan=2, 0+ , rowspan=2, TraceIntermediate decay product of ²³²Th , rowspan=2, , - , CD (1.13×10⁻¹¹%) , ²⁰⁸Pb
²⁰O , - , ²²⁹Th , , style="text-align:right" , 90 , style="text-align:right" , 139 , 229.0317614(26) , 7916(17) y , α , ²²⁵Ra , 5/2+ , TraceIntermediate decay product of ²³⁷Np , , - , style="text-indent:1em" , ^229mTh , , colspan="3" style="text-indent:2em" , 8.355733554021(8) eV , 7(1) μs , ITNeutral ^229mTh decays rapidly by

internal conversion Internal conversion is an 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 conversion (o ...

, ejecting an electron. There is not enough energy to eject a second electron, so ^229mTh⁺ ions live much longer, decaying by gamma emission. See . , ²²⁹Th⁺ , 3/2+ , , , - , style="text-indent:1em" , ^229mTh⁺ , , colspan="3" style="text-indent:2em" , 8.355733554021(8) eV , 29(1) min , γ , ²²⁹Th⁺ , 3/2+ , , , - , rowspan=3, ²³⁰ThUsed in Uranium–thorium dating , rowspan=3, Ionium , rowspan=3 style="text-align:right" , 90 , rowspan=3 style="text-align:right" , 140 , rowspan=3, 230.0331323(13) , rowspan=3, 7.54(3)×10⁴ y , α , ²²⁶Ra , rowspan=3, 0+ , rowspan=3, 0.0002(2)Intermediate decay product of ²³⁸U , rowspan=3, , - , CD (5.8×10⁻¹¹%) , ²⁰⁶Hg
²⁴Ne , - , SF (<4×10⁻¹²%) , (various) , - , ²³¹Th , Uranium Y , style="text-align:right" , 90 , style="text-align:right" , 141 , 231.0363028(13) , 25.52(1) h , β⁻ , ²³¹Pa , 5/2+ , Trace , , - , rowspan=4, ²³²Th Primordial

radionuclide A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable. This excess energy can be used in one of three ...

, rowspan=4, Thorium , rowspan=4 style="text-align:right" , 90 , rowspan=4 style="text-align:right" , 142 , rowspan=4, 232.0380536(15) , rowspan=4, 1.40(1)×10¹⁰ y , αTheorized to also undergo β⁻β⁻ decay to ²³²U , ²²⁸Ra , rowspan=4, 0+ , rowspan=4, 0.9998(2) , rowspan=4, , - , SF (1.1×10⁻⁹%) , (various) , - , CD (<2.78×10⁻¹⁰%) , ²⁰⁸Hg
²⁴Ne , - , CD (<2.78×10⁻¹⁰%) , ²⁰⁶Hg
²⁶Ne , - , ²³³Th , , style="text-align:right" , 90 , style="text-align:right" , 143 , 233.0415801(15) , 21.83(4) min , β⁻ , ²³³Pa , 1/2+ , TraceProduced in neutron capture by ²³²Th , , - , ²³⁴Th , Uranium X₁ , style="text-align:right" , 90 , style="text-align:right" , 144 , 234.0435998(28) , 24.107(24) d , β⁻ , ^234mPa , 0+ , Trace , , -id=Thorium-235 , ²³⁵Th , , style="text-align:right" , 90 , style="text-align:right" , 145 , 235.047255(14) , 7.2(1) min , β⁻ , ²³⁵Pa , 1/2+# , , , -id=Thorium-236 , ²³⁶Th , , style="text-align:right" , 90 , style="text-align:right" , 146 , 236.049657(15) , 37.3(15) min , β⁻ , ²³⁶Pa , 0+ , , , -id=Thorium-237 , ²³⁷Th , , style="text-align:right" , 90 , style="text-align:right" , 147 , 237.053629(17) , 4.8(5) min , β⁻ , ²³⁷Pa , 5/2+# , , , -id=Thorium-238 , ²³⁸Th , , style="text-align:right" , 90 , style="text-align:right" , 148 , 238.05639(30)# , 9.4(20) min , β⁻ , ²³⁸Pa , 0+ , ,

Uses

Thorium has been suggested for use in thorium-based nuclear power. 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 The Eastman Kodak Company, referred to simply as Kodak (), is an American public company that produces various products related to its historic basis in film photography. The company is headquartered in Rochester, New York, and is incorporated i ...

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 chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and ha ...

with 138 neutrons. It was once named Radiothorium, due to its occurrence in the disintegration chain of thorium-232. It has a

half-life Half-life is a mathematical and scientific description of exponential or gradual decay. Half-life, half life or halflife may also refer to: Film * Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang * ''Half Life: ...

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). The parent nucleus transforms or "decays" into a daughter product, with a mass number that is reduced by four and an a ...

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

cluster decay Cluster decay, also named heavy particle radioactivity, heavy ion radioactivity or heavy cluster decay," 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, ...

, 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 ) 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'' , whose name comes from the West Semitic word for ' ...

emission with a

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

uranium-233 Uranium-233 ( or U-233) is a fissile 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 has been used successfully ...

, 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 come into use. Radioactive isotope ...

actinium-225 and bismuth-213.

Thorium-229m

²²⁹Th has a

, , with a remarkably low excitation energy of . Due to this low energy, the lifetime of ^229mTh very much depends on the electronic environment of the nucleus. In neutral ²²⁹Th, the isomer decays by

within a few microseconds. 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 Originally presented as ''Characterization of the elusive ^229mTh isomer – milestones towards a nuclear clock''. 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 syste ...

for

quantum computing A quantum computer is a computer that exploits quantum mechanical phenomena. On small scales, physical matter exhibits properties of wave-particle duality, both particles and waves, and quantum computing takes advantage of this behavior using s ...

. 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 In physics, the Coriolis force is a pseudo force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the left of the motio ...

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 In quantum mechanics, the total angular momentum quantum number parametrises the total angular momentum of a given particle, by combining its orbital angular momentum and its intrinsic angular momentum (i.e., its spin). If s is the particle's ...

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. 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 , 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, 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 were subject to controversial discussions within the community. A direct detection of electrons being emitted in the

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 . 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. 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. In another 2018 experiment, it was possible to perform a first laser-spectroscopic characterization of the nuclear properties of ^229mTh. 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 probing 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 nuclear excited state via synchrotron radiation. 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 clock An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...

Thorium-230

²³⁰Th is a

that can be used to date

coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...

s and determine

ocean current An ocean current is a continuous, directed movement of seawater 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, sh ...

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 a decay chain refers to the predictable series of radioactive disintegrations undergone by the nuclei of certain unstable chemical elements. Radioactive isotopes do not usually decay directly to stable isotopes, but rather ...

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 Uranium-235 ( 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 exists in nat ...

. 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 Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

and has a

of 25.5 hours. When it decays, it emits a beta ray and forms

protactinium-231 Protactinium (91Pa) has no stable isotopes. The four naturally occurring isotopes allow a standard atomic weight to be given. Thirty radioisotopes of protactinium have been characterized, ranging from 210Pa to 239Pa. The most stable isotope is 2 ...

. It has a decay energy of 0.39 MeV. It has a mass of 231.0363043 u.

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; it has chemical symbol, 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. Ura ...

and thorium. The isotope decays by

with a

of 1.405 years, over three times the

and approximately the age of the universe. Its

is the

thorium series In nuclear science a decay chain refers to the predictable series of radioactive disintegrations undergone by the nuclei of certain unstable chemical elements. Radioactive isotopes do not usually decay directly to stable isotopes, but rather ...

, eventually ending in

lead-208 Lead (82Pb) has four observationally 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 ...

. 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. ²³²Th is a fertile material able to absorb a

neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

and undergo transmutation into the

fissile In nuclear engineering, fissile material is material that can undergo nuclear fission when struck by a neutron of low energy. A self-sustaining thermal Nuclear chain reaction#Fission chain reaction, chain reaction can only be achieved with fissil ...

nuclide Nuclides (or nucleides, from nucleus, also known as nuclear species) are 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 the A ...

, 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 ...

. In the form of Thorotrast, a thorium dioxide suspension, it was used as a contrast medium in early

X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

diagnostics. Thorium-232 is now classified as

carcinogen A carcinogen () is any agent that promotes the development of cancer. Carcinogens can include synthetic chemicals, naturally occurring substances, physical agents such as ionizing and non-ionizing radiation, and biologic agents such as viruse ...

ic.

Thorium-233

²³³Th is an isotope of

that decays into protactinium-233 through beta decay. 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

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, known as beta decay. There are two forms of beta decay, β− decay and � ...

, and in doing so, it transmutes into

protactinium Protactinium is a chemical element; it has symbol Pa and atomic number 91. It is a dense, radioactive, silvery-gray actinide metal which readily reacts with oxygen, water vapor, and inorganic acids. It forms various chemical compounds, in which p ...

-234. ²³⁴Th has a mass of 234.0436

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

s, and it has a decay energy of about 270 keV.

Uranium Uranium is a chemical element; it has chemical symbol, 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. Ura ...

-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 in which a heavy atomic nucleus splits into two or more lighter nuclei. In contrast to induced fission, there is no inciting particle to trigger the decay; it is a purely probabilistic proc ...

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