thorium-234
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Thorium Thorium is a weakly radioactive decay, radioactive metallic chemical element with the Symbol (chemistry), symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderatel ...
(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 numbe ...
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 cosmological time, 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 sta ...
. 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 Decay chain, disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nucl ...
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 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 proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive e ...
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 (de ...
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 Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date ...
, 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; transfe ...
, 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, , - , ββ (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 Nuclear fission is a nuclear reaction, reaction in which the atomic nucleus, nucleus of an atom splits into two or more smaller atomic nucleus, nuclei. Th ...
. 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
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 numbe ...
of
thorium Thorium is a weakly radioactive decay, radioactive metallic chemical element with the Symbol (chemistry), symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderatel ...
with 138
neutrons The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a body. It was traditionally believed to be relate ...
. It was once named Radiothorium, due to its occurrence in the disintegration chain 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 radioactivity, 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 fo ...
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
radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive Decay chain, disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nucl ...
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 numbe ...
of
thorium Thorium is a weakly radioactive decay, radioactive metallic chemical element with the Symbol (chemistry), symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderatel ...
that decays by
alpha Alpha (uppercase , lowercase ; grc, ἄλφα, ', or ell, άλφα, álfa) is the first Letter (alphabet), letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician alphabet, P ...
emission with 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 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 actinium-225 and bismuth-213.


Thorium-229m

In 1976, gamma ray spectroscopy first indicated that 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 ...
, 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 nuclear clock 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 quantum system, two-state (or two-level) qu ...
for
quantum computing Quantum computing is a type of computation whose operations can harness the phenomena of quantum mechanics, such as quantum superposition, superposition, Wave interference#Quantum interference, interference, and quantum entanglement, entanglement ...
. Nuclear laser excitation of 229mTh and therefore also the development of a nuclear clock 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 Nucleus (plural, : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom * ...
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 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 nuclear clock. 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
radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive Decay chain, disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nucl ...
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 numbe ...
of
thorium Thorium is a weakly radioactive decay, radioactive metallic chemical element with the Symbol (chemistry), symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderatel ...
that can be used to date
coral Corals are marine invertebrates within the class (biology), class Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important C ...
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 Wind is the natural movement of atmosphere of Earth, air or other gases relative to a planetary surfac ...
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 decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive Decay chain, disintegration, or nuclear disintegration) is the process by which an unsta ...
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 The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a body. It was traditionally believed to be relate ...
. It is the decay product of
uranium-235 Uranium-235 (235U or U-235) is an Isotopes of uranium, 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 ...
. 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 list of largest lakes and seas in the Solar System, volumes of water can be found throughout the Solar System, only water distributi ...
and 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 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 Formation of the Earth, Earth was formed. Primordial nuclid ...
of
thorium Thorium is a weakly radioactive decay, radioactive metallic chemical element with the Symbol (chemistry), symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderatel ...
and makes up effectively all of natural thorium, with other isotopes of thorium appearing only in trace amounts as relatively short-lived
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 (de ...
s of
uranium Uranium is a chemical element with the Symbol (chemistry), 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. ...
and thorium. The isotope decays by
alpha decay Alpha decay or α-decay is a type of radioactivity, 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 fo ...
with 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 1.405 years, over three times 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 approximately the
age of the universe In physical cosmology, the age of the universe is the cosmological time, 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 sta ...
. Its
decay chain In nuclear science, the decay chain refers to a series of radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive Decay chain, disintegration, or nuclear disintegration) is the process by which an unsta ...
is the
thorium series In nuclear science Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter. Nuclear physics should not be confused with atomic phys ...
, 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 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 Uranium-238, 238U (of ...
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 nuclear fission, fissionable by thermal neutrons, can be converted into a fissile material by neutron absorption and subsequent nuclei conversions. Naturally occurring fertile materials Natu ...
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 atomic nucleus, nuclei of atoms. Since protons and ...
and undergo transmutation into the
fissile In nuclear engineering, fissile material is material capable of sustaining a nuclear fission Nuclear fission is a nuclear reaction, reaction in which the atomic nucleus, nucleus of an atom splits into two or more smaller atomic nucleu ...
nuclide A nuclide (or nucleide, from atomic nucleus, 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 co ...
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 ...
, 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 Nuclear transmutation, transmuted into the fissile artificial uranium isotope which is the nuclear fuel. Unlike natura ...
. In the form of Thorotrast, 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, it was used as a
contrast medium A contrast agent (or contrast medium) is a Chemical substance, substance used to increase the Display contrast, contrast of structures or fluids within the body in medical imaging. Contrast agents absorb or alter external electromagnetism or ultras ...
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 Picometre, picometers to 10 Nanometre, nanometers, corresponding to frequency, ...
diagnostics. Thorium-232 is now classified as
carcinogen A carcinogen is any substance, radionuclide, or radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic ...
ic.


Thorium-233

233Th is an isotope of
thorium Thorium is a weakly radioactive decay, radioactive metallic chemical element with the Symbol (chemistry), symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderatel ...
that decays into
protactinium-233 Protactinium Protactinium (formerly protoactinium) is a chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consi ...
through beta decay. It has a half-life of 21.83 minutes.


Thorium-234

234Th is an
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 numbe ...
of
thorium Thorium is a weakly radioactive decay, radioactive metallic chemical element with the Symbol (chemistry), symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderatel ...
whose nuclei contain 144
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 atomic nucleus, nuclei of atoms. Since protons and ...
s. 234Th 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 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 A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of th ...
-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 Mass is an Intrinsic and extrinsic properties, intrinsic property of a body. It was traditionally believed to be related to the physical quantity, quantity ...
s (amu), and it has a decay energy of about 270 keV ( kiloelectronvolts).
Uranium Uranium is a chemical element with the Symbol (chemistry), 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. ...
-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 breakdow ...
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
Thorium Thorium is a weakly radioactive decay, radioactive metallic chemical element with the Symbol (chemistry), symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderatel ...