A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is an atom 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; transferred to one of its electrons to release it as a conversion electron; or used to create and emit a new particle (alpha particle or beta particle) from the nucleus. During those processes, the radionuclide is said to undergo radioactive decay.[1] These emissions are considered ionizing radiation because they are powerful enough to liberate an electron from another atom. The radioactive decay can produce a stable nuclide or will sometimes produce a new unstable radionuclide which may undergo further decay. Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay.[2][3][4][5] However, for a collection of atoms of a single element the decay rate, and thus the half-life (t1/2) for that collection, can be calculated from their measured decay constants. The range of the half-lives of radioactive atoms has no known limits and spans a time range of over 55 orders of magnitude.

Radionuclides occur naturally or are artificially produced in nuclear reactors, cyclotrons, particle accelerators or radionuclide generators. There are about 730 radionuclides with half-lives longer than 60 minutes (see list of nuclides). Thirty-two of those are primordial radionuclides that were created before the earth was formed. At least another 60 radionuclides are detectable in nature, either as daughters of primordial radionuclides or as radionuclides produced through natural production on Earth by cosmic radiation. More than 2400 radionuclides have half-lives less than 60 minutes. Most of those are only produced artificially, and have very short half-lives. For comparison, there are about 252 stable nuclides. (In theory, only 146 of them are stable, and the other 106 are believed to decay via alpha decay, beta decay, double beta decay, electron capture, or double electron capture.)

All chemical elements can exist as radionuclides. Even the lightest element, hydrogen, has a well-known radionuclide, tritium. Elements heavier than lead, and the elements technetium and promethium, exist only as radionuclides. (In theory, elements heavier than dysprosium exist only as radionuclides, but some such elements, like gold and platinum, are observationally stable and their half-lives have not been determined).

Unplanned exposure to radionuclides generally has a harmful effect on living organisms including humans, although low levels of exposure occur naturally without harm. The degree of harm will depend on the nature and extent of the radiation produced, the amount and nature of exposure (close contact, inhalation or ingestion), and the biochemical properties of the element; with increased risk of cancer the most usual consequence. However, radionuclides with suitable properties are used in nuclear medicine for both diagnosis and treatment. An imaging tracer made with radionuclides is called a radioactive tracer. A pharmaceutical drug made with radionuclides is called a radiopharmaceutical.



On Earth, naturally occurring radionuclides fall into three categories: primordial radionuclides, secondary radionuclides, and cosmogenic radionuclides.

  • Radionuclides are produced in stellar nucleosynthesis and supernova explosions along with stable nuclides. Most decay quickly but can still be observed astronomically and can play a part in understanding astronomic processes. Primordial radionuclides, such as uranium and thorium, exist in the present time because their half-lives are so long (>100 million years) that they have not yet completely decayed. Some radionuclides have half-lives so long (many times the age of the universe) that decay has only recently been detected, and for most practical purposes they can be considered stable, most notably bismuth-209: detection of this decay meant that bismuth was no longer considered stable. It is possible decay may be observed in other nuclides, adding to this list of primordial radionuclides.
  • Secondary radionuclides are radiogenic isotopes derived from the decay of primordial radionuclides. They have shorter half-lives than primordial radionuclides. They arise in the decay chain of the primordial isotopes thorium-232, uranium-238, and uranium-235. Examples include the natural isotopes of polonium and radium.
  • Cosmogenic isotopes, such as carbon-14, are present because they are continually being formed in the atmosphere due to cosmic rays.[6]

Many of these radionuclides exist only in trace amounts in nature, including all cosmogenic nuclides. Secondary radionuclides will occur in proportion to their half-lives, so short-lived ones will be very rare. For example, polonium can be found in uranium ores at about 0.1 mg per metric ton (1 part in 1010).[7][8] Further radionuclides may occur in nature in virtually undetectable amounts as a result of rare events such as spontaneous fission or uncommon cosmic ray interactions.

Nuclear fission

Radionuclides are produced as an unavoidable result of nuclear fission and thermonuclear explosions. The process of nuclear fission creates a wide range of fission products, most of which are radionuclides. Further radionuclides can be created from irradiation of the nuclear fuel (creating a range of actinides) and of the surrounding structures, yielding activation products. This complex mixture of radionuclides with different chemistries and radioactivity makes handling nuclear waste and dealing with nuclear fallout particularly problematic.


Artificial nuclide americium-241 emitting alpha particles inserted into a cloud chamber for visualisation

Synthetic radionuclides are deliberately synthesised using nuclear reactors, particle accelerators or radionuclide generators:

  • As well as being extracted from nuclear waste, radioisotopes can be produced deliberately with nuclear reactors, exploiting the high flux of neutrons present. These neutrons activate elements placed within the reactor. A typical product from a nuclear reactor is iridium-192. The elements that have a large propensity to take up the neutrons in the reactor are said to have a high neutron cross-section.
  • Particle accelerators such as cyclotrons accelerate particles to bombard a target to produce radionuclides. Cyclotrons accelerate protons at a target to produce positron-emitting radionuclides, e.g. fluorine-18.
  • Radionuclide generators contain a parent radionuclide that decays to produce a radioactive daughter. The parent is usually produced in a nuclear reactor. A typical example is the technetium-99m generator used in nuclear medicine. The parent produced in the reactor is molybdenum-99.


Radionuclides are used in two major ways: either for their radiation alone (irradiation, nuclear batteries) or for the combination of chemical properties and their radiation (tracers, biopharmaceuticals).

  • In biology, radionuclides of carbon can serve as radioactive tracers because they are chemically very similar to the nonradioactive nuclides, so most chemical, biological, and ecological processes treat them in a nearly identical way. One can then examine the result with a radiation detector, such as a Geiger counter, to determine where the provided atoms were incorporated. For example, one might culture plants in an environment in which the carbon dioxide contained radioactive carbon; then the parts of the plant that incorporate atmospheric carbon would be radioactive. Radionuclides can be used to monitor processes such as DNA replication or amino acid transport.
  • In nuclear medicine, radioisotopes are used for diagnosis, treatment, and research. Radioactive chemical tracers emitting gamma rays or positrons can provide diagnostic information about internal anatomy and the functioning of specific organs, including the human brain.[9][10][11] This is used in some forms of tomography: single-photon emission computed tomography and positron emission tomography (PET) scanning and Cherenkov luminescence imaging. Radioisotopes are also a method of treatment in hemopoietic forms of tumors; the success for treatment of solid tumors has been limited. More powerful gamma sources sterilise syringes and other medical equipment.
  • In food preservation, radiation is used to stop the sprouting of root crops after harvesting, to kill parasites and pests, and to control the ripening of stored fruit and vegetables.
  • In industry, and in mining, radionuclides are used to examine welds, to detect leaks, to study the rate of wear, erosion and corrosion of metals, and for on-stream analysis of a wide range of minerals and fuels.
  • In spacecraft, radionuclides are used to provide power and heat, notably through radioisotope thermoelectric generators (RTGs) and radioisotope heater units (RHUs).
  • In astronomy and cosmology, radionuclides play a role in understanding stellar and planetary process.
  • In particle physics, radionuclides help discover new physics (physics beyond the Standard Model) by measuring the energy and momentum of their beta decay products (for example, neutrinoless double beta decay and the search for weakly interacting massive particles).[12]
  • In ecology, radionuclides are used to trace and analyze pollutants, to study the movement of surface water, and to measure water runoffs from rain and snow, as well as the flow rates of streams and rivers.
  • In geology, archaeology, and paleontology, natural radionuclides are used to measure ages of rocks, minerals, and fossil materials.


The following table lists properties of selected radionuclides illustrating the range of properties and uses.

Mode of formationComments
Tritium (3H) 1212.3 yβ19Cosmogeniclightest radionuclide, used in artificial nuclear fusion, also used for radioluminescence and as oceanic transient tracer. Synthesized from neutron bombardment of lithium-6 or deuterium
Beryllium-10 461,387,000 yβ556 Cosmogenicused to examine soil erosion, soil formation from regolith, and the age of ice cores
Carbon-14 685,700 yβ156 Cosmogenicused for radiocarbon dating
Fluorine-18 99110 minβ+, EC633/1655 Cosmogenicpositron source, synthesised for use as a medical radiotracer in PET scans.
Aluminium-26 1313717,000 yβ+, EC4004 Cosmogenicexposure dating of rocks, sediment
Chlorine-36 1719301,000 yβ, EC709 Cosmogenicexposure dating of rocks, groundwater tracer
Potassium-40 19211.24×109 yβ, EC1330 /1505 Primordialused for potassium-argon dating, source of atmospheric argon, source of radiogenic heat, largest source of natural radioactivity
Calcium-41 202199,400 yEC Cosmogenicexposure dating of carbonate rocks
Cobalt-60 27335.3 yβ2824 Syntheticproduces high energy gamma rays, used for radiotherapy, equipment sterilisation, food irradiation
Krypton-81 3645229,000 yβ+Cosmogenicgroundwater dating
Strontium-90 385228.8 yβ546 Fission productmedium-lived fission product; probably most dangerous component of nuclear fallout
Technetium-99 4356210,000 yβ294 Fission productmost common isotope of the lightest unstable element, most significant of long-lived fission products
Technetium-99m 43566 hrγ,IC141 Syntheticmost commonly used medical radioisotope, used as a radioactive tracer
Iodine-129 537615,700,000 yβ194 Cosmogeniclongest lived fission product; groundwater tracer
Iodine-131 53788 dβ971 Fission productmost significant short-term health hazard from nuclear fission, used in nuclear medicine, industrial tracer
Xenon-135 54819.1 hβ1160 Fission productstrongest known "nuclear poison" (neutron-absorber), with a major effect on nuclear reactor operation.
Caesium-137 558230.2 yβ1176 Fission productother major medium-lived fission product of concern
Gadolinium-153 6489240 dEC SyntheticCalibrating nuclear equipment, bone density screening
Bismuth-209 831262.01×1019yα3137 Primordiallong considered stable, decay only detected in 2003
Polonium-210 84126138 dα5307 Decay productHighly toxic, used in poisoning of Alexander Litvinenko
Radon-222 861363.8 dα5590 Decay productgas, responsible for the majority of public exposure to ionizing radiation, second most frequent cause of lung cancer
Thorium-232 901421.4×1010 yα4083 Primordialbasis of thorium fuel cycle
Uranium-235 921437×108yα4679 Primordialfissile, main nuclear fuel
Uranium-238 921464.5×109 yα4267 PrimordialMain Uranium isotope
Plutonium-238 9414487.7 yα5593 Syntheticused in radioisotope thermoelectric generators (RTGs) and radioisotope heater units as an energy source for spacecraft
Plutonium-239 9414524,110 yα5245 Syntheticused for most modern nuclear weapons
Americium-241 95146432 yα5486 Syntheticused in household smoke detectors as an ionising agent
Californium-252 981542.64 yα/SF6217 Syntheticundergoes spontaneous fission (3% of decays), making it a powerful neutron source, used as a reactor initiator and for detection devices

Key: Z = atomic number; N = neutron number; DM = decay mode; DE = decay energy; EC = electron capture

Household smoke detectors

Americium-241 container in a smoke detector.
Americium-241 capsule as found in smoke detector. The circle of darker metal in the center is americium-241; the surrounding casing is aluminium.

Radionuclides are present in many homes as they are used inside the most common household smoke detectors. The radionuclide used is americium-241, which is created by bombarding plutonium with neutrons in a nuclear reactor. It decays by emitting alpha particles and gamma radiation to become neptunium-237. Smoke detectors use a very small quantity of 241Am (about 0.29 micrograms per smoke detector) in the form of americium dioxide. 241Am is used as it emits alpha particles which ionize the air in the detector's ionization chamber. A small electric voltage is applied to the ionized air which gives rise to a small electric current. In the presence of smoke, some of the ions are neutralized, thereby decreasing the current, which activates the detector's alarm.[13][14]

Impacts on organisms

Radionuclides that find their way into the environment may cause harmful effects as radioactive contamination. They can also cause damage if they are excessively used during treatment or in other ways exposed to living beings, by radiation poisoning. Potential health damage from exposure to radionuclides depends on a number of factors, and "can damage the functions of healthy tissue/organs. Radiation exposure can produce effects ranging from skin redness and hair loss, to radiation burns and acute radiation syndrome. Prolonged exposure can lead to cells being damaged and in turn lead to cancer. Signs of cancerous cells might not show up until years, or even decades, after exposure."[15]

Summary table for classes of nuclides, "stable" and radioactive

Following is a summary table for the total list of nuclides with half-lives greater than one hour. Ninety of these 989 nuclides are theoretically stable, except to proton-decay (which has never been observed). About 252 nuclides have never been observed to decay, and are classically considered stable.

The remaining tabulated radionuclides have half-lives longer than 1 hour, and are well-characterized (see list of nuclides for a complete tabulation). They include 30 nuclides with measured half-lives longer than the estimated age of the universe (13.8 billion years[16]), and another 4 nuclides with half-lives long enough (> 100 million years) that they are radioactive primordial nuclides, and may be detected on Earth, having survived from their presence in interstellar dust since before the formation of the solar system, about 4.6 billion years ago. Another 60+ short-lived nuclides can be detected naturally as daughters of longer-lived nuclides or cosmic-ray products. The remaining known nuclides are known solely from artificial nuclear transmutation.

Numbers are not exact, and may change slightly in the future, as "stable nuclides" are observed to be radioactive with very long half-lives.

This is a summary table[17] for the 989 nuclides with half-lives longer than one hour (including those that are stable), given in list of nuclides.

Stability class Number of nuclides Running total Notes on running total
Theoretically stable to all but proton decay 90 90 Includes first 40 elements. Proton decay yet to be observed.
Theoretically stable to alpha decay, beta decay, isomeric transition, and double beta decay but not spontaneous fission, which is possible for "stable" nuclides ≥ niobium-93 56 146 All nuclides that are possibly completely stable (spontaneous fission has never been observed for nuclides with mass number < 232).
Energetically unstable to one or more known decay modes, but no decay yet seen. All considered "stable" until decay detected. 106 252 Total of classically stable nuclides.
Radioactive primordial nuclides. 34 286 Total primordial elements include uranium, thorium, bismuth, rubidium-87, potassium-40, tellurium-128 plus all stable nuclides.
Radioactive nonprimordial, but naturally occurring on Earth. 61 347 Carbon-14 (and other isotopes generated by cosmic rays) and daughters of radioactive primordial elements, such as radium, polonium, etc. 41 of these have a half life of greater than one hour.
Radioactive synthetic half-life ≥ 1.0 hour). Includes most useful radiotracers. 662 989 These 989 nuclides are listed in the article List of nuclides.
Radioactive synthetic (half-life < 1.0 hour). >2400 >3300 Includes all well-characterized synthetic nuclides.

List of commercially available radionuclides

This list covers common isotopes, most of which are available in very small quantities to the general public in most countries. Others that are not publicly accessible are traded commercially in industrial, medical, and scientific fields and are subject to government regulation.

Gamma emission only

Isotope Activity Half-life Energies (keV)
Barium-133 9694 TBq/kg (262 Ci/g) 10.7 years 81.0, 356.0
Cadmium-109 96200 TBq/kg (2600 Ci/g) 453 days 88.0
Cobalt-57 312280 TBq/kg (8440 Ci/g) 270 days 122.1
Cobalt-60 40700 TBq/kg (1100 Ci/g) 5.27 years 1173.2, 1332.5
Europium-152 6660 TBq/kg (180 Ci/g) 13.5 years 121.8, 344.3, 1408.0
Manganese-54 287120 TBq/kg (7760 Ci/g) 312 days 834.8
Sodium-22 237540 Tbq/kg (6240 Ci/g) 2.6 years 511.0, 1274.5
Zinc-65 304510 TBq/kg (8230 Ci/g) 244 days 511.0, 1115.5
Technetium-99m 1.95×107 TBq/kg (5.27 × 105 Ci/g) 6 hours 140

Beta emission only

Isotope Activity Half-life Energies (keV)
Strontium-90 5180 TBq/kg (140 Ci/g) 28.5 years 546.0
Thallium-204 17057 TBq/kg (461 Ci/g) 3.78 years 763.4
Carbon-14 166.5 TBq/kg (4.5 Ci/g) 5730 years 49.5 (average)
Tritium (Hydrogen-3) 357050 TBq/kg (9650 Ci/g) 12.32 years 5.7 (average)

Alpha emission only

Isotope Activity Half-life Energies (keV)
Polonium-210 166500 TBq/kg (4500 Ci/g) 138.376 days 5304.5
Uranium-238 12580 kBq/kg (0.00000034 Ci/g) 4.468 billion years 4267

Multiple radiation emitters

Isotope Activity Half-life Radiation types Energies (keV)
Caesium-137 3256 TBq/kg (88 Ci/g) 30.1 years Gamma & beta G: 32, 661.6 B: 511.6, 1173.2
Americium-241 129.5 TBq/kg (3.5 Ci/g) 432.2 years Gamma & alpha G: 59.5, 26.3, 13.9 A: 5485, 5443

See also

  • List of nuclides shows all radionuclides with half-life > 1 hour
  • Hyperaccumulators table – 3
  • Radioactivity in biology
  • Radiometric dating
  • Radionuclide cisternogram
  • Uses of radioactivity in oil and gas wells


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  10. Ingvar, David H.; Franzén, Göran (1974). "Distribution of cerebral activity in chronic schizophrenia". The Lancet. 304 (7895): 1484–1486. doi:10.1016/s0140-6736(74)90221-9. PMID 4140398.
  11. Lassen, Niels A.; Ingvar, David H.; Skinhøj, Erik (October 1978). "Brain Function and Blood Flow". Scientific American. 239 (4): 62–71. Bibcode:1978SciAm.239d..62L. doi:10.1038/scientificamerican1078-62. PMID 705327.
  12. Severijns, Nathal; Beck, Marcus; Naviliat-Cuncic, Oscar (2006). "Tests of the standard electroweak model in nuclear beta decay". Reviews of Modern Physics. 78 (3): 991–1040. arXiv:nucl-ex/0605029. Bibcode:2006RvMP...78..991S. doi:10.1103/RevModPhys.78.991. S2CID 18494258.
  13. "Smoke Detectors and Americium". Archived from the original on 2010-11-12.
  14. Office of Radiation Protection – Am 241 Fact Sheet – Washington State Department of Health Archived 2011-03-18 at the Wayback Machine
  15. "Ionizing radiation, health effects and protective measures". World Health Organization. November 2012. Retrieved January 27, 2014.
  16. "Cosmic Detectives". The European Space Agency (ESA). 2013-04-02. Retrieved 2013-04-15.
  17. Table data is derived by counting members of the list; see WP:CALC. References for the list data itself are given below in the reference section in list of nuclides


  • Carlsson, J.; Forssell Aronsson, E; Hietala, SO; Stigbrand, T; Tennvall, J; et al. (2003). "Tumour therapy with radionuclides: assessment of progress and problems". Radiotherapy and Oncology. 66 (2): 107–117. doi:10.1016/S0167-8140(02)00374-2. PMID 12648782.
  • "Radioisotopes in Industry". World Nuclear Association.
  • Martin, James (2006). Physics for Radiation Protection: A Handbook. p. 130. ISBN 978-3527406111.

Further reading

  • Luig, H.; Kellerer, A. M.; Griebel, J. R. (2011). "Radionuclides, 1. Introduction". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a22_499.pub2. ISBN 978-3527306732.
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