Not to be confused with radionucleotide.

A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is an atom that has excess nuclear energy, making it unstable. This excess energy can be either emitted from the nucleus as gamma radiation, or create and emit from the nucleus a new particle (alpha particle or beta particle), or transfer this excess energy to one of its electrons, causing that electron to be ejected as a conversion electron. During those processes, the radionuclide is said to undergo radioactive decay.[1] These emissions constitute ionizing radiation. The unstable nucleus is more stable following the emission, but will sometimes 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 have no known limits and span a time range of over 55 orders of magnitude.

Radionuclides occur naturally and are artificially produced in nuclear reactors, cyclotrons, particle accelerators or radionuclide generators. There are about 650 radionuclides with half-lives longer than 60 minutes (see list of nuclides). Of these, 34 are primordial radionuclides that existed before the creation of the solar system, and there are another 50 radionuclides detectable in nature that are daughters of these, or are produced naturally 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 254 stable nuclides.

All chemical elements have 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.

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.

Many of these radionuclides exist only in trace amounts in nature, including the two shortest-lived primordial nuclides and all cosmogenic nuclides. Secondary radionuclides will occur in proportion to their half-lives, so short-lived ones will be very rare. Thus polonium can be found in uranium ores at about 0.1 mg per metric ton (1 part in 1010),.[7][8] Further radionunclides 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.


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


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).


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

Isotope Z N half life DM DE
Mode of formation Comments
Tritium (3H) 1 2 12.3 y β 19 Cosmogenic lightest 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 4 6 1,387,000 y β 556 Cosmogenic used to examine soil erosion, soil formation from regolith, and the age of ice cores
Carbon-14 6 8 5,700 y β 156 Cosmogenic used for radiocarbon dating
Fluorine-18 9 9 110 min β+,EC 633/1655 Cosmogenic positron source, synthesised for use as a medical radiotracer in PET scans.
Aluminium-26 13 13 717,000 y β+,EC 4004 Cosmogenic exposure dating of rocks, sediment
Chlorine-36 17 19 301,000 y β,EC 709 Cosmogenic exposure dating of rocks, groundwater tracer
Potassium-40 19 21 1.24×109 y β,EC 1330 /1505 Primordial used for potassium-argon dating, source of atmospheric argon, source of radiogenic heat, largest source of natural radioactivity
Calcium-41 20 21 102,000 y EC Cosmogenic exposure dating of carbonate rocks
Cobalt-60 27 33 5.3 y β 2824 Synthetic produces high energy gamma rays, used for radiotherapy, equipment sterilisation, food irradiation
Strontium-90 38 52 28.8 y β 546 Fission product medium-lived fission product; probably most dangerous component of nuclear fallout
Technetium-99 43 56 210,000 y β 294 Fission product commonest isotope of the lightest unstable element, most significant of long-lived fission products
Technetium-99m 43 56 6 hr γ,IC 141 Synthetic most commonly used medical radioisotope, used as a radioactive tracer
Iodine-129 53 76 15,700,000 y β 194 Cosmogenic longest lived fission product; groundwater tracer
Iodine-131 53 78 8 d β 971 Fission product most significant short term health hazard from nuclear fission, used in nuclear medicine, industrial tracer
Xenon-135 54 81 9.1 h β 1160 Fission Product strongest known "nuclear poison" (neutron-absorber), with a major effect on nuclear reactor operation.
Caesium-137 55 82 30.2 y β 1176 Fission Product other major medium-lived fission product of concern
Bismuth-209 83 126 1.9×1019y α 3137 Primordial long considered stable, decay only detected in 2003
Polonium-210 84 126 138 d α 5307 Decay Product Highly toxic, used in poisoning of Alexander Litvinenko
Radon-222 86 136 3.8d α 5590 Decay Product gas, responsible for the majority of public exposure to ionizing radiation, second most frequent cause of lung cancer
Thorium-232 90 142 1.4×1010 y α 4083 Primordial basis of thorium fuel cycle
Uranium-235 92 143 7×108y α 4679 Primordial fissile, main nuclear fuel
Uranium-238 92 146 4.5×109 y α 4267 Primordial Main Uranium isotope
Plutonium-238 94 144 87.7 y α 5593 Synthetic used in radioisotope thermoelectric generators (RTGs) and radioisotope heater units as an energy source for spacecraft
Plutonium-239 94 145 24110 y α 5245 Synthetic used for most modern nuclear weapons
Americium-241 95 146 432 y α 5486 Synthetic used in household smoke detectors as an ionising agent
Californium-252 98 155 2.64 y α/SF 6217 Synthetic undergoes spontaneous fission (3% of decays), making in a powerful neutron source, used as a reactor initiator and for detection devices

Key: Z = no of protons; N = no of Neutrons; DM = Decay Mode; DE = Decay Energy; EC = Electron Capture


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.

Most household smoke detectors contain americium produced in nuclear reactors. The radioisotope used is americium-241. The element americium is created by bombarding plutonium with neutrons in a nuclear reactor. Its isotope americium-241 decays by emitting alpha particles and gamma radiation to become neptunium-237. Most common household smoke detectors use a very small quantity of 241Am (about 0.29 micrograms per smoke detector) in the form of americium dioxide. Smoke detectors use 241Am since the alpha particles it emits collide with oxygen and nitrogen particles in the air. This occurs in the detector's ionization chamber where it produces charged particles or ions. Then, these charged particles are collected by a small electric voltage that will create an electric current that will pass between two electrodes. Then, the ions that are flowing between the electrodes will be neutralized when coming in contact with smoke, thereby decreasing the electric current between the electrodes, which will activate the detector's alarm.[13][14]


The 153Gd isotope is used in X-ray fluorescence and osteoporosis screening. It is a gamma-emitter with an 8-month half-life, making it easier to use for medical purposes. In nuclear medicine, it serves to calibrate the equipment needed like single-photon emission computed tomography systems (SPECT) to make x-rays. It ensures that the machines work correctly to produce images of radioisotope distribution inside the patient. This isotope is produced in a nuclear reactor from europium or enriched gadolinium.[15] It can also detect the loss of calcium in the hip and back bones, allowing the ability to diagnose osteoporosis.[16]

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."[17]

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 905 nuclides are theoretically stable, except to proton-decay (which has never been observed). About 254 nuclides have never been observed to decay, and are classically considered stable.

The remaining 650 radionuclides have half-lives longer than 1 hour, and are well-characterized (see list of nuclides for a complete tabulation). They include 28 nuclides with measured half-lives longer than the estimated age of the universe (13.8 billion years[18]), and another 6 nuclides with half-lives long enough (> 80 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 ~51 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 [19] for the 905 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.
Energetically unstable to one or more known decay modes, but no decay yet seen. Spontaneous fission possible for "stable" nuclides ≥ niobium-93; other mechanisms possible for heavier nuclides. All considered "stable" until decay detected. 164 254 Total of classically stable nuclides.
Radioactive primordial nuclides. 34 288 Total primordial elements include uranium, thorium, bismuth, rubidium-87, potassium-40 plus all stable nuclides.
Radioactive nonprimordial, but naturally occurring on Earth. ~ 51 ~ 339 Carbon-14 (and other isotopes generated by cosmic rays) and daughters of radioactive primordial elements, such as radium, polonium, etc.
Radioactive synthetic (half-life ≥ 1.0 hour). Includes most useful radiotracers. 556 905 These 905 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. For a complete list of all known isotopes for every element (minus activity data), see List of nuclides and Isotope lists. For a table, see Table of nuclides.

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 1.95×104 TBq/g (5.27 × 107 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


  1. R.H. Petrucci, W.S. Harwood and F.G. Herring, General Chemistry (8th ed., Prentice-Hall 2002), p.1025–26
  2. "Decay and Half Life". Retrieved 2009-12-14.
  3. Stabin, Michael G. (2007). "3". Radiation Protection and Dosimetry: An Introduction to Health Physics. Springer. doi:10.1007/978-0-387-49983-3. ISBN 978-0387499826.
  4. Best, Lara; Rodrigues, George; Velker, Vikram (2013). "1.3". Radiation Oncology Primer and Review. Demos Medical Publishing. ISBN 978-1620700044.
  5. Loveland, W.; Morrissey, D.; Seaborg, G.T. (2006). Modern Nuclear Chemistry. Wiley-Interscience. p. 57. ISBN 0-471-11532-0.
  6. Eisenbud, Merril; Gesell, Thomas F (1997-02-25). Environmental Radioactivity: From Natural, Industrial, and Military Sources. p. 134. ISBN 9780122351549.
  7. Bagnall, K. W. (1962). "The Chemistry of Polonium". Advances in Inorganic Chemistry and Radiochemistry 4. New York: Academic Press. pp. 197–226. doi:10.1016/S0065-2792(08)60268-X. ISBN 0-12-023604-4. Retrieved June 14, 2012., p. 746
  8. Bagnall, K. W. (1962). "The Chemistry of Polonium". Advances in Inorganic Chemistry and Radiochemistry 4. New York: Academic Press., p. 198
  9. Ingvar, David H.; Lassen, Niels A. (1961). "Quantitative determination of regional cerebral blood-flow in man". The Lancet. 278 (7206): 806–807. doi:10.1016/s0140-6736(61)91092-3.
  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.
  11. Lassen, Niels A.; Ingvar, David H.; Skinhøj, Erik (October 1978). "Brain Function and Blood Flow" (PDF). Scientific American. 239 (4): 62–71. doi:10.1038/scientificamerican1078-62.
  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. arXiv:nucl-ex/0605029Freely accessible. Bibcode:2006RvMP...78..991S. doi:10.1103/RevModPhys.78.991.
  13. "Smoke Detectors and Americium".
  14. Office of Radiation Protection – Am 241 Fact Sheet – Washington State Department of Health
  15. "PNNL: Isotope Sciences Program – Gadolinium-153".
  16. "Gadolinium". BCIT Chemistry Resource Center. British Columbia Institute of Technology. Retrieved 30 March 2011.
  17. "Ionizing radiation, health effects and protective measures". World Health Organization. November 2012. Retrieved January 27, 2014.
  18. "Cosmic Detectives". The European Space Agency (ESA). 2013-04-02. Retrieved 2013-04-15.
  19. 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


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