A radionuclide is an atom with an unstable nucleus, which is a nucleus characterized by excess energy available to be imparted either to a newly created radiation particle within the nucleus or to an atomic electron. The radionuclide, in this process, undergoes radioactive decay, and emits gamma ray(s) and/or subatomic particles. These particles constitute ionizing radiation. Radionuclides occur naturally, and can also be artificially produced.

The number of radionuclides is uncertain because the number of very short-lived radionuclides that have yet to be characterized is extremely large and potentially unquantifiable. Even the number of long-lived radionuclides is uncertain (to a smaller degree), because many "stable" nuclides are calculated to have half-lives so long that their decay has not been experimentally measured. The total list of nuclides contains 90 nuclides that are theoretically stable, and 255 total stable nuclides that have not been observed to decay. In addition, there exist about 650 radionuclides that have been experimentally observed to decay, with half-lives longer than 60 minutes (see list of nuclides for this list). Of these, about 339 are known from nature (they have been observed on Earth, and not as a consequence of man-made activities).

Including artificially produced nuclides, more than 3300 nuclides are known (including ~3000 radionuclides), including many more (> ~2400) that have decay half-lives shorter than 60 minutes. This list expands as new radionuclides with very short half-lives are characterized.

Radionuclides are often referred to by chemists and physicists as radioactive isotopes or radioisotopes. Radioisotopes with suitable half-lives play an important part in a number of constructive technologies (for example, nuclear medicine). Radionuclides can also present both real and perceived dangers to health.



Naturally occurring radionuclides fall into three categories: primordial radionuclides, secondary radionuclides and cosmogenic radionuclides. Primordial radionuclides originate mainly from the interiors of stars and, like uranium and thorium, are still present because their half-lives are so long that they have not yet completely decayed. Secondary radionuclides are radiogenic isotopes derived from the decay of primordial radionuclides. They have shorter half-lives than primordial radionuclides. Cosmogenic isotopes, such as carbon-14, are present because they are continually being formed in the atmosphere due to cosmic rays.

Artificially produced radionuclides can be produced by nuclear reactors, particle accelerators or by radionuclide generators:

  • Radioisotopes produced with nuclear reactors exploit the high flux of neutrons present. The neutrons activate elements placed within the reactor. A typical product from a nuclear reactor is thallium-201 and 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 radioisotopes, e.g., fluorine-18.
  • Radionuclide generators contain a parent isotope that decays to produce a radioisotope. 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 produced as an unavoidable side effect of nuclear and thermonuclear explosions.

Trace radionuclides are those that occur in tiny amounts in nature either due to inherent rarity, or to half-lives that are significantly shorter than the age of the Earth. Synthetic isotopes are inherently not naturally occurring on Earth, but can be created by nuclear reactions.


Radionuclides are used in two major ways: for their chemical properties and as sources of radiation. Radionuclides of familiar elements such as carbon can serve as tracers because they are chemically very similar to the non-radioactive nuclides, so most chemical, biological, and ecological processes treat them in a near identical way. One can then examine the result with a radiation detector, such as a geiger counter, to determine where the provided atoms ended up. For example, one might culture plants in an environment in which the carbon dioxide contained radioactive carbon; then the parts of the plant that had laid down atmospheric carbon would be radioactive.

In nuclear medicine, radioisotopes are used for diagnosis, treatment, and research. Radioactive chemical tracers emitting gamma rays or positrons can provide diagnostic information about a person's internal anatomy and the functioning of specific organs. This is used in some forms of tomography: single-photon emission computed tomography and positron emission tomography scanning.

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 biochemistry and genetics, radionuclides label molecules and allow tracing chemical and physiological processes occurring in living organisms, such as DNA replication or amino acid transport.

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 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.

Radionuclides are also 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. Natural radionuclides are used in geology, archaeology, and paleontology to measure ages of rocks, minerals, and fossil materials.

Common examples


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 formed 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, Am-241 decays by emitting alpha particles and gamma radiation to become neptunium-237. The most common household smoke detectors use a very small quantity of Am-241 (about 0.29 micrograms per smoke detector) in the form of americium dioxide. The smoke detectors use the Am-241 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. [1][2]

Steps for creating americium-241 The plutonium-241 is formed in any nuclear reactor by neutron capture from uranium-238.

  1. 238
    + neutron => 239
  2. 239
    by beta decay => 239
  3. 239
    by beta decay => 239
  4. 239
    + neutron => 240
  5. 240
    + neutron => 241

This will decay both in the reactor and subsequently to form Am-241 (Half-life: 432.2 years)[3][4]


The Gd-153 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.[5] It can also detect the loss of calcium in the hip and back bones, allowing the ability to diagnose osteoporosis. [6]


Radionuclides that find their way into the environment may cause harmful effects of radioactive contamination. They can also cause damage if they are excessively used during treatment or in other ways applied to living beings, by radiation poisoning.

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 255 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 27 nuclides with measured half-lives longer than the estimated age of the universe (13.7 billion years), 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 [7] 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. 165 255 Total of classically stable nuclides.
Radioactive primordial nuclides. 33 288 Total primordial elements include bismuth, uranium, thorium, plutonium, plus all stable nuclides.
Radioactive non-primordial, but naturally occurring on Earth. ~ 51 ~ 339 Carbon-14 (and other isotopes generated by cosmic rays); daughters of radioactive primordials, such as francium, etc.
Radioactive synthetic (half-life > 1 hour). Includes most useful radiotracers. 556 905 These 905 nuclides are listed in the article List of nuclides.
Radioactive synthetic (half-life < 1 hour). >2400 >3300 Includes all well-characterized synthetic nuclides.

List of commercially available radionuclides

Gamma only

Isotope Activity Half-life Energies (KeV)
Barium-133 1uCi 10.7 years 81.0, 356.0
Cadmium-109 1uCi 453 days 88.0
Cobalt 57 1uCi 270 days 122.1
Cobalt 60 1uCi 5.27 years 1173.2, 1332.5
Europium-152 1uCi 13.5 years 121.8, 344.3, 1408.0
Manganese-54 1uCi 312 days 834.8
Sodium-22 1uCi 2.6 years 511.0, 1274.5
Zinc-65 1uCi 244 days 511.0, 1115.5
Technetium 99m 1uCi 6.01 hours 140

Beta only

Isotope Activity Half-life Energies (KeV)
Strontium-90 0.1uCi 28.5 years 546.0
Thallium-204 1uCi 3.78 years 763.4
Carbon-14 10uCi 5730 years 49.5 (average)

Alpha only

Isotope Activity Half-life Energies (KeV)
Polonium 210 0.1uCi 138 days 5304.5

Multiple radiation emitters

Isotope Activity Half-life Radiation types Energies (KeV)
Caesium-137 1, 5, 10 uCi 30.1 years Gamma & beta G: 32, 661.6 B: 511.6, 1173.2

See also


  1. ^ Smoke Detectors and Americium
  2. ^ Office of Radiation Protection - Am 241 Fact Sheet - Washington State Department of Health
  3. ^ "Smoke Detectors: Uses of Radioactive Isotopes". Chemistry Tutorial : Radioisotopes in Smoke Detectors. AUS-e-TUTE n.d.. Retrieved March,30, 2011. 
  4. ^ Reaction in a Smoke Detector
  5. ^ PNNL: Isotope Sciences Program - Gadolinium-153
  6. ^ "Gadolinium". BCIT Chemistry Resource Center. British Columbia Institute of Technology. Retrieved 30 March, 2011. 
  7. ^ 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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