Radionuclide

A radionuclide is an atom with an unstable nucleus, which is a nucleus characterized by excess energy which is available to be imparted either to a newly-created radiation particle within the nucleus, or else to an atomic electron. The radionuclide, in this process, undergoes radioactive decay, and emits a gamma ray(s) and/or subatomic particles. These particles constitute ionizing radiation. Radionuclides may occur naturally, but 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 nuclide list contain 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). However, radionuclides can also present both real and perceived dangers to health.

Origin

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.

Uses

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 promising method of treatment in hemopoietic forms of tumors, while the success for treatment of solid tumors has been limited so far. More powerful gamma sources sterilise syringes and other medical equipment. About one in two people in Western countries are likely to experience the benefits of nuclear medicine in their lifetime.

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 agriculture and animal husbandry, radionuclides also play an important role. They produce high intake of crops, disease and weather resistant varieties of crops, to study how fertilisers and insecticides work, and to improve the production and health of domestic animals.

Industrially, 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.

Most household smoke detectors contain the radionuclide americium formed in nuclear reactors.

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.

Dangers

If radionuclides are released into the environment, through accident, poor disposal, or other means, they can potentially 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. This is called radiation poisoning. Radionuclides can also cause malfunction of some electrical devices.

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. Only 90 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 with half lifes longer than 1 hour, have half-lives that 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 before the present. 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.

Note that 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 for the 905 nuclides with half lives longer than one hour (including those that are stable), given in list of nuclides. Note that numbers are not exact, and may change slightly in the future, as "stable" nuclides are observed to be radioactive with very long half lives.

{| class="wikitable sortable" width="100%" ! width="300" |Stability class ! Number of nuclides ! Running total ! Notes on running total |- | align="left"| Theoretically stable to all but proton decay | align="center"| 90 | align="center"| 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. | align="center"| 165 | align="center"| 255 | Total of classically stable nuclides. |- | Radioactive primordial nuclides. | align="center"| 33 | align="center"| 288 | Total primordial elements include bismuth, uranium, thorium, plutonium, plus all stable nuclides. |- | Radioactive non-primordial, but naturally occurring on Earth. | align="center"| ~ 51 | align="center"| ~ 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. | align="center"| 556 | align="center"| 905 | These 905 nuclides are listed in the article List of nuclides. |- | Radioactive synthetic (half life < 1 hour). | align="center"| >2400 | align="center"| >3300 | Includes all well-characterized synthetic nuclides. |- |}

List of commercially available radionuclides

Gamma only

http://www.uic.com.au/nip27.htm

See also

List of nuclides shows all radionuclides with half life > 1 hour

Hyperaccumulators table – 3

Radioactivity in biology

Radiometric dating

Radionuclide cisternogram

Notes

References

Carlsson J et al.:"Tumour therapy with radionuclides: assessment of progress and problems". Radiotherapy and Oncology, Volume 66, Issue 2, February 2003, Pages 107–117. PMID 12648782. Available online as full text.

Radioisotopes in Industry, World Nuclear Association.

External links

EPA - Radionuclides - EPA's Radiation Protection Program: Information.

FDA - Radionuclides - FDA's Radiation Protection Program: Information.

Interactive Chart of Nuclides - A chart of all nuclides

The Live Chart of Nuclides - IAEA , in Java or HTML

http://onlinelibrary.wiley.com/doi/10.1002/14356007.a22_499/full

http://www.uic.com.au/nip27.htm

Category:Radioactivity Category:Isotopes Category:Nuclear physics Category:Nuclear chemistry