A radionuclide ( radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that is unstable and known to undergo radioactive decay into a different nuclide, which may be another radionuclide (see decay chain) or be stable. Radiation emitted by radionuclides is almost always ionizing radiation because it is energetic enough to liberate an electron from another atom.
Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay.
Radionuclides occur naturally and are artificially produced in , , particle accelerators or radionuclide generators. There are 735 known radionuclides with half-lives longer than an hour (see list of nuclides); 35 of those are primordial radionuclides whose presence on Earth has persisted from its formation, and another 62 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 251 .
All the have radionuclides - even the lightest element, hydrogen, has one well-known radionuclide, tritium (though helium, lithium, and boron have none with half-life over a second). Elements heavier than lead, and the elements technetium and promethium, have only radionuclides and do not exist in stable forms, though bismuth can generally be treated as stable with the half-life of its natural isotope being extremely long and its decay normally undetectable.
Exposure to radionuclides generally has, due to their radiation, a harmful effect on living organisms including humans, although low levels of exposure occur naturally. 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 considered unavoidable. 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.
Origin
Natural
On Earth, naturally occurring radionuclides fall into three categories: primordial radionuclides, secondary radionuclides, and
cosmogenic radionuclides.
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Radionuclides are produced in stellar nucleosynthesis and supernova explosions along with stable nuclides. Most decay quickly, but some can be observed astronomically and can play a part in understanding astrophysical processes. Primordial radionuclides, such as uranium and thorium, still exist because their half-life are so long (>100 million years) that the Earth's initial content has 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 that decay may be observed in other nuclides now considered stable, adding to the list of primordial radionuclides.
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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 - such as the natural isotopes of polonium and radium - some are also produced by natural fission and other nucleogenic processes.
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Cosmogenic isotopes, such as carbon-14, are present because they are continually being formed on Earth, typically in the atmosphere, due to the action of .
Many of these radionuclides exist only in trace amounts in nature, including all cosmogenic nuclides. Secondary radionuclides in a decay chain 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 a concentration about 1 part 1010 of uranium (0.1 mg per metric ton) by calculating the ratio of half-lives of polonium-210 to uranium-238, its ultimate parent.
Nuclear fission
Radionuclides are produced as an unavoidable result of
nuclear fission and
nuclear weapon. The process of nuclear fission creates a wide range of
fission products, most of which are radionuclides. Further radionuclides are 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
Synthetic radionuclides are created in
or by particle accelerators (not necesssarily on purpose) or as decay products of such:
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As well as being extracted from nuclear waste, radioisotopes can be produced deliberately with nuclear reactors, exploiting the high flux of present. These neutrons activate elements placed within the reactor. A typical product from a nuclear reactor is iridium-192, from activation of iridium targets. The elements that have a large propensity to take up neutrons in the reactor are said to have a high neutron cross-section, but even at low cross-sections this process is generally economical.
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Particle accelerators such as accelerate particles to bombard a target to produce radionuclides. Cyclotrons accelerate (most often) protons at a target to produce positron-emitting radionuclides, e.g. fluorine-18.
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Radionuclide generators, standard for many medical isotopes, contain a parent radionuclide that decays to produce a shorter-lived radioactive daughter. A typical example is the technetium-99m generator, which employs molybdenum-99 produced in a reactor.
Uses
Radionuclides are used in two major ways: either for their radiation alone (
irradiation,
nuclear battery) or for the combination of chemical properties and their radiation (tracers, biopharmaceuticals). For scientific study they may be used for their chemical properties alone when there is no stable form of that element.
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In biology, radionuclides (most often 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.
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in physics and biology radionuclide X-ray fluorescence (conventional X-ray sources may also be used) is used to determine chemical composition of the compound. X-ray from a radionuclide source hits the sample and excites characteristic X-rays in the sample. This radiation is registered and the chemical composition of the sample can be determined from the analysis of the measured spectrum. By measuring the energy of the characteristic radiation lines, it is possible to determine the Atomic number of the chemical element that emits the radiation, and by measuring the number of emitted , it is possible to determine the concentration of individual chemical elements.
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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.
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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. Food irradiation usually uses strong gamma emitters like cobalt-60 or caesium-137.
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In industry, and in mining, radiation from radionuclides may be 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.
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In spacecraft, radionuclides are used to provide power and heat, notably through radioisotope thermoelectric generators (RTGs) and radioisotope heater units (RHUs).
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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).
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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.
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In geology, archaeology, and paleontology, natural radionuclides are used to measure ages of rocks, minerals, and fossil materials. This is called radiometric dating.
Examples
The following table lists properties of selected radionuclides illustrating the range of properties and uses.
Key:
Z =
atomic number;
N =
neutron number; DM = decay mode; DE = decay energy; EC =
electron capture
Household smoke detectors
Radionuclides are present in many homes as they are used inside the most common household
. The radionuclide used is americium-241, which is created by bombarding plutonium with neutrons in a nuclear reactor. It decays by emitting
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.
[ Office of Radiation Protection – Am 241 Fact Sheet – Washington State Department of Health ]
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
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."
Summary table for classes of nuclides, stable and radioactive
Following is a summary table for the list of 986 nuclides with half-lives greater than one hour. A total of 251 nuclides have never been observed to decay, and are classically considered stable. Of these, 90 are believed to be absolutely stable except to
proton decay (which has never been observed), while the rest are "observationally stable" and theoretically can undergo radioactive decay with extremely long half-lives.
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 31 nuclides with measured half-lives longer than the estimated age of the universe (13.8 billion years
Numbers may change slightly in the future as some nuclides now classified as stable are observed to be radioactive with very long half-lives.
This is a summary table[Table data is derived by counting members of the list; see . References for the list data itself are given below in the reference section in list of nuclides] for the 986 nuclides with half-lives longer than one hour (including those that are stable), given in list of nuclides.
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| 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. | 105 | 251 | Total of classically . |
| Radioactive primordial nuclides | 35 | 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 | 62 | 348 | Carbon-14 (and other isotopes generated by cosmic rays) and daughters of radioactive primordial elements, such as radium and polonium, of which 32 have a half-life of greater than one hour, also long-lived fission products. |
| Radioactive synthetic half-life ≥ 1.0 hour). Includes most useful . | 638 | 986 | These comprise the remainder of the list of nuclides. |
| Radioactive synthetic (half-life < 1.0 hour). | >2400 | >3300 | Includes all well-characterized synthetic nuclides. |
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See also
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List of nuclides shows all radionuclides with half-life > 1 hour
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Hyperaccumulators table – 3 (includes organisms noted for accumulating radionuclides)
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Radioactivity in biology
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Radiometric dating
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Radionuclide cisternogram
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Uses of radioactivity in oil and gas wells
Notes
Further reading
External links
