Radionuclide

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. For a collection of atoms of a single nuclide, their average decay rate can be measured, and its half-life (t1/2) calculated. Half-lives vary by 55 orders of magnitude and have no known limits. All chemical elements have radionuclides - even the lightest, hydrogen, has a well-known radionuclide, tritium (though helium, lithium, and boron have none with half-life over a second). Elements heavier than lead (Z > 82), and the elements technetium and promethium, have only radionuclides and do not exist in stable forms, though bismuth can be treated as stable with the half-life of its natural isotope being over a trillion times longer than the current age of the universe.

Production and effects Artificial production methods of radionuclides include neutron sources such as nuclear reactors, as well as particle accelerators such as cyclotrons. The radiation from radionuclides generally has a harmful effect on organisms including humans, although low levels of exposure occur naturally. The degree of harm depends on the nature and extent of the radiation (alpha, beta, gamma, or neutron), the amount and nature of exposure (close contact, inhalation or ingestion), and the element's biochemical properties (toxicity). Increased risk of cancer is unavoidable, and worse cases induce cancer, chronic radiation syndrome or acute radiation syndrome. Radionuclides can be used as weapons by the fallout effects of nuclear weapons and by radiological weapons. Radionuclides are used in nuclear medicine for both diagnosis and treatment. An imaging tracer made with radionuclides is a radioactive tracer. Radionuclide therapy is a form of radiotherapy. A pharmaceutical drug made with radionuclides is called a radiopharmaceutical.

Origins

Overview Radionuclides occur naturally and are artificially produced in nuclear reactors, cyclotrons, 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, continuously produced either as daughter products of primordial radionuclides or 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 stable nuclides.

Natural 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 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-lives 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. 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. 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 cosmic rays. 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 explosions. 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 nuclear reactors or by particle accelerators (not necesssarily on purpose) or as decay products of such:

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