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.^[1][2] However, for a collection of atoms of a single nuclide, the decay rate (considered as a statistical average), and thus the half-life (t_1/2) for that nuclide, can be calculated from the measurement of the decay. The range of the half-lives of radioactive atoms has no known limits and spans 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 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.

All the chemical elements 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 (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.

Artificial production methods of radionuclides include neutron sources such as nuclear reactors, as well as particle accelerators such as cyclotrons.

Exposure to radionuclides generally has, due to their radiation, a harmful effect on 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 (alpha, beta, gamma, or neutron), the amount and nature of exposure (close contact, inhalation or ingestion), and the biochemical properties of the element (toxicity). Increased risk of cancer is considered unavoidable, and worse cases experience radiation-induced cancer, chronic radiation syndrome or acute radiation syndrome. Radionuclides are weaponized by the fallout effects of nuclear weapons and by radiological weapons.

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. Radionuclide therapy is a form of radiotherapy. A pharmaceutical drug made with radionuclides is called a radiopharmaceutical.