Radionuclide

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.

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.

Synthetic

Synthetic radionuclides are created in nuclear reactors or by particle accelerators (not necesssarily on purpose) or as decay products of such:^[3]

• As well as being extracted from nuclear waste, radioisotopes can be produced deliberately with nuclear reactors, exploiting the high flux of neutrons 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.
• Particle accelerators such as cyclotrons 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.
• 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.

Radionuclides are used in two major ways: either for their radiation alone (irradiation, nuclear batteries) 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.

• 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.^{[citation needed]}
• 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. Radiation 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 proton number of the chemical element that emits the radiation, and by measuring the number of emitted photons, it is possible to determine the concentration of individual chemical elements.^{[citation needed]}
• 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.^[4][5][6] This is used in some forms of tomography: single-photon emission computed tomography and positron emission tomography (PET) scanning and Cherenkov luminescence imaging. 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 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.^{[citation needed]}
• 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.
• In spacecraft, radionuclides are used to provide power and heat, notably through radioisotope thermoelectric generators (RTGs) and radioisotope heater units (RHUs).^{[citation needed]}
• 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).^[7]
• 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.^{[citation needed]}
• In geology, archaeology, and paleontology, natural radionuclides are used to measure ages of rocks, minerals, and fossil materials. This is called radiometric dating.

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 smoke detectors. The radionuclide used is americium-241, which is created by bombarding plutonium with neutrons in a nuclear reactor. It decays by emitting alpha particles 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.^[8][9]

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 radiation burns 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."^[10]

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.^{[citation needed]}

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^[11]), and another four nuclides with half-lives long enough (> 100 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 60+ 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.^{[citation needed]}

Numbers may change slightly in the future as some nuclides now classified as stable are observed to be radioactive with very long half-lives.^{[citation needed]}

This is a summary table^[12] for the 986 nuclides with half-lives longer than one hour (including those that are stable), given in list of nuclides.

• List of nuclides shows all radionuclides with half-life > 1 hour
• Hyperaccumulators table – 3 (includes organisms noted for accumulating radionuclides)
• Radioactivity in biology
• Radiometric dating
• Radionuclide cisternogram
• Uses of radioactivity in oil and gas wells