Chart of knownnuclides as of 2013[update]. The vast majority are radionuclides.
Aradionuclide (radioactive nuclide,radioisotope orradioactive isotope) is anuclide that is unstable and known to undergoradioactive decay into a different nuclide, which may be another radionuclide (seedecay chain) or be stable.Radiation emitted by radionuclides is almost alwaysionizing radiation because it is energetic enough to liberate an electron from another atom.
Radioactive decay is a randomprocess at the level of singleatoms: it is impossible topredict 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 thehalf-life (t1/2) 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.
All thechemical elements have radionuclides - even the lightest element,hydrogen, has one well-known radionuclide,tritium (thoughhelium,lithium, andboron have none with half-life over a second). Elements heavier thanlead (Z > 82), and the elementstechnetium andpromethium, have only radionuclides and do not exist in stable forms, thoughbismuth can be treated as stable with the half-life ofits natural isotope being over a trillion times longer than the currentage of the universe.
Radionuclides occur naturally and are artificially produced innuclear reactors,cyclotrons,particle accelerators orradionuclide generators. There are 735 known radionuclides with half-lives longer than an hour (seelist of nuclides); 35 of those areprimordial radionuclides whose presence on Earth has persisted from its formation, and another 62 are detectable in nature, continuously produced either asdaughter products of primordial radionuclides or bycosmic radiation. More than 2400 radionuclides have half-lives less than 60minutes. Most of those are only produced artificially, and have very short half-lives. For comparison, there are 251stable nuclides.
On Earth, naturally occurring radionuclides fall into three categories: primordial radionuclides, secondary radionuclides, andcosmogenic radionuclides.
Radionuclides are produced instellar nucleosynthesis andsupernova 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 asuranium andthorium, still exist because theirhalf-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 notablybismuth-209: detection of this decay meant thatbismuth 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.[citation needed]
Secondary radionuclides are radiogenic isotopes derived from the decay of primordial radionuclides. They have shorter half-lives than primordial radionuclides. They arise in thedecay chain of the primordial isotopesthorium-232,uranium-238, anduranium-235 - such as the natural isotopes ofpolonium andradium - some are also produced by naturalfission and othernucleogenic processes.[citation needed]
Many of these radionuclides exist only in trace amounts in nature, including all cosmogenic nuclides. Secondary radionuclides in adecay chain will occur in proportion to their half-lives, so short-lived ones will be very rare. For example, polonium can be found inuranium ores at a concentration about 1 part 1010 of uranium (0.1 mg permetric ton) by calculating the ratio of half-lives ofpolonium-210 touranium-238, its ultimate parent.[citation needed]
Radionuclides are produced as an unavoidable result ofnuclear fission andnuclear explosions. The process of nuclear fission creates a wide range offission products, most of which are radionuclides. Further radionuclides are created from irradiation of the nuclear fuel (creating a range ofactinides) and of the surrounding structures, yieldingactivation products. This complex mixture of radionuclides with different chemistries and radioactivity makes handlingnuclear waste and dealing withnuclear fallout particularly problematic.[citation needed]
As well as being extracted from nuclear waste, radioisotopes can be produced deliberately with nuclear reactors, exploiting the high flux ofneutrons present. These neutrons activate elements placed within the reactor. A typical product from a nuclear reactor isiridium-192, from activation ofiridium targets. The elements that have a large propensity to take up neutrons in the reactor are said to have a highneutron cross-section, but even at low cross-sections this process is generally economical.
Particle accelerators such ascyclotrons 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 thetechnetium-99m generator, which employsmolybdenum-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.
Inbiology, radionuclides (most often ofcarbon) can serve asradioactive 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 aGeiger counter, to determine where the provided atoms were incorporated. For example, one might culture plants in an environment in which thecarbon 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 asDNA replication oramino acid transport.[citation needed]
inphysics andbiology radionuclideX-ray fluorescence (conventional X-ray sources may also be used) is used to determinechemical composition of thecompound.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 theproton number of thechemical element that emits the radiation, and by measuring the number of emittedphotons, it is possible to determine theconcentration of individual chemical elements.[citation needed]
Innuclear 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 thehuman brain.[4][5][6] This is used in some forms of tomography:single-photon emission computed tomography andpositron emission tomography (PET) scanning andCherenkov luminescence imaging. Radioisotopes are also a method of treatment inhemopoietic forms of tumors; the success for treatment of solid tumors has been limited. More powerful gamma sourcessterilise syringes and other medical equipment.
Infood 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 likecobalt-60 orcaesium-137.[citation needed]
Inindustry, and inmining, 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.
Inecology, 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]
used predominantly in targeted radionuclide therapy (TRT) against somatostatin receptor-positive gastroenteropancreatic neuroendocrine tumors (GEP-NETs)
Americium-241 container in a smoke detector.Americium-241 capsule as found in smoke detector. The circle of darker metal in the center is americium-241; the surrounding casing is aluminium.
Radionuclides are present in many homes as they are used inside the most common householdsmoke detectors. The radionuclide used isamericium-241, which is created by bombarding plutonium with neutrons in a nuclear reactor. It decays by emittingalpha particles andgamma radiation to becomeneptunium-237. Smoke detectors use a very small quantity of241Am (about 0.29 micrograms per smoke detector) in the form ofamericium dioxide.241Am is used as it emits alpha particles which ionize the air in the detector'sionization 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 asradioactive contamination. They can also cause damage if they are excessively used during treatment or in other ways exposed to living beings, byradiation 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, toradiation burns andacute 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]
Summary table for classes of nuclides, stable and radioactive
Following is a summary table for thelist 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 toproton 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 (seelist 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 radioactiveprimordial nuclides, and may be detected on Earth, having survived from their presence in interstellar dust since before the formation of theSolar 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 artificialnuclear 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 inlist of nuclides.
Radioactive nonprimordial, but naturally occurring on Earth
62
348
Carbon-14 (and other isotopes generated bycosmic rays) and daughters of radioactive primordial elements, such asradium andpolonium, of which 32 have a half-life of greater than one hour, also long-livedfission products.
Radioactive synthetic half-life ≥ 1.0 hour). Includes most usefulradiotracers.
^"Cosmic Detectives". The European Space Agency (ESA). 2 April 2013. Retrieved15 April 2013.
^Table data is derived by counting members of the list; seeWP:CALC. References for the list data itself are given below in the reference section inlist of nuclides
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