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A radionuclide is an atom with an unstable nucleus, characterized by excess energy available to be imparted either to a newly created radiation particle within the nucleus or via internal conversion. During this process, the radionuclide is said to undergo radioactive decay, resulting in the emission of gamma ray(s) and/or subatomic particles such as alpha or beta particles. These emissions constitute ionizing radiation. Radionuclides occur naturally, or can be produced artificially.
Radionuclides are often referred to by chemists and physicists as radioactive isotopes or radioisotopes. Radioisotopes with suitable half-lives play an important part in a number of technologies (for example, nuclear medicine). Radionuclides can also present both real and perceived dangers to health.
The number of radionuclides is uncertain because the number of very short-lived radionuclides that have yet to be characterized is extremely large and potentially unquantifiable. Even the number of long-lived radionuclides is uncertain (to a lesser degree), because many "stable" nuclides are calculated to have half-lives so long that their decay has not been experimentally measured. The total list of nuclides contains 90 nuclides that are theoretically stable, and 255 total stable nuclides that have not been observed to decay. In addition, there exist about 650 radionuclides that have been experimentally observed to decay, with half-lives longer than 60 minutes (see list of nuclides for this list). Of these, about 339 are known from nature (they have been observed on Earth, and not as a consequence of man-made activities).
Including artificially produced nuclides, more than 3300 nuclides are known (including ~3000 radionuclides), many of which (> ~2400) have decay half-lives shorter than 60 minutes. This list expands as new radionuclides with very short half-lives are characterized.
All elements form a number of radionuclides, although the half lives of many are too short for them to be observed in nature. Even the lightest element, hydrogen, has a well-known radioisotope, tritium. The heaviest elements (heavier than bismuth) exist only as radionuclides. For every chemical element, many radioisotopes that do not occur in nature (due to short half lives or the lack of an ongoing natural production mechanism), have been produced artificially.
Naturally occurring radionuclides fall into three categories: primordial radionuclides, secondary radionuclides, and cosmogenic radionuclides. Primordial radionuclides, such as uranium and thorium, originate mainly from the interiors of stars and are still present as their half-lives are so long they have not yet completely decayed. Secondary radionuclides are radiogenic isotopes derived from the decay of primordial radionuclides. They have shorter half-lives than primordial radionuclides. Cosmogenic isotopes, such as carbon-14, are present because they are continually being formed in the atmosphere due to cosmic rays.
Artificially produced radionuclides can be produced by nuclear reactors, particle accelerators or by radionuclide generators:
Trace radionuclides are those that occur in tiny amounts in nature either due to inherent rarity, or to half-lives that are significantly shorter than the age of the Earth. Synthetic isotopes are inherently not naturally occurring on Earth, but can be created by nuclear reactions.
Radionuclides are used in two major ways: for their chemical properties and as sources of radiation. Radionuclides of familiar elements such as carbon can serve as tracers because they are chemically very similar to the non-radioactive nuclides, so most chemical, biological, and ecological processes treat them in a near identical way. One can then examine the result with a radiation detector, such as a geiger counter, to determine where the provided atoms ended up. For example, one might culture plants in an environment in which the carbon dioxide contained radioactive carbon; then the parts of the plant that had laid down atmospheric carbon would be radioactive.
In nuclear medicine, radioisotopes are used for diagnosis, treatment, and research. Radioactive chemical tracers emitting gamma rays or positrons can provide diagnostic information about a person's internal anatomy and the functioning of specific organs. This is used in some forms of tomography: single-photon emission computed tomography and positron emission tomography scanning and Cerenkov 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.
Radionuclides are also 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. Natural radionuclides are used in geology, archaeology, and paleontology to measure ages of rocks, minerals, and fossil materials.
Most household smoke detectors contain americium formed in nuclear reactors. The radioisotope used is americium-241. The element americium is created by bombarding plutonium with neutrons in a nuclear reactor. Its isotope, Am-241 decays by emitting alpha particles and gamma radiation to become neptunium-237. The most common household smoke detectors use a very small quantity of Am-241 (about 0.29 micrograms per smoke detector) in the form of americium dioxide. The smoke detectors use the Am-241 since the alpha particles it emits collide with oxygen and nitrogen particles in the air. This occurs in the detector's ionization chamber where it produces charged particles or ions. Then, these charged particles are collected by a small electric voltage that will create an electric current that will pass between two electrodes. Then, the ions that are flowing between the electrodes will be neutralized when coming in contact with smoke, thereby decreasing the electric current between the electrodes, which will activate the detector's alarm. 
The Gd-153 isotope is used in X-ray fluorescence and osteoporosis screening. It is a gamma-emitter with an 8-month half-life, making it easier to use for medical purposes. In nuclear medicine, it serves to calibrate the equipment needed like single-photon emission computed tomography systems (SPECT) to make x-rays. It ensures that the machines work correctly to produce images of radioisotope distribution inside the patient. This isotope is produced in a nuclear reactor from europium or enriched gadolinium. It can also detect the loss of calcium in the hip and back bones, allowing the ability to diagnose osteoporosis. 
Radionuclides that find their way into the environment may cause harmful effects of radioactive contamination. They can also cause damage if they are excessively used during treatment or in other ways applied to living beings, by radiation poisoning.
Following is a summary table for the total list of nuclides with half-lives greater than one hour. Ninety of these 905 nuclides are theoretically stable, except to proton-decay (which has never been observed). About 255 nuclides have never been observed to decay, and are classically considered stable.
The remaining 650 radionuclides have half-lives longer than 1 hour, and are well characterized (see list of nuclides for a complete tabulation). They include 27 nuclides with measured half-lives longer than the estimated age of the universe (13.7 billion years), and another 6 nuclides with half-lives long enough (> 80 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 ~51 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.
Numbers are not exact, and may change slightly in the future, as "stable nuclides" are observed to be radioactive with very long half-lives.
|Stability class||Number of nuclides||Running total||Notes on running total|
|Theoretically stable to all but proton decay||90||90||Includes first 40 elements. Proton decay yet to be observed.|
|Energetically unstable to one or more known decay modes, but no decay yet seen. Spontaneous fission possible for "stable" nuclides > niobium-93; other mechanisms possible for heavier nuclides. All considered "stable" until decay detected.||165||255||Total of classically stable nuclides.|
|Radioactive primordial nuclides.||33||288||Total primordial elements include bismuth, uranium, thorium, plutonium, plus all stable nuclides.|
|Radioactive non-primordial, but naturally occurring on Earth.||~ 51||~ 339||Carbon-14 (and other isotopes generated by cosmic rays); daughters of radioactive primordials, such as francium, etc.|
|Radioactive synthetic (half-life > 1 hour). Includes most useful radiotracers.||556||905||These 905 nuclides are listed in the article List of nuclides.|
|Radioactive synthetic (half-life < 1 hour).||>2400||>3300||Includes all well-characterized synthetic nuclides.|
This list covers common isotopes, most of which are available in very small quantities to the general public in the US and other countries. Others which are not publicly accessible are traded commercially in industrial, medical, and scientific fields and are subject to government regulation. For a complete list of all known isotopes for every element (minus activity data), see Isotope Lists and Table of Nuclides.
|Barium-133||9694 TBq/Kg (262 Ci/g)||10.7 years||81.0, 356.0|
|Cadmium-109||96200 TBq/Kg (2600 Ci/g)||453 days||88.0|
|Cobalt-57||312280 TBq/Kg (8440 Ci/g)||270 days||122.1|
|Cobalt-60||40700 TBq/Kg (1100 Ci/g)||5.27 years||1173.2, 1332.5|
|Europium-152||6660 TBq/Kg (180 Ci/g)||13.5 years||121.8, 344.3, 1408.0|
|Manganese-54||287120 TBq/Kg (7760 Ci/g)||312 days||834.8|
|Sodium-22||237540 Tbq/Kg (6240 Ci/g)||2.6 years||511.0, 1274.5|
|Zinc-65||304510 TBq/Kg (8230 Ci/g)||244 days||511.0, 1115.5|
|Technetium-99m||1.95x10^4 TBq/g (5.27x10^7 Ci/g)||6 hours||140|
|Strontium-90||5180 TBq/Kg (140 Ci/g)||28.5 years||546.0|
|Thallium-204||17057 TBq/Kg (461 Ci/g)||3.78 years||763.4|
|Carbon-14||166.5 TBq/Kg (4.5 Ci/g)||5730 years||49.5 (average)|
|Tritium (Hydrogen-3)||357050 TBq/Kg (9650 Ci/g)||12.32 years||5.7 (average)|
|Polonium-210||166500 TBq/Kg (4500 Ci/g)||138 days||5304.5|
|Uranium-238||12580 KBq/Kg (0.00000034 Ci/g)||4.468 billion years||4267|
|Isotope||Activity||Half-life||Radiation types||Energies (KeV)|
|Caesium-137||3256 TBq/Kg (88 Ci/g)||30.1 years||Gamma & beta||G: 32, 661.6 B: 511.6, 1173.2|
|Americium-241||129.5 TBq/Kg (3.5 Ci/g)||432.2 years||Gamma & alpha||G: 59.5, 26.3, 13.9 A: 5485, 5443|
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