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A radionuclide or radioactive nuclide is a nuclide that is radioactive. Also referred to as a radioisotope or radioactive isotope, it is an isotope 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. Many radionuclides occur naturally, and others are produced artificially, for example in nuclear reactors and cyclotrons.
There are about 650 radionuclides with half-lives longer than 60 minutes (see list of nuclides). Of these, 34 are primordial radionuclides that existed before the creation of the solar system, and there are another 50 radionuclides detectable in nature as daughters of these, or produced naturally on Earth by cosmic radiation. There is a much larger number of radionuclides, more than 2400, with decay half-lives shorter than 60 minutes. Most of these are only produced artificially, and have very short half-lives. For comparison, there are about 254 stable nuclides.
All chemical elements have radionuclides. Even the lightest element, hydrogen, has a well-known radionuclide, tritium. Elements heavier than lead, and the elements technetium and promethium, exist only as radionuclides.
Radionuclides with suitable half-lives play an important part in a number of technologies, for example ionization smoke detectors and nuclear medicine. A pharmaceutical drug made with radionuclides is called a radiopharmaceutical, and an imaging tracer made with radionuclides is called a radiotracer. Nuclear medicine makes use of these drugs and tracers for radiation therapy such as brachytherapy and imaging.
Radionuclides can also present both real and perceived dangers to health.
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 due 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 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 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 produced 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 americium-241 decays by emitting alpha particles and gamma radiation to become neptunium-237. Most common household smoke detectors use a very small quantity of 241Am (about 0.29 micrograms per smoke detector) in the form of americium dioxide. Smoke detectors use 241Am 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 153Gd 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 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."
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 254 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 28 nuclides with measured half-lives longer than the estimated age of the universe (13.8 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.||164||254||Total of classically stable nuclides.|
|Radioactive primordial nuclides.||34||288||Total primordial elements include uranium, thorium, bismuth, rubidium-87, potassium-40 plus all stable nuclides.|
|Radioactive nonprimordial, but naturally occurring on Earth.||~ 51||~ 339||Carbon-14 (and other isotopes generated by cosmic rays) and daughters of radioactive primordial elements, such as radium, polonium, etc.|
|Radioactive synthetic (half-life ≥ 1.0 hour). Includes most useful radiotracers.||556||905||These 905 nuclides are listed in the article List of nuclides.|
|Radioactive synthetic (half-life < 1.0 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 most countries. Others that 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 list of nuclides, and Isotope Lists. For a table see 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||1.95×104 TBq/g (5.27 × 107 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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