出典(authority):フリー百科事典『ウィキペディア(Wikipedia)』「2016/08/19 11:24:48」(JST)
A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is an atom that has excess nuclear energy, making it unstable. This excess energy can either create and emit, from the nucleus, new radiation (gamma radiation) or a new particle (alpha particle or beta particle), or transfer this excess energy to one of its electrons, causing it to be ejected (conversion electron). During this process, the radionuclide is said to undergo radioactive decay.[1] These emissions constitute ionizing radiation. The unstable nucleus is more stable following the emission, but will sometimes undergo further decay. Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay.[2][3][4][5] However, for a collection of atoms of a single element the decay rate, and thus the half-life (t1/2) for that collection can be calculated from their measured decay constants. The duration of the half-lives of radioactive atoms have no known limits; the time range is over 55 orders of magnitude.
Radionuclides both occur naturally and are artificially made using nuclear reactors, cyclotrons, particle accelerators or radionuclide generators. 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. More than 2400 radionuclides have half-lives less 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.
Unplanned exposure to radionuclides generally has a harmful effect on living organisms including humans, although low levels of exposure occur naturally without harm. The degree of harm will depend on the nature and extent of the radiation produced, the amount and nature of exposure (close contact, inhalation or ingestion), and the biochemical properties of the element; with increased risk of cancer the most usual consequence. However, 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. A pharmaceutical drug made with radionuclides is called a radiopharmaceutical.
On Earth, naturally occurring radionuclides fall into three categories: primordial radionuclides, secondary radionuclides, and cosmogenic radionuclides.
Many of these radionuclides exist only in trace amounts in nature, including the two shortest-lived primordial nuclides and all cosmogenic nuclides. Secondary radionuclides will occur in proportion to their half-lives, so short-lived ones will be very rare. Thus polonium can be found in uranium ores at about 0.1 mg per metric ton (1 part in 1010),.[7][8] Further radionunclides may occur in nature in virtually undetectable amounts as a result of rare events such as spontaneous fission or uncommon cosmic ray interactions.
Radionuclides are produced as an unavoidable result of nuclear fission and thermonuclear explosions. The process of nuclear fission creates a wide range of fission products, most of which are radionuclides. Further radionuclides can be created from irradiation of the nuclear fuel (creating a range of actinides) and of the surrounding structures, yielding activation products. This complex mixture of radionuclides with different chemistries and radioactivity makes handling nuclear waste and dealing with nuclear fallout particularly problematic.
Synthetic radionuclides are deliberately synthesised using nuclear reactors, particle accelerators or radionuclide generators:
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).
The following table lists properties of selected radionuclides illustrating the range of properties and uses.
Isotope | Z | N | half life | DM | DE keV |
Mode of formation | Comments |
---|---|---|---|---|---|---|---|
Tritium (3H) | 1 | 2 | 12.3 y | β− | 19 | Cosmogenic | lightest radionuclide, used in artificial nuclear fusion, also used for radioluminescence and as oceanic transient tracer. Synthesized from neutron bombardment of lithium-6 or deuterium |
Beryllium-10 | 4 | 6 | 1,387,000 y | β− | 556 | Cosmogenic | used to examine soil erosion, soil formation from regolith, and the age of ice cores |
Carbon-14 | 6 | 8 | 5,700 y | β− | 156 | Cosmogenic | used for radiocarbon dating |
Fluorine-18 | 9 | 9 | 110 min | β+,EC | 633/1655 | Cosmogenic | positron source, synthesised for use as a medical radiotracer in PET scans. |
Aluminium-26 | 13 | 13 | 717,000 y | β+,EC | 4004 | Cosmogenic | exposure dating of rocks, sediment |
Chlorine-36 | 17 | 19 | 301,000 y | β−,EC | 709 | Cosmogenic | exposure dating of rocks, groundwater tracer |
Potassium-40 | 19 | 21 | 1.24×109 y | β−,EC | 1330 /1505 | Primordial | used for potassium-argon dating, source of atmospheric argon, source of radiogenic heat, largest source of natural radioactivity |
Calcium-41 | 20 | 21 | 102,000 y | EC | Cosmogenic | exposure dating of carbonate rocks | |
Cobalt-60 | 27 | 33 | 5.3 y | β− | 2824 | Synthetic | produces high energy gamma rays, used for radiotherapy, equipment sterilisation, food irradiation |
Strontium-90 | 38 | 52 | 28.8 y | β− | 546 | Fission product | medium-lived fission product; probably most dangerous component of nuclear fallout |
Technetium-99 | 43 | 56 | 210,000 y | β− | 294 | Fission product | commonest isotope of the lightest unstable element, most significant of long-lived fission products |
Technetium-99m | 43 | 56 | 6 hr | γ,IC | 141 | Synthetic | most commonly used medical radioisotope, used as a radioactive tracer |
Iodine-129 | 53 | 76 | 15,700,000 y | β− | 194 | Cosmogenic | longest lived fission product; groundwater tracer |
Iodine-131 | 53 | 78 | 8 d | β− | 971 | Fission product | most significant short term health hazard from nuclear fission, used in nuclear medicine, industrial tracer |
Xenon-135 | 54 | 81 | 9.1 h | β− | 1160 | Fission Product | strongest known "nuclear poison" (neutron-absorber), with a major effect on nuclear reactor operation. |
Caesium-137 | 55 | 82 | 30.2 y | β− | 1176 | Fission Product | other major medium-lived fission product of concern |
Bismuth-209 | 83 | 126 | 1.9×1019y | α | 3137 | Primordial | long considered stable, decay only detected in 2003 |
Polonium-210 | 84 | 126 | 138 d | α | 5307 | Decay Product | Highly toxic, used in poisoning of Alexander Litvinenko |
Radon-222 | 86 | 136 | 3.8d | α | 5590 | Decay Product | gas, responsible for the majority of public exposure to ionizing radiation, second most frequent cause of lung cancer |
Thorium-232 | 90 | 142 | 1.4×1010 y | α | 4083 | Primordial | basis of thorium fuel cycle |
Uranium-235 | 92 | 143 | 7×108y | α | 4679 | Primordial | fissile, main nuclear fuel |
Uranium-238 | 92 | 146 | 4.5×109 y | α | 4267 | Primordial | Main Uranium isotope |
Plutonium-238 | 94 | 144 | 87.7 y | α | 5593 | Synthetic | used in radioisotope thermoelectric generators (RTGs) and radioisotope heater units as an energy source for spacecraft |
Plutonium-239 | 94 | 145 | 24110 y | α | 5245 | Synthetic | used for most modern nuclear weapons |
Americium-241 | 95 | 146 | 432 y | α | 5486 | Synthetic | used in household smoke detectors as an ionising agent |
Californium-252 | 98 | 155 | 2.64 y | α/SF | 6217 | Synthetic | undergoes spontaneous fission (3% of decays), making in a powerful neutron source, used as a reactor initiator and for detection devices |
Key: Z = no of protons; N = no of Neutrons; DM = Decay Mode; DE = Decay Energy
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.[13][14]
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[compared to?] 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.[15] It can also detect the loss of calcium in the hip and back bones, allowing the ability to diagnose osteoporosis.[16]
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."[17]
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[18]), 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.
This is a summary table [19] for the 905 nuclides with half-lives longer than one hour (including those that are stable), given in list of nuclides.
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.
Isotope | Activity | Half-life | Energies (keV) |
---|---|---|---|
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 | 7004195000000000000♠1.95×104 TBq/g (5.27 × 107 Ci/g) | 6 hours | 140 |
Isotope | Activity | Half-life | Energies (keV) |
---|---|---|---|
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) |
Isotope | Activity | Half-life | Energies (keV) |
---|---|---|---|
Polonium-210 | 166500 TBq/kg (4500 Ci/g) | 138.376 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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