Radionuclide
A radionuclide is an atom that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferred to one of its electrons to release it as a conversion electron; or used to create and emit a new particle from the nucleus. During those processes, the radionuclide is said to undergo radioactive decay. These emissions are considered ionizing radiation because they are powerful enough to liberate an electron from another atom. The radioactive decay can produce a stable nuclide or will sometimes produce a new unstable radionuclide which may 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. However, for a collection of atoms of a single element the decay rate, and thus the half-life for that collection, can be calculated from their measured decay constants. The range of the half-lives of radioactive atoms has no known limits and spans a time range of over 55 orders of magnitude.
Radionuclides occur naturally or are artificially produced in nuclear reactors, cyclotrons, particle accelerators or radionuclide generators. There are about 730 radionuclides with half-lives longer than 60 minutes. Thirty-two of those are primordial radionuclides that were created before the earth was formed. At least another 60 radionuclides are detectable in nature, either as daughters of primordial radionuclides or as radionuclides produced through natural production on Earth 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 about 252 stable nuclides.
All chemical elements can exist as 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, 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.
Origin
Natural
On Earth, naturally occurring radionuclides fall into three categories: primordial radionuclides, secondary radionuclides, and cosmogenic radionuclides.- Radionuclides are produced in stellar nucleosynthesis and supernova explosions along with stable nuclides. Most decay quickly but can still be observed astronomically and can play a part in understanding astronomic processes. Primordial radionuclides, such as uranium and thorium, exist in the present time because their half-lives are so long that they have not yet completely decayed. Some radionuclides have half-lives so long that decay has only recently been detected, and for most practical purposes they can be considered stable, most notably bismuth-209: detection of this decay meant that bismuth was no longer considered stable. It is possible decay may be observed in other nuclides, adding to this list of primordial radionuclides.
- Secondary radionuclides are radiogenic isotopes derived from the decay of primordial radionuclides. They have shorter half-lives than primordial radionuclides. They arise in the decay chain of the primordial isotopes thorium-232, uranium-238, and uranium-235. Examples include the natural isotopes of polonium and radium.
- Cosmogenic isotopes, such as carbon-14, are present because they are continually being formed in the atmosphere due to cosmic rays.
Nuclear fission
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 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
Synthetic radionuclides are deliberately synthesised using nuclear reactors, particle accelerators or radionuclide generators:- 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. The elements that have a large propensity to take up the neutrons in the reactor are said to have a high neutron cross-section.
- Particle accelerators such as cyclotrons accelerate particles to bombard a target to produce radionuclides. Cyclotrons accelerate protons at a target to produce positron-emitting radionuclides, e.g. fluorine-18.
- Radionuclide generators contain a parent radionuclide that decays to produce a radioactive daughter. The parent is usually produced in a nuclear reactor. A typical example is the technetium-99m generator used in nuclear medicine. The parent produced in the reactor is molybdenum-99.
Uses
- In biology, radionuclides 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.
- 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. This is used in some forms of tomography: single-photon emission computed tomography and positron emission tomography 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.
- In industry, and in mining, radionuclides are 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.
- In astronomy and cosmology, radionuclides play a role in understanding stellar and planetary process.
- In particle physics, radionuclides help discover new physics by measuring the energy and momentum of their beta decay products.
- 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.
- In geology, archaeology, and paleontology, natural radionuclides are used to measure ages of rocks, minerals, and fossil materials.
Examples
| Isotope | Z | N | half-life | DM | DE keV | Mode of formation | Comments |
| Tritium | 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 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 | 99,400 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", 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 |
| Gadolinium-153 | 64 | 89 | 240 d | EC | Synthetic | Calibrating nuclear equipment, bone density screening | |
| Bismuth-209 | 83 | 126 | 2.01y | 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 | 3.8 d | 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 y | α | 4083 | Primordial | basis of thorium fuel cycle |
| Uranium-235 | 92 | 143 | 7y | α | 4679 | Primordial | fissile, main nuclear fuel |
| Uranium-238 | 92 | 146 | 4.5 y | α | 4267 | Primordial | Main Uranium isotope |
| Plutonium-238 | 94 | 144 | 87.7 y | α | 5593 | Synthetic | used in radioisotope thermoelectric generators and radioisotope heater units as an energy source for spacecraft |
| Plutonium-239 | 94 | 145 | 24,110 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 | 154 | 2.64 y | α/SF | 6217 | Synthetic | undergoes spontaneous fission, making it a powerful neutron source, used as a reactor initiator and for detection devices |
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 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.Impacts on organisms
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."Summary table for classes of nuclides, "stable" and radioactive
Following is a summary table for the total list of nuclides with half-lives greater than one hour. Ninety of these 989 nuclides are theoretically stable, except to proton-decay. About 252 nuclides have never been observed to decay, and are classically considered stable.The remaining tabulated radionuclides have half-lives longer than 1 hour, and are well-characterized. They include 30 nuclides with measured half-lives longer than the estimated age of the universe, and another 4 nuclides with half-lives long enough 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.
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 for the 989 nuclides with half-lives longer than one hour, 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. | |||
| Theoretically stable to alpha decay, beta decay, isomeric transition, and double beta decay but not spontaneous fission, which is possible for "stable" nuclides ≥ niobium-93 | 56 | 146 | All nuclides that are possibly completely stable. | |||
| Energetically unstable to one or more known decay modes, but no decay yet seen. All considered "stable" until decay detected. | 106 | 252 | Total of classically stable nuclides. | |||
| Radioactive primordial nuclides. | 34 | 286 | Total primordial elements include uranium, thorium, bismuth, rubidium-87, potassium-40, tellurium-128 plus all stable nuclides. | |||
| Radioactive nonprimordial, but naturally occurring on Earth. | 61 | 347 | Carbon-14. Includes most useful radiotracers. | 662 | 989 | These 989 nuclides are listed in the article List of nuclides. |
| Radioactive synthetic. | >2400 | >3300 | Includes all well-characterized synthetic nuclides. |