Industrial radiography is a method of non-destructive testing where many types of manufactured components can be examined to verify the internal structure and integrity of the specimen. Industrial Radiography can be performed utilizing either X-rays or gamma rays. Both are forms of electromagnetic radiation; the difference between various forms of electromagnetic energy is related to the wavelength. X and gamma rays have the shortest wavelength and this property leads to the ability to penetrate, travel through, exit various materials such as carbon steel and other metals. Radiography started in 1895 with the discovery of a type of electromagnetic radiation. Soon after the discovery of X-rays, radioactivity was discovered. By using radioactive sources such as radium, far higher photon energies could be obtained than those from normal X-ray generators. Soon these found various applications, with one of the earliest users being Loughborough College. X-rays and gamma rays were put to use early, before the dangers of ionizing radiation were discovered.
After World War II new isotopes such as caesium-137, iridium-192 and cobalt-60 became available for industrial radiography, the use of radium and radon decreased. Gamma radiation sources, most iridium-192 and cobalt-60, are used to inspect a variety of materials; the vast majority of radiography concerns the testing and grading of welds on pressurized piping, pressure vessels, high-capacity storage containers and some structural welds. Other tested materials include concrete, welder's test coupons, machined parts, plate metal, or pipewall. Non-metal components such as ceramics used in the aerospace industries are regularly tested. Theoretically, industrial radiographers could radiograph any solid, flat material or any hollow cylindrical or spherical object; the beam of radiation must be directed to the middle of the section under examination and must be normal to the material surface at that point, except in special techniques where known defects are best revealed by a different alignment of the beam.
The length of weld under examination for each exposure shall be such that the thickness of the material at the diagnostic extremities, measured in the direction of the incident beam, does not exceed the actual thickness at that point by more than 6%. The specimen to be inspected is placed between the source of radiation and the detecting device the film in a light tight holder or cassette, the radiation is allowed to penetrate the part for the required length of time to be adequately recorded; the result is a two-dimensional projection of the part onto the film, producing a latent image of varying densities according to the amount of radiation reaching each area. It is known as a radio graph, as distinct from a photograph produced by light; because film is cumulative in its response weak radiation can be detected by prolonging the exposure until the film can record an image that will be visible after development. The radiograph is examined without printing as a positive as in photography; this is because, no useful purpose is served.
Before commencing a radiographic examination, it is always advisable to examine the component with one's own eyes, to eliminate any possible external defects. If the surface of a weld is too irregular, it may be desirable to grind it to obtain a smooth finish, but this is to be limited to those cases in which the surface irregularities may make detecting internal defects difficult. After this visual examination, the operator will have a clear idea of the possibilities of access to the two faces of the weld, important both for the setting up of the equipment and for the choice of the most appropriate technique. Defects such as delaminations and planar cracks are difficult to detect using radiography to the untrained eye. Without overlooking the negatives of radiographic inspection, Radiography does hold many significant benefits over ultrasonics insomuch that as a'picture' is produced keeping a semi permanent record for the life cycle of the film, more accurate identification of the defect can be made, by more interpreters.
Important as most construction standards permit some level of defect acceptance, depending on the type and size of the defect. To the trained Radiographer, subtle variations in visible film density provide the technician the ability to not only locate a defect, but identify its type and location. For purposes of inspection, including weld inspection, there exist several exposure arrangements. First, there is one of the four single-wall exposure/single-wall view arrangements; this exposure is created when the radiographer places the source of radiation at the center of a sphere, cone, or cylinder. Depending upon client requirements, the radiographer would place film cassettes on the outside of the surface to be examined; this exposure arrangement is nearly ideal – when properly arranged and exposed, all portions of all exposed film will be of the same approximate density. It has the advantage of taking less time than other arrangements since the source must only penetrate the total wall thickness once and must only travel the radius of th
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or from a combination of fission and fusion reactions. Both bomb types release large quantities of energy from small amounts of matter; the first test of a fission bomb released an amount of energy equal to 20,000 tons of TNT. The first thermonuclear bomb test released energy equal to 10 million tons of TNT. A thermonuclear weapon weighing little more than 2,400 pounds can release energy equal to more than 1.2 million tons of TNT. A nuclear device no larger than traditional bombs can devastate an entire city by blast and radiation. Since they are weapons of mass destruction, the proliferation of nuclear weapons is a focus of international relations policy. Nuclear weapons have been used twice in war, both times by the United States against Japan near the end of World War II. On August 6, 1945, the U. S. Army Air Forces detonated a uranium gun-type fission bomb nicknamed "Little Boy" over the Japanese city of Hiroshima.
S. Army Air Forces detonated a plutonium implosion-type fission bomb nicknamed "Fat Man" over the Japanese city of Nagasaki; these bombings caused injuries that resulted in the deaths of 200,000 civilians and military personnel. The ethics of these bombings and their role in Japan's surrender are subjects of debate. Since the atomic bombings of Hiroshima and Nagasaki, nuclear weapons have been detonated over two thousand times for testing and demonstration. Only a few nations are suspected of seeking them; the only countries known to have detonated nuclear weapons—and acknowledge possessing them—are the United States, the Soviet Union, the United Kingdom, China, India and North Korea. Israel is believed to possess nuclear weapons, though, in a policy of deliberate ambiguity, it does not acknowledge having them. Germany, Turkey and the Netherlands are nuclear weapons sharing states. South Africa is the only country to have independently developed and renounced and dismantled its nuclear weapons.
The Treaty on the Non-Proliferation of Nuclear Weapons aims to reduce the spread of nuclear weapons, but its effectiveness has been questioned, political tensions remained high in the 1970s and 1980s. Modernisation of weapons continues to this day. There are two basic types of nuclear weapons: those that derive the majority of their energy from nuclear fission reactions alone, those that use fission reactions to begin nuclear fusion reactions that produce a large amount of the total energy output. All existing nuclear weapons derive some of their explosive energy from nuclear fission reactions. Weapons whose explosive output is from fission reactions are referred to as atomic bombs or atom bombs; this has long been noted as something of a misnomer, as their energy comes from the nucleus of the atom, just as it does with fusion weapons. In fission weapons, a mass of fissile material is forced into supercriticality—allowing an exponential growth of nuclear chain reactions—either by shooting one piece of sub-critical material into another or by compression of a sub-critical sphere or cylinder of fissile material using chemically-fueled explosive lenses.
The latter approach, the "implosion" method, is more sophisticated than the former. A major challenge in all nuclear weapon designs is to ensure that a significant fraction of the fuel is consumed before the weapon destroys itself; the amount of energy released by fission bombs can range from the equivalent of just under a ton to upwards of 500,000 tons of TNT. All fission reactions generate the remains of the split atomic nuclei. Many fission products are either radioactive or moderately radioactive, as such, they are a serious form of radioactive contamination. Fission products are the principal radioactive component of nuclear fallout. Another source of radioactivity is the burst of free neutrons produced by the weapon; when they collide with other nuclei in surrounding material, the neutrons transmute those nuclei into other isotopes, altering their stability and making them radioactive. The most used fissile materials for nuclear weapons applications have been uranium-235 and plutonium-239.
Less used has been uranium-233. Neptunium-237 and some isotopes of americium may be usable for nuclear explosives as well, but it is not clear that this has been implemented, their plausible use in nuclear weapons is a matter of dispute; the other basic type of nuclear weapon produces a large proportion of its energy in nuclear fusion reactions. Such fusion weapons are referred to as thermonuclear weapons or more colloquially as hydrogen bombs, as they rely on fusion reactions between isotopes of hydrogen. All such weapons derive a significant portion of their energy from fission reactions used to "trigger" fusion reactions, fusion reactions can themselves trigger additional fission reactions. Only six countries—United States, United Kingdom, China and India—have conducted thermonuclear weapon tests. North Korea claims to have tested a fusion weapon as of January 2016. Thermonuclear weapons a
Uranium-238 is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it is fissionable by fast neutrons, is fertile, meaning it can be transmuted to fissile plutonium-239. 238U cannot support a chain reaction because inelastic scattering reduces neutron energy below the range where fast fission of one or more next-generation nuclei is probable. Doppler broadening of U-238's neutron absorption resonances, increasing absorption as fuel temperature increases, is an essential negative feedback mechanism for reactor control. Around 99.286% of natural uranium's mass is uranium-238, which has a half-life of 1.41×1017 seconds. Due to its natural abundance and half-life relative to other radioactive elements, 238U produces ~40% of the radioactive heat produced within the Earth. 238U decay contributes 6 electron anti-neutrinos per decay, resulting in a large detectable geoneutrino signal when decays occur within the Earth.
The decay of 238U to daughter isotopes is extensively used in radiometric dating for material older than ~ 1 million years. Depleted uranium has an higher concentration of the 238U isotope, low-enriched uranium, while having a higher proportion of the uranium-235 isotope, is still 238U. Reprocessed uranium is mainly 238U, with about as much uranium-235 as natural uranium, a comparable proportion of uranium-236, much smaller amounts of other isotopes of uranium such as uranium-234, uranium-233, uranium-232. In a fission nuclear reactor, uranium-238 can be used to generate 239Pu, which itself can be used in a nuclear weapon or as a nuclear-reactor fuel supply. In a typical nuclear reactor, up to one-third of the generated power does come from the fission of 239Pu, not supplied as a fuel to the reactor, but rather, produced from 238U. 238U is not usable directly as nuclear fuel. In this process, a neutron that has a kinetic energy in excess of 1 MeV can cause the nucleus of 238U to split in two.
Depending on design, this process can contribute some one to ten percent of all fission reactions in a reactor, but too few of the average 2.5 neutrons produced in each fission have enough speed to continue a chain reaction. 238U can be used as a source material for creating plutonium-239, which can in turn be used as nuclear fuel. Breeder reactors carry out such a process of transmutation to convert the fertile isotope 238U into fissile Pu-239, it has been estimated that there is anywhere from 10,000 to five billion years worth of 238U for use in these power plants. Breeder technology has been used in several experimental nuclear reactors. By December 2005, the only breeder reactor producing power was the 600-megawatt BN-600 reactor at the Beloyarsk Nuclear Power Station in Russia. Russia has planned to build another unit, BN-800, at the Beloyarsk nuclear power plant. Japan's Monju breeder reactor is planned to be started, having been shut down since 1995, both China and India have announced plans to build nuclear breeder reactors.
However, after safety and design hazards were uncovered, in 2016 the Japanese government ordered the decommissioning of the Monju reactor which may be completed by 2047. The breeder reactor as its name implies creates larger quantities of Pu-239 than the fission nuclear reactor; the Clean And Environmentally Safe Advanced Reactor, a nuclear reactor concept that would use steam as a moderator to control delayed neutrons, will be able to use 238U as fuel once the reactor is started with LEU fuel. This design is still in the early stages of development. 238U is used as a radiation shield – its alpha radiation is stopped by the non-radioactive casing of the shielding and the uranium's high atomic weight and high number of electrons are effective in absorbing gamma rays and x-rays. It is not as effective as ordinary water for stopping fast neutrons. Both metallic depleted uranium dioxide are used for radiation shielding. Uranium is about five times better as a gamma ray shield than lead, so a shield with the same effectiveness can be packed into a thinner layer.
DUCRETE, a concrete made with uranium dioxide aggregate instead of gravel, is being investigated as a material for dry cask storage systems to store radioactive waste. The opposite of enriching is downblending. Surplus enriched uranium can be downblended with depleted uranium or natural uranium to turn it into low enriched uranium suitable for use in commercial nuclear fuel. 238U from depleted uranium and natural uranium is used with recycled Pu-239 from nuclear weapons stockpiles for making mixed oxide fuel, now being redirected to become fuel for nuclear reactors. This dilution called downblending, means that any nation or group that acquired the finished fuel would have to repeat the expensive and complex chemical separation of uranium and plutonium process before assembling a weapon. Most modern nuclear weapons utilize 238U as a "tamper" material. A tamper which surrounds a fissile core works to reflect neutrons and to add inertia to the compression of the Pu-239 charge; as such, it reduces the critical mass required.
In the case of a thermonuclear weapon 238U can be used to encase the fusion fuel, the high flux of energetic neutrons from the resulting fusion reaction causes 238U nuclei to split and adds more energy to the "yield" of the weapon. Such weapons are referred to as fission-fusion-fission weapons after the order in which e
Table of nuclides
A table of nuclides or chart of nuclides is a two-dimensional graph in which one axis represents the number of neutrons and the other represents the number of protons in an atomic nucleus. Each point plotted on the graph thus represents the nuclide of a real or hypothetical chemical element; this system of ordering nuclides can offer a greater insight into the characteristics of isotopes than the better-known periodic table, which shows only elements instead of each of their isotopes. A chart or table of nuclides is a simple map to the nuclear, or radioactive, behaviour of nuclides, as it distinguishes the isotopes of an element, it contrasts with a periodic table, which only maps their chemical behavior, since isotopes do not differ chemically to any significant degree, with the exception of hydrogen. Nuclide charts organize nuclides along the X axis by their numbers of neutrons and along the Y axis by their numbers of protons, out to the limits of the neutron and proton drip lines; this representation was first published by Kurt Guggenheimer in 1934 and expanded by Giorgio Fea in 1935, Emilio Segrè in 1945 or G. Seaborg.
In 1958, Walter Seelmann-Eggebert and Gerda Pfennig published the first edition of the Karlsruhe Nuclide Chart. Its 7th edition was made available in 2006. Today, there are several nuclide charts, four of which have a wide distribution: the Karlsruhe Nuclide Chart, the Strasbourg Universal Nuclide Chart, the Chart of the Nuclides from the JAEA and the Nuclide Chart from Knolls Atomic Power Laboratory, it has become a basic tool of the nuclear community. The nuclide table below shows nuclides, including all with half-life of at least one day, they are arranged with increasing atomic numbers from left to right and increasing neutron numbers from top to bottom. Cell color denotes the half-life of each nuclide. In graphical browsers, each nuclide has a tool tip indicating its half-life; each color represents a certain range of length of half-life, the color of the border indicates the half-life of its nuclear isomer state. Some nuclides have multiple nuclear isomers, this table notes the longest one.
Dotted borders mean that a nuclide has a nuclear isomer, their color is represented the same way as for their normal counterparts. Isotopes are nuclides with the same number of protons but differing numbers of neutrons. Isotopes neighbor each other vertically, e.g. carbon-12, carbon-13, carbon-14 or oxygen-15, oxygen-16, oxygen-17. Isotones are nuclides with the same number of neutrons but differing number of protons. Isotones neighbor each other horizontally. Example: carbon-14, nitrogen-15, oxygen-16 in the sample table above. Isobars are nuclides with the same number of nucleons, i.e. mass number, but different numbers of protons and different number of neutrons. Isobars neighbor each other diagonally from lower-left to upper-right. Example: carbon-14, nitrogen-14, oxygen-14 in the sample table above. Isodiaphers are nuclides with the same difference between protons. Like isobars, they at right angles to the isobar lines. Examples: boron-10, carbon-12, nitrogen-14 where N−Z=0. Beyond the neutron drip line along the lower left, nuclides decay by neutron emission.
Beyond the proton drip line along the upper right, nuclides decay by proton emission. Drip lines have only been established for some elements; the island of stability is a hypothetical region of the table of nuclides that contains isotopes far more stable than other transuranic elements. There are no stable nuclides having an equal number of protons and neutrons in their nuclei with atomic number greater than 20 as can be "read" from the chart. Nuclei of greater atomic number require an excess of neutrons for stability; the only stable nuclides having an odd number of protons and an odd number of neutrons are hydrogen-2, lithium-6, boron-10, nitrogen-14 and tantalum-180m. This is because the mass-energy of such atoms is higher than that of their neighbors on the same isobaric chain, so most of them are unstable to beta decay. There are no stable nuclides with mass numbers 5 or 8. There are stable nuclides with all other mass numbers up to 208 with the exceptions of 147 and 151. With the possible exception of the pair tellurium-123 and antimony-123, odd mass numbers are never represented by more than one stable nuclide.
This is because the mass-energy is a convex function of atomic number, so all nuclides on an odd isobaric chain except one have a lower-energy neighbor to which they can decay by beta decay. There are no stable nuclides having atomic number greater than Z=82, although bismuth is stable for all practical human purposes. Elements with atomic numbers from 1 to 82 all have stable isotopes, with the exceptions of technetium and promethium. Interactive Chart of Nuclides app for mobiles: Android or Apple - for PC use The Live Chart of Nuclides - IAEA Another example of a Chart of Nuclides from Korea Data up to Jan 1999 only
A radioactive source is a known quantity of a radionuclide which emits ionizing radiation. Sources can be used for irradiation, where the radiation performs a significant ionising function on a target material, or as a radiation metrology source, used for the calibration of radiometric process and radiation protection instrumentation, they are used for industrial process measurements, such as thickness gauging in the paper and steel industries. Sources can be sealed in a container or deposited on a surface; as an irradiation source they are used in medicine for radiation therapy and in industry for such as industrial radiography, food irradiation, vermin disinfestation, irradiation crosslinking of PVC. Radionuclides are chosen according to the type and character of the radiation they emit, intensity of emission, the half-life of their decay. Common source radionuclides include cobalt-60, iridium-192, strontium-90; the SI measurement quantity of source activity is the Becquerel, though the historical unit Curies is still in partial use, such as in the USA, despite the USA NIST advising the use of the SI unit.
The SI unit for health purposes is mandatory in the EU. An irradiation source lasts for between 5 and 15 years before its activity drops below useful levels; however sources with long half-life radionuclides when utilised as calibration sources can be used for much longer. Many radioactive sources are sealed, meaning they are permanently either contained in a capsule or bonded solid to a surface. Capsules are made of stainless steel, platinum or another inert metal; the use of sealed sources removes all risk of dispersion of radioactive material into the environment due to mishandling, but the container is not intended to attenuate radiation, so further shielding is required for radiation protection. Sealed sources are used in all applications where the source does not need to be chemically or physically included in a liquid or gas. Sealed sources are categorised by the IAEA according to their activity in relation to a minimum dangerous source; the ratio used is A/D, where A is the activity of the source and D is the minimum dangerous activity.
Note that sources with sufficiently low radioactive output as to not cause harm to humans are not categorised. Calibration sources are used for the calibration of radiometric instrumentation, used on process monitoring or in radiological protection. Capsule sources, where the radiation emits from a point, are used for beta, gamma and X-ray instrument calibration. High level sources are used in a calibration cell: a room with thick walls to protect the operator and the provision of remote operation of the source exposure; the plate source is in common use for the calibration of radioactive contamination instruments. This has a known amount of radioactive material fixed to its surface, such as an alpha and/or beta emitter, to allow the calibration of large area radiation detectors used for contamination surveys and personnel monitoring; such measurements are counts per unit time received by the detector, such as counts per minute or counts per second. Unlike the capsule source, the plate source emitting material must be on the surface to prevent attenuation by a container or self-shielding due to the material itself.
This is important with alpha particles which are stopped by a small mass. The Bragg curve shows the attenuation effect in free air. Unsealed sources are sources that are not in a permanently sealed container, are used extensively for medical purposes, they are used when the source needs to be dissolved in a liquid for injection into a patient or ingestion by the patient. Unsealed sources are used in industry in a similar manner for leak detection as a Radioactive tracer. Disposal of expired radioactive sources presents similar challenges to the disposal of other nuclear waste, although to a lesser degree. Spent low level sources will sometimes be sufficiently inactive that they are suitable for disposal via normal waste disposal methods — landfill. Other disposal methods are similar to those for higher-level radioactive waste, using various depths of borehole depending on the activity of the waste. A notorious incident of neglect in disposing of a high level source was the Goiânia accident, which resulted in several fatalities.
Common beta emitters Commonly used gamma-emitting isotopes Geiger counter Ionizing radiation Neutron source
In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of thermal energy; the predominant neutron energy may be typified by fast neutrons. Fissile material can be used to fuel thermal-neutron reactors, fast-neutron reactors and nuclear explosives. According to the Ronen fissile rule, for a heavy element with 90 ≤ Z ≤ 100, its isotopes with 2 × Z − N = 43 ± 2, with few exceptions, are fissile."Fissile" is distinct from "fissionable". A nuclide capable of undergoing fission after capturing a neutron of high or low energy is referred to as "fissionable". A fissionable nuclide that can be induced to fission with low-energy thermal neutrons with a high probability is referred to as "fissile". Although the terms were synonymous, fissionable materials include those that can be fissioned only with high-energy neutrons; as a result, fissile materials are a subset of fissionable materials.
Uranium-235 fissions with low-energy thermal neutrons because the binding energy resulting from the absorption of a neutron is greater than the critical energy required for fission. By contrast, the binding energy released by uranium-238 absorbing a thermal neutron is less than the critical energy, so the neutron must possess additional energy for fission to be possible. Uranium-238 is a fissionable material but not a fissile material. An alternative definition defines fissile nuclides as those nuclides that can be made to undergo nuclear fission and produce neutrons from such fission that can sustain a nuclear chain reaction in the correct setting. Under this definition, the only nuclides that are fissionable but not fissile are those nuclides that can be made to undergo nuclear fission but produce insufficient neutrons, in either energy or number, to sustain a nuclear chain reaction; as such, while all fissile isotopes are fissionable, not all fissionable isotopes are fissile. In the arms control context in proposals for a Fissile Material Cutoff Treaty, the term "fissile" is used to describe materials that can be used in the fission primary of a nuclear weapon.
These are materials. Under all definitions above, uranium-238 is fissionable, but because it cannot sustain a neutron chain reaction, it is not fissile. Neutrons produced by fission of 238U have lower energies than the original neutron below 1 MeV, the fission threshold to cause subsequent fission of 238U, so fission of 238U does not sustain a nuclear chain reaction. Fast fission of 238U in the secondary stage of a nuclear weapon contributes to yield and to fallout; the fast fission of 238U makes a significant contribution to the power output of some fast-neutron reactors. In general, most actinide isotopes with an odd neutron number are fissile. Most nuclear fuels have an odd atomic mass number, an atomic number Z; this implies an odd number of neutrons. Isotopes with an odd number of neutrons gain an extra 1 to 2 MeV of energy from absorbing an extra neutron, from the pairing effect which favors numbers of both neutrons and protons; this energy is enough to supply the needed extra energy for fission by slower neutrons, important for making fissionable isotopes fissile.
More nuclides with an number of protons and an number of neutrons, located near a well-known curve in nuclear physics of atomic number vs. atomic mass number are more stable than others. They are more to "ignore" the neutron and let it go on its way, or else to absorb the neutron but without gaining enough energy from the process to deform the nucleus enough for it to fission; these "even-even" isotopes are less to undergo spontaneous fission, they have much longer partial half-lives for alpha or beta decay. Examples of these isotopes are uranium-238 and thorium-232. On the other hand, other than the lightest nuclides, nuclides with an odd number of protons and an odd number of neutrons are short-lived because they decay by beta-particle emission to their isobars with an number of protons and an number of neutrons becoming much more stable; the physical basis for this phenomenon comes from the pairing effect in nuclear binding energy, but this time from both proton–proton and neutron–neutron pairing.
The short half-life of such odd-odd heavy isotopes means that they are not available in quantity and are radioactive. To be a useful fuel for nuclear fission chain reactions, the material must: Be in the region of the binding energy curve where a fission chain reaction is possible Have a high probability of fission on neutron capture Release more than one neutron on average per neutron capture. Have a reasonably long half-life Be available in suitable quantitiesFissile nuclides in nuclear fuels include: Uranium-235 which occurs in natural uranium and enriched uranium Plutonium-239 bred from uranium-238 by neutron capture Plutonium-241 bred from plutonium-240 by neutron capture; the 240Pu comes from 239Pu by the same process. Uranium-2
Isotopes are variants of a particular chemical element which differ in neutron number, in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom; the term isotope is formed from the Greek roots isos and topos, meaning "the same place". It was coined by a Scottish doctor and writer Margaret Todd in 1913 in a suggestion to chemist Frederick Soddy; the number of protons within the atom's nucleus is called atomic number and is equal to the number of electrons in the neutral atom. Each atomic number identifies a specific element, but not the isotope; the number of nucleons in the nucleus is the atom's mass number, each isotope of a given element has a different mass number. For example, carbon-12, carbon-13, carbon-14 are three isotopes of the element carbon with mass numbers 12, 13, 14, respectively; the atomic number of carbon is 6, which means that every carbon atom has 6 protons, so that the neutron numbers of these isotopes are 6, 7, 8 respectively.
A nuclide is a species of an atom with a specific number of protons and neutrons in the nucleus, for example carbon-13 with 6 protons and 7 neutrons. The nuclide concept emphasizes nuclear properties over chemical properties, whereas the isotope concept emphasizes chemical over nuclear; the neutron number has large effects on nuclear properties, but its effect on chemical properties is negligible for most elements. In the case of the lightest elements where the ratio of neutron number to atomic number varies the most between isotopes it has only a small effect, although it does matter in some circumstances; the term isotopes is intended to imply comparison, for example: the nuclides 126C, 136C, 146C are isotopes, but 4018Ar, 4019K, 4020Ca are isobars. However, because isotope is the older term, it is better known than nuclide, is still sometimes used in contexts where nuclide might be more appropriate, such as nuclear technology and nuclear medicine. An isotope and/or nuclide is specified by the name of the particular element followed by a hyphen and the mass number.
When a chemical symbol is used, e.g. "C" for carbon, standard notation is to indicate the mass number with a superscript at the upper left of the chemical symbol and to indicate the atomic number with a subscript at the lower left. Because the atomic number is given by the element symbol, it is common to state only the mass number in the superscript and leave out the atomic number subscript; the letter m is sometimes appended after the mass number to indicate a nuclear isomer, a metastable or energetically-excited nuclear state, for example 180m73Ta. The common pronunciation of the AZE notation is different from how it is written: 42He is pronounced as helium-four instead of four-two-helium, 23592U as uranium two-thirty-five or uranium-two-three-five instead of 235-92-uranium; some isotopes/nuclides are radioactive, are therefore referred to as radioisotopes or radionuclides, whereas others have never been observed to decay radioactively and are referred to as stable isotopes or stable nuclides.
For example, 14C is a radioactive form of carbon, whereas 12C and 13C are stable isotopes. There are about 339 occurring nuclides on Earth, of which 286 are primordial nuclides, meaning that they have existed since the Solar System's formation. Primordial nuclides include 32 nuclides with long half-lives and 253 that are formally considered as "stable nuclides", because they have not been observed to decay. In most cases, for obvious reasons, if an element has stable isotopes, those isotopes predominate in the elemental abundance found on Earth and in the Solar System. However, in the cases of three elements the most abundant isotope found in nature is one long-lived radioisotope of the element, despite these elements having one or more stable isotopes. Theory predicts that many "stable" isotopes/nuclides are radioactive, with long half-lives; some stable nuclides are in theory energetically susceptible to other known forms of decay, such as alpha decay or double beta decay, but no decay products have yet been observed, so these isotopes are said to be "observationally stable".
The predicted half-lives for these nuclides greatly exceed the estimated age of the universe, in fact there are 27 known radionuclides with half-lives longer than the age of the universe. Adding in the radioactive nuclides that have been created artificially, there are 3,339 known nuclides; these include 905 nuclides that are either stable or have half-lives