1.
Alpha particle
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Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus. They are generally produced in the process of decay. Alpha particles are named after the first letter in the Greek alphabet, the symbol for the alpha particle is α or α2+. Because they are identical to helium nuclei, they are sometimes written as He2+ or 4 2He2+ indicating a helium ion with a +2 charge. If the ion gains electrons from its environment, the particle can be written as a normal helium atom 4 2He. Some science authors may use doubly ionized helium nuclei and alpha particles as interchangeable terms, the nomenclature is not well defined, and thus not all high-velocity helium nuclei are considered by all authors to be alpha particles. As with beta and gamma rays/particles, the used for the particle carries some mild connotations about its production process and energy. Thus, alpha particles may be used as a term when referring to stellar helium nuclei reactions. A higher energy version of alphas than produced in alpha decay is a product of an uncommon nuclear fission result called ternary fission. However, helium nuclei produced by particle accelerators are less likely to be referred to as alpha particles, alpha particles, like helium nuclei, have a net spin of zero. Due to the mechanism of their production in standard alpha radioactive decay, alpha particles generally have an energy of about 5 MeV. They are a highly ionizing form of radiation, and have low penetration depth. They are able to be stopped by a few centimeters of air, however, so-called long range alpha particles from ternary fission are three times as energetic, and penetrate three times as far. To a lesser extent, this is true of very high-energy helium nuclei produced by particle accelerators. The best-known source of particles is alpha decay of heavier atoms. When an atom emits an alpha particle in alpha decay, the mass number decreases by four due to the loss of the four nucleons in the alpha particle. The atomic number of the atom goes down by two, as a result of the loss of two protons – the atom becomes a new element. Examples of this sort of nuclear transmutation are when uranium becomes thorium, or radium becomes radon gas, alpha particles are commonly emitted by all of the larger radioactive nuclei such as uranium, thorium, actinium, and radium, as well as the transuranic elements
2.
Helium
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Helium is a chemical element with symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and its boiling point is the lowest among all the elements. Its abundance is similar to figure in the Sun and in Jupiter. This is due to the high nuclear binding energy of helium-4 with respect to the next three elements after helium. This helium-4 binding energy also accounts for why it is a product of nuclear fusion and radioactive decay. Most helium in the universe is helium-4, and is believed to have formed during the Big Bang. Large amounts of new helium are being created by fusion of hydrogen in stars. Helium is named for the Greek god of the Sun, Helios and it was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by French astronomer Jules Janssen. Janssen is jointly credited with detecting the element along with Norman Lockyer, Janssen observed during the solar eclipse of 1868 while Lockyer observed from Britain. Lockyer was the first to propose that the line was due to a new element, the formal discovery of the element was made in 1895 by two Swedish chemists, Per Teodor Cleve and Nils Abraham Langlet, who found helium emanating from the uranium ore cleveite. In 1903, large reserves of helium were found in gas fields in parts of the United States. Liquid helium is used in cryogenics, particularly in the cooling of superconducting magnets, a well-known but minor use is as a lifting gas in balloons and airships. As with any gas whose density differs from that of air, inhaling a small volume of helium temporarily changes the timbre, on Earth it is relatively rare—5.2 ppm by volume in the atmosphere. Most terrestrial helium present today is created by the radioactive decay of heavy radioactive elements. Previously, terrestrial helium—a non-renewable resource, because once released into the atmosphere it readily escapes into space—was thought to be in short supply. The first evidence of helium was observed on August 18,1868, the line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India. This line was assumed to be sodium. He concluded that it was caused by an element in the Sun unknown on Earth, Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος
3.
Electron
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The electron is a subatomic particle, symbol e− or β−, with a negative elementary electric charge. Electrons belong to the first generation of the lepton particle family, the electron has a mass that is approximately 1/1836 that of the proton. Quantum mechanical properties of the include a intrinsic angular momentum of a half-integer value, expressed in units of the reduced Planck constant. As it is a fermion, no two electrons can occupy the same state, in accordance with the Pauli exclusion principle. Like all elementary particles, electrons exhibit properties of particles and waves, they can collide with other particles and can be diffracted like light. Since an electron has charge, it has an electric field. Electromagnetic fields produced from other sources will affect the motion of an electron according to the Lorentz force law, electrons radiate or absorb energy in the form of photons when they are accelerated. Laboratory instruments are capable of trapping individual electrons as well as electron plasma by the use of electromagnetic fields, special telescopes can detect electron plasma in outer space. Electrons are involved in applications such as electronics, welding, cathode ray tubes, electron microscopes, radiation therapy, lasers, gaseous ionization detectors. Interactions involving electrons with other particles are of interest in fields such as chemistry. The Coulomb force interaction between the positive protons within atomic nuclei and the negative electrons without, allows the composition of the two known as atoms, ionization or differences in the proportions of negative electrons versus positive nuclei changes the binding energy of an atomic system. The exchange or sharing of the electrons between two or more atoms is the cause of chemical bonding. In 1838, British natural philosopher Richard Laming first hypothesized the concept of a quantity of electric charge to explain the chemical properties of atoms. Irish physicist George Johnstone Stoney named this charge electron in 1891, electrons can also participate in nuclear reactions, such as nucleosynthesis in stars, where they are known as beta particles. Electrons can be created through beta decay of isotopes and in high-energy collisions. The antiparticle of the electron is called the positron, it is identical to the electron except that it carries electrical, when an electron collides with a positron, both particles can be totally annihilated, producing gamma ray photons. The ancient Greeks noticed that amber attracted small objects when rubbed with fur, along with lightning, this phenomenon is one of humanitys earliest recorded experiences with electricity. In his 1600 treatise De Magnete, the English scientist William Gilbert coined the New Latin term electricus, both electric and electricity are derived from the Latin ēlectrum, which came from the Greek word for amber, ἤλεκτρον
4.
Positron
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The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has a charge of +1 e, a spin of 1/2. When a low-energy positron collides with an electron, annihilation occurs. Positrons may be generated by positron emission radioactive decay, or by production from a sufficiently energetic photon which is interacting with an atom in a material. In 1928, Paul Dirac published a paper proposing that electrons can have both a positive charge and negative energy and this paper introduced the Dirac equation, a unification of quantum mechanics, special relativity, and the then-new concept of electron spin to explain the Zeeman effect. The paper did not explicitly predict a new particle but did allow for electrons having either positive or negative energy as solutions, hermann Weyl then published a paper discussing the mathematical implications of the negative energy solution. The positive-energy solution explained experimental results, but Dirac was puzzled by the equally valid negative-energy solution that the model allowed. However, no such transition had yet been observed experimentally and he referred to the issues raised by this conflict between theory and observation as difficulties that were unresolved. Dirac wrote a paper in December 1929 that attempted to explain the unavoidable negative-energy solution for the relativistic electron. An electron with negative energy moves in a field as though it carries a positive charge. The paper also explored the possibility of the proton being an island in this sea, Dirac acknowledged that the proton having a much greater mass than the electron was a problem, but expressed hope that a future theory would resolve the issue. Robert Oppenheimer argued strongly against the proton being the negative-energy electron solution to Diracs equation and he asserted that if it were, the hydrogen atom would rapidly self-destruct. Feynman, and earlier Stueckelberg, proposed an interpretation of the positron as an electron moving backward in time, electrons moving backward in time would have a positive electric charge. Wheeler invoked this concept to explain the properties shared by all electrons, suggesting that they are all the same electron with a complex. Dmitri Skobeltsyn first observed the positron in 1929, carl David Anderson discovered the positron on August 2,1932, for which he won the Nobel Prize for Physics in 1936. Anderson did not coin the term positron, but allowed it at the suggestion of the Physical Review journal editor to which he submitted his paper in late 1932. The positron was the first evidence of antimatter and was discovered when Anderson allowed cosmic rays to pass through a cloud chamber, a magnet surrounded this apparatus, causing particles to bend in different directions based on their electric charge. The ion trail left by each positron appeared on the plate with a curvature matching the mass-to-charge ratio of an electron
5.
Gamma ray
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Gamma ray, denoted by the lower-case Greek letter gamma, is penetrating electromagnetic radiation of a kind arising from the radioactive decay of atomic nuclei. It consists of photons in the highest observed range of photon energy, paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium. In 1903, Ernest Rutherford named this radiation gamma rays, Rutherford had previously discovered two other types of radioactive decay, which he named alpha and beta rays. Gamma rays are able to ionize atoms, and are thus biologically hazardous. The decay of a nucleus from a high energy state to a lower energy state. Natural sources of gamma rays on Earth are observed in the decay of radionuclides. There are rare terrestrial natural sources, such as lightning strikes and terrestrial gamma-ray flashes, However, a large fraction of such astronomical gamma rays are screened by Earths atmosphere and can only be detected by spacecraft. Gamma rays typically have frequencies above 10 exahertz, and therefore have energies above 100 keV and wavelengths less than 10 picometers, However, this is not a strict definition, but rather only a rule-of-thumb description for natural processes. Electromagnetic radiation from radioactive decay of nuclei is referred to as gamma rays no matter its energy. This radiation commonly has energy of a few hundred keV, in astronomy, gamma rays are defined by their energy, and no production process needs to be specified. The energies of gamma rays from astronomical sources range to over 10 TeV, a notable example is the extremely powerful bursts of high-energy radiation referred to as long duration gamma-ray bursts, of energies higher than can be produced by radioactive decay. These bursts of gamma rays are thought to be due to the collapse of stars called hypernovae, the first gamma ray source to be discovered historically was the radioactive decay process called gamma decay. In this type of decay, a nucleus emits a gamma ray almost immediately upon formation. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, However, Villard did not consider naming them as a different fundamental type. Rutherford also noted that gamma rays were not deflected by a field, another property making them unlike alpha. Gamma rays were first thought to be particles with mass, like alpha, Rutherford initially believed that they might be extremely fast beta particles, but their failure to be deflected by a magnetic field indicated that they had no charge. In 1914, gamma rays were observed to be reflected from crystal surfaces, Rutherford and his coworker Edward Andrade measured the wavelengths of gamma rays from radium, and found that they were similar to X-rays, but with shorter wavelengths and higher frequency. This was eventually recognized as giving them more energy per photon
6.
Radioactive decay
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A material containing such unstable nuclei is considered radioactive. Certain highly excited short-lived nuclear states can decay through neutron emission, or more rarely, however, for a collection of atoms, the collections expected decay rate is characterized in terms of their measured decay constants or half-lives. This is the basis of radiometric dating, the half-lives of radioactive atoms have no known upper limit, spanning a time range of over 55 orders of magnitude, from nearly instantaneous to far longer than the age of the universe. A radioactive nucleus with spin can have no defined orientation. If there are multiple particles produced during a single decay, as in decay, their relative angular distribution. Such a parent process could be a previous decay, or a nuclear reaction, the decaying nucleus is called the parent radionuclide, and the process produces at least one daughter nuclide. Except for gamma decay or internal conversion from an excited state. When the number of changes, an atom of a different chemical element is created. The first decay processes to be discovered were alpha decay, beta decay, alpha decay occurs when the nucleus ejects an alpha particle. This is the most common process of emitting nucleons, but highly excited nuclei can eject single nucleons, or in the case of cluster decay, specific light nuclei of other elements. Beta decay occurs when the nucleus emits an electron or positron, the nucleus may capture an orbiting electron, causing a proton to convert into a neutron in a process called electron capture. All of these result in a well-defined nuclear transmutation. By contrast, there are radioactive decay processes that do not result in a nuclear transmutation, another type of radioactive decay results in products that vary, appearing as two or more fragments of the original nucleus with a range of possible masses. For a summary table showing the number of stable and radioactive nuclides in each category, there are 29 naturally occurring chemical elements on Earth that are radioactive. They are those that contain 34 radionuclides that date before the time of formation of the solar system, well-known examples are uranium and thorium, but also included are naturally occurring long-lived radioisotopes, such as potassium-40. Radioactivity was discovered in 1896 by the French scientist Henri Becquerel and these materials glow in the dark after exposure to light, and he suspected that the glow produced in cathode ray tubes by X-rays might be associated with phosphorescence. He wrapped a photographic plate in black paper and placed various phosphorescent salts on it, all results were negative until he used uranium salts. The uranium salts caused a blackening of the plate in spite of the plate being wrapped in black paper and these radiations were given the name Becquerel Rays
7.
Atomic nucleus
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After the discovery of the neutron in 1932, models for a nucleus composed of protons and neutrons were quickly developed by Dmitri Ivanenko and Werner Heisenberg. Almost all of the mass of an atom is located in the nucleus, protons and neutrons are bound together to form a nucleus by the nuclear force. The diameter of the nucleus is in the range of 6985175000000000000♠1.75 fm for hydrogen to about 6986150000000000000♠15 fm for the heaviest atoms and these dimensions are much smaller than the diameter of the atom itself, by a factor of about 23,000 to about 145,000. The branch of physics concerned with the study and understanding of the nucleus, including its composition. The nucleus was discovered in 1911, as a result of Ernest Rutherfords efforts to test Thomsons plum pudding model of the atom, the electron had already been discovered earlier by J. J. Knowing that atoms are electrically neutral, Thomson postulated that there must be a charge as well. In his plum pudding model, Thomson suggested that an atom consisted of negative electrons randomly scattered within a sphere of positive charge, to his surprise, many of the particles were deflected at very large angles. This justified the idea of an atom with a dense center of positive charge. The term nucleus is from the Latin word nucleus, a diminutive of nux, in 1844, Michael Faraday used the term to refer to the central point of an atom. The modern atomic meaning was proposed by Ernest Rutherford in 1912, the adoption of the term nucleus to atomic theory, however, was not immediate. In 1916, for example, Gilbert N, the nuclear strong force extends far enough from each baryon so as to bind the neutrons and protons together against the repulsive electrical force between the positively charged protons. The nuclear strong force has a short range, and essentially drops to zero just beyond the edge of the nucleus. The collective action of the charged nucleus is to hold the electrically negative charged electrons in their orbits about the nucleus. The collection of negatively charged electrons orbiting the nucleus display an affinity for certain configurations, which chemical element an atom represents is determined by the number of protons in the nucleus, the neutral atom will have an equal number of electrons orbiting that nucleus. Individual chemical elements can create more stable electron configurations by combining to share their electrons and it is that sharing of electrons to create stable electronic orbits about the nucleus that appears to us as the chemistry of our macro world. Protons define the entire charge of a nucleus, and hence its chemical identity, neutrons are electrically neutral, but contribute to the mass of a nucleus to nearly the same extent as the protons. Neutrons explain the phenomenon of isotopes – varieties of the chemical element which differ only in their atomic mass. They are sometimes viewed as two different quantum states of the particle, the nucleon
8.
Beta decay
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In nuclear physics, beta decay is a type of radioactive decay in which a beta ray, and a neutrino are emitted from an atomic nucleus. Neither the beta particle nor its associated neutrino exist within the prior to beta decay. By this process, unstable atoms obtain a more stable ratio of protons to neutrons, the probability of a nuclide decaying due to beta and other forms of decay is determined by its binding energy. The binding energies of all existing nuclides form what is called the valley of stability. Beta decay is a consequence of the force, which is characterized by relatively lengthy decay times. Nucleons are composed of up or down quarks, and the force allows a quark to change type by the exchange of a W boson. For example, a neutron, composed of two quarks and an up quark, decays to a proton composed of a down quark. Decay times for many nuclides that are subject to beta decay can be thousands of years, electron capture is sometimes included as a type of beta decay, because the basic nuclear process, mediated by the weak force, is the same. In electron capture, an atomic electron is captured by a proton in the nucleus, transforming it into a neutron. The two types of decay are known as beta minus and beta plus. β+ decay is known as positron emission. Beta decay conserves a number known as the lepton number, or the number of electrons. These particles have lepton number +1, while their antiparticles have lepton number −1, since a proton or neutron has lepton number zero, β+ decay must be accompanied with an electron neutrino, while β− decay must be accompanied by an electron antineutrino. This new element has a mass number A, but an atomic number Z that is increased by one. As in all nuclear decays, the element is known as the parent nuclide while the resulting element is known as the daughter nuclide. The beta spectrum, or distribution of values for the beta particles, is continuous. The total energy of the process is divided between the electron, the antineutrino, and the recoiling nuclide. In the figure to the right, an example of an electron with 0.40 MeV energy from the decay of 210Bi is shown
9.
Ionizing radiation
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Ionizing radiation is radiation that carries enough energy to free electrons from atoms or molecules, thereby ionizing them. Ionizing radiation is made up of energetic particles, ions or atoms moving at high speeds. The boundary between ionizing and non-ionizing electromagnetic radiation that occurs in the ultraviolet is not sharply defined, since different molecules, conventional definition places the boundary at a photon energy between 10 eV and 33 eV in the ultraviolet. Typical ionizing subatomic particles from radioactivity include alpha particles, beta particles, almost all products of radioactive decay are ionizing because the energy of radioactive decay is typically far higher than that required to ionize. Cosmic rays are generated by stars and certain events such as supernova explosions. Cosmic rays may also produce radioisotopes on Earth, which in turn decay, cosmic rays and the decay of radioactive isotopes are the primary sources of natural ionizing radiation on Earth referred to as background radiation. Ionizing radiation can also be generated artificially using X-ray tubes, particle accelerators, ionizing radiation is invisible and not directly detectable by human senses, so radiation detection instruments such as Geiger counters are required to detect it. However, it can cause emission of light upon interaction with matter, such as in Cherenkov radiation. Exposure to ionizing radiation damage to living tissue, and can result in mutation, radiation sickness, cancer. Ionizing radiation is categorized by the nature of the particles or electromagnetic waves that create the ionizing effect and these have different ionization mechanisms, and may be grouped as directly or indirectly ionizing. Any charged massive particle can ionize atoms directly by fundamental interaction through the Coulomb force if it carries sufficient kinetic energy and this includes atomic nuclei, electrons, muons, charged pions, protons, and energetic charged nuclei stripped of their electrons. When moving at relativistic speeds these particles have enough energy to be ionizing. For example, an alpha particle is ionizing, but moves at about 5% c. Natural cosmic rays are made up primarily of protons but also include heavier atomic nuclei like helium ions. Pions can also be produced in large amounts in particle accelerators, alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus. Alpha particle emissions are produced in the process of alpha decay. Alpha particles are named after the first letter in the Greek alphabet, the symbol for the alpha particle is α or α2+. Because they are identical to helium nuclei, they are sometimes written as He2+ or 4 2He2+ indicating a Helium ion with a +2 charge
10.
Radiation protection
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The IAEA also states The accepted understanding of the term radiation protection is restricted to protection of people. Suggestions to extend the definition to include the protection of species or the protection of the environment are controversial. Ionizing radiation is used in industry and medicine, and can present a significant health hazard. It causes microscopic damage to living tissue, which can result in burns and radiation sickness at high exposures. Fundamental to radiation protection is the reduction of expected dose and the measurement of dose uptake. The ICRP recommends, develops and maintains the International System of Radiological Protection based on evaluation of the body of scientific studies available, the recommendations it makes flow down to national regulators, which have the opportunity to incorporate them into law. This is shown in the accompanying diagram, the types of exposure, as well as government regulations and legal exposure limits are different for each of these groups, so they must be considered separately. There are three factors that control the amount, or dose, of radiation received from a source, Radiation exposure can be managed by a combination of these factors, Time, Reducing the time of an exposure reduces the effective dose proportionally. An example of reducing radiation doses by reducing the time of exposures might be improving operator training to reduce the time they take to handle a source, distance, Increasing distance reduces dose due to the inverse square law. Distance can be as simple as handling a source with forceps rather than fingers, Shielding, The term biological shield refers to a mass of absorbing material placed around a reactor, or other radioactive source, to reduce the radiation to a level safe for humans. Hence, shielding strength or thickness is measured in units of g/cm2. The radiation that manages to get through falls exponentially with the thickness of the shield, in x-ray facilities, walls surrounding the room with the x-ray generator may contain lead sheets, or the plaster may contain barium sulfate. Operators view the target through a glass screen, or if they must remain in the same room as the target. Almost any material can act as a shield from gamma or x-rays if used in sufficient amounts, practical radiation protection tends to be a job of juggling the three factors to identify the most cost effective solution. These use the overall principles, Justification, No unnecessary use of radiation is permitted. Limitation, Each individual must be protected against risks that are far too large through individual radiation dose limits, optimization, Radiation doses should all be kept as low as reasonably achievable. This means that it is not enough to remain under the radiation dose limits, as permit holder, you are responsible for ensuring that radiation doses are as low as reasonably achievable, which means that the actual radiation doses are often much lower than the permitted limit. ALARP is an acronym for an important principle in exposure to radiation and other health risks
11.
Neutron
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The neutron is a subatomic particle, symbol n or n0, with no net electric charge and a mass slightly larger than that of a proton. Protons and neutrons, each with approximately one atomic mass unit, constitute the nucleus of an atom. Their properties and interactions are described by nuclear physics, the nucleus consists of Z protons, where Z is called the atomic number, and N neutrons, where N is the neutron number. The atomic number defines the properties of the atom. The terms isotope and nuclide are often used synonymously, but they are chemical and nuclear concepts, the atomic mass number, symbol A, equals Z+N. For example, carbon has atomic number 6, and its abundant carbon-12 isotope has 6 neutrons, some elements occur in nature with only one stable isotope, such as fluorine. Other elements occur with many stable isotopes, such as tin with ten stable isotopes, even though it is not a chemical element, the neutron is included in the table of nuclides. Within the nucleus, protons and neutrons are bound together through the nuclear force, neutrons are produced copiously in nuclear fission and fusion. They are a contributor to the nucleosynthesis of chemical elements within stars through fission, fusion. The neutron is essential to the production of nuclear power, in the decade after the neutron was discovered in 1932, neutrons were used to induce many different types of nuclear transmutations. These events and findings led to the first self-sustaining nuclear reactor, free neutrons, or individual neutrons free of the nucleus, are effectively a form of ionizing radiation, and as such, are a biological hazard, depending upon dose. A small natural background flux of free neutrons exists on Earth, caused by cosmic ray showers. Dedicated neutron sources like neutron generators, research reactors and spallation sources produce free neutrons for use in irradiation, neutrons and protons are both nucleons, which are attracted and bound together by the nuclear force to form atomic nuclei. The nucleus of the most common isotope of the atom is a lone proton. The nuclei of the hydrogen isotopes deuterium and tritium contain one proton bound to one. All other types of nuclei are composed of two or more protons and various numbers of neutrons. The most common nuclide of the chemical element lead, 208Pb has 82 protons and 126 neutrons. The free neutron has a mass of about 1. 675×10−27 kg, the neutron has a mean square radius of about 0. 8×10−15 m, or 0.8 fm, and it is a spin-½ fermion
12.
Proton
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A proton is a subatomic particle, symbol p or p+, with a positive electric charge of +1e elementary charge and mass slightly less than that of a neutron. Protons and neutrons, each with masses of one atomic mass unit, are collectively referred to as nucleons. One or more protons are present in the nucleus of every atom, the number of protons in the nucleus is the defining property of an element, and is referred to as the atomic number. Since each element has a number of protons, each element has its own unique atomic number. The word proton is Greek for first, and this name was given to the nucleus by Ernest Rutherford in 1920. In previous years, Rutherford had discovered that the nucleus could be extracted from the nuclei of nitrogen by atomic collisions. Protons were therefore a candidate to be a particle, and hence a building block of nitrogen. In the modern Standard Model of particle physics, protons are hadrons, and like neutrons, although protons were originally considered fundamental or elementary particles, they are now known to be composed of three valence quarks, two up quarks and one down quark. The rest masses of quarks contribute only about 1% of a protons mass, the remainder of a protons mass is due to quantum chromodynamics binding energy, which includes the kinetic energy of the quarks and the energy of the gluon fields that bind the quarks together. At sufficiently low temperatures, free protons will bind to electrons, however, the character of such bound protons does not change, and they remain protons. A fast proton moving through matter will slow by interactions with electrons and nuclei, the result is a protonated atom, which is a chemical compound of hydrogen. In vacuum, when electrons are present, a sufficiently slow proton may pick up a single free electron, becoming a neutral hydrogen atom. Such free hydrogen atoms tend to react chemically with other types of atoms at sufficiently low energies. When free hydrogen atoms react with other, they form neutral hydrogen molecules. Protons are spin-½ fermions and are composed of three quarks, making them baryons. Protons have an exponentially decaying positive charge distribution with a mean square radius of about 0.8 fm. Protons and neutrons are both nucleons, which may be together by the nuclear force to form atomic nuclei. The nucleus of the most common isotope of the atom is a lone proton
13.
Antiparticle
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Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite charge. For example, the antiparticle of the electron is the positron, the opposite is also true, the antiparticle of the positron is the electron. Some particles, such as the photon, are their own antiparticle, otherwise, for each pair of antiparticle partners, one is designated as normal matter, and the other as antimatter. Particle–antiparticle pairs can annihilate each other, producing photons, since the charges of the particle and antiparticle are opposite, for example, the positrons produced in natural radioactive decay quickly annihilate themselves with electrons, producing pairs of gamma rays, a process exploited in positron emission tomography. The laws of nature are very nearly symmetrical with respect to particles and antiparticles, for example, an antiproton and a positron can form an antihydrogen atom, which is believed to have the same properties as a hydrogen atom. The discovery of Charge Parity violation helped to shed light on this problem by showing that this symmetry, antiparticles are produced naturally in beta decay, and in the interaction of cosmic rays in the Earths atmosphere. Because charge is conserved, it is not possible to create an antiparticle without either destroying a particle of the charge or by creating a particle of the opposite charge. The latter is seen in many processes in both a particle and its antiparticle are created simultaneously, as in particle accelerators. This is the inverse of the annihilation process. Although particles and their antiparticles have opposite charges, electrically neutral particles need not be identical to their antiparticles. The neutron, for example, is out of quarks, the antineutron from antiquarks. However, other particles are their own antiparticles, such as photons, hypothetical gravitons. The electric charge-to-mass ratio of a particle can be measured by observing the radius of curling of its track in a magnetic field. Positrons, because of the direction that their paths curled, were at first mistaken for electrons travelling in the opposite direction, the antiproton and antineutron were found by Emilio Segrè and Owen Chamberlain in 1955 at the University of California, Berkeley. Since then, the antiparticles of many subatomic particles have been created in particle accelerator experiments. In recent years, complete atoms of antimatter have been assembled out of antiprotons and positrons, solutions of the Dirac equation contained negative energy quantum states. As a result, an electron could always radiate energy and fall into an energy state. Even worse, it could keep radiating infinite amounts of energy because there were many negative energy states available
14.
Neutrino
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A neutrino is a fermion that interacts only via the weak subatomic force and gravity. The mass of the neutrino is much smaller than that of the known elementary particles. The neutrino is so named because it is neutral and because its rest mass is so small that it was originally thought to be zero. The weak force has a short range, gravity is extremely weak on the subatomic scale. Thus, neutrinos pass through normal matter unimpeded and undetected. Weak interactions create neutrinos in one of three flavors, electron neutrinos, muon neutrinos, or tau neutrinos, in association with the corresponding charged lepton. Although neutrinos were long believed to be massless, it is now known there are three discrete neutrino masses with different tiny values, but they dont correspond uniquely to the three flavors. A neutrino created with a specific flavor is in a specific quantum superposition of all three mass states. Neutrinos oscillate between different flavors in flight, for example, an electron neutrino produced in a beta decay reaction may interact in a distant detector as a muon or tau neutrino. For each neutrino, there exists a corresponding antiparticle, called an antineutrino. They are distinguished from the neutrinos by having opposite signs of lepton number, the majority of neutrinos in the vicinity of the Earth are from nuclear reactions in the Sun. About 65 billion solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun in the region of the Earth, neutrinos are also finding practical applications, such as tomography of the interior of the earth. The neutrino was postulated first by Wolfgang Pauli in 1930 to explain how beta decay could conserve energy, momentum and he considered that the new particle was emitted from the nucleus together with the electron or beta particle in the process of beta decay. James Chadwick discovered a more massive nuclear particle in 1932 and also named it a neutron. However, the journal Nature rejected Fermis paper, saying that the theory was too remote from reality. He submitted the paper to an Italian journal, which accepted it, however, by 1934 there was experimental evidence against Bohrs idea that energy conservation is invalid for beta decay. Such a limit is not expected if the conservation of energy is invalid, Pauli made use of the occasion to publicly emphasize that the still-undetected neutrino must be an actual particle. In 1942, Wang Ganchang first proposed the use of beta capture to experimentally detect neutrinos, in the 20 July 1956 issue of Science, Clyde Cowan, Frederick Reines, F. B
15.
Weak interaction
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In particle physics, the weak interaction is one of the four known fundamental interactions of nature, alongside the strong interaction, electromagnetism, and gravitation. The weak interaction is responsible for radioactive decay, which plays an role in nuclear fission. The theory of the interaction is sometimes called quantum flavourdynamics, in analogy with the terms QCD dealing with the strong interaction. However the term QFD is rarely used because the force is best understood in terms of electro-weak theory. The Standard Model of particle physics, which does not address gravity, provides a framework for understanding how the electromagnetic, weak. An interaction occurs when two particles, typically but not necessarily half-integer spin fermions, exchange integer-spin, force-carrying bosons, the fermions involved in such exchanges can be either elementary or composite, although at the deepest levels, all weak interactions ultimately are between elementary particles. In the case of the interaction, fermions can exchange three distinct types of force carriers known as the W+, W−, and Z bosons. The mass of each of these bosons is far greater than the mass of a proton or neutron, the force is in fact termed weak because its field strength over a given distance is typically several orders of magnitude less than that of the strong nuclear force or electromagnetic force. During the quark epoch of the universe, the electroweak force separated into the electromagnetic. Important examples of the weak interaction include beta decay, and the fusion of hydrogen into deuterium that powers the Suns thermonuclear process, most fermions will decay by a weak interaction over time. Such decay makes radiocarbon dating possible, as carbon-14 decays through the interaction to nitrogen-14. It can also create radioluminescence, commonly used in tritium illumination, quarks, which make up composite particles like neutrons and protons, come in six flavours – up, down, strange, charm, top and bottom – which give those composite particles their properties. The weak interaction is unique in that it allows for quarks to swap their flavour for another, the swapping of those properties is mediated by the force carrier bosons. Also, the interaction is the only fundamental interaction that breaks parity-symmetry, and similarly. In 1933, Enrico Fermi proposed the first theory of the weak interaction and he suggested that beta decay could be explained by a four-fermion interaction, involving a contact force with no range. However, it is described as a non-contact force field having a finite range. The existence of the W and Z bosons was not directly confirmed until 1983, the weak interaction is unique in a number of respects, It is the only interaction capable of changing the flavour of quarks. It is the interaction that violates P or parity-symmetry
16.
Virtual particle
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In physics, a virtual particle is a transient fluctuation that exhibits many of the characteristics of an ordinary particle, but that exists for a limited time. The concept of virtual particles arises in perturbation theory of field theory where interactions between ordinary particles are described in terms of exchanges of virtual particles. Any process involving virtual particles admits a schematic representation known as a Feynman diagram, virtual particles do not necessarily carry the same mass as the corresponding real particle, although they always conserve energy and momentum. The longer the virtual particle exists, the closer its characteristics come to those of ordinary particles and they are important in the physics of many processes, including particle scattering and Casimir forces. As such the accuracy and use of particles in calculations is firmly established. Antiparticles have been proven to exist and should not be confused with particles or virtual antiparticles. Such calculations are performed using schematic representations known as Feynman diagrams. A virtual particle does not precisely obey the energy–momentum relation m2c4 = E2 − p2c2 and its kinetic energy may not have the usual relationship to velocity–indeed, it can be negative. This is expressed by the phrase off mass shell, the probability amplitude for a virtual particle to exist tends to be canceled out by destructive interference over longer distances and times. As a consequence, a photon is massless and thus has only two polarization states, whereas a virtual one, being effectively massive, has three polarization states. Quantum tunnelling may be considered a manifestation of virtual particle exchanges, written in the usual mathematical notations, in the equations of physics, there is no mark of the distinction between virtual and actual particles. In the quantum field theory view, actual particles are viewed as being detectable excitations of underlying quantum fields, virtual particles are also viewed as excitations of the underlying fields, but appear only as forces, not as detectable particles. They are temporary in the sense that they appear in calculations, thus, in mathematical terms, they never appear as indices to the scattering matrix, which is to say, they never appear as the observable inputs and outputs of the physical process being modelled. There are two ways in which the notion of virtual particles appears in modern physics. They appear as terms in Feynman diagrams, that is. They also appear as a set of states to be summed or integrated over in the calculation of a semi-non-perturbative effect. In the latter case, it is said that virtual particles contribute to a mechanism that mediates the effect. There are many observable physical phenomena that arise in interactions involving virtual particles, examples of such short-range interactions are the strong and weak forces, and their associated field bosons
17.
Quark
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A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, due to a phenomenon known as color confinement, quarks are never directly observed or found in isolation, they can be found only within hadrons, such as baryons and mesons. For this reason, much of what is known about quarks has been drawn from observations of the hadrons themselves, Quarks have various intrinsic properties, including electric charge, mass, color charge, and spin. There are six types of quarks, known as flavors, up, down, strange, charm, top, up and down quarks have the lowest masses of all quarks. The heavier quarks rapidly change into up and down quarks through a process of particle decay, the transformation from a higher mass state to a lower mass state. Because of this, up and down quarks are generally stable and the most common in the universe, whereas strange, charm, bottom, and top quarks can only be produced in high energy collisions. For every quark flavor there is a type of antiparticle, known as an antiquark. The quark model was proposed by physicists Murray Gell-Mann and George Zweig in 1964. Accelerator experiments have provided evidence for all six flavors, the top quark was the last to be discovered at Fermilab in 1995. The Standard Model is the theoretical framework describing all the known elementary particles. This model contains six flavors of quarks, named up, down, strange, charm, bottom, antiparticles of quarks are called antiquarks, and are denoted by a bar over the symbol for the corresponding quark, such as u for an up antiquark. As with antimatter in general, antiquarks have the mass, mean lifetime, and spin as their respective quarks. Quarks are spin- 1⁄2 particles, implying that they are fermions according to the spin-statistics theorem and they are subject to the Pauli exclusion principle, which states that no two identical fermions can simultaneously occupy the same quantum state. This is in contrast to bosons, any number of which can be in the same state, unlike leptons, quarks possess color charge, which causes them to engage in the strong interaction. The resulting attraction between different quarks causes the formation of composite particles known as hadrons, there are two families of hadrons, baryons, with three valence quarks, and mesons, with a valence quark and an antiquark. The most common baryons are the proton and the neutron, the blocks of the atomic nucleus. A great number of hadrons are known, most of them differentiated by their quark content, the existence of exotic hadrons with more valence quarks, such as tetraquarks and pentaquarks, has been conjectured but not proven. However, on 13 July 2015, the LHCb collaboration at CERN reported results consistent with pentaquark states, elementary fermions are grouped into three generations, each comprising two leptons and two quarks
18.
Nuclear fission product
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Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release of heat energy. The two smaller nuclei are the fission products, about 0. 2% to 0. 4% of fissions are ternary fissions, producing a third light nucleus such as helium-4 or tritium. The fission products themselves are often unstable and radioactive, due to being relatively neutron-rich for their atomic number and this releases additional energy in the form of beta particles, antineutrinos, and gamma rays. Thus, fission events normally result in radiation and antineutrinos. Many of these nuclides have a very short half-life, and therefore are very radioactive, for instance, strontium-90, strontium-89 and strontium-94 are all fission products, they are produced in similar quantities, and each nucleus decays by shooting off one beta particle. But Sr-90 has a 30-year half-life, Sr-89 a 50. 5-day half-life, the same number of atoms of Sr-89 will decay 10,600 times faster than Sr-90, and Sr-94 will do so 915 million times faster. It is these short-half-life nuclides that make spent fuel so dangerous, in addition to generating much heat, immediately after the reactor itself has been shut down. The most radioactive also decay the fastest, after 50 days, Sr-94 has had 58,000 half-lives and is gone, Sr-89 is at half its original quantity. As there are hundreds of different nuclides created, the radioactivity level fades quickly. The sum of the mass of the two atoms produced by the fission of one fissile atom is always less than the atomic mass of the original atom. Since the nuclei that can undergo fission are particularly neutron-rich. The initial fission products therefore may be unstable and typically undergo beta decay to move towards a stable configuration, a few neutron-rich and short-lived initial fission products decay by ordinary beta decay, followed by immediate emission of a neutron by the excited daughter-product. This process is the source of so-called delayed neutrons, which play an important role in control of a nuclear reactor, the first beta decays are rapid and may release high energy beta particles or gamma radiation. However, as the fission products approach stable nuclear conditions, the last one or two decays may have a long half-life and release less energy. Fission products have half-lives of 90 years or less, except for seven long-lived fission products that have lives of 211,100 years. This behavior of pure fission products with actinides removed, contrasts with the decay of fuel that still contains actinides and this fuel is produced in the so-called open nuclear fuel cycle. A number of these actinides have half lives in the range of about 100 to 200,000 years
19.
Nuclear reactor
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This article is a subarticle of Nuclear power. A nuclear reactor, formerly known as a pile, is a device used to initiate. Nuclear reactors are used at power plants for electricity generation. Heat from nuclear fission is passed to a fluid, which runs through steam turbines. These either drive a ships propellers or turn electrical generators, Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, some are run only for research. As of April 2014, the IAEA reports there are 435 nuclear power reactors in operation, when a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more nuclei, releasing kinetic energy, gamma radiation, and free neutrons. A portion of neutrons may later be absorbed by other fissile atoms and trigger further fission events, which release more neutrons. This is known as a chain reaction. To control such a chain reaction, neutron poisons and neutron moderators can change the portion of neutrons that will go on to cause more fission. Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if monitoring detects unsafe conditions, commonly-used moderators include regular water, solid graphite and heavy water. Some experimental types of reactor have used beryllium, and hydrocarbons have been suggested as another possibility, the reactor core generates heat in a number of ways, The kinetic energy of fission products is converted to thermal energy when these nuclei collide with nearby atoms. The reactor absorbs some of the rays produced during fission. Heat is produced by the decay of fission products and materials that have been activated by neutron absorption. This decay heat-source will remain for some even after the reactor is shut down. A kilogram of uranium-235 converted via nuclear processes releases approximately three times more energy than a kilogram of coal burned conventionally. A nuclear reactor coolant — usually water but sometimes a gas or a metal or molten salt — is circulated past the reactor core to absorb the heat that it generates
20.
Positron emission
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Positron emission is mediated by the weak force. The positron is a type of particle, the other beta particle being the electron emitted from the β− decay of a nucleus. Positron decay results in nuclear transmutation, changing an atom of one element into an atom of an element with an atomic number that is less by one unit. Positron emission should not be confused with electron emission or beta decay, which occurs when a neutron turns into a proton and the nucleus emits an electron. This was the first example of β+ decay, the Curies termed the phenomenon artificial radioactivity, since 30 15P is a short-lived nuclide which does not exist in nature. The discovery of radioactivity would be cited when the husband. Isotopes which undergo this decay and thereby emit positrons include carbon-11, potassium-40, nitrogen-13, oxygen-15, aluminium-26, sodium-22, fluorine-18, the two most common types of quarks are up quarks, which have a charge of +2/3, and down quarks, with a −1/3 charge. Quarks arrange themselves in sets of three such that they make protons and neutrons, in a proton, whose charge is +1, there are two up quarks and one down quark. Neutrons, with no charge, have one up quark and two down quarks, via the weak interaction, quarks can change flavor from down to up, resulting in electron emission. Positron emission happens when an up quark changes into a down quark, nuclei which decay by positron emission may also decay by electron capture. For low-energy decays, electron capture is energetically favored by 2mec2 =1.022 MeV, as the energy of the decay goes up, so does the branching ratio towards positron emission. However, if the difference is less than 2mec2, then positron emission cannot occur. Certain isotopes are stable in galactic cosmic rays, because the electrons are stripped away, a positron is ejected from the parent nucleus, and the daughter atom must shed an orbital electron to balance charge. Isotopes which increase in mass under the conversion of a proton to a neutron, or which decrease in mass by less than 2me and these isotopes are used in positron emission tomography, a technique used for medical imaging. Note that the energy emitted depends on the isotope that is decaying, the short-lived positron emitting isotopes 11C, 13N, 15O and 18F used for positron emission tomography are typically produced by proton irradiation of natural or enriched targets. The LIVEChart of Nuclides - IAEA with filter on β+ decay
21.
Binding energy
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Binding energy is the energy required to disassemble a whole system into separate parts. A bound system typically has a potential energy than the sum of its constituent parts. Often this means that energy is released upon the creation of a bound state and this definition corresponds to a positive binding energy. In bound systems, if the energy is removed from the system, it must be subtracted from the mass of the unbound system. Thus, if energy is removed from the system at the time it is bound, system mass is not conserved in this process because the system is open during the binding process. There are several types of binding energy, each operating over a different distance, the smaller the scale of a bound system, the higher its associated binding energy. In astrophysics, the binding energy of a celestial body is the energy required to expand the material to infinity.66 eV per atom. If a carbon-12 body had the mass and radius of the Sun, at the molecular level, bond energy and bond-dissociation energy are measures of the binding energy between the atoms in a chemical bond. It is the required to disassemble a molecule into its constituent atoms. This energy appears as chemical energy, such as that released in chemical explosions, bond energies and bond-dissociation energies are typically in the range of few eV per bond. For example, the energy of a carbon-carbon bond is about 3.6 eV. At the atomic level, the binding energy of the atom derives from electromagnetic interaction. It is the required to disassemble an atom into free electrons. Electron binding energy is a measure of the required to free electrons from their atomic orbits. This is more known as ionization energy. Among the chemical elements, the range of energies is from 3.8939 eV for the first electron in an atom of caesium to 11.567617 keV for the 29th electron in an atom of copper. At the nuclear level, nuclear binding energy is the required to disassemble a nucleus into the free. It is the equivalent of the mass defect, the difference between the mass number of a nucleus and its true measured mass
22.
Electronvolt
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In physics, the electronvolt is a unit of energy equal to approximately 1. 6×10−19 joules. By definition, it is the amount of energy gained by the charge of an electron moving across an electric potential difference of one volt. Thus it is 1 volt multiplied by the elementary charge, therefore, one electronvolt is equal to 6981160217662079999♠1. 6021766208×10−19 J. The electronvolt is not a SI unit, and its definition is empirical, like the elementary charge on which it is based, it is not an independent quantity but is equal to 1 J/C √2hα / μ0c0. It is a unit of energy within physics, widely used in solid state, atomic, nuclear. It is commonly used with the metric prefixes milli-, kilo-, in some older documents, and in the name Bevatron, the symbol BeV is used, which stands for billion electronvolts, it is equivalent to the GeV. By mass–energy equivalence, the electronvolt is also a unit of mass and it is common in particle physics, where units of mass and energy are often interchanged, to express mass in units of eV/c2, where c is the speed of light in vacuum. It is common to express mass in terms of eV as a unit of mass. The mass equivalent of 1 eV/c2 is 1 eV / c 2 = ⋅1 V2 =1.783 ×10 −36 kg. For example, an electron and a positron, each with a mass of 0.511 MeV/c2, the proton has a mass of 0.938 GeV/c2. In general, the masses of all hadrons are of the order of 1 GeV/c2, the unified atomic mass unit,1 gram divided by Avogadros number, is almost the mass of a hydrogen atom, which is mostly the mass of the proton. To convert to megaelectronvolts, use the formula,1 u =931.4941 MeV/c2 =0.9314941 GeV/c2, in high-energy physics, the electronvolt is often used as a unit of momentum. A potential difference of 1 volt causes an electron to gain an amount of energy and this gives rise to usage of eV as units of momentum, for the energy supplied results in acceleration of the particle. The dimensions of units are LMT−1. The dimensions of units are L2MT−2. Then, dividing the units of energy by a constant that has units of velocity. In the field of particle physics, the fundamental velocity unit is the speed of light in vacuum c. Thus, dividing energy in eV by the speed of light, the fundamental velocity constant c is often dropped from the units of momentum by way of defining units of length such that the value of c is unity
23.
Poly(methyl methacrylate)
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The same material can be utilised as a casting resin, in inks and coatings, and has many other uses. Chemically, it is the polymer of methyl methacrylate. PMMA is an alternative to polycarbonate when extreme strength is not necessary. Additionally, PMMA does not contain the potentially harmful bisphenol-A subunits found in polycarbonate and it is often preferred because of its moderate properties, easy handling and processing, and low cost. The first acrylic acid was created in 1843, methacrylic acid, derived from acrylic acid, was formulated in 1865. The reaction between acid and methanol results in the ester methyl methacrylate. In 1877 the German chemist Wilhelm Rudolph Fittig discovered the process that turns methyl methacrylate into polymethyl methacrylate. In 1933, the brand name Plexiglas was patented and registered by another German chemist, in 1936 Imperial Chemical Industries began the first commercially viable production of acrylic safety glass. During World War II both Allied and Axis forces used acrylic glass for submarine periscopes and aircraft windshields, canopies, common orthographic stylings include polymethyl methacrylate and polymethylmethacrylate. The full chemical name is poly, although often called simply acrylic, acrylic can also refer to other polymers or copolymers containing polyacrylonitrile. The other notable names include, Acrylite, a trademark of Evonik Cyro since 1976 Lucite, a trademark of DuPont, first registered in 1937 R-Cast. Founded in 1987 after spinning off from Reynolds & Taylor and they specialize in large scale and thick monolithic acrylic. Plexiglas, a trademark of ELF Atochem, now a subsidiary of Arkema in the US, generally, radical initiation is used, but anionic polymerization of PMMA can also be performed. To produce 1 kg of PMMA, about 2 kg of petroleum is needed, PMMA produced by radical polymerization is atactic and completely amorphous. The glass transition temperature of atactic PMMA is 105 °C, PMMA is thus an organic glass at room temperature, i. e. it is below its Tg. The forming temperature starts at the transition temperature and goes up from there. All common molding processes may be used, including molding, compression molding. The highest quality PMMA sheets are produced by casting, but in this case
24.
Cherenkov radiation
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The characteristic blue glow of an underwater nuclear reactor is due to Cherenkov radiation. It is named after Soviet scientist Pavel Alekseyevich Cherenkov, the 1958 Nobel Prize winner who was the first to detect it experimentally. A theory of this effect was developed within the framework of Einsteins special relativity theory by Igor Tamm and Ilya Frank. Cherenkov radiation had been predicted by the English polymath Oliver Heaviside in papers published in 1888–89. While electrodynamics holds that the speed of light in a vacuum is a universal constant, for example, the speed of the propagation of light in water is only 0. 75c. Matter can be accelerated beyond this speed during nuclear reactions and in particle accelerators, Cherenkov radiation results when a charged particle, most commonly an electron, travels through a dielectric medium with a speed greater than that at which light propagates in the same medium. Moreover, the velocity that must be exceeded is the velocity of light rather than the group velocity of light. In a more complex periodic medium, such as a crystal, one can also obtain a variety of other anomalous Cherenkov effects. As a charged particle travels, it disrupts the electromagnetic field in its medium. In particular, the medium becomes electrically polarized by the electric field. If the particle travels slowly then the disturbance elastically relaxes back to equilibrium as the particle passes. A common analogy is the boom of a supersonic aircraft or bullet. In a similar way, a particle can generate a light shock wave as it travels through an insulator. In the figure, the travels in a medium with speed v p such that c / n < v p < c, where c is speed of light in vacuum. We define the ratio between the speed of the particle and the speed of light as β = v p / c, the emitted light waves travel at speed v em = c / n. The left corner of the triangle represents the location of the particle at some initial moment. The right corner of the triangle is the location of the particle at some time t. In the given time t, the particle travels the distance x p = v p t = β c t whereas the emitted electromagnetic waves are constricted to travel the distance x em = v em t = c n t
25.
TRIGA
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TRIGA is a class of small nuclear reactor designed and manufactured by General Atomics. The design team for TRIGA, which included Edward Teller, was led by the physicist Freeman Dyson and it is thus highly unlikely, though not impossible for a nuclear meltdown to occur. Because of this feature, it can be safely pulsed at a power of 22,000 megawatts. The TRIGA was developed to be a reactor that, in the words of Frederic de Hoffmann, Edward Teller headed a group of young nuclear physicists in San Diego in the summer of 1956 to design a reactor which could not, by its design, suffer from a meltdown. The design was largely the suggestion of Freeman Dyson, the prototype for the TRIGA nuclear reactor was commissioned on 3 May 1958 in San Diego and operated until shut down in 1997. It has been designated as a historic landmark by the American Nuclear Society. Mark II, Mark III and other variants of the TRIGA design have subsequently been produced, a further 35 reactors have been installed in other countries. Many of these installations were prompted by US President Eisenhowers 1953 Atoms for Peace policy, TRIGA International, a joint venture between General Atomics and CERCA — a subsidiary of AREVA of France — was established in 1996. Since then, TRIGA fuel assemblies have been manufactured at CERCAs plant in Romans-sur-Isère, new TRIGA installations by General Atomics are underway in Morocco, Thailand and Romania. Some of the competitors to General Atomics in the supply of research reactors are Areva of Europe. Memoirs, A Twentieth-Century Journey in Science and Politics, General Atomics official TRIGA website TRIGA history webpage Technical Specifications
26.
Pool-type reactor
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Pool-type reactors, also called swimming pool reactors, are a type of nuclear reactor that has a core immersed in an open pool of usually water. Some sodium-cooled reactors like the BN-600 have sodium pools instead, the rest of this article will assume that water is being used. The water acts as moderator, cooling agent and radiation shield. The layer of water directly above the reactor core shields the radiation so completely that operators may work above the reactor safely. This design has two advantages, the reactor is easily accessible and the whole primary cooling system, i. e. the pool water, is under normal pressure. This avoids the high temperatures and great pressures of nuclear power plants, pool reactors are used as a source of neutrons and for training, and in rare instances for processing heat but not for electrical generation. Open pools range in height from 6m to 9m and diameter from 1. 8m to 3. 6m, some pools, like the one at the Canadian MAPLE reactor, are rectangular instead of cylindrical and often contain as much as 416,000 litres of water. Most pools are built above floor level but some are completely or partially below ground, ordinary water- and heavy water-only types exist as well as so-called tank in pool designs that use heavy water moderation in a small tank situated in a larger light water pool for cooling. Life preservers are sometimes located around the facility to rescue personnel that may fall into the pool, normally the reactor is charged with low enriched uranium fuel consisting of less than 20% U-235 alloyed with a matrix such as aluminium or zirconium. Highly enriched uranium was the fuel of choice since it had a longer lifetime, however most often 19. 75% enrichment is used, falling just under the 20% level that would make it highly enriched. Fuel elements may be plates or rods with 8. 5% to 45% uranium, beryllium and graphite blocks or plates may be added to the core as neutron reflectors and neutron absorbing rods pierce the core for control. General Atomics of La Jolla, CA manufactures TRIGA reactor fuel elements in France for the majority of types of reactors around the world. Core cooling is accomplished either by convection induced by the hot core or in larger reactors by forced coolant flow, various stations for holding items to be irradiated are located inside the core or directly adjacent to the core. Samples may be lowered into the core from above or delivered pneumatically via horizontal tubes from outside the tank at core level, evacuated, or helium filled horizontal tubes may also be installed to direct a beam of neutrons to targets situated at a distance from the reactor hall. Most research reactors are of the pool type and these tend to be low power, low maintenance designs. For example AECLs SLOWPOKE is licensed to run unattended for up to 18 hours, boron neutron capture therapy is another, medical use. Pressurized water reactor CANDU reactor NRX TRIGA
27.
Aluminium
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Aluminium or aluminum is a chemical element in the boron group with symbol Al and atomic number 13. It is a silvery-white, soft, nonmagnetic, ductile metal, Aluminium metal is so chemically reactive that native specimens are rare and limited to extreme reducing environments. Instead, it is combined in over 270 different minerals. The chief ore of aluminium is bauxite, Aluminium is remarkable for the metals low density and its ability to resist corrosion through the phenomenon of passivation. Aluminium and its alloys are vital to the industry and important in transportation and structures, such as building facades. The oxides and sulfates are the most useful compounds of aluminium, despite its prevalence in the environment, no known form of life uses aluminium salts metabolically, but aluminium is well tolerated by plants and animals. Because of these salts abundance, the potential for a role for them is of continuing interest. Aluminium is a soft, durable, lightweight, ductile. It is nonmagnetic and does not easily ignite, a fresh film of aluminium serves as a good reflector of visible light and an excellent reflector of medium and far infrared radiation. The yield strength of aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa. Aluminium has about one-third the density and stiffness of steel and it is easily machined, cast, drawn and extruded. Aluminium atoms are arranged in a cubic structure. Aluminium has an energy of approximately 200 mJ/m2. Aluminium is a thermal and electrical conductor, having 59% the conductivity of copper. Aluminium is capable of superconductivity, with a critical temperature of 1.2 kelvin. Aluminium is the most common material for the fabrication of superconducting qubits, the strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper. This corrosion resistance is reduced by aqueous salts, particularly in the presence of dissimilar metals. In highly acidic solutions, aluminium reacts with water to form hydrogen, primarily because it is corroded by dissolved chlorides, such as common sodium chloride, household plumbing is never made from aluminium
28.
Bremsstrahlung
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Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon, the term is also used to refer to the process of producing the radiation. Bremsstrahlung has a spectrum, which becomes more intense and whose peak intensity shifts toward higher frequencies as the change of the energy of the decelerated particles increases. However, the term is used in the more narrow sense of radiation from electrons slowing in matter. Bremsstrahlung emitted from plasma is referred to as free/free radiation. A charged particle accelerating in a vacuum radiates power, as described by the Larmor formula, although the term, bremsstrahlung, is usually reserved for charged particles accelerating in matter, not vacuum, the formulas are similar. This is commonly written in the equivalent form using 2 = β →2 ⋅ β → ˙2 −2, P = q 2 γ66 π ε0 c. For the case of acceleration perpendicular to the velocity, the power radiated reduces to P a ⊥ v = q 2 a 2 γ46 π ε0 c 3. Radiated in the two limiting cases is proportional to γ4 or γ6 and this is the reason a TeV energy electron-positron collider cannot use a circular tunnel, while a proton-proton collider can utilize a circular tunnel. The electrons lose energy due to bremsstrahlung at a rate 4 ≈1013 times higher than protons do, NOTE, this article currently gives formulas that apply in the Rayleigh-Jeans limit ℏ ω ≪ k B T e, and does not use a quantized treatment of radiation. Thus a usual factor like exp does not appear, the appearance of ℏ ω / k B T e in y below is due to the quantum-mechanical treatment of collisions. In a plasma, the free electrons collide with the ions. A complete analysis requires accounting for both binary Coulomb collisions as well as collective behavior, a detailed treatment is given by Bekefi, while a simplified one is given by Ichimaru. In this section we follow Bekefis dielectric treatment, with collisions included approximately via the cutoff wavenumber, consider a uniform plasma, with thermal electrons distributed according to the Maxwell–Boltzmann distribution with the temperature T e. The second bracketed factor is the index of refraction of a wave in a plasma. This formula thus only applies for ω > ω p and this formula should be summed over ion species in a multi-species plasma. Otherwise, k m ∝1 / l c where l c is the classical Coulomb distance of closest approach, for the usual case k m =1 / λ B, we find y =122. The formula for d P B r / d ω is approximate, in the limit y ≪1, we can approximate E1 as E1 ≈ − ln + O where γ ≈0.577 is the Euler–Mascheroni constant
29.
X-ray
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X-radiation is a form of electromagnetic radiation. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz, X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays. Spelling of X-ray in the English language includes the variants x-ray, xray, X-rays with high photon energies are called hard X-rays, while those with lower energy are called soft X-rays. Due to their ability, hard X-rays are widely used to image the inside of objects, e. g. in medical radiography. The term X-ray is metonymically used to refer to an image produced using this method. Since the wavelengths of hard X-rays are similar to the size of atoms they are useful for determining crystal structures by X-ray crystallography. By contrast, soft X-rays are easily absorbed in air, the length of 600 eV X-rays in water is less than 1 micrometer. There is no consensus for a definition distinguishing between X-rays and gamma rays, one common practice is to distinguish between the two types of radiation based on their source, X-rays are emitted by electrons, while gamma rays are emitted by the atomic nucleus. This definition has problems, other processes also can generate these high-energy photons. One common alternative is to distinguish X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10−11 m and this criterion assigns a photon to an unambiguous category, but is only possible if wavelength is known. Occasionally, one term or the other is used in specific contexts due to precedent, based on measurement technique. Thus, gamma-rays generated for medical and industrial uses, for radiotherapy, in the ranges of 6–20 MeV. X-ray photons carry enough energy to ionize atoms and disrupt molecular bonds and this makes it a type of ionizing radiation, and therefore harmful to living tissue. A very high radiation dose over a period of time causes radiation sickness. In medical imaging this increased risk is generally greatly outweighed by the benefits of the examination. The ionizing capability of X-rays can be utilized in treatment to kill malignant cells using radiation therapy. It is also used for material characterization using X-ray spectroscopy, hard X-rays can traverse relatively thick objects without being much absorbed or scattered. For this reason, X-rays are widely used to image the inside of visually opaque objects, the most often seen applications are in medical radiography and airport security scanners, but similar techniques are also important in industry and research
30.
Cloud chamber
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The cloud chamber, also known as the Wilson chamber, is a particle detector used for detecting ionizing radiation. In its most basic form, a chamber is a sealed environment containing a supersaturated vapor of water or alcohol. When a charged particle interacts with the mixture, the fluid is ionized, the resulting ions act as condensation nuclei, around which a mist will form. The high energies of alpha and beta particles mean that a trail is left, Cloud chambers played a prominent role in the experimental particle physics from the 1920s to the 1950s, until the advent of the bubble chamber. In particular, the discoveries of the positron in 1932, the muon in 1936, Anderson detected the positron and muon in cosmic rays. Charles Thomson Rees Wilson, a Scottish physicist, is credited with inventing the cloud chamber, inspired by sightings of the Brocken spectre while working on the summit of Ben Nevis in 1894, he began to develop expansion chambers for studying cloud formation and optical phenomena in moist air. Very rapidly he discovered that ions could act as centers for water droplet formation in such chambers and he pursued the application of this discovery and perfected the first cloud chamber in 1911. When an ionizing particle passes through the chamber, water condenses on the resulting ions. Wilson, along with Arthur Compton, received the Nobel Prize in Physics in 1927 for his work on the cloud chamber and this kind of chamber is also called a Pulsed Chamber because the conditions for operation are not continuously maintained. Further developments were made by Patrick Blackett who utilised a stiff spring to expand and compress the chamber very rapidly, a cine film was used to record the images. The diffusion cloud chamber was developed in 1936 by Alexander Langsdorf, instead of water vapor, alcohol is used because of its lower freezing point. Cloud chambers cooled by dry ice are a demonstration and hobbyist device. There are also water-cooled diffusion cloud chambers, using ethylene glycol, a simple cloud chamber consists of the sealed environment, radioactive source, dry ice or a cold plate and some kind of alcohol source. The alcohol falls as it cools down and the cold condenser provides a steep temperature gradient, the result is a supersaturated environment. The alcohol vapor condenses around ion trails left behind by the travelling ionizing particles, the result is cloud formation, seen in the cloud chamber by the presence of droplets falling down to the condenser. As particles pass through they leave ionization trails and because the vapor is supersaturated it condenses onto these trails. Since the tracks are emitted radially out from the source, their point of origin can easily be determined, just above the cold condenser plate there is an area of the chamber which is sensitive to radioactive tracks. At this height, most of the alcohol has not condensed and this means that the ion trail left by the radioactive particles provides an optimal trigger for condensation and cloud formation
31.
Ionization chamber
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Ion chambers have a good uniform response to radiation over a wide range of energies and are the preferred means of measuring high levels of gamma radiation. They are widely used in the power industry, research labs, radiography, radiobiology. An ionization chamber measures the charge from the number of ion pairs created within a gas caused by incident radiation and it consists of a gas-filled chamber with two electrodes, known as anode and cathode. The electrodes may be in the form of plates, or a cylinder arrangement with a coaxially located internal anode wire. A voltage potential is applied between the electrodes to create a field in the fill gas. This generates a current which is measured by an electrometer circuit. The electrometer must be capable of measuring the small output current which is in the region of femtoamperes to picoamperes, depending on the chamber design, radiation dose. This continual generation of charge produces a current, which is a measure of the total ionizing dose entering the chamber. However, the chamber cannot discriminate between radiation types and cannot produce a spectrum of radiation. This is in distinction to the Geiger-Müller tube or the proportional counter whereby secondary electrons, the following chamber types are commonly used. This is a chamber open to atmosphere, where the fill gas is ambient air. The domestic smoke detector is an example of this, where a natural flow of air through the chamber is necessary so that smoke particles can be detected by the change in ion current. Other examples are applications where the ions are created outside the chamber but are carried in by a flow of air or gas. These chambers are normally cylindrical and operate at atmospheric pressure, and this is to stop moisture building up in the interior of the chamber, which would otherwise be introduced by the pump effect of changing atmospheric air pressure. These chambers have a body made of aluminium or plastic a few millimetres thick. The material is selected to have a number similar to that of air so that the wall is said to be air equivalent over a range of radiation beam energies. The higher the number of the wall material, the greater the chance of interaction. The wall thickness is a trade-off between maintaining the air effect with a wall, and increasing sensitivity by using a thinner wall
32.
Geiger counter
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It detects ionizing radiation such as alpha particles, beta particles and gamma rays using the ionization effect produced in a Geiger–Müller tube, which gives its name to the instrument. In wide and prominent use as a radiation survey instrument. Since then it has very popular due to its robust sensing element. However, there are limitations in measuring high radiation rates and the energy of incident radiation, a Geiger counter consists of a Geiger-Müller tube, the sensing element which detects the radiation, and the processing electronics, which displays the result. The Geiger-Müller tube is filled with a gas such as helium, neon, or argon at low pressure. The tube briefly conducts electrical charge when a particle or photon of incident radiation makes the gas conductive by ionization, the ionization is considerably amplified within the tube by the Townsend discharge effect to produce an easily measured detection pulse, which is fed to the processing and display electronics. This large pulse from the tube makes the G-M counter relatively cheap to manufacture, the electronics also generates the high voltage, typically 400–600 volts, that has to be applied to the Geiger-Müller tube to enable its operation. There are two types of radiation readout, counts or radiation dose, the counts display is the simplest and is the number of ionizing events displayed either as a count rate, commonly counts per second, or as a total over a set time period. The counts readout is normally used when alpha or beta particles are being detected, more complex to achieve is a display of radiation dose rate, displayed in a unit such as the sievert which is normally used for measuring gamma or X-ray dose rates. A G-M tube can detect the presence of radiation, but not its energy influences the radiations ionising effect. Consequently, instruments measuring dose rate require the use of an energy compensated G-M tube, the electronics will apply known factors to make this conversion, which is specific to each instrument and is determined by design and calibration. The readout can be analog or digital, and increasingly, modern instruments are offering serial communications with a host computer or network, there is usually an option to produce audible clicks representing the number of ionization events detected. This is the distinctive sound associated with hand held or portable Geiger counters. The purpose of this is to allow the user to concentrate on manipulation of the instrument whilst retaining auditory feedback on the radiation rate, there are two main limitations of the Geiger counter. Because the output pulse from a Geiger-Müller tube is always the same regardless of the energy of the incident radiation. A further limitation is the inability to measure radiation rates due to the dead time of the tube. This is a period after each ionization of the gas during which any further incident radiation will not result in a count. Typically the dead time will reduce indicated count rates above about 104 to 105 counts per second depending on the characteristic of the tube being used, whilst some counters have circuitry which can compensate for this, for accurate measurements ion chamber instruments are preferred for high radiation rates
33.
Scintillator
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A scintillator is a material that exhibits scintillation — the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by a particle, absorb its energy. A scintillation detector or scintillation counter is obtained when a scintillator is coupled to a light sensor such as a photomultiplier tube, photodiode. PMTs absorb the light emitted by the scintillator and re-emit it in the form of electrons via the photoelectric effect, the subsequent multiplication of those electrons results in an electrical pulse which can then be analyzed and yield meaningful information about the particle that originally struck the scintillator. Vacuum photodiodes are similar but do not amplify the signal while silicon photodiodes, on the other hand, the first device which used a scintillator was built in 1903 by Sir William Crookes and used a ZnS screen. The scintillations produced by the screen were visible to the eye if viewed by a microscope in a darkened room. The technique led to a number of important discoveries but was obviously tedious, scintillators gained additional attention in 1944, when Curran and Baker replaced the naked eye measurement with the newly developed PMT. This was the birth of the scintillation detector. Scintillators are used by the American government as Homeland Security radiation detectors, scintillators can also be used in particle detectors, new energy resource exploration, X-ray security, nuclear cameras, computed tomography and gas exploration. Other applications of scintillators include CT scanners and gamma cameras in medical diagnostics, scintillators generate light in fluorescent tubes, to convert the ultra-violet of the discharge into visible light. Scintillation detectors are used in the petroleum industry as detectors for Gamma Ray logs. There are many desired properties of scintillators, such as density, fast operation speed, low cost, radiation hardness. High density reduces the size of showers for high-energy γ-quanta. The range of Compton scattered photons for lower energy γ-rays is also decreased via high density materials and this results in high segmentation of the detector and leads to better spatial resolution. Usually high density materials have heavy ions in the lattice, significantly increasing the photo-fraction, the increased photo-fraction is important for some applications such as positron emission tomography. High stopping power for electromagnetic component of the ionizing radiation needs greater photo-fraction, high operating speed is needed for good resolution of spectra. Precision of time measurement with a detector is proportional to √τsc. Short decay times are important for the measurement of time intervals, high density and fast response time can allow detection of rare events in particle physics
34.
Scintillation counter
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The modern electronic scintillation counter was invented in 1944 by Sir Samuel Curran whilst he was working on the Manhattan Project at the University of California at Berkeley. This built upon the work of researchers such as Antoine Henri Becquerel. Previously scintillation events had to be detected by eye using a spinthariscope which was a simple microscope to observe light flashes in the scintillator. When an ionizing particle passes into the material, atoms are ionized along a track. For charged particles the track is the path of the particle itself, for gamma rays, their energy is converted to an energetic electron via either the photoelectric effect, Compton scattering or pair production. The chemistry of atomic de-excitation in the scintillator produces a multitude of low-energy photons, the number of such photons is in proportion to the amount of energy deposited by the ionizing particle. Some portion of these low-energy photons arrive at the photocathode of a photomultiplier tube. The photocathode emits at most one electron for each arriving photon by the photoelectric effect and this group of primary electrons is electrostatically accelerated and focused by an electrical potential so that they strike the first dynode of the tube. The impact of an electron on the dynode releases a number of secondary electrons which are in turn accelerated to strike the second dynode. Each subsequent dynode impact releases further electrons, and so there is a current amplifying effect at each dynode stage, each stage is at a higher potential than the previous to provide the accelerating field. The resultant output signal at the anode is in the form of a pulse for each group of photons that arrived at the photocathode. The pulse carries information about the energy of the incident radiation on the scintillator. The number of pulses per unit time gives information about the intensity of the radiation. In some applications individual pulses are not counted, but rather only the current at the anode is used as a measure of radiation intensity. The scintillator must be shielded from all ambient light so that photons do not swamp the ionization events caused by incident radiation. To achieve this a thin foil, such as aluminized mylar, is often used. The article on the photomultiplier tube carries a description of the tubes operation. The scintillator consists of a transparent crystal, usually a phosphor, cesium iodide in crystalline form is used as the scintillator for the detection of protons and alpha particles
35.
International System of Units
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The International System of Units is the modern form of the metric system, and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units, the system also establishes a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. The system was published in 1960 as the result of an initiative began in 1948. It is based on the system of units rather than any variant of the centimetre-gram-second system. The motivation for the development of the SI was the diversity of units that had sprung up within the CGS systems, the International System of Units has been adopted by most developed countries, however, the adoption has not been universal in all English-speaking countries. The metric system was first implemented during the French Revolution with just the metre and kilogram as standards of length, in the 1830s Carl Friedrich Gauss laid the foundations for a coherent system based on length, mass, and time. In the 1860s a group working under the auspices of the British Association for the Advancement of Science formulated the requirement for a coherent system of units with base units and derived units. Meanwhile, in 1875, the Treaty of the Metre passed responsibility for verification of the kilogram, in 1921, the Treaty was extended to include all physical quantities including electrical units originally defined in 1893. The units associated with these quantities were the metre, kilogram, second, ampere, kelvin, in 1971, a seventh base quantity, amount of substance represented by the mole, was added to the definition of SI. On 11 July 1792, the proposed the names metre, are, litre and grave for the units of length, area, capacity. The committee also proposed that multiples and submultiples of these units were to be denoted by decimal-based prefixes such as centi for a hundredth, on 10 December 1799, the law by which the metric system was to be definitively adopted in France was passed. Prior to this, the strength of the magnetic field had only been described in relative terms. The technique used by Gauss was to equate the torque induced on a magnet of known mass by the earth’s magnetic field with the torque induced on an equivalent system under gravity. The resultant calculations enabled him to assign dimensions based on mass, length, a French-inspired initiative for international cooperation in metrology led to the signing in 1875 of the Metre Convention. Initially the convention only covered standards for the metre and the kilogram, one of each was selected at random to become the International prototype metre and International prototype kilogram that replaced the mètre des Archives and kilogramme des Archives respectively. Each member state was entitled to one of each of the prototypes to serve as the national prototype for that country. Initially its prime purpose was a periodic recalibration of national prototype metres. The official language of the Metre Convention is French and the version of all official documents published by or on behalf of the CGPM is the French-language version
36.
Becquerel
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The becquerel is the SI derived unit of radioactivity. One becquerel is defined as the activity of a quantity of material in which one nucleus decays per second. The becquerel is therefore equivalent to a second, s−1. The becquerel is named after Henri Becquerel, who shared a Nobel Prize in Physics with Pierre, as with every International System of Units unit named for a person, the first letter of its symbol is uppercase. 1 Bq =1 s−1 A special name was introduced for the second to represent radioactivity to avoid potentially dangerous mistakes with prefixes. For example,1 µs−1 could be taken to mean 106 disintegrations per second, other names considered were hertz, a special name already in use for the reciprocal second, and fourier. The hertz is now used for periodic phenomena. Whereas 1 Hz is 1 cycle per second,1 Bq is 1 aperiodic radioactivity event per second, the gray and the becquerel were introduced in 1975. Between 1953 and 1975, absorbed dose was often measured in rads, decay activity was measured in curies before 1946 and often in rutherfords between 1946 and 1975. Like any SI unit, Bq can be prefixed, commonly used multiples are kBq, MBq, GBq, TBq, for practical applications,1 Bq is a small unit, therefore, the prefixes are common. For example, the roughly 0.0169 g of potassium-40 present in a human body produces approximately 4,400 disintegrations per second or 4.4 kBq of activity. The global inventory of carbon-14 is estimated to be 8. 5×1018 Bq, the nuclear explosion in Hiroshima is estimated to have produced 8×1024 Bq. The becquerel succeeded the curie, an older, non-SI unit of radioactivity based on the activity of 1 gram of radium-226, the curie is defined as 3. 7·1010 s−1, or 37 GBq. The following table shows radiation quantities in SI and non-SI units
37.
Roentgen (unit)
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The roentgen or röntgen is a legacy unit of measurement for the exposure of X-rays and gamma rays up to several megaelectronvolts. It is a measure of the ionization produced in air by X-rays or gamma radiation and it is named after the German physicist Wilhelm Röntgen, who discovered X-rays. Originating in 1908, this unit has been redefined and renamed over the years. It was last defined by the US National Institute of Standards and Technology in 1998 as 2. 58×10−4 C/kg, one roentgen of air kerma deposits 0.00877 grays of absorbed dose in dry air, or 0.0096 Gy in soft tissue. One roentgen of X-rays may deposit anywhere from 0.01 to 0.04 Gy in bone depending on the beam energy and this tissue-dependent conversion from kerma to absorbed dose is called the F-factor in radiotherapy contexts. The conversion depends on the energy of a reference medium. Even where the medium is fully defined, the ionizing energy of the calibration. Using 1 esu ≈3. 33564×10−10 C and the air density of ~1.293 kg/m³ at 0 °C and 101 kPa, this converts to 2.58 × 10−4 C/kg, which is the modern value given by NIST. 1 esu/cm3 ×3.33564 × 10−10 C/esu ×1,000,000 cm3/m3 ÷1.293 kg/m3 =2.58 × 10−4 C/kg This definition was used under different names for the next 20 years. In the meantime, the French Roentgen was given a different definition which amounted to 0.444 German R. The stated 1 cc of air would have a mass of 1.293 mg at the given, so in 1937 the ICR rewrote this definition in terms of this mass of air instead of volume. The 1937 definition was extended to gamma rays, but later capped at 3 MeV in 1950. The USSR all-union committee of standards had meanwhile adopted a different definition of the roentgen in 1934. The distinction of physical dose from dose caused confusion, some of which may have led Cantrill and they named this derivative quantity the roentgen equivalent physical to distinguish it from the ICR roentgen. Towards the middle of the 20th century, roentgens were used for the purpose of radiation protection and this replaced earlier practices that relied on time, film exposure, or fluorescence. The National Council on Radiation Protection established the first formal dose limit in 1931 as 0.1 roentgen per day. The International X-ray and Radium Protection Committee, now known as the International Commission on Radiological Protection soon followed with a limit of 0.2 roentgen per day in 1934. The 3 MeV cap was no part of the definition
38.
Absorbed dose
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Absorbed dose is a physical dose quantity D representing the mean energy imparted to matter per unit mass by ionizing radiation. In the SI system of units, the unit of measure is joules per kilogram, the non-SI CGS unit rad is sometimes also used, predominantly in the USA. Absorbed dose is used in the calculation of dose uptake in living tissue in both radiation protection and radiology and it is also used to directly compare the effect of radiation on inanimate matter. The quantity absorbed dose is of importance in radiological protection for calculating radiation dose. However, absorbed dose is a quantity and used unmodified is not an adequate indicator of the likely health effects in humans. It has been found that for stochastic radiation risk consideration must be given to the type of radiation and the sensitivity of the irradiated tissues, which requires the use of modifying factors. Conventionally therefore, unmodified absorbed dose is not used for comparing stochastic risks, to represent stochastic risk the equivalent dose H T and effective dose E are used, and appropriate dose factors and coefficients are used to calculate these from the absorbed dose. Equivalent and effective dose quantities are expressed in units of the sievert or rem which implies that biological effects have been taken into account and these are usually in accordance with the recommendations of the International Committee on Radiation Protection and International Commission on Radiation Units and Measurements. The coherent system of protection quantities developed by them is shown in the accompanying diagram. Absorbed dose is used to rate the survivability of devices such as components in ionizing radiation environments. Absorbed dose is the physical quantity used to ensure irradiated food has received the correct dose to ensure effectiveness. Variable doses are used depending on the application and can be as high as 70 kGy, the absorbed dose is equal to the radiation exposure of the radiation beam multiplied by the ionization energy of the medium to be ionized. For example, the energy of dry air at 20 °C and 101.325 kPa of pressure is 33. 97±0.06 J/C. Therefore, an exposure of 2. 58×10−4 C/kg would deposit an absorbed dose of 8. 76×10−3 J/kg in dry air at those conditions, self-shielding means that the absorbed dose will be higher in the tissues facing the source than deeper in the body. The mass average can be important in evaluating the risks of radiotherapy treatments, since they are designed to very specific volumes in the body. For example, if 10% of a bone marrow mass is irradiated with 10 Gy of radiation locally. Bone marrow makes up 4% of the mass, so the whole-body absorbed dose would be 0.04 Gy. The first figure is indicative of the effects on the tumour, while the second
39.
Gray (unit)
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The gray is a derived unit of ionizing radiation dose in the International System of Units. It is defined as the absorption of one joule of energy per kilogram of matter. It is used as a measure of absorbed dose, specific energy and it is a physical quantity, and does not take into account any biological context. Unlike the pre-1971 non-SI roentgen unit of exposure, the gray when used for absorbed dose is defined independently of any target material. However, when measuring kerma the reference target material must be defined explicitly, usually as dry air at standard temperature and pressure. The corresponding cgs unit, the rad, remains common in the United States, though strongly discouraged in the guide for U. S. National Institute of Standards. The gray was named after British physicist Louis Harold Gray, a pioneer in the measurement of X-ray and radium radiation and it was adopted as part of the International System of Units in 1975. One gray is the absorption of one joule of energy, in the form of ionizing radiation, measuring and controlling the value of absorbed dose is vital to ensuring correct operation of these processes. Kerma is a measure of the energy of ionisation due to irradiation. Importantly, kerma dose is different from absorbed dose, depending on the energies involved. The measurement of absorbed dose is a problem, and so many different dosimeters are available for these measurements. These dosimeters cover measurements that can be done in 1-D, 2-D and 3-D, in radiation therapy, the amount of radiation applied varies depending on the type and stage of cancer being treated. For curative cases, the dose for a solid epithelial tumor ranges from 60 to 80 Gy. Preventive doses are typically around 45–60 Gy in 1. 8–2 Gy fractions, the absorbed dose also plays an important role in radiation protection, as it is the starting point for calculating the effect of low levels of radiation. In radiation protection dose assessment, the health risk is defined as the probability of cancer induction. This probability is related to the equivalent dose in sieverts, which has the dimensions as the gray. It is related to the gray by weighting factors described in the articles on equivalent dose, to avoid any risk of confusion between the absorbed dose and the equivalent dose, the absorbed dose is stated in grays and the equivalent dose is stated in sieverts. The accompanying diagrams show how absorbed dose is first obtained by computational techniques, radiation poisoning - The gray is conventionally used to express the severity of what are known as tissue effects from doses received in acute exposure to high levels of ionizing radiation
