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.
Beta
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Beta is the second letter of the Greek alphabet. In the system of Greek numerals it has a value of 2, in Ancient Greek, beta represented the voiced bilabial plosive /b/. In Modern Greek, it represents the voiced labiodental fricative /v/, letters that arose from beta include the Roman letter ⟨B⟩ and the Cyrillic letters ⟨Б⟩ and ⟨В⟩. Like the names of most other Greek letters, the name of beta was adopted from the name of the corresponding letter in Phoenician. In Greek, the name was βῆτα bêta, pronounced in Ancient Greek and it is spelled βήτα in modern monotonic orthography, and pronounced. In US English, the name is pronounced /ˈbeɪtə/, while in British English it is pronounced /ˈbiːtə/, the letter beta was derived from the Phoenician letter beth. The letter Β had the largest number of highly divergent local forms, besides the standard form, there were forms as varied as, and. In the system of Greek numerals, beta has a value of 2, such use is denoted by a number mark, Β′. In computing, the term beta is used as the last testing release in the release life cycle before the release version. Beta versions often have a number of the form 0. n. Typically it is the last version before a version of a software is released to all its actual customers. Beta is used in finance as a measure of investment portfolio risk, Beta in this context is calculated as the covariance of the portfolios returns with its benchmarks returns, divided by the variance of the benchmarks returns. A beta of 1.5 means that for every 1% change in the value of the benchmark, in the International Phonetic Alphabet, Greek minuscule beta denotes a voiced bilabial fricative. The name Beta was used as a name during the 2005 Atlantic hurricane season as Hurricane Beta, Beta is often used to denote a variable in mathematics and physics, where it often has specific meanings for certain applications. In physics a stream of unbound energetic electrons is referred to as beta radiation or beta rays. β is sometimes used as a placeholder for a number if α is already used. In spaceflight, beta angle describes the angle between the plane of a spacecraft or other body and the vector from the sun. The term beta refers to advice on how to complete a particular climbing route, boulder problem
7.
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
8.
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
9.
Potassium-40
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Potassium-40 is a radioactive isotope of potassium which has a very long half-life of 1. 251×109 years. It makes up 0. 012% of the amount of potassium found in nature. Potassium-40 is an example of an isotope that undergoes all three types of beta decay. About 89. 28% of the time, it decays to calcium-40 with emission of a particle with a maximum energy of 1.33 MeV. About 10. 72% of the time it decays to argon-40 by electron capture, with the emission of a 1.460 MeV gamma ray, the radioactive decay of this particular isotope explains the large abundance of argon in the earths atmosphere. Very rarely it will decay to 40Ar by emitting a positron, potassium-40 is especially important in potassium–argon dating. Argon is a gas that does not ordinarily combine with other elements, so, when a mineral forms – whether from molten rock, or from substances dissolved in water – it will be initially argon-free, even if there is some argon in the liquid. However, if the mineral contains any potassium, then decay of the 40K isotope present will create fresh argon-40 that will remain locked up in the mineral. Since the rate at which this occurs is known, it is possible to determine the elapsed time since the mineral formed by measuring the ratio of 40K. It follows that most of the terrestrial argon derives from potassium-40 that decayed into argon-40, the radioactive decay of 40K in the Earths mantle ranks third, after 232Th and 238U, as the source of radiogenic heat. The core also likely contains radiogenic sources, although how much is uncertain and it has been proposed that significant core radioactivity may be caused by high levels of U, Th, and K. 40K is the largest source of natural radioactivity in animals including humans
10.
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
11.
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
12.
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
13.
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
14.
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
15.
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
16.
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
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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
18.
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
19.
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
20.
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
21.
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
22.
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
23.
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
24.
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
25.
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
26.
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
27.
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
28.
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
29.
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
30.
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
31.
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
32.
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
33.
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
34.
Paper
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Paper is a thin material produced by pressing together moist fibres of cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets. It is a material with many uses, including writing, printing, packaging, cleaning. The modern pulp and paper industry is global, with China leading its production, the oldest known archaeological fragments of the immediate precursor to modern paper, date to the 2nd century BC in China. The pulp papermaking process is ascribed to Cai Lun, a 2nd-century AD Han court eunuch, with paper as an effective substitute for silk in many applications, China could export silk in greater quantity, contributing to a Golden Age. Because of papers introduction to the West through the city of Baghdad, in the 19th century, industrial manufacture greatly lowered its cost, enabling mass exchange of information and contributing to significant cultural shifts. In 1844, the Canadian inventor Charles Fenerty and the German F. G. Keller independently developed processes for pulping wood fibres, before the industrialisation of the paper production the most common fibre source was recycled fibres from used textiles, called rags. The rags were from hemp, linen and cotton, a process for removing printing inks from recycled paper was invented by German jurist Justus Claproth in 1774. Today this method is called deinking and it was not until the introduction of wood pulp in 1843 that paper production was not dependent on recycled materials from ragpickers. The word paper is etymologically derived from Latin papyrus, which comes from the Greek πάπυρος, although the word paper is etymologically derived from papyrus, the two are produced very differently and the development of the first is distinct from the development of the second. Papyrus is a lamination of natural plant fibres, while paper is manufactured from fibres whose properties have changed by maceration. To make pulp from wood, a chemical pulping process separates lignin from cellulose fibres and this is accomplished by dissolving lignin in a cooking liquor, so that it may be washed from the cellulose, this preserves the length of the cellulose fibres. Paper made from chemical pulps are also known as wood-free papers–not to be confused with paper, this is because they do not contain lignin. The pulp can also be bleached to produce paper, but this consumes 5% of the fibres, chemical pulping processes are not used to make paper made from cotton. There are three main chemical pulping processes, the process dates back to the 1840s and it was the dominant method extent before the second world war. Most pulping operations using the process are net contributors to the electricity grid or use the electricity to run an adjacent paper mill. Another advantage is that this process recovers and reuses all inorganic chemical reagents, soda pulping is another specialty process used to pulp straws, bagasse and hardwoods with high silicate content. There are two major mechanical pulps, thermomechanical pulp and groundwood pulp, in the TMP process, wood is chipped and then fed into steam heated refiners, where the chips are squeezed and converted to fibres between two steel discs. In the groundwood process, debarked logs are fed into grinders where they are pressed against rotating stones to be made into fibres
35.
Tritium radioluminescence
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Tritium lumination is the use of gaseous tritium, a radioactive isotope of hydrogen, to create visible light. Tritium emits electrons through beta decay, and, when they interact with a material, fluorescent light is created. As tritium illumination requires no energy, it found wide use in applications such as emergency exit signs. More recently, many applications using radioactive materials have been replaced with photoluminescent materials, tritium lighting is made using glass tubes with a phosphor layer in them and tritium gas inside the tube. Such a tube is known as a gaseous tritium light source, the tritium in a gaseous tritium light source undergoes beta decay, releasing electrons that cause the phosphor layer to fluoresce. During manufacture, a length of glass tube that has had the inside surface coated with a phosphor-containing material is filled with the radioactive tritium. The tube is then fused with a carbon dioxide laser at the desired length, borosilicate is preferred for its strength and resistance to breakage. In the tube, the tritium gives off a stream of electrons due to beta decay. These particles excite the phosphor, causing it to emit a low, tritium is not the only material that can be used for self-powered lighting. Other beta particle-emitting radioisotopes can also serve, radium was used to make self-luminous paint from the early years of the 20th Century until approximately 1970, but it has been replaced by tritium, which is less hazardous. Various preparations of the compound can be used to produce different colors of light. Some of the colors that have manufactured in addition to the common phosphors are green, red, blue, yellow, purple, orange. The types of GTLS used in watches give off a small amount of light, not enough to be seen in daylight, the average such GTLS has a useful life of 10–20 years. As the tritium component of the lighting is more expensive than the rest of the watch itself. Being an unstable isotope with a half-life of 12.32 years, the more tritium that is initially placed in the tube, the brighter it is to begin with, and the longer its useful life. Tritium exit signs usually come in three brightness levels guaranteed for 10-, 15-, or 20-year useful life expectancies, the difference between the signs is how much tritium the manufacturer installs. The light produced by GTLSs varies in colour and size, green is usually the brightest color and white the least bright. Sizes can be found ranging from tiny tubes small enough to fit on the hand of a watch to ones the size of a pencil and these light sources are most often seen as permanent illumination for the hands of wristwatches intended for diving, nighttime, or combat use
36.
Tritium
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Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons, naturally occurring tritium is extremely rare on Earth, where trace amounts are formed by the interaction of the atmosphere with cosmic rays. It can be produced by irradiating lithium metal or lithium bearing ceramic pebbles in a nuclear reactor, the name of this isotope is derived from the Greek word τρίτος meaning third. While tritium has several different experimentally determined values of its half-life and it decays into helium-3 by beta decay as in this nuclear equation, and it releases 18.6 keV of energy in the process. The electrons kinetic energy varies, with an average of 5.7 keV, beta particles from tritium can penetrate only about 6.0 mm of air, and they are incapable of passing through the dead outermost layer of human skin. The unusually low energy released in the beta decay makes the decay appropriate for absolute neutrino mass measurements in the laboratory. The low energy of tritiums radiation makes it difficult to detect tritium-labeled compounds except by using liquid scintillation counting, Tritium is produced in nuclear reactors by neutron activation of lithium-6. This is possible with neutrons of any energy, and is an exothermic reaction yielding 4.8 MeV, in comparison, the fusion of deuterium with tritium releases about 17.6 MeV of energy. High-energy neutrons can produce tritium from lithium-7 in an endothermic reaction. This was discovered when the 1954 Castle Bravo nuclear test produced a high yield. High-energy neutrons irradiating boron-10 will also produce tritium, A more common result of boron-10 neutron capture is 7Li. The reactions requiring high neutron energies are not attractive production methods for peaceful applications, Tritium is also produced in heavy water-moderated reactors whenever a deuterium nucleus captures a neutron. This reaction has a quite small absorption cross section, making water a good neutron moderator. Even so, cleaning tritium from the moderator may be desirable after several years to reduce the risk of its escaping to the environment. Ontario Power Generations Tritium Removal Facility processes up to 2,500 tonnes of water a year. Deuteriums absorption cross section for thermal neutrons is about 0.52 millibarns, whereas that of oxygen-16 is about 0.19 millibarns and that of oxygen-17 is about 240 millibarns. Tritium is a product of the nuclear fission of uranium-235, plutonium-239. The release or recovery of tritium needs to be considered in the operation of reactors, especially in the reprocessing of nuclear fuels
37.
Phosphor
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A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness, and fluorescent materials, where the emission decay takes place over tens of nanoseconds, Phosphors are often transition metal compounds or rare earth compounds of various types. The most common uses of phosphors are in CRT displays and fluorescent lights, CRT phosphors were standardized beginning around World War II and designated by the letter P followed by a number. Phosphorus, the element named for its light-emitting behavior, emits light due to chemiluminescence. A material can emit light either through incandescence, where all atoms radiate, or by luminescence, in inorganic phosphors, these inhomogeneities in the crystal structure are created usually by addition of a trace amount of dopants, impurities called activators. The wavelength emitted by the center is dependent on the atom itself. The scintillation process in inorganic materials is due to the band structure found in the crystals. An incoming particle can excite an electron from the band to either the conduction band or the exciton band. This leaves a hole behind, in the valence band. Impurities create electronic levels in the forbidden gap, the excitons are loosely bound electron-hole pairs that wander through the crystal lattice until they are captured as a whole by impurity centers. The latter then rapidly de-excite by emitting scintillation light, in case of inorganic scintillators, the activator impurities are typically chosen so that the emitted light is in the visible range or near-UV where photomultipliers are effective. The holes associated with electrons in the band are independent from the latter. Those holes and electrons are captured successively by impurity centers exciting certain metastable states not accessible to the excitons, the delayed de-excitation of those metastable impurity states, slowed down by reliance on the low-probability forbidden mechanism, again results in light emission. Many phosphors tend to lose efficiency gradually by several mechanisms, the degradation of electroluminescent devices depends on frequency of driving current, the luminance level, and temperature, moisture impairs phosphor lifetime very noticeably as well. Harder, high-melting, water-insoluble materials display lower tendency to lose luminescence under operation, examples, BaMgAl10O17, Eu2+, a plasma display phosphor, undergoes oxidation of the dopant during baking. Thin coating of aluminium phosphate or lanthanum phosphate is effective in creation a barrier layer blocking access of oxygen to the BAM phosphor, for the cost of reduction of phosphor efficiency. Addition of hydrogen, acting as an agent, to argon in the plasma displays significantly extends the lifetime of BAM, Eu2+ phosphor. Y2O3, Eu phosphors under electron bombardment in presence of oxygen form a non-phosphorescent layer on the surface, the traps also damage the crystal lattice
38.
Photon
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A photon is an elementary particle, the quantum of the electromagnetic field including electromagnetic radiation such as light, and the force carrier for the electromagnetic force. The photon has zero rest mass and is moving at the speed of light. Like all elementary particles, photons are currently best explained by quantum mechanics and exhibit wave–particle duality, exhibiting properties of both waves and particles. For example, a photon may be refracted by a lens and exhibit wave interference with itself. The quanta in a light wave cannot be spatially localized, some defined physical parameters of a photon are listed. The modern concept of the photon was developed gradually by Albert Einstein in the early 20th century to explain experimental observations that did not fit the classical model of light. The benefit of the model was that it accounted for the frequency dependence of lights energy. The photon model accounted for observations, including the properties of black-body radiation. In that model, light was described by Maxwells equations, in 1926 the optical physicist Frithiof Wolfers and the chemist Gilbert N. Lewis coined the name photon for these particles. After Arthur H. Compton won the Nobel Prize in 1927 for his studies, most scientists accepted that light quanta have an independent existence. In the Standard Model of particle physics, photons and other particles are described as a necessary consequence of physical laws having a certain symmetry at every point in spacetime. The intrinsic properties of particles, such as charge, mass and it has been applied to photochemistry, high-resolution microscopy, and measurements of molecular distances. Recently, photons have been studied as elements of quantum computers, in 1900, the German physicist Max Planck was studying black-body radiation and suggested that the energy carried by electromagnetic waves could only be released in packets of energy. In his 1901 article in Annalen der Physik he called these packets energy elements, the word quanta was used before 1900 to mean particles or amounts of different quantities, including electricity. In 1905, Albert Einstein suggested that waves could only exist as discrete wave-packets. He called such a wave-packet the light quantum, the name photon derives from the Greek word for light, φῶς. Arthur Compton used photon in 1928, referring to Gilbert N. Lewis, the name was suggested initially as a unit related to the illumination of the eye and the resulting sensation of light and was used later in a physiological context. Although Wolferss and Lewiss theories were contradicted by many experiments and never accepted, in physics, a photon is usually denoted by the symbol γ
39.
Cathode ray tube
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The cathode ray tube is a vacuum tube that contains one or more electron guns and a phosphorescent screen, and is used to display images. It modulates, accelerates, and deflects electron beam onto the screen to create the images, the images may represent electrical waveforms, pictures, radar targets, or others. CRTs have also used as memory devices, in which case the visible light emitted from the fluorescent material is not intended to have significant meaning to a visual observer. In television sets and computer monitors, the front area of the tube is scanned repetitively and systematically in a fixed pattern called a raster. An image is produced by controlling the intensity of each of the three beams, one for each additive primary color with a video signal as a reference. A CRT is constructed from an envelope which is large, deep, fairly heavy. The interior of a CRT is evacuated to approximately 0.01 Pa to 133 nPa. evacuation being necessary to facilitate the flight of electrons from the gun to the tubes face. That it is evacuated makes handling an intact CRT potentially dangerous due to the risk of breaking the tube and causing a violent implosion that can hurl shards of glass at great velocity. As a matter of safety, the face is made of thick lead glass so as to be highly shatter-resistant and to block most X-ray emissions. Flat panel displays can also be made in large sizes, whereas 38 to 40 was about the largest size of a CRT television, flat panels are available in 60. Cathode rays were discovered by Johann Hittorf in 1869 in primitive Crookes tubes and he observed that some unknown rays were emitted from the cathode which could cast shadows on the glowing wall of the tube, indicating the rays were traveling in straight lines. In 1890, Arthur Schuster demonstrated cathode rays could be deflected by electric fields, the earliest version of the CRT was known as the Braun tube, invented by the German physicist Ferdinand Braun in 1897. It was a diode, a modification of the Crookes tube with a phosphor-coated screen. In 1907, Russian scientist Boris Rosing used a CRT in the end of an experimental video signal to form a picture. He managed to display simple geometric shapes onto the screen, which marked the first time that CRT technology was used for what is now known as television. The first cathode ray tube to use a hot cathode was developed by John B. Johnson and Harry Weiner Weinhart of Western Electric and it was named by inventor Vladimir K. Zworykin in 1929. RCA was granted a trademark for the term in 1932, it released the term to the public domain in 1950. The first commercially made electronic television sets with cathode ray tubes were manufactured by Telefunken in Germany in 1934, in oscilloscope CRTs, electrostatic deflection is used, rather than the magnetic deflection commonly used with television and other large CRTs
40.
Half-life
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Half-life is the time required for a quantity to reduce to half its initial value. The term is used in nuclear physics to describe how quickly unstable atoms undergo. The term is used more generally to characterize any type of exponential or non-exponential decay. For example, the medical sciences refer to the biological half-life of drugs, the converse of half-life is doubling time. The original term, half-life period, dating to Ernest Rutherfords discovery of the principle in 1907, was shortened to half-life in the early 1950s. Rutherford applied the principle of a radioactive elements half-life to studies of age determination of rocks by measuring the period of radium to lead-206. Half-life is constant over the lifetime of an exponentially decaying quantity, the accompanying table shows the reduction of a quantity as a function of the number of half-lives elapsed. A half-life usually describes the decay of discrete entities, such as radioactive atoms, in that case, it does not work to use the definition that states half-life is the time required for exactly half of the entities to decay. For example, if there are 3 radioactive atoms with a half-life of one second, instead, the half-life is defined in terms of probability, Half-life is the time required for exactly half of the entities to decay on average. In other words, the probability of a radioactive atom decaying within its half-life is 50%, for example, the image on the right is a simulation of many identical atoms undergoing radioactive decay. Note that after one half-life there are not exactly one-half of the remaining, only approximately. Nevertheless, when there are many identical atoms decaying, the law of large numbers suggests that it is a good approximation to say that half of the atoms remain after one half-life. There are various simple exercises that demonstrate probabilistic decay, for example involving flipping coins or running a computer program. The three parameters t1⁄2, τ, and λ are all related in the following way. Amount approaches zero as t approaches infinity as expected, some quantities decay by two exponential-decay processes simultaneously. There is a half-life describing any exponential-decay process, for example, The current flowing through an RC circuit or RL circuit decays with a half-life of RCln or lnL/R, respectively. For this example, the half time might be used instead of half life. In a first-order chemical reaction, the half-life of the reactant is ln/λ, in radioactive decay, the half-life is the length of time after which there is a 50% chance that an atom will have undergone nuclear decay
41.
Isotope
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Isotopes are variants of a particular chemical element which differ in neutron number. All isotopes of an element have the same number of protons in each atom. The number of protons within the nucleus is called atomic number and is equal to the number of electrons in the neutral atom. Each atomic number identifies a specific element, but not the isotope, the number of nucleons in the nucleus is the atoms mass number, and each isotope of a given element has a different mass number. For example, carbon-12, carbon-13 and carbon-14 are three isotopes of the element carbon with mass numbers 12,13 and 14 respectively. The atomic number of carbon is 6, which means that carbon atom has 6 protons. Nuclide refers to a rather than to an atom. Identical nuclei belong to one nuclide, for each nucleus of the carbon-13 nuclide is composed of 6 protons and 7 neutrons. The nuclide concept emphasizes nuclear properties over chemical properties, whereas the isotope concept emphasizes chemical over nuclear, the neutron number has large effects on nuclear properties, but its effect on chemical properties is negligible for most elements. Because isotope is the term, it is better known than nuclide. An isotope and/or nuclide is specified by the name of the particular element followed by a hyphen, when a chemical symbol is used, e. g. C for carbon, standard notation is to indicate the number with a superscript at the upper left of the chemical symbol. Because the atomic number is given by the element symbol, it is common to only the mass number in the superscript. The letter m is sometimes appended after the number to indicate a nuclear isomer. For example, 14C is a form of carbon, whereas 12C. There are about 339 naturally occurring nuclides on Earth, of which 286 are primordial nuclides, primordial nuclides include 32 nuclides with very long half-lives and 254 that are formally considered as stable nuclides, because they have not been observed to decay. In most cases, for reasons, if an element has stable isotopes. Theory predicts that many apparently stable isotopes/nuclides are radioactive, with extremely long half-lives, of the 254 nuclides never observed to decay, only 90 of these are theoretically stable to all known forms of decay
