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In nuclear physics, beta decay (''β''-decay) is a type of radioactive decay in which a proton is transformed into a neutron, or vice versa, inside an atomic nucleus. This process allows the atom to move closer to the optimal ratio of protons and neutrons. As a result of this transformation, the nucleus emits a detectable beta particle, which is an electron or positron. Beta decay is mediated by the weak force. There are two types of beta decay, known as ''beta minus'' and ''beta plus''. In beta minus (β−) decay a neutron is lost and a proton appears and the process produces an electron and electron antineutrino, while in beta plus (β+) decay a proton is lost and a neutron appears and the process produces a positron and electron neutrino; β+ decay is thus also known as positron emission. An example of electron emission (β− decay) is the decay of carbon-14 into nitrogen-14: : → + + In this form of decay, the original element becomes a new chemical element in a process known as nuclear transmutation. This new element has an unchanged mass number , but an atomic number that is increased by one. As in all nuclear decays, the decaying element (in this case ) is known as the ''parent nuclide'' while the resulting element (in this case ) is known as the ''daughter nuclide''. The emitted electron or positron is known as a beta particle. An example of positron emission (β+ decay) is the decay of magnesium-23 into sodium-23: : → + + In contrast to β− decay, β+ decay is accompanied by the emission of an electron neutrino and a positron. β+ decay also results in nuclear transmutation, with the resulting element having an atomic number that is decreased by one. 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 inner atomic electron is captured by a proton in the nucleus, transforming it into a neutron, and an electron neutrino is released. An example of electron capture is the decay of krypton-81 into bromine-81: : + → + Electron capture is a competing (simultaneous) decay process for all nuclei that can undergo β+ decay. The converse, however, is not true: electron capture is the ''only'' type of decay that is allowed in proton-rich nuclides that do not have sufficient energy to emit a positron and neutrino. == β− decay == In decay, the weak interaction converts an atomic nucleus into a nucleus with atomic number increased by one, while emitting an electron () and an electron antineutrino (). The generic equation is: : → + + 〔 where and are the mass number and atomic number of the decaying nucleus, and X and X' are the initial and final elements, respectively. Another example is when the free neutron () decays by decay into a proton (): : → + + . At the fundamental level (as depicted in the Feynman diagram on the left), this is caused by the conversion of the negatively charged (− e) down quark to the positively charged (+ e) up quark by emission of a ; the boson subsequently decays into an electron and an electron antineutrino: : → + + . The beta spectrum is a continuous spectrum: the total decay energy is divided between the electron and the antineutrino. In the figure to the right, this is shown, by way of example, for an electron of 0.4 MeV energy. In this example, the antineutrino then gets the remainder: 0.76 MeV, since the total decay energy is assumed to be 1.16 MeV. decay generally occurs in neutron-rich nuclei. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Beta decay」の詳細全文を読む スポンサード リンク
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