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In nuclear engineering, a delayed neutron is a neutron emitted after a nuclear fission event, by one of the fission products (or actually, a fission product daughter after beta decay), any time from a few milliseconds to a few minutes after the fission event. Neutrons born within 10−14 seconds of the fission are termed "prompt neutrons". In a nuclear reactor large nuclides fission into two neutron-rich fission products (i.e. unstable nuclides). Many of these fission products then undergo radioactive decay (usually beta decay) and the resulting nuclides are left in an excited state. These usually immediately undergo gamma decay but a small fraction of them are excited enough to be able to decay by emitting a neutron in addition. The moment of beta decay of the precursor nuclides - which are the precursors of the delayed neutrons - happens orders of magnitude later compared to the emission of the prompt neutrons. Hence the neutron that originates from the precursor's decay is termed a delayed neutron. However, the "delay" in the neutron emission is due to the delay in beta decay, since neutron emission, like gamma emission, happens almost immediately after the beta decay. The various half lives of these decays that finally result in neutron emission, are thus the beta decay half lives of the precursor radionuclides. Delayed neutrons play an important role in nuclear reactor control and safety analysis. ==Principle== Delayed neutrons are associated with the beta decay of the fission products. After prompt fission neutron emission the residual fragments are still neutron rich and undergo a beta decay chain. The more neutron rich the fragment, the more energetic and faster the beta decay. In some cases the available energy in the beta decay is high enough to leave the residual nucleus in such a highly excited state that neutron emission instead of gamma emission occurs. Using U-235 as an example, this nucleus absorbs thermal neutrons, and the immediate mass products of a fission event are two large fission fragments, which are remnants of the formed U-236 nucleus. These fragments emit, on average, two or three free neutrons (in average 2.47), called "prompt" neutrons. A subsequent fission fragment occasionally undergoes a stage of radioactive decay (which is a beta minus decay) that yields a new nucleus (the emitter nucleus) in an excited state that emits an additional neutron, called a "delayed" neutron, to get to ground state. These neutron-emitting fission fragments are called delayed neutron precursor atoms. Delayed Neutron Data for Thermal Fission in U-235〔Lamarsh, Introduction to Nuclear Engineering〕 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「Delayed neutron」の詳細全文を読む スポンサード リンク
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