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Delayed neutron
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<p>In <a href="/facts/Nuclear_engineering/wu26D25C">nuclear engineering</a>, a delayed neutron is a <a href="/facts/Neutron/oRCM1geb">neutron</a> emitted after a <a href="/facts/Nuclear_fission/LfkIylvm">nuclear fission</a> event, by one of the <a href="/facts/Fission_product/xJd9MXXH">fission products</a> (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". </p><p>In a <a href="/facts/Nuclear_reactor/8xqjIbge">nuclear reactor</a> large nuclides fission into two neutron-rich fission products (i.e. unstable <a href="/facts/Nuclides/97Ykvzr3">nuclides</a>) and free neutrons (prompt neutrons). Many of these fission products then undergo <a href="/facts/Radioactive_decay/Ya6FVjt4">radioactive decay</a> (usually beta decay) and the resulting nuclides are unstable with respect to <a href="/facts/Beta_decay/vxTBlckp">beta decay</a>. A small fraction of them are excited enough to be able to beta-decay by emitting a delayed neutron in addition to the beta. The moment of <a href="/facts/Beta_decay/vxTBlckp">beta decay</a> of the precursor nuclides – which are the precursors of the delayed neutrons – happens orders of magnitude later compared to the emission of the <a href="/facts/Prompt_neutron/0wPEUS6h">prompt neutrons</a>. Hence the neutron that originates from the <a href="/facts/Neutron_emission/lXLQRDCQ">precursor's decay</a> is termed a delayed neutron. The "delay" in the neutron emission is due to the delay in beta decay (which is slower since controlled by the <a href="/facts/Weak_force/f6upmKMs">weak force</a>), since neutron emission, like gamma emission, is controlled by the <a href="/facts/Strong_nuclear_force/JjRsKcLg">strong nuclear force</a> and thus either happens at fission, or nearly simultaneously with the beta decay, immediately after it. The various half lives of these decays that finally result in neutron emission, are thus the beta decay half lives of the precursor radionuclides. </p><p>Delayed neutrons play an important role in <a href="/facts/Nuclear_reactor_control/lgJG9pJB">nuclear reactor control</a> and safety analysis. </p>