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Prompt neutron
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<p>In <a href="/facts/Nuclear_engineering/wu26D25C">nuclear engineering</a>, a prompt neutron is a <a href="/facts/Neutron/oRCM1geb">neutron</a> immediately emitted (<a href="/facts/Neutron_emission/lXLQRDCQ">neutron emission</a>) by a <a href="/facts/Nuclear_fission/LfkIylvm">nuclear fission</a> event, as opposed to a <a href="/facts/Delayed_neutron/TrlfD8V9">delayed neutron decay</a> which can occur within the same context, emitted after <a href="/facts/Beta_decay/vxTBlckp">beta decay</a> of one of the <a href="/facts/Fission_product/xJd9MXXH">fission products</a> anytime from a few milliseconds to a few minutes later. </p><p>Prompt neutrons emerge from the fission of an unstable <a href="/facts/Fissionable/XJR9qoxW">fissionable</a> or <a href="/facts/Fissile/XJR9qoxW">fissile</a> heavy nucleus almost instantaneously. There are different definitions for how long it takes for a prompt neutron to emerge. For example, the <a href="/facts/United_States_Department_of_Energy/PRp6Cz8g">United States Department of Energy</a> defines a prompt neutron as a neutron born from fission within 10−13 seconds after the fission event. The U.S. <a href="/facts/Nuclear_Regulatory_Commission/tbC56grX">Nuclear Regulatory Commission</a> defines a prompt neutron as a neutron emerging from fission within 10−14 seconds. This emission is controlled by the <a href="/facts/Nuclear_force/A2MyJoWE">nuclear force</a> and is extremely fast. By contrast, so-called delayed neutrons are delayed by the time delay associated with beta decay (mediated by the <a href="/facts/Weak_interaction/f6upmKMs">weak force</a>) to the precursor excited nuclide, after which neutron emission happens on a prompt time scale (i.e., almost immediately). </p>