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In nuclear engineering, a prompt neutron is a neutron immediately emitted (neutron emission) by a nuclear fission event, as opposed to a delayed neutron decay which can occur within the same context, emitted after beta decay of one of the fission products anytime from a few milliseconds to a few minutes later.
In nuclear engineering, prompt criticality describes a nuclear fission event in which criticality (the threshold for an exponentially growing nuclear fission chain reaction) is achieved with prompt neutrons alone and does not rely on delayed neutrons. As a result, prompt supercriticality causes a much more rapid growth in the rate of energy ...
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".
The prompt neutron lifetime, , is the average time between the emission of a neutron and either its absorption or escape from the system. [17] The neutrons that occur directly from fission are called prompt neutrons, and the ones that are a result of radioactive decay of fission fragments are called delayed neutrons.
The neutrons are usually classified in 6 delayed neutron groups. [4] The average neutron lifetime considering delayed neutrons is approximately 0.1 sec, which makes the chain reaction relatively easy to control over time. The remaining 993 prompt neutrons are released very quickly, approximately 1 μs after the fission event.
However, without addition of a neutron poison or active neutron-absorber, decreases in fission rate are limited in speed, because even if the reactor is taken deeply subcritical to stop prompt fission neutron production, delayed neutrons are produced after ordinary beta decay of fission products already in place, and this decay-production of ...
[7]: 19 Finally, internal conversion and x-ray emission complete the prompt emissions. [2]: 53–4 Daughter products created by prompt decays are often unstable against beta-decay, and further photon and neutron emissions are also expected. Such emissions are termed 'delayed emissions' and take place with half-lives ranging from picoseconds to ...
Physicists recognize two points in the gradual increase of neutron flux which are significant: critical, where the chain reaction becomes self-sustaining thanks to the contributions of both kinds of neutron generation, [15] and prompt critical, where the immediate "prompt" neutrons alone will sustain the reaction without need for the decay ...