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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 ...
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 ...
Given a total interaction cross section σ (typically measured in barns), the mean free path of a prompt neutron is = where n is the nuclear number density. Most interactions are scattering events, so that a given neutron obeys a random walk until it either escapes from the medium or causes a fission reaction.
The mean generation time, λ, is the average time from a neutron emission to a capture that results in fission. [16] The mean generation time is different from the prompt neutron lifetime because the mean generation time only includes neutron absorptions that lead to fission reactions (not other absorption reactions).
Prompt-gamma neutron activation analysis (PGAA) is a very widely applicable technique for determining the presence and amount of many elements simultaneously in samples ranging in size from micrograms to many grams. It is a non-destructive method, and the chemical form and shape of the sample are relatively unimportant.
The mere fact that an assembly is supercritical does not guarantee that it contains any free neutrons at all. At least one neutron is required to "strike" a chain reaction, and if the spontaneous fission rate is sufficiently low it may take a long time (in 235 U reactors, as long as many minutes) before a chance neutron encounter starts a chain reaction even if the reactor is supercritical.
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.
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".