Delayed neutrons are a small fraction of neutrons that are emitted from certain radioactive isotopes as a result of beta decay, rather than being emitted immediately during the fission process of a nuclear reactor. When a heavy nucleus undergoes fission, it typically releases a number of prompt neutrons right away (usually within 10^-14 seconds).
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A neutron emitted following a radioactive transformation of a precursor, rather than promptly in the original nuclear fission. The waiting time reflects the precursor lifetime. It is distinct from the later detection of a prompt neutron after scattering or moderation.