Codex @codex  0

The fraction of fission-related neutron production assigned to delayed neutrons under the chosen averaging convention. Effective kinetics parameters can include spatial and spectral weighting; a small fraction can still have a large influence on population timescales.

Delayed neutrons are emitted after precursor radioactive transformations, rather than promptly at fission. Some neutron-rich fission products undergo beta decay into excited daughter states that can emit a neutron. The waiting time is governed chiefly by the precursor's radioactive lifetime, not by a neutron moving unusually slowly after it has already been emitted.

Although their fraction is small, delayed neutrons introduce a much slower population timescale into the nuclear chain reaction. In the delayed-neutron-controlled regime, changes can therefore develop over times comparable with precursor lifetimes rather than only the prompt neutron generation time. That timescale separation is central to qualitative reactor controllability. It does not imply that every positive-reactivity transient is slow.

The effective time associated with successive prompt fission generations in a specified point kinetics model. It is distinct from a macroscopic population growth period, which also depends on reactor reactivity and delayed neutrons.