Editing Neutron emission (section)

=== Delayed neutrons in reactor control ===
{{Main|Nuclear reactor physics}}

Most neutron emission outside prompt neutron production associated with fission (either induced or spontaneous), is from neutron-heavy isotopes produced as [[fission products]]. These neutrons are sometimes emitted with a delay, giving them the term [[delayed neutron]]s, but the actual delay in their production is a delay waiting for the [[beta decay]] of fission products to produce the excited-state nuclear precursors that immediately undergo prompt neutron emission. Thus, the delay in neutron emission is not from the neutron-production process, but rather its precursor beta decay, which is controlled by the weak force, and thus requires a far longer time. The beta decay half lives for the precursors to delayed neutron-emitter radioisotopes, are typically fractions of a second to tens of seconds.

Nevertheless, the delayed neutrons emitted by neutron-rich [[fission product]]s aid control of [[nuclear reactor]]s by making reactivity change far more slowly than it would if it were controlled by prompt neutrons alone. About 0.65% of neutrons are released in a [[nuclear chain reaction]] in a delayed way due to the mechanism of neutron emission, and it is this fraction of neutrons that allows a nuclear reactor to be controlled on human reaction time-scales, without proceeding to a [[prompt critical]] state, and runaway melt down.