Neutron emission usually happens from nuclei that are in an
excited state, such as the excited
17O* produced from the beta decay of
17N. The neutron emission process itself is controlled by the
nuclear force and therefore is extremely fast, sometimes referred to as "nearly instantaneous". This process allows unstable atoms to become more stable. The ejection of the neutron may be as a product of the movement of many nucleons, but it is ultimately mediated by the repulsive action of the nuclear force that exists at extremely short-range distances between nucleons.
Delayed neutrons in reactor control 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 neutrons, 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 products aid control of
nuclear reactors 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 meltdown. == Neutron emission in fission ==