In nuclear engineering, a prompt neutron is a neutron immediately emitted by a nuclear fission event. Prompt neutrons emerge from the fission of an unstable fissionable or fissile heavy nucleus almost instantaneously. Delayed neutron decay can occur within the same context, emitted after beta decay of one of the fission products. Delayed neutron decay can occur at times from a few milliseconds to a few minutes. The U.S. Nuclear Regulatory Commission defines a prompt neutron as a neutron emerging from fission within 10 seconds.