Prompt neutron

A "prompt neutron" refers to neutrons that are emitted immediately during a nuclear fission event, as opposed to delayed neutrons, which are emitted by the beta decay of fission products some time after the initial fission reaction. In a fission event, when a heavy nucleus (such as Uranium-235 or Plutonium-239) absorbs a neutron and becomes unstable, it splits into two smaller nuclei, releasing a significant amount of energy and several neutrons almost instantaneously.