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delayed neutron[di′lād ′nü‚trän]
A neutron emitted spontaneously from a nucleus as a consequence of excitation remaining from a preceding radioactive decay event. Analogously, delayed emission of protons and alpha particles is also observed, but the known delayed neutron emitters are more numerous, and some of them have practical implications. In particular, they are of importance in the control of nuclear chain reactors.
In a 235U nuclear reactor, about 0.7% of the neutrons are delayed, the others being prompt. In a conventional, moderated reactor, the prompt neutrons are born, slowed down, and reabsorbed to produce the next fissions in a cycling time of about 1 millisecond. (In a fast-neutron reactor, the time is much shorter.) Consequently, if the reactor were to become overcritical (more neutrons generated per millisecond than are absorbed or leak out), the chain reaction would exponentiate or “run away,” and the reactor might overheat itself and possibly cause a dangerous accident unless the control rods could respond within a few milliseconds to correct the situation. The fortunate presence of the delayed neutrons eases the situation, because so long as the reactor operates within the margin of 0.7% (“delayed critical”), the control rods can take as long as several seconds to respond, and thus the chain reaction comes within the range of easy and leisurely control. See Reactor physics