Prompt neutron
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In nuclear engineering, a prompt neutron is a neutron immediately emitted by a nuclear fission event, as opposed to a delayed neutron which is emitted by one of the fission products anytime from a few milliseconds to a few minutes later.
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[edit] Principle
Using U-235 as an example, the immediate mass products of a fission event are two large fission fragments, which are remnants of the original U-235 nucleus, plus, on average, two or three free neutrons (in average 2.47), called "prompt" neutrons. A subsequent fission fragment occasionally, but rarely, undergoes a stage of radioactive decay that yields an additional neutron, called a "delayed" neutron. These neutron-emitting fission fragments are called delayed neutron precursor atoms.
Delayed neutrons are associated with the beta decay of the fission products. Indeed, after prompt fission neutron emission the residual fragments are still neutron rich. They undergo a beta decay chain. The more neutron rich the fragment, the more energetic and faster the beta decay. In some cases the available energy in the beta decay is high enough to leave the residual nucleus in such a highly excited state that neutron emission instead of gamma emission occurs.
[edit] Importance in nuclear reactors
If a nuclear reactor happened to be prompt critical - even very slightly - the number of neutrons would increase exponentially and very quickly the reactor would become uncontrollable. However, thanks to the delayed neutrons, it is possible to run a reactor subcritically as far as only prompt neutrons are concerned: the delayed neutrons come a moment later, just in time to sustain the chain reaction when it is going to die out. Thus, allowing to reach criticality with a larger marginal and to dispose of more time to regulate the reactor, the delayed neutrons are essential to inherent reactor safety.
[edit] Fraction definitions
The factor β is defined as:
and it is equal to 0.0064 for U-235.
The delayed neutron fraction (DNF) is defined as:
These two factors, β and DNF, are not the same thing in case of a rapid change in the number of neutrons in the reactor.
Another concept, especially useful in the case where there are isotopes with threshold fission cross sections in the nuclear fuel, is the effective fraction of delayed neutrons, which is the fraction of delayed neutrons weighted with their probability to induce a new fission in the chain reaction.
