Prompt neutron 1221448 225731817 2008-07-15T03:26:56Z RJFJR 141808 remove cleanup tag 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 product]]s anytime from a few milliseconds to a few minutes later. This is not [[neutron decay|delayed neutron decay]], which can occur within the same context, but is a different process. ==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 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 and 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 ray|gamma emission]] occurs. '''Delayed Neutron Data for Thermal Fission in U-235''' {| class="wikitable" |- ! Group ! Half-Life (s) ! Decay Constant (s<sup>-1</sup>) ! Energy (keV) ! Yield, Neutrons per Fission ! Fraction |- | 1 | 55.72 | 0.0124 | 250 | 0.00052 | 0.000215 |- | 2 | 22.72 | 0.0305 | 560 | 0.00546 | 0.001424 |- | 3 | 6.22 | 0.111 | 405 | 0.00310 | 0.001274 |- | 4 | 2.30 | 0.301 | 450 | 0.00624 | 0.002568 |- | 5 | 0.614 | 1.14 | - | 0.00182 | 0.000748 |- | 6 | 0.230 | 3.01 | - | 0.00066 | 0.000273 |} <ref>Lamarsh, Introduction to Nuclear Engineering</ref> ==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 [[subcritical]]ly 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, by widening the margins of non-operation and supercriticality and allowing more time to regulate the reactor, the delayed neutrons are essential to [[Passive nuclear safety|inherent reactor safety]]. ==Fraction definitions== The factor β is defined as: :<math> \beta = \frac{\mbox{precursor atoms}} {\mbox{prompt neutrons}+\mbox{precursor atoms}}. </math> and it is equal to 0.0064 for U-235. The delayed neutron fraction (DNF) is defined as: :<math> DNF = \frac{\mbox{delayed neutrons}} {\mbox{prompt neutrons}+\mbox{delayed neutrons}}. </math> 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 section (physics)|cross section]]s 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. ==See also== *[[prompt critical]] *[[Critical mass (nuclear)|critical mass]] *[[nuclear chain reaction]] ==External links== *[http://lpsc.in2p3.fr/gpr/PPNPport/node47.html Hybrid nuclear reactors:delayed neutrons] *[http://www.pipeline.com/~rstater/nuke1a.html Beta is not the delayed neutron (population) fraction] [[Category:Nuclear technology]] [[Category:Neutron|Prompt]] [[de:Promptes Neutron]] [[ru:Мгновенные нейтроны]]