Editing Prompt neutron (section)

==Principle==

Using [[uranium-235]] as an example, this nucleus absorbs a [[thermal neutron]], and the immediate mass products of a fission event are two large fission fragments, which are remnants of the formed uranium-236 nucleus. These fragments emit two or three free neutrons (2.5 on average), 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. 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.

{| class="wikitable sortable"
|+Delayed Neutron Data for Thermal Fission in Uranium-235<ref>{{Cite book |last=Lamarsh |first=John R. |url=https://archive.org/details/introductiontonu0000lama/page/73 |title=Introduction to nuclear engineering |date=1975 |publisher=Addison-Wesley Publishing Company |isbn=978-0-201-04160-6 |edition=1 |location=Reading, MA |page=73 |oclc=643602942 |ol=5044744M |url-access=limited}}</ref><ref>{{Cite book |last1=McKown |first1=D. M. |url=https://pubs.usgs.gov/bul/1770/report.pdf |title=Methods for Geochemical Analysis |last2=Millard Jr. |first2=H. T. |publisher=[[United States Geological Survey]] |year=1987 |editor-last=Baedecker |editor-first=Philip A. |series=U.S. Geological Survey Bulletin |volume=1770 |page=H12 |chapter=Determination of Uranium and Thorium by Delayed Neutron Counting}}</ref>
|-
! rowspan="2" | Group
! rowspan="2" | Half-Life<br/>(s)
! rowspan="2" | Decay Constant<br/>(s<sup>−1</sup>)
! rowspan="2" | Energy<br/>(keV)
! rowspan="2" | Fraction 
! colspan="2" | Yield of delayed neutrons
|-
! of all fissions
! of this group
|-
| 1
| 55.72
| 0.0124
| 250
| {{val|0.000215}}
| {{val|0.00052}}
| 2.4
|-
| 2
| 22.72
| 0.0305
| 560
| {{val|0.001424}}
| {{val|0.00346}}
| 2.4
|-
| 3
| 6.22
| 0.111
| 405
| {{val|0.001274}}
| {{val|0.00310}}
| 2.4
|-
| 4
| 2.30
| 0.301
| 450
| {{val|0.002568}}
| {{val|0.00624}}
| 2.4
|-
| 5
| 0.610
| 1.14
| {{sdash}}
| {{val|0.000748}}
| {{val|0.00182}}
| 2.4
|-
| 6
| 0.230
| 3.01
| {{sdash}}
| {{val|0.000273}}
| {{val|0.00066}}
| 2.4
|- class="sortbottom"
|colspan=4 | Total
| {{val|0.0065}}
| {{val|0.0158}}
| 2.4
|}