Editing Delayed neutron (section)

==Principle==
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.

Using [[Uranium-235|U-235]] as an example, this nucleus absorbs [[thermal neutron]]s, and the immediate mass products of a fission event are two large fission fragments, which are remnants of the formed U-236 nucleus. These fragments emit, on average, two or three free neutrons (in average 2.47), called [[prompt neutron|"prompt" neutrons]]. A subsequent fission fragment occasionally undergoes a stage of radioactive decay (which is a [[beta decay|beta minus decay]]) that yields a new nucleus (the emitter nucleus) in an excited state that emits an additional neutron, called a "delayed" neutron, to get to ground state. These neutron-emitting fission fragments are called delayed neutron precursor atoms.

'''Delayed Neutron Data for Thermal Fission in U-235'''<ref>J. R. Lamarsh, ''Introduction to Nuclear Engineering,'' Addison-Wesley, 2nd Edition, 1983, page 76.</ref><ref>G. R. Keepin, ''Physics of Nuclear Kinetics'', Addison-Wesley, 1965.</ref>

{| 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
| 0.00346
| 0.001424
|-
| 3
| 6.22
| 0.111
| 405
| 0.00310
| 0.001274
|-
| 4
| 2.30
| 0.301
| 450
| 0.00624
| 0.002568
|-
| 5
| 0.610
| 1.14
| –
| 0.00182
| 0.000748
|-
| 6
| 0.230
| 3.01
| –
| 0.00066
| 0.000273
|}