Prompt neutron
In
Prompt neutrons emerge from the fission of an unstable
Principle
Using
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 emission occurs.
Group | Half-Life (s) |
Decay Constant (s−1) |
Energy (keV) |
Fraction | Yield of delayed neutrons | |
---|---|---|---|---|---|---|
of all fissions | of this group | |||||
1 | 55.72 | 0.0124 | 250 | 0.000215 | 0.00052 | 2.4 |
2 | 22.72 | 0.0305 | 560 | 0.001424 | 0.00346 | 2.4 |
3 | 6.22 | 0.111 | 405 | 0.001274 | 0.00310 | 2.4 |
4 | 2.30 | 0.301 | 450 | 0.002568 | 0.00624 | 2.4 |
5 | 0.610 | 1.14 | — | 0.000748 | 0.00182 | 2.4 |
6 | 0.230 | 3.01 | — | 0.000273 | 0.00066 | 2.4 |
Total | 0.0065 | 0.0158 | 2.4 |
Importance in nuclear fission basic research
The standard deviation of the final kinetic energy distribution as a function of mass of final fragments from low energy fission of uranium 234 and uranium 236, presents a peak around light fragment masses region and another on heavy fragment masses region. Simulation by Monte Carlo method of these experiments suggests that those peaks are produced by prompt neutron emission.[5][6][7][8] This effect of prompt neutron emission does not provide a primary mass and kinetic distribution which is important to study fission dynamics from saddle to scission point.
Importance in nuclear reactors
If a
However, thanks to the delayed neutrons, it is possible to leave the reactor in a
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, is the effective fraction of delayed neutrons, which is the fraction of delayed neutrons weighted (over space, energy, and angle) on the adjoint neutron flux. This concept arises because delayed neutrons are emitted with an energy spectrum more thermalized relative to prompt neutrons. For low enriched uranium fuel working on a thermal neutron spectrum, the difference between the average and effective delayed neutron fractions can reach 50 pcm (1 pcm = 1e-5).[9]
See also
- Prompt criticality
- Critical mass
- Nuclear chain reaction
References
- U.S. Department of Energy, January 1993, p. 29 (p. 133 of .pdf format)
- ^ Mihalczo, John T. (November 19, 2004), "Radiation Detection From Fission" (PDF), Radiation Detection From Fission, ORNL/TM-2004/234, Oak Ridge National Laboratory, p. 1 (p. 11 of .pdf format)
- OL 5044744M.
- ^ McKown, D. M.; Millard Jr., H. T. (1987). "Determination of Uranium and Thorium by Delayed Neutron Counting". In Baedecker, Philip A. (ed.). Methods for Geochemical Analysis (PDF). U.S. Geological Survey Bulletin. Vol. 1770. United States Geological Survey. p. H12.
- ^ R. Brissot, J.P. Boucquet, J. Crançon, C.R. Guet, H.A. Nifenecker. and Montoya, M., "Kinetic-Energy Distribution for Symmetric Fission of 235U", Proc. of a Symp. On Phys. And Chem. Of Fission, IAEA. Vienna, 1980 (1979)
- S2CID 9831107.
- Bibcode:2007RMxF...53..366M.
- ^ Montoya, M.; Rojas, J.; Lobato, I. "Neutron emission effects on final fragments mass and kinetic energy distribution from low energy fission of U 234" (PDF). Revista Mexicana de Física. 54 (6): 440. Archived from the original (PDF) on 2009-02-05. Retrieved 2009-02-20.
- ^ Deterministic and Monte Carlo Analyses of YALINA Thermal Subcritical Assembly