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Prompt neutron
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In nuclear engineering, a prompt neutron is a neutron immediately emitted (neutron emission) by a nuclear fission event, as opposed to a delayed neutron decay which can occur within the same context, emitted after beta decay of one of the fission products anytime from a few milliseconds to a few minutes later.
Prompt neutrons emerge from the fission of an unstable fissionable or fissile heavy nucleus almost instantaneously. There are different definitions for how long it takes for a prompt neutron to emerge. For example, the United States Department of Energy defines a prompt neutron as a neutron born from fission within 10−13 seconds after the fission event.[1] The U.S. Nuclear Regulatory Commission defines a prompt neutron as a neutron emerging from fission within 10−14 seconds.[2] This emission is controlled by the nuclear force and is extremely fast. By contrast, so-called delayed neutrons are delayed by the time delay associated with beta decay (mediated by the weak force) to the precursor excited nuclide, after which neutron emission happens on a prompt time scale (i.e., almost immediately).
Principle
[edit]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 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
[edit]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
[edit]
If a nuclear reactor happened to be prompt critical — even very slightly — the number of neutrons and power output would increase exponentially at a high rate. The response time of mechanical systems like control rods is far too slow to moderate this kind of power surge. The control of the power rise would then be left to its intrinsic physical stability factors, like the thermal dilatation of the core, or the increased resonance absorptions of neutrons, that usually tend to decrease the reactor's reactivity when temperature rises; but the reactor would run the risk of being damaged or destroyed by heat.
However, thanks to the delayed neutrons, it is possible to leave the reactor in a subcritical state 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. In that regime, neutron production overall still grows exponentially, but on a time scale that is governed by the delayed neutron production, which is slow enough to be controlled (just as an otherwise unstable bicycle can be balanced because human reflexes are quick enough on the time scale of its instability). Thus, by widening the margins of non-operation and supercriticality and allowing more time to regulate the reactor, the delayed neutrons are essential to inherent reactor safety and even in reactors requiring active control.
Fraction definitions
[edit]The factor 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
[edit]References
[edit]- ^ "DOE Fundamentals Handbook - Nuclear Physics and Reactor Theory" (PDF), DOE Fundamentals Handbook - Nuclear Physics and Reactor Theory, DOE-HDBK-1019/1-93, 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)
- ^ Lamarsh, John R. (1975). Introduction to nuclear engineering (1 ed.). Reading, MA: Addison-Wesley Publishing Company. p. 73. ISBN 978-0-201-04160-6. OCLC 643602942. 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)
- ^ Montoya, M.; Saettone, E.; Rojas, J. (2007). "Effects of Neutron Emission on Fragment Mass and Kinetic Energy Distribution from Thermal Neutron-Induced Fission of 235U". AIP Conference Proceedings. 947: 326–329. arXiv:0711.0954. Bibcode:2007AIPC..947..326M. doi:10.1063/1.2813826. S2CID 9831107.
- ^ Montoya, M.; Saettone, E.; Rojas, J. (2007). "Monte Carlo Simulation for fragment mass and kinetic energy distribution from neutron-induced fission of U 235" (PDF). Revista Mexicana de Física. 53 (5): 366–370. arXiv:0709.1123. Bibcode:2007RMxF...53..366M. Archived from the original (PDF) on 2022-05-31. Retrieved 2009-02-20.
- ^ 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
External links
[edit]Prompt neutron
View on Grokipediafrom Grokipedia
Fundamentals
Definition and Emission Mechanism
Prompt neutrons are neutrons emitted instantaneously from the fissioning nucleus during nuclear fission, occurring within a timeframe of approximately seconds after the fission event, distinguishing them from delayed neutrons produced by subsequent radioactive decays of fission fragments. These neutrons are a direct byproduct of the fission process, released as the nucleus splits into two lighter fragments, and they play a critical role in sustaining chain reactions in nuclear systems.[4] The emission mechanism of prompt neutrons begins with the formation of an excited compound nucleus upon absorption of a neutron by a fissile isotope, such as uranium-235. As the nucleus approaches the fission barrier, it passes through the saddle point, where deformation increases, and reaches the scission point, at which the nucleus divides into two fission fragments. The resulting fragments are highly excited with a total excitation energy (TXE) of approximately 20 MeV—and de-excite primarily through the evaporation of neutrons from their surfaces as they accelerate apart, a process governed by the compound nucleus evaporation model. This evaporation occurs rapidly because the fragments' excitation energy exceeds the neutron binding energy, leading to the prompt release before any significant beta decays can take place. The concept of prompt neutrons was first theoretically predicted in the seminal work by Niels Bohr and John Archibald Wheeler in 1939, who modeled fission as a liquid drop deformation process and anticipated the emission of neutrons accompanying the splitting of heavy nuclei. Experimental confirmation followed shortly thereafter, with initial observations in fission experiments during the early 1940s, and theoretical models refined in the mid-1940s incorporated statistical treatments of neutron evaporation to better describe the yields. The average number of prompt neutrons emitted per fission event is denoted by , which represents the prompt neutron multiplicity and is typically around 2.4 to 2.5 for thermal fission of uranium-235, derived from adjustments to the semi-empirical mass formula that account for the neutron excess in heavy fissile nuclei. This parameter is essential for quantifying the efficiency of fission chain reactions but varies slightly with the incident neutron energy and fissile isotope.Distinction from Delayed Neutrons
Prompt neutrons are emitted almost instantaneously during the fission process, typically within approximately seconds following the splitting of the nucleus, arising directly from the de-excitation of the highly energetic fission fragments.[4] In contrast, delayed neutrons are released much later, on timescales ranging from fractions of a second to several minutes, as a result of the beta decay of specific neutron-rich fission product precursors that accumulate after the initial fission event; these precursors are conventionally grouped into six categories based on their decay half-lives for thermal fission of uranium-235, with representative half-lives of about 0.23 seconds, 0.61 seconds, 2.3 seconds, 6.2 seconds, 23 seconds, and 56 seconds.[5] The fundamental origin of prompt neutrons lies in the immediate evaporation from the excited states of the fission fragments produced during scission, whereas delayed neutrons originate from subsequent neutron emission following the beta-minus decay of unstable fission products, such as bromine-87 (with a half-life of 55.7 seconds) or iodine-137 (with a half-life of 24.5 seconds), which transform into neutron-unbound excited states in their daughter nuclei.[6] This two-step process for delayed neutrons—beta decay followed by neutron emission—distinguishes them mechanistically from the direct emission of prompt neutrons. In terms of abundance, prompt neutrons overwhelmingly dominate, comprising the vast majority of neutrons produced in fission; for example, in thermal neutron-induced fission of uranium-235, prompt neutrons account for approximately 99.4% of the total neutron yield, while delayed neutrons represent only about 0.6%. These fractions vary slightly depending on the fissile isotope, as shown in the table below for common thermal fission cases:| Fissile Isotope | Delayed Neutron Fraction (β) | Prompt Neutron Fraction (1 - β) |
|---|---|---|
| U-233 | 0.0027 | 0.9973 |
| U-235 | 0.0064 | 0.9936 |
| Pu-239 | 0.0021 | 0.9979 |