Prompt neutron
Prompt neutrons are neutrons released directly and instantaneously during the nuclear fission process, typically within approximately $10^{-14} to $10^{-13} seconds of the fission event, and they comprise over 99% of the total neutrons produced in fission reactions.[1][2] These neutrons are born as fast neutrons with a fission spectrum energy distribution, where the average energy is about 2 MeV, the most probable energy is around 0.7 MeV, and most fall between 1 and 2 MeV, though some exceed 10 MeV.[1][2] In thermal fission of uranium-235, the total average number of neutrons per fission (\nu) is approximately 2.43, of which the prompt neutron multiplicity (\nu_p) is about 2.41, leaving a small delayed neutron fraction (\beta) of roughly 0.0065.[2] For plutonium-239, \beta is lower at about 0.0021, making prompt neutrons an even larger proportion.[2] The prompt fission neutron spectrum follows a Maxwellian-like distribution modified by nuclear physics effects, and it is crucial for modeling neutron transport in reactors.[1] In nuclear reactors, prompt neutrons drive the rapid chain reaction, with their short generation time—on the order of $10^{-4} seconds in a typical thermal reactor—enabling quick power excursions if not controlled.[2] A reactor achieves prompt criticality when the effective multiplication factor k_{eff} exceeds $1 + \beta, leading to exponential power growth governed solely by prompt neutrons, which poses significant safety challenges.[3] The presence of the small delayed neutron fraction allows for manageable control through reactivity insertions, as delayed neutrons extend the overall neutron generation time and provide a buffer against uncontrolled prompt-driven transients.[2][1] Without delayed neutrons, reactor operation would be impractical due to the extreme speed of power changes induced by prompt neutrons alone.[2]Fundamentals
Definition and Emission Mechanism
Prompt neutrons are neutrons emitted instantaneously from the fissioning nucleus during nuclear fission, occurring within a timeframe of approximately $10^{-14} 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 due to the available energy from the fission process—typically around 200 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 \nu_p, 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 $10^{-14} 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 |