Background: The development of GenIV nuclear reactors will take advantage of fast neutrons. They will necessitate nuclear data of low uncertainties over a wide range of neutron energies, notably above 1 MeV. As far as ${}^{235}\mathrm{U}$ is concerned, current experimental data on average prompt neutron multiplicities $\left({\overline{\nu }}_{\text{prompt}}\right)$ of qualified uncertainties show some discrepancies and do not always cover the full incoming neutron energy range of interest.

Purpose: Measure the average prompt neutron multiplicity in the ${}^{235}\mathrm{U}(n,f)$ reaction as a function of incoming neutron energy between 1 and 700 MeV.

Methods: The double time-of-flight technique was used at the Los Alamos Neutron Science Center to measure incoming and prompt fission neutron energies. The data were measured relative to the very well-known ${}^{252}\mathrm{Cf}\left(\mathrm{sf}\right)$ prompt fission neutron spectrum (PFNS) and ${\overline{\nu }}_{\text{prompt}\text{Cf}}$. The integral over the reconstructed PFNS energy and angle distributions in the ${}^{235}\mathrm{U}(n,f)$ reaction provided ${\overline{\nu }}_{\text{prompt}}$.

Results: ${\overline{\nu }}_{\text{prompt}}$ was obtained between 1 and 700 MeV with uncertainties below 0.5% above neutron incoming energies of 3 MeV. There are discrepancies between the data and the evaluations JEFF 4.0, ENDF/B-VIII.1, and JENDL-5.

Conclusions: The results are the most precise measurements of ${\overline{\nu }}_{\text{prompt}}$ in the ${}^{235}\mathrm{U}$($n,f$) reaction so far and cover well the region of interest for nuclear applications between 1 and 25 MeV. In addition, the data are precise enough to highlight the impact of second-chance fission on the increase rate of ${\overline{\nu }}_{\text{prompt}}$ as a function of the incoming neutron energy.

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• Fission
• Neutron physics
• Nuclear reactors
• Nucleon induced nuclear reactions
• A ≥ 220