The neutron-induced fission cross section of ${}^{239}\mathrm{Pu}$ was measured relative to ${}^{235}\mathrm{U}(n,f)$ at the back-streaming white neutron beam line (Back-n) of the China Spallation Neutron Source (CSNS). A multicell fast fission ionization chamber was used to perform the measurement. The reliability of the measurement was verified by the high consistency of ${}^{235}\mathrm{U}$'s resonances between the measurement and the evaluation data. The ${}^{239}\mathrm{Pu}$ fission cross sections from $4\mathrm{keV}$ to $100\mathrm{MeV}$ are obtained with 1.7–5.8 % uncertainty when unfolding uncertainties are excluded. The total uncertainties including the unfolding errors, which reflect the effect of the double-bunch unfolding method, are 2.6–15 % from $10\mathrm{keV}$ to $100\mathrm{MeV}$.

Neutron energy of deuterium-tritium (D-T) generator plays an important role for the application of nuclear energy and nuclear technology. Based on the deuterium energy losing in tritiated titanium (T-Ti) target and differential cross sections of D-T reaction, an improved theoretical formula for calculating the mean neutron energy irradiating a large solid angle sample was proposed. Besides that, the mean neutron energy was discussed under the actual experimental conditions. The results were compared to the reported ratio values of ⁹⁰Zr(n,2 n)^89m+gZr to ⁹³Nb(n,2 n)^92mNb reaction cross sections (‘Zr/Nb’ ratio methods) and published theoretical calculated results, respectively. The relative deviations between present work and the ‘Zr/Nb’ ratio methods are below 0.5% and a higher precision was obtained compared with previous published theoretical method. The present work lays a foundation for the application of D-T neutron source in nuclear technology such as nuclear reaction cross section measurement, calibration of neutron detector, and neutron irradiation tests, etc. Graphical Abstract