Measurement of the Pu239(n,f) cross section from 4 keV to 100 MeV using the white neutron source at the CSNS Back-n facility

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 $4\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$ to $100\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ 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 $10\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$ to $100\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$.