Measurement of the U-236,U-238(n,f) cross sections from the threshold to 200 MeV at CSNS Back-n

Neutron-induced uranium fission reactions play an essential role in advanced nuclear energy research, basic and applied nuclear science. Based on the China Spallation Neutron Source (CSNS) Back-streaming white neutron source (Back-n), the measurement of the U-236,U-238 fission cross sections relative to U-235 from the threshold to 200 MeV was carried out by using time-of-flight (TOF) method with a multi-cell fast fission ionization chamber in double-bunch mode. The TOF spectra in double-bunch mode were unfolded by using Bayesian method. The fission cross section ratios of U-238/U-235 and U-236/U-235 were obtained. The experimental data of the U-236,U-238/U-235 fission cross section ratios measured in single-bunch mode were re-analyzed in high energy region and the upper limit was extended to 200 MeV for comparison. The experimental results in double-bunch mode are in good agreement with those in single-bunch mode and the main evaluation data within experimental uncertainties. The U-236,U-238(n,f) cross sections were obtained from the measured ratios and the standard U-235(n,f) cross sections and they agree well with the theoretical calculation, previous experiments, main evaluations and IAEA NDS. The experimental results can provide technical support for further experimental researches in double-bunch mode and relevant nuclear data evaluations.