Measurement of the 232Th(n,f) cross section in the 1–200 MeV range at the CSNS Back-n

The 232Th(n,f) cross section is very important in basic nuclear physics and applications based on the Th/U fuel cycle. Using the time-of-flight method and a multi-cell fast-fission ionization chamber, a novel measurement of the 232Th(n,f) cross section relative to 235U in the 1–200 MeV range was performed at the China Spallation Neutron Source Back-n white neutron source (Back-n). The fission event-neutron energy spectra of 232Th and 235U fission cells were measured in the single-bunch mode. Corrected 232Th/235U fission cross-sectional ratios were obtained, and the measurement uncertainties were 2.5–3.7% for energies in the 2–20 MeV range and 3.6–6.2% for energies in the 20–200 MeV range. The 232Th(n,f) cross section was obtained by introducing the standard cross section of 235U(n,f). The results were compared with those of previous theoretical calculations, measurements, and evaluations. The measured 232Th fission cross section agreed with the main evaluation results in terms of the experimental uncertainty, and 232Th fission resonances were observed in the 1–3 MeV range. The present results provide 232Th(n,f) cross-sectional data for the evaluation and design of Th/U cycle nuclear systems.