Cross section of the 232Th(n, f) reaction in the MeV neutron energy region

Accurate cross section of the Th-232(n, f) reaction are demanded in the design of advanced nuclear systems and in the development of fission theory. However, the existing measurement data are relatively sparse comparing with those of the U-238(n, f) reaction, with big uncertainties and obvious discrepancies. Furthermore, analysis shows that systematic deviations exist between the results measured with mono-energetic neutron sources and white neutron sources, which is the main reason for the differences among different evaluation libraries. This work is dedicated to the clarification of this discrepancy. Based on mono-energetic d-d neutron sources and using back-to-back Th/U-238 samples, cross section of the Th-232(n, f) reaction were measured at 12 energies in the 4.2-11.5 MeV region. Elaborated measures were taken in the measurement procedure including the exchange of the forward and the backward direction of the samples, as well as in the data processing containing the correction of interference fission counts from low-energy neutrons and the detailed Monte Carlo simulations for the determination of detection efficiencies for fission events. In addition, theoretical analysis was also performed using TALYS-1.9 and UNF codes. The present results agree with existing measurement data using white neutron sources, showing that previous cross section of the Th-232(n, f) reaction measured using mono-energetic neutron sources are systematically overestimated on average. The present results are in accordance with the latest measurement data of Michalopoulou et al., which is helpful in the improvement of nuclear data evaluations of the Th-232(n, f) reaction.