Neutron-induced fission cross sections of ²³²Th and ²³³U up to 1 GeV using parallel plate avalanche counters at the CERN n_TOF facility

Abstract

The neutron-induced fission cross sections of ²³²Th and ²³³U were measured relative to ²³5U in a wide neutron energy range up to 1 GeV (and from fission threshold in the case of ²³²Th, and from 0.7 eV in case of ²³³U), using the white-spectrum neutron source at the CERN Neutron Time-of-Flight (n_TOF) facility. Parallel plate avalanche counters (PPACs) were used, installed at the Experimental Area 1 (EAR1), which is located at 185 m from the neutron spallation target. The anisotropic emission of fission fragments were taken into account in the detection efficiency by using, in the case of ²³³U, previous results available in EXFOR, whereas in the case of ²³²Th these data were obtained from our measurement, using PPACs and targets tilted 45◦ with respect to the neutron beam direction. Finally, the obtained results are compared with past measurements and major evaluated nuclear data libraries. Calculations using the high-energy reaction models INCL++ and ABLA07 were performed and some of their parameters were modified to reproduce the experimental results. At high energies, where no other neutron data exist, our results are compared with experimental data on proton-induced fission. Moreover, the dependence of the fission cross section at 1 GeV with the fissility parameter of the target nucleus is studied by combining those (p, f) data with our (n, f) data on ²³²Th and ²³³U and on other isotopes studied earlier at n_TOF using the same experimental setup.