The reaction cross section of neutron-induced fission on 235U is the most important standard for neutron measurements at the neutron thermal point and between 0.15 MeV and 200 MeV. It is therefore no surprise that several measurements serving diverse purposes are still being carried out to date. Two recent measurements of the 235U(n,f) cross section will be presented : one was carried out at NIST at sub-thermal neutron energies and the other focused on high-energy neutrons, was performed at n_TOF, the CERN neutron spallation source. In the thermal and cold neutron energy region, the fission reaction cross section is known with a sub-percent uncertainty, and the measurement at NIST was organised as a means of validation of the cross section evaluation and of the Alpha-Gamma device, developed to calibrate the neutron monitor used in the neutron lifetime measurement. Above 50 MeV, instead, only a few measurements are available, and there are no experimental data above 200 MeV. For this reason, the objective of the experiment at CERN, a joint effort of INFN, IPN and PTB, is the determination of the 235U(n,f) cross section from 20 MeV to 1 GeV of neutron energy. The experiment concepts and status of the data analysis will be presented, and the challenges faced when performing high-precision measurements or when covering broad energy ranges will be discussed.

Contact : tellier@ill.fr