Speakers
Description
The $^{235}$U(n,f) cross section is one of the most important cross-sections in many applications, and it is a standard at the thermal neutron energy point (0.025 eV), and from 0.15 MeV to 200 MeV. Above 100 MeV, it also plays an important role in understanding fundamental nuclear physics processes, as it can be related to the properties of excited nuclear matter. Above 50 MeV, however, the evaluation recommended by the IAEA is based on only two measurements, and above 200 MeV no data are available. For these reasons, a new measurement was carried out in 2018 at n_TOF, the neutron time-of-flight facility of CERN, aimed at measuring the $^{235}$U(n,f) cross section relative to $^1$H(n,n) in the high-energy neutron range, from 10 MeV up to several hundred MeV.
The setup consisted of ten $^{235}$U samples mounted in two reaction chambers, and two polyethylene samples, which were all irradiated simultaneously. The protons emitted from the polyethylene radiators after elastic neutron scattering on hydrogen were detected and discriminated from other particles using three particle telescopes placed outside the beam at the laboratory angles of 20 and 25 degrees. Considering the complexity of the experiment and of its analysis, this contribution will focus on the data covering the incident neutron energy range up to 200 MeV, and the results above 200 MeV will be discussed in the contribution of A. Manna for the n_TOF Collaboration, submitted to ND2022. The challenges faced when carrying out an absolute measurement will be addressed, especially those related to the determination of the incident neutron fluence, and the normalization procedure relative to n-p scattering, the primary standard for neutron cross sections.