Measurement of the 235 U(n,f) cross section relative to the neutron-proton elastic scattering up to 500 MeV at n_TOF

Abstract

Neutron cross-section standards are fundamental ingredients for both measurements and evaluations of neutron-induced reaction cross sections. This is the case of 235U(n,f) cross section: one of the most important standard cross sections at thermal neutron energy and between 0.15 MeV and 200 MeV. Above 200 MeV this reaction plays an important role for several applications, from biological effectiveness, via nuclear astrophysics, to nuclear technology, as well as for fundamental nuclear physics. However, no measurement exists for energies above 200 MeV. This led to a request for new absolute measurements of 235U(n,f) cross section, in order to extend the precision and possibly to establish it as a standard up to 1 GeV. The n_TOF facility at CERN offers the possibility to study such reaction thanks to the wide neutron energy spectrum available in its experimental area, from thermal to 1 GeV. A dedicated measurement campaign was carried out to provide accurate and precise cross-section data of the 235U(n,f) reaction in the energy region from 10 MeV to 500 MeV. The experimental setup consisted of two chambers to detect the 235U fission events, while the neutron flux was simultaneously measured by exploiting the neutron-proton scattering process using three Proton Recoil Telescopes. In this PhD thesis the development, implementation and characterisation of the two telescopes under the responsibility of the INFN are discussed, as well as the analysis of the data acquired during the campaign. In this measurement, the n_TOF neutron flux in the energy interval between 10 and 500 MeV was measured for the first time. In addition, the analysis of one of the two chambers dedicated to the measurement of fission events is presented in detail. From these data, the 235U(n,f) cross section was determined and represents, at this time, the unique measure of this kind.