Speakers
Description
The $^{235}$U(n,f) cross section is one of the most important neutron cross-section standards, and therefore it is recognized as a convenient reference for several other reaction cross-section measurements at thermal energy and between 0.15 MeV and 200 MeV. Outside these energy ranges the cross section is either not well known nor measured yet, as for example above 200 MeV, where evaluations can only rely on theoretical calculations. In this energy region, $^{235}$U(n,f) plays an important role for fundamental nuclear physics as well as for several applications, ranging from biological effectiveness, via nuclear astrophysics, to nuclear technology.
Therefore, in spite of the continuous effort for the improvement of the standards by the International Atomic Energy Agency (IAEA), together with ENDF/B, the longstanding quest for $^{235}$U(n,f) cross section data turns out to be still pending above 20 MeV (only 2 data sets are present in the 20-200 MeV energy region). In particular, the request is for fission measurements relative to the neutron-proton elastic scattering, which is considered the primary reference for neutron cross section measurements.
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 up to 1 GeV. Recently, a dedicated measurement campaign was carried out to provide accurate and precise cross section data of the $^{235}$U(n,f) reaction in the energy region from 10 MeV to 500 MeV.
The experimental apparatus is essentially developed by INFN (Italy), PTB (Germay) and IPN (France) with support from CERN. It consists, which involves the efforts of several institutes (INFN, PTB and IPN in addition to CERN), consisted of three flux and two fission detectors, thus allowing to simultaneously record the number of neutrons impinging on the $^{235}$U samples (neutron flux) and of fission events, as a function of the neutron energy. The neutron flux measurement is based on the neutron-proton elastic scattering reaction, and it exploits the detection of the recoil protons from the n-p reaction in a polyethylene target using three Proton Recoil Telescopes.
The experimental apparatus and data analysis will be presented. Due to the complexity of the experiment and its challenging data analysis, the results obtained in the energy range between 10 to 200 MeV are shown in the E. Pirovano for the n_TOF Collaboration contribution to ND2022, while I will focus on the results obtained in the high-energy region, above 200 MeV.