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Measurement of the neutron-induced fission cross section of 235U relative to neutron-proton elastic scattering at CERN-nTOF

Kolloquium der Abteilung 6

The 235U(n,f) cross section is one of the most important cross-section standards at the thermal neutron energy point (0.025 eV) and between 0.15 MeV and 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 (such as viscosity). Above 50 MeV, however, the 235U(n,f) cross-section recommended by the IAEA is based on two measurements only, and above 200 MeV no data are available. For these reasons, in October 2018, a 5-week-long measurement was carried out at the CERN neutron source nTOF, in a joint effort of INFN, IPN, and PTB, for the determination 235U fission cross section up to 1 GeV of neutron energy.

The challenges faced when covering such broad energy range will be discussed, especially those related to the measurement of the incident neutron fluence on the irradiated uranium samples, and the normalization relative to n-p scattering, the primary neutron standard for neutron cross section measurement. At PTB, a parallel-plate ionization chamber (for fission) and a recoil proton telescope (for normalization) were developed with the objective of covering the low energy part, below 150 MeV, which is the most relevant for metrology. The detector concept and status of the data analysis will be presented.