A Proton Recoil Telescope for the characterisation of the neutron beam at n_TOF

The neutron induced fission cross section of U-235 is extensively used as a reference for neutron fluence measurements in various applications. At intermediate energies, the U-235(n,f) cross section plays an important role also for fundamental nuclear physics. Despite its widespread use, no data exist on neutron-induced fission of U-235 above 200 MeV. Hence, there is a clear and long-standing demand from the International Atomic Energy Agency (IAEA) to complement the experimental database from 20 MeV to 1 GeV. For this purpose at the neutron facility n TOF at CERN the measurement of U-235(n,f) cross section was planned in October 2018, taking advantage of the intense neutron beam with a wide energy spectrum available in the experimental area. The cross section measurement will be performed relative to the elastic neutron-proton scattering, a very well known reaction generally accepted as primary reference. A prototype of the Proton Recoil Telescope detector, that will be used to measure the incident neutron flux, has been built and tested at n TOF in 2017.