An Anisotropic Neutron Collimating System for Proton Accelerator–Generated Secondary Particles

Proton–stainless steel interactions occurring at the first collimator of the Middle East Technical University Defocusing Beamline generate high-energy secondary particles like neutrons (23 MeV), gamma rays (14 MeV), and electrons and positrons ( 7.0 MeV) with particle fluxes between 10⁷ to 10⁹ particles/(cm²∙s). A neutron collimating system aiming to reduce most of these secondaries and obtain a moderate flux of fast neutrons was designed and constructed. The collimating structure consists of a moderating unit aiming to shield the outside of the system, a neutron funnel to redirect the neutrons to the desired beam geometry, and a testing station. This system funnels neutrons into a 10-cm-diameter nonuniform beam and directs them to a testing area capable of hosting up to six samples of 7.3-cm diameter and up to 3.0-cm thickness. Simulation results show neutrons with energies up to 5.0 MeV and a flux of 10⁶ neutrons/(cm²∙s) at the testing unit, while the experimental result gives a neutron dose rate of about 22 mSv/h.