A fast neutron imaging telescope for inner heliosphere missions

We describe a Fast Neutron Imaging Telescope (FNIT) that is sensitive to neutrons in the energy range 3-100 MeV, optimized to study solar neutrons in the innermost heliosphere. The detection principle is based on multiple elastic neutron-proton scatterings in stacked organic scintillator layers. Plastic scintillator provides the proton target and is an efficient detector of charged particles. At the upper and lower face of each scintillator plate orthogonally oriented wavelength shifting fibers (∅ ∼ 1 mm) collect and guide the scintillation light to a multi-anode photomultiplier tube (MAPMT). The relative anode signals of the MAPMT register the interaction location while the total intensity of the signal is a measure of the recoil proton energy. Elastic scattering kinematics allows one to determine direction and energy spectrum of the primary neutron flux. A performance evaluation prototype has been built, and we show results from first laboratory measurements. We then present Monte Carlo simulation results to outline the performance of an optimized design.