Characterization of the response of Fast Ion Loss Detectors to fusion neutrons for applications at JT-60SA and ITER

One of the main configuration of Fast Ion Loss Detectors (FILD) installed in present day tokamaks and stellarators consists of a collimator and a scintillator coupled to a suitable optical system. In view of their use at the JT-60SA and ITER tokamaks, the impact of the background radiation induced by fusion born neutrons on the instrument must be quantified. In JT-60SA the interaction is predominantly due to 2.5 MeV neutrons born from D –D reactions while, at ITER, 14 MeV neutrons born from D –T are of additional concern, as their flux is expected to be the same as the one from the escaping ions at the position of the FILD. In particular, the generation of background charged particles when neutrons interact with the FILD supporting structure is of most relevance, both at JT-60SA and ITER. In this work we present the results of a study on the neutron sensitivity of the whole FILD setup to 2.5 MeV and 14 MeV neutrons. A set of GEANT4 simulations with a detector geometry derived from the current CAD model of the proposed FILD design has been carried out. Modelling has been validated at the Frascati Neutron Generator, where aspects of the interaction of MeV range neutrons with the FILD setup have been tested. Based on our simulations, we predict that neutrons will induce a measurable background on the FILD, both at JT-60SA and ITER, but they will also not impede measurements.