The future Swiss FEL facility - challenges from a radiation protection point of view

The Swiss Free Electron Laser (SwissFEL) is a new large-scale facility currently under construction at the Paul Scherrer Institute. SwissFEL comprises an injector area (gun and booster), three linear accelerating sections, two parallel undulator lines (Aramis and Athos) and several experimental areas, for a total length of 720 m. Accessible areas surrounding the 720 m long accelerator tunnel, together with the pulsed time structure of the primary beam, lead to new challenges for ensuring that the radiation level in these areas remains in compliance with the legal constraints. For this purpose an online survey system (DRPS) based on the monitoring of the ambient dose rate arising from neutrons inside the accelerator tunnel and adequately calibrated to indicate the total dose rate outside of the tunnel, will be installed. The major contribution to the total dose rate outside of the accelerator tunnel is expected from secondary neutrons. The DRPS will therefore employ extended range neutron REM counters placed along the full accelerator length, which will measure the neutron dose rate inside the tunnel. The LUPIN-BF₃-NP detector was selected as main measurement device for the DRPS. It consists of a BF₃ neutron proportional counter, which is inserted in a cylindrical polyethylene moderator with a lead layer