$^3He$-free triple GEM thermal neutron detector

A novel type of thermal neutron detector based on the gas electron multiplier (GEM) technology is presented in the framework of the research and development activity on the 3He replacement for neutron detection. The device relies on a series of boron-coated alumina sheets placed perpendicularly to the incident neutron beam direction. The detector, named side-on GEM (S-GEM), was tested on beam at the High Flux Isotope Reactor at the Oak Ridge National Laboratory (US) to assess its performance in terms of beam position resolution, efficiency and signal-to-background (S/B) ratio as compared to a 10 bar 3He tube for sub-thermal neutrons. Using 3 mm wide PADs, a sub-millimeter position resolution was obtained. The achieved efficiency is about 30% with a quite good S/B ratio. The obtained results demonstrate the effectiveness of the proposed detector configuration to achieve a good spatial resolution and, in the perspective, a higher thermal neutron efficiency, comparable to 3He tubes typically used for diagnostic in nuclear reactors. The main issues to be addressed to reach the goal, mostly related to boron coating procedures and characterization, are also pointed out.