FGS, a multi-mission space gamma-ray spectrometer: Design optimization and first results

Following the failure of the TARANIS (Tool for the Analysis of RAdiation from lightNIng and Sprites) mission launch[1], the French space agency (CNES) funded a R&D program with APC and LESIA laboratories to develop a new gamma-ray spectrometer. The first studies started in 2021 and lead to a new design called FGS (Fast Gamma-ray Spectrometer) based on new GAGG scintillators readout by SiPM and analysed by the IDEAS/APOCAT fast ASIC. This program's main objective is to build a space qualified FGS prototype before 2024. FGS scientific specifications are based on Terrestrial Gamma-ray Flashes (TGF) science studies but this spectrometer could also be used or optimized for other missions: solar science, astrophysical observations (Gamma-Ray Bursts) or planetology studies. The main instrumental objectives are to detect gamma rays in the [20keV-20MeV] energy range with high count rate abilities and a large detection surface. In this paper, we will present our FGS concept and the first spectroscopic results we obtained, consistent on ground with our scientific requirements.