GermanIum Array for Non-destructive Testing (GIANT) setup for Muon Induced X-ray Emission (MIXE) at the Paul Scherrer Institute

Abstract: The usage of muonic X-rays to study elemental properties like nuclear radii ranges back to the seventies. This triggered the pioneering work at the Paul Scherrer Institute (PSI), during the eighties, on the Muon Induced X-ray Emission (MIXE) technique for a non-destructive assessment of elemental compositions. In the recent years, this method has seen a rebirth, improvement and adoption at most muon facilities around the world. Hereby, the PSI offers unique capabilities with its high-rate continuous muon beam at the Swiss Muon Source (S$\mu$S). We report here the decision making, construction and commissioning of a dedicated MIXE spectrometer at PSI, the GermanIum Array for Non-destructive Testing (GIANT) setup. Multiple campaigns highlighted the outstanding capabilities of MIXE at PSI, e.g. resolving down to 1 at% elemental concentrations with as little as 1 h data taking, measuring isotopic ratios for elements from iron to lead, and characterizing gamma rays induced by muon nuclear capture. On-target beamspots were characterized with a dedicated charged particle tracker to be 22.06$\pm$0.18 and 14.45$\pm$0.06 mm for 25 and 45 MeV/c, respectively. Advanced analysis of the High Purity Germanium (HPGe) signals further allows to improve energy and timing resolutions to ~1 keV and 20 ns at 1 MeV, respectively. Within the GIANT setup, an average detector has a photopeak efficiency of $\overline{\epsilon_E}$ =0.11% and an energy resolution of $\overline{\sigma_E}$ = 0.8 keV at E=1000 keV. The overall performance of the GIANT setup at S$\mu$S allowed to start a rich user program with archaeological samples, Li-ion battery research, and collaboration with industry. Future improvements will include a simulation based analysis and a higher degree of automation, e.g. automatic scans of a series of muon momenta and automatic sample changing.