The new versatile general purpose surface-muon instrument (GPS) based on silicon photomultipliers for $μ$SR measurements on a continuous-wave beam
We report on the design and commissioning of a new spectrometer for muon-spin relaxation/rotation studies installed at the Swiss Muon Source (S$μ$S) of the Paul Scherrer Institute (PSI, Switzerland). This new instrument is essentially a new design and replaces the old general-purpose surface-muon instrument (GPS) which has been for long the workhorse of the $μ$SR user facility at PSI. By making use of muon and positron detectors made of plastic scintillators read out by silicon photomultipliers (SiPMs), a time resolution of the complete instrument of about 160 ps (standard deviation) could be achieved. In addition, the absence of light guides, which are needed in traditionally built $μ$SR instrument to deliver the scintillation light to photomultiplier tubes located outside magnetic fields applied, allowed us to design a compact instrument with a detector set covering an increased solid angle compared to the old GPS.