We report on the design and commissioning of a new spectrometer for muon-spin relaxation/rotation studies installed at the Swiss Muon Source (SμS) of the Paul Scherrer Institute (PSI, Switzerland). This new instrument is essentially a new design and replaces the old general-purpose surface-muon (GPS) instrument that has been for long the workhorse of the μSR user facility at PSI. By making use of muon and positron detectors made of plastic scintillators read out by silicon photomultipliers, 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 μSR 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 with the old GPS.
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As the muon events are Poisson distributed, note that to maintain the pile-up events at the same level as in the case of a 10 μs time-window, an incoming-muon rate of 140 kHz is in principle allowed with a 2.5 μs time-window. In reality a reduction to about 100 kHz is necessary to maintain the veto-event rate in the pyramid below about 1 MHz, which represents the limit per channel for the TDC.
