A novel VME based μSR data acquisition system at PSI

A new data acquisition system (DAQ) for bulk μSR and low-energy μSR (LE-μSR) has been developed at PSI. It is based on commercial and in-house VME modules, and on the MIDAS DAQ software library. The system is able to cope with the different needs of the various PSI μSR spectrometers, which was not possible with the existing CAMAC and ORTEC's pTA-clock based DAQ systems. The VME clock is a 64-channel CAEN V1190 TDC, using the CERN HPTDC chip, with programmable time resolution of 25, 100, 200 or 800 ps. The TDC onboard memory is continuously read through the VME bus by standard PC's or dedicated servers, using a 1-Gbit/s SIS3100/1100 VME-PCI interface. Detector rates are independently monitored using a 32-channel SIS3820 scaler module. In-house developed modules comprise an 8-channel constant fraction discriminator CFD950, a CD950 clock divider, an 8-channel linear fan-out SP950, a 16-channel NIM-ECL level converter LC950, and a programmable coincidence module FC950. All modules feature a superior performance compared to commercially available devices which allows their use also in the planned high-field μSR spectrometer where a time resolution of about 100 ps is envisaged. A special external hardware logic is no longer required, since the system can deal with a rate total of 5 MHz which is sufficient for μSR spectrometers with high event rate in their active veto systems.