Testing of triple-GEM chambers for CBM experiment at FAIR using self-triggered readout electronics

Triple GEM chamber has been selected to be used for the muon detection system (MUCH) in the Compressed Baryonic Matter (CBM) experiment at the upcoming Facility for the Antiproton and Ion Research (FAIR). Two prototype triple-GEM chambers filled with the mixture of argon and CO2 gases in the ratio of 70:30 were made of small-size (10cm×10cm) single-mask GEM foils and read by a self-triggered readout ASIC called nXYTER have been tested with pion beams at CERN. The readout electronics records the arrival times of GEM hits and of corresponding coincidence trigger signals separately. The distribution of time difference between GEM hits and respective triggers shows a peak demonstrating the time correlation between GEM hits and the trigger. The width (σ) of the time-correlation peak, which is related to the time resolution of the chamber, reduces with voltage approaching ≈12ns at ΔVGEM of 335V. Considering the hits inside the time-correlation peak to correspond to the beam trigger, the efficiency reaches a plateau at 95% at ΔGEM above 330V. For a readout plane segmented by 3mm×3mm pads, on an average 1.2 pads are fired per trigger for pion beams at the operating voltage. The chamber shows a gain of ≈4000 at ΔVGEM of 350V.