Testing of triple GEM prototypes for the CBM Muon Chamber system in the mCBM experiment at the SIS18 facility of GSI

Gaseous tracking chambers based on the Gas Electron Multiplier (GEM) technology will be used for the first two stations of the Muon Chamber system of the Compressed Baryonic Matter (CBM) experiment at FAIR . Large size trapezoidal modules each having 2D pad-readout with $\sim 2.2$k pads were designed and fabricated for data taking in the mCBM (mini-CBM) experiment set-up at the SIS18 facility of GSI as a part of the FAIR Phase-0 program. In this contribution, we report on the testing of two such GEM modules within nucleus $+$ nucleus collisions at 1 - 2A GeV beam energies in the mCBM experiment. Data with the first version of a self-triggered readout ASIC has been used for the first time. The response of the GEM modules in terms of spill structure, cluster-size characteristics, time correlation between detector sub-systems, among others, have been studied. Preliminary results obtained during the mCBM data campaign in March 2019, displaying the detector characteristics, which include time-based event reconstruction, are discussed.