Stability and detection performance of a GEM-based Optical Readout TPC with He/CF$_4$ gas mixtures

The performance and long term stability of an optically readout TimeProjection Chamber with an electron amplification structure based on three GasElectron Multipliers was studied. He/CF$_4$ based gas mixtures were used in twodifferent proportions (60/40 and 70/30) in a CYGNO prototype with 7 litressensitive volume. With electrical configurations providing very similarelectron gains, an almost full detection efficiency in the whole detectorvolume was found with both mixtures, while a light yield about 20% larger forthe 60/40 was found. The electrostatic stability was tested by monitoringvoltages and currents during 25 days. The detector worked in very stable andsafe condition for the whole period. In the presence of less CF$_4$, a largerprobability of unstable events was clearly detected.