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 Time Projection Chambers with an electron amplification structure based on three Gas Electron Multipliers was studied. He/CF$_4$ based gas mixtures were used in two different proportions (60/40 and 70/30) in a CYGNO prototype with 7 litre sensitive volume. With electrical configurations providing very similar electron gains, an almost full detection efficiency in the whole detector volume was found with both mixtures, while a light yield about 20\% larger for the 60/40 was found. The electrostatic stability was tested by monitoring voltages and currents %provided by the high voltage system for 25 days. The detector worked in very stable and safe condition for the whole period. Anyway, in the presence of less CF$_4$, a larger probability of unstable events was clearly detected.