The performance and long term stability of an optically readout Time Projection Chamber with an electron amplification structure based on three Gas Electron Multipliers was studied. He/CF4 based gas mixtures were used in two different proportions (60/40 and 70/30) in a CYGNO prototype with 7 litres 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 during 25 days. The detector worked in very stable and safe condition for the whole period. In the presence of less CF4, a larger probability of unstable events was clearly detected.
@article{arxiv.2007.00608,
title = {Stability and detection performance of a GEM-based Optical Readout TPC with He/CF$_4$ gas mixtures},
author = {E. Baracchini and L. Benussi and S. Bianco and C. Capoccia and M. Caponero and G. Cavoto and A. Cortez and I. A. Costa and E. Di Marco and G. D'Imperio and G. Dho and F. Iacoangeli and G. Maccarrone and M. Marafini and G. Mazzitelli and A. Messina and R. A. Nobrega and A. Orlandi and E. Paoletti and L. Passamonti and F. Petrucci and D. Piccolo and D. Pierluigi and D. Pinci and F. Renga and F. Rosatelli and A. Russo and G. Saviano and S. Tomassini},
journal= {arXiv preprint arXiv:2007.00608},
year = {2020}
}