Operation of a triple GEM detector with CsI photocathode in pure CF4

Results obtained with a triple GEM detector operated in pure CF₄ with and without a reflective CsI photocathode are presented. The detector operates in a stable mode at gains up to 10⁴. A deviation from exponential growth starts to develop when the total charge exceeds ∼ 4 × 10⁶ e leading to gain saturation when the total charge is ∼ 2 × 10⁷ e and making the structure relatively robust against discharges. No aging effects are observed in the GEM foils after a total accumulated charge of ∼ 10 mC/cm² at the anode. The ion back-flow current to the reflective photocathode is comparable to the electron current to the anode. However, no significant degradation of the CA photocathode is observed for a total ion back-flow charge of ∼ 7 mC/cm².