A fast multi-GEM-based detector for high-rate charged-particle triggering

In this paper, results of a time performance study of gas electron multiplier (GEM)-based detectors are discussed. This study was driven by an R&D activity on detectors for the Level 0 LHCb muon trigger. Results presented in this paper are of more general interest, i.e., for experiments in which high-rate charged-particle triggering is needed. Little interest was given so far to time performance of GEM-based detectors, with the exception of one paper reporting the measurement of a double-GEM detector time resolution with an Ar/CO₂ (70/30) gas mixture where the authors quoted a time resolution such that high-efficiency muon triggering at LHCb would be impossible. The results reported here, obtained with the addition of CF₄ and isobutane to the Ar/CO₂ standard mixture, considerably improve the time performance discussed in the above-mentioned paper, allowing one to reach a time distribution root mean square of 5 ns with an isobutane-based mixture. In these conditions, a spark probability per incoming hadron has been preliminary measured to be in the range 5·10^-12÷10^-11 at 95% confidence level. These facts make the triple-GEM detector a promising option for high-rate charged-particle triggering.