Fast triggering of high-rate charged particles with a triple-GEM detector

A 3 year long R&D activity on triple gas electron multiplier (GEM) detectors is reported. This activity was made in the framework of the LHCb experiment in order to find the technology to instrument the central region of the first muon station (M1R1) where a high particle rate is expected. Detector geometry, gas mixture and electric field configuration have been optimized in order to achieve the performance required by the experiment. The use of a very fast, CF//4 based, gas mixture provides a time resolution of about 4.5 ns (r.m.s.) with a single chamber with gain less than 10**4. In addition, an optimized gain sharing between the three GEMs allows to keep the discharge probability per incident hadron below $10^{-12}$. The average number of firing pads per crossing particle have been found to be lower than 1.2. In a global aging test two detectors were exposed to a dose rate of 16 Gy/h. Each detector integrated about 2 C/cm**2 equivalent to more than 10 years of operation at LHCb. Good aging properties were measured. These results make the triple-GEM detectors a good solution for M1R1 and, in general, for a fast trigger in the presence of a high rate of charged particles.