The Gas Electron Multiplier, developed by Sauli at CERN, consists of a thin (50 mm) kapton foil, copper clad on each side, chemically perforated by a high density of holes having bi-conical structure with external (internal) diameter of 70 mm (50 mm) and a pitch of 140 mm. Under a suitable voltage application, 400-500 V, between the two GEM electrodes, electric fields up to 100 kV/cm are reached into the hole which acts as an electron
multiplication channel for the ionisation released by the radiation in the gas.

We report the results of a systematic study of the time performance, efficiency and aging of triple-GEM detectors operated with several gas mixtures which are new for this kind of detectors.
The detector has been tested with a high intensity (up to 60 MHz/cm2) 5.9 keV X-ray tube, with 3-10 GeV pion beam at CERN and with a high intensity (up to 30 MHz/cm2)low energy hadron beam at PSI.
In safe condition, gas gain up to 104-105, time resolution of about 5 ns, high rate capability and very good radiation hard have been observed.
Various detector geometries and maximum detector size of the order 30x50 cm2, characterized by low material budget(down to 1-2%), can be easily realized, making this detector suitable for many applications.