Design and construction of the triple GEM detector for TOTEM

We describe the design and construction of the first triple-GEM chamber prototypes for the TOTEM detector at CERN LHC. The chambers are semicircular, with an inner and outer radius of the active area of about 40 and 150 mm; six to eight detectors will be mounted on each arm. Each chamber has analogue readout of the charge on concentric circular strips, at 400 /spl mu/m pitch, to obtain the radial coordinates, and about 1000 pads of sizes between 3/spl times/3 and 7/spl times/7 mm/sup 2/ with digital readout, used to generate the triggering in combination with the other chambers.