A novel idea for an ultra-light cylindrical GEM based vertex detector

An ultra-light cylindrical triple-GEM detector for vertex purposes is being developed. The cathode, the three GEMs and the anode are five concentric cylinders, obtained winding parallelogram-shaped foils, with a resulting helicoidal junction overlap ($<$3 mm wide). The result is a wholly cylindrical GEM, almost free of dead-zones and without support frames inside the active area, fruitfully usable as a vertex detector. The detetector is also extremely light: considering that the material used is mainly kapton, and the possibility to reduce the copper layer thickness to 2$\mu$m, a single tracking layer can have a material budget as low as 0.2\% of X$_0$. The readout of the detector is performed with large stereo angle U-V strip layers: one directly realized on the anode electrode, and the other onto the bottom side of the third GEM foil. Considering a large stereo angle of about 40 degrees and a strip pitch of 400$\mu$m, the spatial resolutions achievable are $\sigma$$_{r\phi} \simeq \sigma_z \sim 200 \mu$m. In order to avoid ambiguity in the position reconstruction the right strips crossing angle, corresponding to the winding angle, must be tg$\theta$ = L/$\pi$R (where L is the lenght of the vertex detector and R its radius). The mechanical assembly of one single layer is described and preliminary results obtained with an Ar/CO$_2$ = 70/30 gas mixture are discussed.