A high resolution TPC based on GEM optical readout

The R&D work of a gas detector for high precision particle tracking over large gas volumes is presented. The scintillation light accompanying the electronic avalanches in a triple GEM structure is detected by a CMOS-based camera. The sensor provides a high granularity along with low noise and a very high sensitivity (70% of quantum efficiency). Once operated with a large aperture and suitable focal length lens, large areas can be imaged at reduced costs. The performance of a prototype (called LEMOn), with a 7 litre sensitive volume, operated with a He/CF 4(60/40) mixture was studied on a 450 MeV electron beam and preliminary results are presented. A space resolution on the plane parallel to the GEM $(X, Y)$ of $90 \mu m$ was achieved in the whole sensitive volume. The effect of the electron diffusion on the drift was studied and exploited in order to evaluate the position in $Z$ of the tracks. An innovative element is the concurrent readout of the light with a suitable photomultiplier system. It will complement the CMOS-readout by providing the time resolution necessary to the reconstruction of third coordinate of each cluster. By exploiting the time information, a resolution of $60 \mu m$ on $Z$ was measured. Such a detector can be an interesting candidate for future large scale experiments searching for Dark Matter (DM) searches with directional sensitivity and for measurements of coherent neutrino scattering on nuclei. Additional applications of this detector might be in the realm of neutron detection, X-ray polarimetry and particle cancer therapy.