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In a previous work it bas been pointed out that a CCD readout systcrn associated to a microstructure based gaseous detector (microstrip, microgap, GEM, ctc.) can be used for non destructive testing OS these detectors. The choicc of the gas mixture is an important issue, in so far as its emission spectrum should overlap efficiently the scnsitivity region of the CCD (400-1100 nm). In the present work we repott on a systematic study for several gas mixtures which includes mcasuremenls of the total light yields as a function of the electric field and of the spectrometric distribution of the light emitted, in the wavelength region between 250 and 930 nm. Results are presented for pure argon and argon and xenon based gas mixtures. A comparison is made between the results obtained with the CCD coupled to a GEM detector and with a gaseous scintillation proportional counter.

We report on the optical readout of the gas electron multiplier (GEM) operated with a gaseous mixture suitable for the detection of thermal neutrons: 3 He-CF 4 . A CCD system operating in the 400-1000 nm band was used to collect the light. Spectroscopic data on the visible and NIR scintillation of He-CF 4 are presented. Images of the tracks of the proton and triton recorded with a triple GEM detector are also shown. r

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We report on the operation of a GEM-based small TPC using an optical readout. The detector was operated with a mixture of Ar+CF 4 using 5.48 MeV alpha particles obtained from a 241 Am source and the GEM scintillation was concurrently read by a CCD camera and a photomultiplier. Precision collimators were used to define the track orientation. Qualitative results on the accuracy of the track angle, length and charge deposition measurements are presented. r

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Secondary scintillation (defined here as photon emission originating from electron avalanches) was studied for two gaseous micropattern detectors: MSGC (MicroStrip Gas Chamber) and GEM (Gas Electron Multiplier) operated in pure CF 4 . For MSGC, the study was performed in the pressure range from 1 to 5 bar; for GEM all experiments were carried out at a fixed pressure of 1 bar. Charge gains from ∼10 to ∼150 were used in both cases. The primary ionization of the gas was produced by alpha particles from an Am-241 source. Emission spectra of the secondary scintillation were recorded in the wavelength range from 200 to 800 nm and corrected for the response of the detection system. Photon yields (number of photons generated per electron collected at MSGC or GEM) were measured for the integrated UV (200-500 nm) and visible (500-800 nm) emission bands. The obtained emission spectra and photon-per-electron ratios were compared to the corresponding data for the primary scintillation.

We present an overview of results from our recent studies on the use of the visible and NIR scintillation emitted by the gas electron multiplier (GEM) and on the possibility of using detectors operated with cascaded GEMs to build tracking chambers.