Long electron drift in TMAE-doped gas in the Barrel RICH detector

The DELPHI Barrel RICH (ring-imaging Cherenkov) detector is based on the principle of the drift of single electrons over distances up to 1.5 m in TPC-like drift tubes made of quartz. The drift is performed in a longitudinal electric field of 0.53 kV/cm and in a gas mixture of 80% CH4, 20% C2H6 and ≅ 1000 ppm of TMAE as photosensitive additive. The electron loss during the drift has been measured for different TMAE concentrations and different TMAE qualities. It will be shown that an absorption length λ>10 m can be obtained even for high TMAE concentrations. This could only be achieved by taking extreme care in the purity of the TMAE and the drift gas. We will also report on the systematic errors in the reconstruction of the photon conversion point due to distortions in the electron drift. Small focusing effects have been observed close to the walls. (orig.)