P. Fauland

University of Bielefeld, Bielefeld, Germany
on behalf of the COMPASS RICH group

   COMPASS RICH-1 is a large acceptance gaseous RICH using 3 m of C4F10 as radiator and MWPCs with CsI photocathodes as VUV Photon Detectors (PD), with a total active surface of about 5.5 m2. The focusing onto the photon detectors, which are placed outside the COMPASS spectrometer acceptance, is obtained with two spherical mirror surfaces (total area: ~20 m2 with focal length of 3.3 m).
   The choice of VUV PDs has allowed to instrument a large detection surface, but has imposed to match all the requirements related to the useful photon wavelength region (from 165 to 200 nm). In particular, the VUV radiator transmission is a critical parameter: it can only be obtained by ensuring very low concentration of UV-light absorbing impurities in the gas. This is achieved via (j) good level of tightness of the radiator vessel and (ii) removal of impurities present in the radiator. The vessel has been design to get good tightness; it is checked locally with He-leak detection system and the global leakage rate has been measured to be ~3 x 10-2 mbar x l/s. The radiator gas is continuously filtered through purifiers during operation using a closed re-circulation system (similar to others already in operation: Hera-B, HARP) which also ensures the control of the pressure inside the vessel: only few mbar pressure difference between the two faces of the quartz plates separating the vessel and the photon detectors volume is allowed to avoid mechanical stress. With its refrigeration and storage capabilities the gas system allows, in addition, a separation of C4F10 from N2 which is needed in the filling and emptying processes of the vessel. It was necessary to pre-clean the radiator gas in dedicated pre-cleaning installations until a transmission of 75% at 165 nm through 35 mm of liquid (corresponding to 5 m of gas at atmospheric pressure) is obtained. The gas transparency can be measured in a 2,87 m long cell on-line, performing an integral measurement with a spectral response matching the effective quantum efficiency of CsI photo-cathode and quartz window with a system equipped with a deuterium lamp and a solar-blind-PM. A second set-up with monochromator for differential measurements is in preparation.
   The description of the vessel design, of the different gas systems (radiator gas system, pre-cleaning installations) and of the transmission measurement setups is given, as well as results performances obtained so far.