S. Costa

University of Torino and Sezione INFN di Torino, Torino, Italy
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. Two spherical reflecting surfaces (total area of about 20 m2, focal length 3.3 m) are tilted with respect to the detector (beam) axis in order to focalise the ring images on two photon detector planes placed outside the COMPASS spectrometer acceptance. The reflecting surfaces are formed by 116 individual mirror segments, combined in a mosaic type composition. To allow the achievement of continuous spherical surfaces using separated spherical mirror units, the mechanical support of each mirror piece allows for individual angular alignment with a resolution of 0.1 mrad.
   Among the most relevant characteristics of the RICH detector, there are the large acceptance (resulting in very large transversal dimensions) and the use of VUV photon detectors. These features have dictated the main parameters of the optical system, namely its large size and the request of a very high reflectance (~80%) for wavelengths down to 160 nm.
   In the COMPASS spectrometer, electromagnetic and hadron calorimeters are placed downstream of RICH-1; this results in the request to minimise the mirror wall material. The resulting achievements are 5.5% radiation length for the mirror substrate and 2.5% equivalent for the supporting mechanical structure.
   The detailed description of the project is given, as well as the result of the mirror characterisation, the mirror alignment procedure and performances. Some information about the mirror prototyping activity is also presented.