The HERA-B ring imaging Cherenkov system – design and performance

The Ring Imaging Cherenkov detector of the HERA-B experiment at DESY has been completed in January of 1999. Data in its final configuration were taken recently. The HERA-B RICH uses C4F10 as radiator gas and a large 24 m2 spherical mirror for imaging. The photon detector employs 2240 Hamamatsu multi-anode photomultipliers with a total of 27 000 channels. To match anode dimension (4.5 mm) and dispersion error and to solve the PMT packing problem we used a 2 : 1 reducing two-lens telescope in front of each PMT. The design performance of the RICH was fully reached: the average number of photons for a β=1 particle detected in the RICH was found to be 35.4; a single-photon resolution of 0.65 mrad was reached. We used stand-alone ring finding algorithms to measure the angle and β of tracks. With the position information of the target and that of clusters in the electromagnetic calorimeter we were able to identify electrons, pions, kaons and protons.