The HERMES dual-radiator RICH detector 

H. E. Jackson

Argonne National Laboratory, Argonne, Illinois. 
On behalf of the HERMES Collaboration

Abstract

   A dual radiator Ring Imaging Cherenkov (RICH) detector for the identification of hadrons has been in operation as part of the HERMES spectrometer since 1998. The HERMES experiment emphasizes measurements of semi-inclusive deep-inelastic scattering. However, most of the hadrons produced lie between 2 and 10 GeV, a region in which it had not previously been feasible to separate pions, kaons, and protons with standard particle identification (PID) techniques. The recent development of new clear, large, homogeneous and hydrophobic silica aerogel material with a low index of refraction offered the means to apply RICH PID techniques to this difficult momentum region for the first time. The HERMES instrument uses two radiators, C4F10, a heavy fluorocarbon gas, and a wall of silica aerogel tiles. A lightweight spherical mirror constucted using a newly perfected technique to make resin-coated carbon-fiber surfaces of optical quality provides optical focusing on a photon detector consisting of 1934 photomultipilier tubes {PMT) for each detector half. The PMT array is held in a soft steel matrix to provide shielding against the residual field of the main spectrometer magnet. Ring reconstruction is accomplished with pattern recognition techniques based on a combination of inverse and direct ray tracing. The efficiencies and purities of hadron identification have been determined by using particles generated in the decay of independently identified mesons. Details of the detector design and performance will be presented.


RICH2002, Pylos, Greece