Determination of the efficiency of an aerogel Cerenkov-detector

The CB-TAPS experiment at ELSA in Bonn is optimized for measuring neutral, photonic final states. The identification of charged particles is normally done by measuring time-of-flight and energy loss. Already at moderately high particle energies the discrimination becomes impossible. Using the Cerenkov-effect is a good method to improve the identification of the particles. Aerogel is a very appropriate radiator-material due to its very low density, its high transparency and its refraction-index of about n ∼ 1.05. Exploiting the Cerenkov-effect on aerogel, electrons and most charged pions can be discriminated from protons. Therefore a diffusively reflecting aerogel Cerenkov-detector was built. Its performance was tested at the ELSA accelerator in Bonn with an electron-beam. In addition a test with a 1.1 GeV π--beam at GSI, Darmstadt, was performed. Furthermore, the aerogel Cerenkov-detector has been employed in an experiment at ELSA searching for ω-mesic states. Results of these measurements are presented