Prototype tests for a DIRC detector for the WASA-at-COSY experiment

The WASA-at-COSY experiment allows the study of production and decay of η and η′ mesons in proton-proton reactions. At the moment the Forward Range Hodoscope (FRH) in the forward angle spectrometer region determines the identity of the particles by measuring the energy loss ΔE−E. Simulations concerning the estimated background have shown that an additional ring imaging Cherenkov detector in front of the FRH would significantly improve the particle identification and the energy resolution as well. Due to the very limited space available at the intended detector position, the development of a DIRC (Detection of Internally Reflected Cherenkov light) detection system based on Plexiglas (PMMA) radiators is under discussion. In order to show the feasibility, two different prototypes, consisting of a square PMMA radiator bar and a PMMA focusing element were tested using the COSY proton beam. One focusing element is based on an internal reflecting polynomial shape surface, the other is based on a mirrored circular shape surface. The photon readout in the focal plane is done by Hamamatsu H8500C 64 channel multianode photomultipliertubes (MAPMT). In addition, new MAPMTs with higher quantum efficiency were studied in order to increase the number of detected photons.