Photon Detector Performance and Radiator Scintillation in the HADES RICH ⋆ R. Gernh¨auser, B.Bauer, J. Friese, J. Homolka, A. Kastenm¨ uller, P. Kienle, H.-J.K¨orner,P. Maier-Komor, M. M¨ unch, R. Schneider, K. Zeitelhack. For the HADES collaboration Physik-Department E12, TU M¨ unchen, 85748 Garching, Germany We report the performance of a VUV-sensitive MWPC with a pad cathode covered by a solid CsI photon converter. Cherenkov photons produced by relativistic 12 C-ions in a MgF 2 radiator have been detected with 95% single photo electron efficiency. In the HADES design a resulting figure of merit N 0 > 66 promises proper operation of the RICH as a fast hadron blind trigger device. In addition, we have investigated the scintillation properties of several fluoro-carbon radiator gases in the VUV wavelength region. A significant emission around λ ∼ 170 nm is found for CF 4 . 1 Introduction In the dilepton spectrometer HADES [1], presently under construction, e + e - -pairs produced in heavy ion collisions will be investigated for pro- jectile energies up to E = 2 A·GeV at the heavy ion accelerator facility GSI (Darmstadt). A fast Ring Imag- ing CHerenkov (RICH) detector sur- rounding the target is designed to act as a hadron blind trigger de- vice. Leptons emitted from central collisions will produce Cherenkov radiation when passing a gaseous C 4 F 10 radiator, while hadrons have ⋆ Supported by BMBF and GSI. velocities far below the threshold. As shown in Fig.1 the emitted radia- tion is reflected by a spherical mirror and focused to rings onto a position sensitive photon detector with CaF 2 entrance window. To match the ex- pected collision rates of up to 10 6 s -1 , this device is presently designed as a sixfold segmented, fast Multi Wire Proportional Chamber (MWPC) with a solid photon converter evap- orated onto the cathode pad plane. Reflective layers of CsI have proven to allow for efficient photon conver- sion in the VUV wavelength region even in the presence of standard detector gases [2][3][4]. A detailed description of the whole HADES project and its detector components Preprint submitted to Elsevier Preprint 8 August 1995