Diamond Start Detectors

In this paper we describe the operation principles and the in-beam performance of Start Detector (SD) assemblies consisting of various diamond detectors (DDs) and special Front End Electronics (FEE) adapted to the requirements of different applications. In the framework of the European Joint Research Activity NoRHDia (Novel Radiation Hard Diamond detectors for hadron research) we optimized different sensor and FEE designs, particularly aiming at the development of relativistic proton SDs for the GSI experiments FOPI (Four PI detector) and HADES (High Acceptance Di-electron Spectrometer) as well as for the upcoming FAIR experiment CBM (Compressed Baryonic Matter). The limiting parameter in minimum-ionizing particles (mip) is the high pair-production energy of diamond (?_r ~ 13eV/e-h pair), which leads to a rather small signal amplitude (a factor of ~2.5 smaller than the one of silicon sensors). We present the result from various SD assemblies obtained with relativistic heavy ions and protons. An excellent heavy-ion time resolution (?_i <30ps) was obtained using the broadband preamplifier-discriminator plate FEE-1 with both, polycrystalline (pc) and single crystal (sc) devices. Good results are obtained with PADI-1, a preamplifier-discriminator ASIC designed for timing RPCs. In contrast, due to the small signal amplitude in the case of relativistic protons a more elaborated solution is needed to increase the S/N ratio and to improve the time resolution by the decrease of the total capacitance of the detector-FEE ensemble. The segmentation of the detector area and hence lowering of the input capacitance of the amplifier led to a proton time resolution of ?_i =100ps. For this application a microwave BJT in common-emitter configuration was used for the FEE, which was mounted near the detecting sector of the DD. For micro-patterned DD applications the PADI-4 ASIC was designed, a four-channels variant of PADI-1 optimized to match low capac- - itance detectors.