No abstract

The future Compressed Baryonic Matter experiment to be built at the Facility for Antiproton and Ion Research in Darmstadt, Germany, will create the highest particle densities ever created in a laboratory. One of its components, the Time-of-Flight Wall, will be comprised of Resistive Plate Chambers. The high particle fluxes expected at the most inner region, close to the beam pipe, have led to the research of new low-resistive materials to be used as electrodes. Si 3 N 4 /SiC composites are a very good candidate for this function. Their bulk resistivity, in the order of 10 9 -10 10 Ω cm, allows for increased rate capabilities up to 10 6 s −1 cm −2 . In this report, the properties of these new materials will be discussed. Also, the performance of resistive plate chambers with ceramic electrodes under irradiation in electron and proton beams will be presented.

The main goals of a new test facility at Helmholtz-Zentrum Dresden-Rossendorf are precision measurements of the electron drift velocity and the Townsend coefficient of gases at atmospheric pressure in the strongest ever used homogenous electrical fields and the search for new RPC gas mixtures to substitute the climate harmful Freon. Picosecond UV laser pulses were focused into a sub-millimeter gas gap to initialize a defined tiny charge. These gaps are formed by electrodes of low-resistive ceramics or high-resistive float glass. The charge multiplication occurs in a strong homogeneous electric field of up to 100 kV/cm. Electron-ion pairs were generated in a cylindrical micro-volume by multi-photon ionization. The laser-pulse repetition rate ranges from 1 Hz to a few kHz. The RPC time resolution has been measured for different gases. First results of the Townsend coefficient at 100 kV/cm show a strong disagreement between the present measurement and Magboltz simulations for the typical timing RPC gas mixture C2F4H2/SF6/i-C4H10, while the measured electron drift velocities are in a good agreement with the model predictions.

Multi-gap resistive plate chambers (RPCs) are proposed to build the Time-of-Flight wall of the Compressed Baryonic Matter experiment with a time resolution better than 80 ps. The high fluxes expected at the innermost part of the detector, ∼ 20 × 10 3 cm −2 s −1 require the development of new materials capable of withstanding such fluxes. At Helmholtz-Zentrum Dresden-Rossendorf, several RPC prototypes of 10×10 cm 2 and 20×20 cm 2 have been built with ceramic plates with bulk resistivities in the range of 10 9 -10 10 Ω cm [1]. They have been tested at the superconducting electron accelerator facility ELBE with 30 MeV electrons. We present the characteristics of the ceramic electrodes and the latest results concerning the performance of these prototypes in electron and protons beams up to fluxes of 10 6 cm −2 s −1 .