A New Type of Spark-protected Parallel Mesh Chamber

We developed a very low resistivity RPC-type detector, the anode of which was a plate made from materials with resistivity up to 5x107 Ωcm, the cathode being a metallic mesh preceded by a drift region. In such detector it was actually possible to combine the versatility and high counting-rate capability of metallic PPACs with the extreme robustness and "protectiveness" of Resistive Plate Chambers. Occasional discharges triggered by large deposits of primary ionisation or by extreme counting rates are quenched by the resistive anode and are constrained to the glow discharge phase of the sparking process. The study shows that this discharge affects the detector only locally and the charge released is limited to a few tens of nC. Proportional counting rates up to 105 Hz/mm2 were achieved at gains above 104. The energy resolution at 6 keV was 20% FWHM. The observed gain-rate trade-off is well described by an analytic model and further improvements may be expected by lowering the resistivity of the anode material. The properties of several custom-made, controllable resistivity, anode materials are described and perspectives of improvement in the performance of the detector are discussed.