Operation of microstrip gas counters with DME-based gas mixtures

Charged particle detection and track reconstruction in high energy physics experiments, that have to cope with high particle rates, is a challenge requiring a careful detector optimization. The choice of the gas mixture plays a major role in gaseous detectors and recently, it has been subject to intensive studies in the case of micro strip gas counters (MSGCs). Due to its high primary and secondary ionization, dimethylether (DME) is often envisaged for MSGC operation. Addition of a noble gas leading to large gas gain, like neon, has the advantage of reducing the cathode strip voltage at which full efficiency is reached, keeping a long efficiency plateau. An efficiency plateau starting at low cathode voltage is an advantage as the risk of discharge and the energy dissipated in a discharge both increase with the cathode voltage. A long efficiency plateau is important to maintain a good performance of the detector after a long exposure to high radiation levels. However we present measurements of the electron drift velocity in these Ne/DME mixtures and show it is too low for operation at high rate experiments. In addition the price of neon is higher than that of other commonly used gases. Therefore we investigated the possibility to replace neon by helium or argon and to add CO2 as it is well known for its high electron drift velocity. A detailed description of this study is given in our paper published in Nucl. Instr. and Meth. A 413 (1998) 105-118. For these gases we present measurements of signal amplitude and detection efficiency for minimum ionizing particles as well as measurements of the drift velocity and transverse diffusion. From these results, it appears that Ne/DME/CO2 gas mixture seems to be a good step to an ideal gas for high rate operations. (author)