Performance of high rate MRPC with different gas mixtures

A Time Of Flight system (TOF) based on MRPC technology is widely used and plays an important role in modern high energy and nuclear physics experiments. With the increase of the energy and luminosity of the accelerators, many experiments have higher demand for a better resolution TOF detectors working in an high data rate environment, which undoubtedly represents a big challenge for this technology. FAIR-CBM is a typical high rate experiment and the particle rate of its TOF can reach 30 kHz/cm(2). A new type of MRPC, built with low resistive glass with bulk resistivity of about 10(10) Ohm cm, has been developed. The rate capability of this high rate MRPC can reach 70 kHz/cm(2). The standard gas mixture for the high rate MRPC used for the FAIR-CBM-TOF is 90% C2H2F4, 5% i-C4H10 and 5% SF6. For the high rate MRPC used for the STAR-eTOF the gas mixture contains only two components, namely 95% C2H2F4 and 5% i-C4H10. In order to ensure the long time stability of the high rate MRPC and its use with different gas mixtures, we have studied the performance with different gas mixtures. The first measurements were performed with the standard gas mixture and different gas mixtures were tested by reducing the content of SF6. Three high rate MRPCs were tested at the same time for a cross-check, using a cosmic ray test-stand. The result of the test shows that the working point of the high rate MRPC moves toward a lower value of the high voltage by reducing the SF6 content in the gas mixture. The working point for a chamber efficiency of 95% without SF6 is about 800 V lower than the one with the standard gas mixture. Other properties such as time resolution, noise rate, dark current and cluster size have also been measured with different gas mixtures.