Gas mixture dependence of the performance for multigap resistive plate chambers in the avalanche mode

We have investigated the performance of prototype multigap resistive plate chambers in the avalanche mode using cosmic ray muons for various gas mixtures. In the operational high-voltage plateau region, the efficiency is larger than 90%, the best time resolution is 123 ps, and the fraction of the non-Gaussian tail in the time spectrum is about 15%. The slewing corrections significantly improve the time resolution by about 40% at 6.4 kV, which is the operational high voltage. Although the addition of SF6 to the gas mixture is necessary for avalanche mode operation, a relatively small fraction is preferrable for a better time resolution. The present results demonstrate that the current multigap resistive plate chambers can be used for charged particle identification at high momentum in real collision experiments.