Performance of resistive plate counters with freonless gas mixtures for measurements of cosmic ray muons

Various operational features of bakelite Resistive Plate Counters with a bulk resistivity of 1012 Ω cm are studied with different argon, methane and isobutane mixtures, but avoiding freon as counter gas contribution. The efficiency and the time resolution of the detector as well as the cross-talk behavior of neighbored read-out pads are investigated for different voltages by use of cosmic ray muons. The signals of the detector are classified by number and shape of the afterpulses. Relative abundances of these classes are measured and the timely appearance of the afterpulses is investigated. An efficiency of more than 90% is found for argon–isobutane mixtures and less than 27% for argon–methane mixtures. The time resolution differs between 1.27 and 4.19 ns, depending on the kind of quenching gas. A strong asymmetry of the time resolution spectra of the various gas mixtures has been observed. The shape of the signals depends on the used quenching gas.