Study of Resistive Plate Chamber with high rate capability

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A resistive plate chamber (RPC) is one of detectors of charged particles. In Japan, it was the first time that the large RPCs system was used to detect KL and muons in BELLE experiment at KEK B-factory. Those RPCs were made from glass. A gas mixture for BELLE experiment is 30% Ar, 8 % Butane, 62% freon (134a) as a standard gas mixture. The RPCs have been operated in streamer mode with many advantages, however a rate capability is worse as an efficiency of detecting charge particles is less than 90% at about 0.3Hz/cm^2. In BELLE, as an increasing of beam currents and luminosity, the RPCs of outer-layer at endcap have been affected by background of ~ 1Hz/cm^2 due to neutrons from beam tunnel, so that the efficiency drooped to about 60% from 98%. A purpose of this study is to improve the rate capability.We tested a performance of 10 glass RPCs by measuring cosmic rays, when single count rates of RPCs were changed manually by using ^<60>Co. We found a relation between the efficiency and single count rate to be a formula, η= 1-τ・f, where η is an efficiency, τ an effective dead time (s・cm^2), f a single count rate (Hz/cm^2). If τ is smaller, η will be higher even if the background rate (f) is higher. The τ is 0.47 (s・cm^2) for the standard gas mixture. As a result of studying the gas mixture, the τ becomes 0.15 (s・cm^2) for the gas mixture of 50% Ar, 8% Butane, 37% freon, 5% SF_6. This is three times better improvement in the effective dead time. If we supply high voltage to keep plateau efficiency, the τ becomes 0.05 (s・ cm^2) which is ~ 10 times better. This improvements is the efficiency to be ~ 95 % at 1Hz/cm^2.These results were presented at JPS meeting in Sept. 2004, International conference of IEEE2004 in Oct.2004.