Performance

The system has been operating over one year. When we first installed the modules, we used inch diameter flexible polyolefin tubing from the gas distribution manifolds to the RPCs. After several weeks of operation, we noticed an increase in the dark current drawn by some of the RPCs and the corresponding decrease in efficiency. This was found to be due to water vapor in the air migrating through the tubing and entering the RPC active volume. Some of these tubes were as long as 12 m and we measured concentrations of O as high as 2000 ppm in some RPC exhaust lines. Approximately 50 % of the barrel modules were affected. The efficiency of some barrel RPCs dropped below 50 % before corrective measures were taken. We replaced the plastic tubing with copper tubing to prevent additional water vapor entering the RPCs. The contaminated RPCs eventually dried out and have recovered most of their lost efficiency. Other than the initial water vapor problem, which has been solved, the RPCs have been operated reliably and with an average efficiency of better than 97 %.