The resistive plate chamber system for the CMS experiment at LHC

The resistive plate chambers system for the CMS experiment at LHC will be one of the largest, most complex and elaborated apparatus ever built, based on RPCs. It involves the construction of more than 1000 chambers of different types, distributed both in the barrel and the endcap of the detector, to be individually tested to guarantee they can meet the necessary requirements during at least a ten year period in a hostile environment, equipped with a sophisticated front-end electronics and whose data flow has to be carefully processed and integrated with the one coming from the rest of CMS. It will have the crucial role of providing a first level muon trigger and identification, unambiguous bunch crossing assignment, and, via also dedicated track finder algorithm, a cut on particle transverse momentum even at this stage. The long series of tests and trials performed, before starting the construction, to decide the best possible configuration, among single, double or multigap design, narrow or wide gap, different possible gas mixtures to be employed, is briefly outlined. In these tests, great care was put to find the right solutions so that the chambers will not suffer any significant degradation due, directly, to high particle fluxes, or, indirectly, to gas impurities produced by the discharges in the gas gap. Here an overview of the system is presented; the new technical solutions adopted during the different stages of the chamber construction, starting from bakelite procurement, to single, double gap and chamber assembly, is described. To assure elevate standards and uniformity in the whole set of produced chamber, multistage-quality control procedures, for instance on mechanical robustness or current stability, have been elaborated, and, after final assembly, chambers are furthermore tested with cosmic rays, to characterize them in terms of efficiency and operating plateau. The chambers are then sent to CERN/ISR where additional tests are performed, they are installed together with the other detectors of the muon system, and the commissioning phase, which just started, can take place. Results and statistics on all the relevant parameter measured on the chambers, will be presented, and some conclusion about the optimal way to operate the system durin- g CMS run will be drawn. Experience gained facing the problems encountered during all construction steps, which began three years ago and is still going on, will be precious for future experiments.