The OPERA spectrometer RPC system

OPERA is part of CNGS project and it is an experiment dedicated to the observation of long-baseline ν_μ into ν_τ oscillations through τ appearance. Resistive plate chambers, RPCs, with bakelite electrodes are used for instrumenting the 2 cm gaps between the magnetized iron slabs of the two spectrometers. The RPCs installation in the first spectrometer ended in May 2004. Before the installation, every RPC is subject to a complete test chain for rejecting the worst quality detectors. The tests are performed in dedicated facilities for ensuring the proper RPC gluing, for measuring its electrical properties and for verifying the response to cosmic rays and the intrinsic noisiness. We have also tested the long term stability of real size OPERA RPC prototypes operated at cosmic ray fluxes for more than one year, which is equivalent, in terms of detector´s countings, to about 20 OPERA years. On small size prototypes we are performing studies on the gas mixtures in order to reduce the total charge released in the gas for each detector count. The validation of the installed RPCs has been performed with pure nitrogen. A small part of them has been also tested with the gas mixture Ar/C₂H₂F₄/i-C₄H₁₀/SF₆ = 75.4/20/4/0.6.