Performance of the quadri-gap LHCb Muon Chambers studied with cosmic rays

The LHCb muon system is composed of five detection stations (M1-M5) mainly equipped with a total of 1368 multi-wire proportional chambers (MWPC). In order to allow a fast evaluation of the transverse momentum of muons, the MWPC have to provide a high detection efficiency, a fast response and a good space resolution. We present the results of a detailed study of the performance of different chambers, completely equipped with the final front-end electronics, measured with cosmic rays. A stand able to house 6 MWPCs and to acquire 600 TDC channels a time was used in order to measure the detection efficiency, the time resolution and the hit multiplicity as a function of the gas gain. For the first time, both the chambers with anode-wire-readout and chambers with cathode-pad-readout were tested and the results are compared. The large amount of acquired data results to be very useful to get a better knowledge of the behaviour of the chambers allowing to individuate the optimised working conditions of the apparatus and to make a realistic detector description in the LHCb Monte Carlo simulation.