Validation and performance of the CMS barrel Muon drift chambers with cosmic rays

The production of the 250 muon barrel Drift Tube chambers for the CMS experiment at the LHC has started in 2001. In each assembly site the produced chambers equipped with the final front-end electronics and the Low and High Voltage boards are checked in a cosmic rays facility. While the wire position is measured and recorded at construction time, all electrical connections and the quality and performance of the chambers and of the local trigger devices can only 9be checked with real tracks. The methods used and the behavior of the first 22 chambers built in Legnaro (Padova) INFN National Lab (LNL) are reported and compared with Test Beam results. The mean noise level at nominal conditions is 20Hz/m/wire, with an RMS of 5Hz/m/wire. The detection efficiency of each layer is above 98%. The wires with noise above 2 S.D. is less than 0.1% of the total. Due to HV problems the number of wire not connected is less than 0.1%. The wire position accuracy is better than 70mm. The resolution is 200-250mm per layer.