CMS

The CMS barrel muon detection system is based on 4 concentric shells of wire drift chambers (drift tubes, DT), called MB1 (the innermost), MB2, MB3 and MB4 (the outermost). Longitudinally they are arranged in five rings about 2.5 m wide; within each wheel the chambers are divided in 12 azimuthal sectors. The chambers are made up of three independent units, called Super Layers (SL), each one composed of four layers of drift tubes with parallel wires. Two SLs measure the coordinate in the bending plane, the third one (not present in MB1) the one along the beam direction. Within a SL, even layers are staggered by half a cell with respect to odd layers. The single tube consists of a central anode wire and two C-shaped cathodes deposited on the aluminium bars (and called I-beams) that separate neighbouring cells. Two strip electrodes above and below the wire improve the uniformity of the electric field over all the drift lenght. The gas mixture is made of Ar(85%)+CO2(15%). The nominal voltages are Vwire=3600 V, Vstrip=1800 V and Vcath=1200 V.