The drift tube (DT) system measures and identifies muon tracks in the barrel part of the detector. Each 4cm-wide tube contains a stretched wire within a gas volume. When a muon or any charged particle passes through the volume, it knocks electrons off the atoms of the gas. These electrons “drift” towards the anode following the electric field, ending up at the positively-charged wire where they are amplified and produce a measurable charge pulse. Each unit or “superlayer” contains four layers of staggered drift cells.

By registering the time taken by the electrons to reach the wire, and since the drift velocity along the cell is a rather constant value, we can identify the exact crossing point of the muon along the cell. Each superlayer provides 4 points in 2D for the muons position.

Sketch of the Drift Tube cell

The sizes of DT chambers range from  2m x 2.5m to 4m x 2.5m, approximately. Each chamber consists of 8 or 12 aluminium layers, arranged in two or three superlayers, each up with up to 90 tubes: the middle superlayer measures the coordinate along the direction parallel to the beam and the two outside superlayers measure the perpendicular coordinate, thus providing a combined 3D measurement of the muon track.

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