Wing Sheung Chan

40 The Large Hadron Collider and the ATLAS detector 2.2.5. The muon spectrometer The outermost subsystem, which also defines the outline and dimensions of the ATLAS detector, is the muon spectrometer (MS). While most particles typically deposit all of their energy into the calorimeters, and are therefore contained, muons are an exception. At relativistic speed, muons are minimally ionising. As a result, the calorimeters are mostly transparent to muons produced from hard-scattering processes. Therefore, having detectors built outside of the calorimeters are very useful for identifying and reconstructing muons. The MS is built exactly for this reason. The MS consists of two types of chambers: tracking chambers that perform precision tracking and trigger chambers that provide rapid read-out. An overview of the MS is shown in Figure 2.7. Figure 2.7.: A computer-generated cutaway view of the ATLAS muon spectrometer [67] . Tracking chambers The tracking chambers enable precision tracking of muons over the range | η | < 2 . 7 . They make use of monitored drift tubes (MDT) and cathode strip chambers (CSC). The MDTs are pressurised drift tubes filled with 93% argon and 7% carbon dioxide. Tungsten-rhenium wires are used as the anodes of the MDTs. MDTs are used in most part of the tracking chambers. In the barrel region, three layers of MDTs are situated radially 5 m , 7 . 5 m and 10 m away from the beam axis. In the end-cap region, MDTs are emplaced on four wheels that are located 7 . 4 m , 10 . 8 m , 14 m and 21 . 5 m away from the nominal collision point. The MDTs have a limited counting rate above which they could be damaged. In the forward region where | η | > 2 , the expected particle flux is high enough to exceed such a limit. Therefore, the MDTs on the innermost wheel in the region | η | > 2 are replaced