Performance of the ATLAS Precision Muon Chambers under LHC Operating Conditions

Abstract For the muon spectrometer of the ATLAS detector at the Large Hadron Collider (LHC), large drift chambers consisting of 6–8 layers of pressurized drift tubes are used for precision tracking covering an active area of 5000 m 2 in the toroidal field of superconducting air-core magnets. The chambers have to provide a spatial resolution of 41 μm with Ar:CO 2 (93:7) gas mixture at an absolute pressure of 3 bar and gas gain of 2×10 4 . The environment in which the chambers will be operated is characterized by high neutron and γ background with counting rates of up to 100 s −1 cm −2 . The resolution and efficiency of a chamber from the serial production for ATLAS has been investigated in a 100 GeV muon beam at photon irradiation rates as expected during LHC operation. A silicon strip detector telescope was used as external reference in the beam. The spatial resolution of a chamber is degraded by 4 μm at the highest background rate. The detection efficiency of the drift tubes is unchanged under irradiation. A tracking efficiency of 98% at the highest rates has been demonstrated.