Performance of fast high-resolution Muon drift tube chambers for LHC upgrades

Monitored drift tube chambers are used as precision tracking detectors in the muon spectrometer of the ATLAS experiment at the LHC at CERN. These chambers provide a spatial resolution of 35 mm and a tracking efficiency of close to 100 % up to background rates of 0.5 kHz/cm², the former being limited at higher rates mainly due to space charge effects and the latter due to the maximum drift time of 700 ns. For LHC upgrades, a faster drift tube chamber has been developed, using drift tubes with a diameter of 15mm instead of 30 mm. The increased channel density and shorter drift time of about 200 ns raise the rate capability to about 10 kHz/cm², while retaining the spatial resolution. A prototype chamber with trapezoidal shape consisting of 2×8 layers of 15mm diameter drift tubes with an active surface of 0.8 m² has been constructed. This chamber has been tested at CERN with a 180 GeV muon beam (H8) and with cosmic ray muons at the Gamma Irradiation Facility (GIF) at high γ radiation rates.