Optimization of the ATLAS muon drift-tube chambers at high background rates and in magnetic fields

In the ATLAS muon spectrometer, large drift-tube chambers are used for precision tracking. These chambers will he operated at a high neutron and γ background resulting in count rates of up to 500 counts s^-1 cm^-2 corresponding to 300 kHz per tube. The spatial resolution of the drift tubes is degraded from 82 μm without background to 108 μm at 500 Hz cm^-2 background count rate. In order to limit the background count rate, the drift tubes are read out with an artificial dead time of 790 ns which causes an efficiency loss of 23% at a rate of 300 kHz per tube. The space-to-drift-time relationships of the tubes vary with the background rate, the temperature, and the magnetic field strength. They must be recalibrated in short time intervals with an accuracy better than 20 μm which is guaranteed by an autocalibration procedure using muon tracks and by applying measured magnetic field corrections to the relationship.