ATLAS monitored drift tube chambers for super-LHC

After the high-luminosity upgrade of the Large Hadron Collider (LHC), the ATLAS muon spectrometer is expected to work at 10-fold background rates of gammas and neutrons. As the momentum resolution of the spectrometer is expected to be 10% a single tube resolution of the muon drift tubes of 80μm is required. At background rates around 1000 Hz/cm ² space charge effects will lead in the slow and non-linear AR:CO₂=93:7 gas mixture degrade the drift-tube spatial resolution. While this was studied for gammas and low energetic neutrons, almost no information exists for fast neutrons. This paper shows that:11 MeV neutrons at flux densities of 4 and 16 kHz/cm ² degrade the position resolution by only 10μm at a detection efficiency of 4×10^-4.two modified inert gas mixtures selected by fastness and linearity of their position-drifttime (rt) relation show similar resolution as the standard gas in agreement with Garfield simulations.the resolution of newly developed 15 mm ∅ drift tubes are mostly unaffected by space charge effects due to gamma irradiation up to 1400 Hz/cm² according to Garfield simulations.