The muon spectrometer of the ATLAS experiment at the Large Hadron Collider (LHC) is instrumented with three layers of precision tracking detectors each consisting of 6 or 8 layers of pressurized aluminum drift tubes of 30 mm diameter. The magnetic field of the spectrometer is generated by superconducting air-core toroid magnets. Already at the LHC design luminosity of 10{sup}34 cm{sup}2s{sup}(-1), the ATLAS muon chambers have to cope with unprecedentedly high neutron and gamma ray background rates of up to 500 Hz/cm{sup}2 in the inner and middle chamber layers in the forward regions of the spectrometer. At a high-luminosity upgrade of the LHC (S-LHC), the background rates are expected to increase by an order of magnitude. The resulting high occupancies lead to a significant deterioration of the muon detection efficiency compromising the physics goals. The possibility to improve the muon detection efficiency by reducing the diameter of the drift tubes has been investigated. We report about the design and test results of prototype drift-tube detectors with thin-walled aluminum tubes of 15 mm diameter.
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