  Pressurized drift-tube chambers are efficient detectors for high-precision
tracking over large areas. The Monitored Drift-Tube (MDT) chambers of the muon
spectrometer of the ATLAS detector at the Large Hadron Collider (LHC) reach a
spatial resolution of 35 micons and almost 100% tracking efficiency with 6
layers of 30 mm diameter drift tubes operated with Ar:CO2 (93:7) gas mixture at
3 bar and a gas gain of 20000. The ATLAS MDT chambers are designed to cope with
background counting rates due to neutrons and gamma-rays of up to about 300 kHz
per tube which will be exceeded for LHC luminosities larger than the design
value of 10-34 per square cm and second. Decreasing the drift-tube diameter to
15 mm while keeping the other parameters, including the gas gain, unchanged
reduces the maximum drift time from about 700 ns to 200 ns and the drift-tube
occupancy by a factor of 7. New drift-tube chambers for the endcap regions of
the ATLAS muon spectrometer have been designed. A prototype chamber consisting
of 12 times 8 layers of 15 mm diameter drift tubes of 1 m length has been
constructed with a sense wire positioning accuracy of 20 microns. The 15 mm
diameter drift-tubes have been tested with cosmic rays in the Gamma Irradiation
Facility at CERN at counting rates of up to 1.85 MHz.
