Upgrades of the ATLAS Muon Spectrometer with sMDT Chambers 

H.Kroha

Max-Planck-Institut fuer Physik, Munich

for the ATLAS Muon Collaboration


The Monitored Drift Tube (MDT) chambers of the ATLAS muon spectrometer demonstrated that
they provide very precise and robust tracking over large areas. Goals of ATLAS muon detector upgrades 
are to increase the acceptance for precision muon momentum measurement and triggering and to improve 
the rate capability of the muon chambers in the high-background regions when the LHC luminosity increases.
Small-diameter Muon Drift Tube (sMDT) chambers have been developed for these purposes.
With half the drift-tube diameter of the MDT chambers and otherwise unchanged operating parameters, 
sMDT chambers share the advantages with the MDTs, but have more than ten times higher rate capability and 
can be installed in detector regions where MDT chambers do not fit in. 
The chamber assembly methods have been optimized for mass production, reducing
cost and construction time considerably and improving the the sense wire positioning accuracy to better than ten microns. 
Two sMDT chambers have been installed in 2014 to improve the momentum resolution in the barrel part of the spactrometer
taking cosmic muon data. Construction of another twelve chambers to be installed 
in the feet of the ATLAS detector in the LHC winter shutdown 2016/17 has started. A just recently approved 
upgrade project forsees the replacement of the MDT chambers at the ends of the inner barrel detector layer
with sMDTs making space for additional RPC trigger chambers and improving the rate performance as well as the
optical alignment in these regions. The design, construction and installation of the new sMDT chambers
for ATLAS will be discussed as well as their performance, especially from tests at high background counting rates
in the Gamma Ray Facility at CERN. Aging tests have been extended by a factor of 15 beyond the ATLAS MDT
specifications.