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% Abstract IEEE NSS 2004
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% Contact: Hubert Kroha
%          Max-Planck-Institut fuer Physik
%          Foehringer Ring 6
%          D-80805 Muenchen
%          Tel.: +49 (89) 32354-435
%          Fax:  +49 (89) 32354-305
%          Email: kroha@mppmu.mpg.de
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Operation of the ATLAS Muon Drift-Tube Chambers at High Irradiation Rates
and in Magnetic Fields


S. Horvat, O. Kortner, H. Kroha, S. Mohrdieck-Moeck,
R. Richter, W. Stiller, Ch. Valderanis

Max-Planck-Institut fuer Physik, Foehringer Ring 6, D-80805 Munich, Germany

J. Dubbert, F. Rauscher, A. Staude

Ludwig-Maximilians-Universitaet Muenchen, Am Coulombwall 1, D-85748 Garching, Germany


In the muon spectrometer of the ATLAS detector at the Large Hadron Collider
(LHC), large drift tube chambers (Monitored Drift Tube chambers, MDT) consisting
of 6 to 8 layers of drift tubes will be used for precision tracking in the toroidal
field of superconducting air core magnets. The chambers have to provide a
spatial resolution of 40 microns with a sense wire positioning accurcay
of 20 microns and using an Ar:CO_2 (93:7) gas mixture at 3 bar.
The ATLAS muon chambers will experience unprecedentedly high neutron and photon 
background count rates of up to 500 Hz/cm^2 at the LHC design luminosity. 
The space-to-drift time relationship of the drift tubes varies with the 
operating conditions, the background rate and the magnetic field strength. 
Efficient algorithms have been developed to calibrate the chambers in short 
time intervals with an accuracy of better than 20 micron using muon tracks.
The performance of the MDT chambers with final readout electronics
was studied in a muon beam at the Gamma Irradiation Facility at CERN for
varying magnetic fields and photon irradiation rates of up to 990 Hz/cm^2.
A silicon strip detector telescope served as external reference in the beam.
Without photon irradiation, an average drift tube resolution of 80 microns is 
achieved employing time-slewing corrections. At the maximum background rates,
the spatial resolution deteriorates by 25 micron due to space charge fluctuations.
The results are reproduced by simulations of the drift tube response.
The consequences for the operation of the chambers at even higher luminosities 
have been investigated.