Sergey Kotov

Operation of the ATLAS muon drift-tube chambers at high background rates and in magnetic fields

In the ATLAS muon spectrometer, large drift-tube chambers are used for precision tracking. The chambers will be operated at a high neutron and /spl gamma/ background resulting in count rates of up to 500 Hz cm/sup -2/ corresponding to 300 kHz per tube. The spatial resolution of the drift tubes is degraded from 82 /spl mu/m without background to 108 /spl mu/m at 500 Hz cm/sup -2/ background count rate. Due to afterpulsing in the Ar/CO/sub 2/ gas mixture used in the drift tubes, ionizing radiation causes more than one hit in a tube within the maximum drift time of about 800 ns which is expected for magnetic field strengths around 1.2 T. In order to limit the count rate, the drift tubes are read out with an artificial dead time of 790 ns which causes an efficiency loss of 23% at a rate of 300 kHz per tube. The space-to-drift-time relationship of the tubes varies with background rate, temperature, and magnetic field strength. The mean magnetic field strength in a muon chamber is 0.4 T on the average, but may vary by up to 0.4 T within a chamber. The space-to-drift-time relationship must therefore be determined in short time intervals with an accuracy better than 20 /spl mu/m using muon tracks and applying corrections for measured magnetic field variations.

Construction and test of the precision drift chambers for the ATLAS muon spectrometer

The monitored drift tube (MDT) chambers for the muon spectrometer of the ATLAS detector at the Large Hadron Collider (LHC) consist of three or four layers of pressurised drift tubes on either side of a space frame carrying an optical deformation monitoring system. The chambers have to provide a track position resolution of 40 /spl mu/m with a single-tube resolution of at least 80 /spl mu/m and a sense wire positioning accuracy of 20 /spl mu/m (rms). The feasibility was demonstrated with the full-scale prototype of one of the largest MDT chambers with 432 drift tubes of 3.8 m length. For the ATLAS muon spectrometer, 88 chambers of this type have to be built. The first chamber has been completed with a wire positioning accuracy of 14 /spl mu/m (rms).ATLAS is a detector under construction to explore the physics at the Large Hadron Collider at CERN. It has a muon spectrometer with an excellent momentum resolution of 3-10%, provided by three layers of precision monitored-drift-tube chambers in a toroidal magnetic field. A single drift tube measures a track point with a mean resolution close to 100 micron, even at the expected high neutron and gamma background rates. The tubes are positioned within the chamber with an accuracy of 20 microns, achieved by elaborate construction and assembly monitoring procedures.

Optimization of the ATLAS muon drift-tube chambers at high background rates and in magnetic fields

In the ATLAS muon spectrometer, large drift-tube chambers are used for precision tracking. These chambers will he operated at a high neutron and /spl gamma/ background resulting in count rates of up to 500 counts s/sup -1/ cm/sup -2/ corresponding to 300 kHz per tube. The spatial resolution of the drift tubes is degraded from 82 /spl mu/m without background to 108 /spl mu/m at 500 Hz cm/sup -2/ background count rate. In order to limit the background count rate, the drift tubes are read out with an artificial dead time of 790 ns which causes an efficiency loss of 23% at a rate of 300 kHz per tube. The space-to-drift-time relationships of the tubes vary with the background rate, the temperature, and the magnetic field strength. They must be recalibrated in short time intervals with an accuracy better than 20 /spl mu/m which is guaranteed by an autocalibration procedure using muon tracks and by applying measured magnetic field corrections to the relationship.

Alignment of the ATLAS muon spectrometer with Muon Tracks

The ATLAS muon spectrometer uses three layers of precision drift-tube chambers to measure muon momenta accurately up to the TeV scale in an air-core toroid magnet system providing a field integral of 2.5 to 7 Tm. In order to achieve the required momentum resolution of better than 4% for transverse momenta below 400 GeV/c and of 10% at 1 TeV/c, the relative positions of the muon chambers must be known with 30 μm accuracy. A system of optical alignment sensors monitors relative movements of the chambers with a few micrometers accuracy. It is capable of measuring the relative chamber positions with an accuracy better than 30 μm after it has been calibrated with straight tracks. These will be provided by special runs with the magnets turned off. We present results of Monte-Carlo studies for the alignment with straight tracks recorded with proton-proton collisions at the LHC and cosmic muon data.

New methods for the alignment of the ATLAS muon spectrometer

The ATLAS muon spectrometer consists of three layers of precision drift-tube chambers in an air-core toroid magnet system with an average field of 0.4 T. The muon momenta are determined with high accuracy from the measurement of the sagitta of the muon tracks in the three chamber layers. In order to achieve the required momentum resolution of the muon spectrometer of better than 4% for transverse momenta below 400 GeV/c and of 10% at 1 TeV/c, the relative positions of the muon chambers are measured by a system of optical sensors with an accuracy of 30 mum. In order to verify the correctness of the optical alignment, a method has been developed to measure the relative chamber positions with muon tracks which are recorded during the operation of the experiment. For this purpose, an independent estimate of the muon momenta is needed. This is not provided with sufficient accuracy by the track measurement in the inner detector because of energy loss fluctuations in the calorimeters. For muons of PT < 40 GeV/c, however, the momenta can be determined with high-enough precision independently of the relative misalignment of the chambers from comparison of the local track direction measurements in the individual chamber layers. This method allows for monitoring of the chamber positions with an accuracy of about 30 mum in time intervals of a few hours during LHC operation.

Test, integration, commissioning and installation of large drift tube chambers of the ATLAS barrel muon spectrometer

The ATLAS experiment at the Large Hadron CoIlider (LHC) at CERN is currently being assembled and to be ready to take data in 2007. In the barrel part of the muon spectrometer a toroidal air-core magnet is instrumented with three layers of monitored drift tube (MDT) chambers as precision tracking detectors. The installation of the muon detectors has started in January 2005. At the Max-Planck-Institut fur Physik and the Ludwig-Maximilians-University in Munich, 88 MDT chambers, each covering an area of 8 m/sup 2/, are being built for the outermost barrel region. The MDT chambers have to pass a series of stringent tests before installation to ensure their proper operation in the experiment. At the production site in Munich, these tests include gas tightness, high voltage stability and measurements of the noise rate, and the response to cosmic muons. In addition, the individual wire positions and electronic time offsets of the drift tubes are determined from the cosmic ray data. At CERN, a subset of the tests is repeated and the MDT chambers are integrated on a common support frame with resistive plate chambers (RPCs) of the trigger system. The results of the tests are stored in the ATLAS commissioning database and form an important basis for LHC data taking. We present the test methods and results, an overview on the integration work of the muon detectors and report on the experience with their installation in the ATLAS experiment.

Dependence of the space-to-drift-time relationship of monitored drift-tube chambers on the magnetic field in the ATLAS muon spectrometer

The ATLAS detector is a multi-purpose detector which is built for the search for the standard model Higgs particle and new physics at the Large Hadron Collider (LHC). A striking feature of its design is the muon spectrometer, which is able to measure muon momenta with an accuracy of 3% over a wide momentum range; at 1 TeV/c an accuracy of 10% will be achieved. In the muon spectrometer, the muon trajectories will be measured by three stations of monitored drift-tube (MDT) chambers. The MDT chambers are operated in an average magnetic field of 0.4 T which is generated by 8 superconducting air-core toroid coils. The MDT chambers of the inner and outer layers in the barrel region are mounted outside the magnet coils and therefore experience a highly non-uniform magnetic field with field variations of up to 0.4 T. Due to the bending of the paths of the drift electrons in the magnetic field, the space-to-drift-time relationship r(t) depends on the magnetic field inside a tube. The maximum drift-time of 700 ns in absence of a magnetic field is extended by about 70 ns/T. The dependence of r(t) must be corrected for with an accuracy of 1 ns in order to achieve a chamber resolution of 50 /mum. The dependence of r(t) on the magnitude and the orientation of the magnetic field with respect to the anode wires of the tubes and the muon incident angle was measured in a test-beam. These measurements allow for a parameterization of the magnetic-field dependence of r(t) with the required accuracy of 1 ns