Performance of the ATLAS precision muon chambers under LHC operating conditions

Abstract

For the muon spectrometer of the ATLAS detector at the large hadron collider (LHC), large drift chambers consisting of 6 to 8 layers of pressurized drift tubes are used for precision tracking covering an active area of 5000 m 2 in the toroidal field of superconducting air core magnets. The chambers have to provide a spatial resolution of 41 µm with Ar:CO2 (93:7) gas mixture at an absolute pressure of 3 bar and gas gain of 2·10 4 . The environment in which the chambers will be operated is characterized by high neutron and γ background with counting rates of up to 100 s −1 cm −2 . The resolution and efficiency of a chamber from the serial production for ATLAS has been investigated in a 100 GeV muon beam at photon irradiation rates as expected during LHC operation. A silicon strip detector telescope was used as external reference in the beam. The spatial resolution of a chamber is degraded by 4 µm at the highest background rate. The detection efficiency of the drift tubes is unchanged under irradiation. A tracking efficiency of 98% at the highest rates has been demonstrated.

Performance of fast high-resolution Muon drift tube chambers for LHC upgrades

“…Space charge effects [5], [6] are degrading the resolution of the single drift tubes and the muon detection efficiency due to the increased drift tube occupancy [3]. Both effects can be suppressed by reducing the tube diameter while leaving the other operational parameters of the detector, in particular gas mixture, pressure and gain, unchanged.…”

“…In addition, the fluctuations of the space charge lead to fluctuations of the electric field in the tubes. Since the drift velocity depends on the electric field in non-linear drift gases like Ar:CO 2 , this causes a deterioration of the spatial resolution, which increases rapidly with the drift distance above a value of 7.5 mm [5], [11]. Fig.…”

“…Since the drift tube spatial resolution at low background rates improves with the drift distance, the average singletube resolution deteriorates from 80 m for 30 mm diameter tubes [3], [5] to about 100 m for 15 mm diameter tubes. For 30 mm diameter drift tubes, the resolution has been measured to deteriorate approximately linearly with the counting rate to about 100 m at 500 Hz/cm 2 [3], [5].…”

ATLAS Muon Drift Tube Electronics

“…The degradation of position resolution due to space charges could thus be precisely measured and turned out to be in excellent agreement with simulation results. First tests with a few tubes were reported in [9], while results on full chambers are presented in [19,20]. At the nominal luminosity and the simulated background rate (including the safety factor 5 for simulation errors 3 ) the average resolution of a MDT tube degrades from about 80 to 108 µm, still inside the acceptable limits.…”

Precision drift-tube chambers for the ATLAS muon spectrometer at super-LHC

“…At high background rates, the MDT chambers are known to suffer from a degradation of the spatial resolution due to space-charge fluctuations [6], [7] and of the muon detection efficiency due to increased occupancy of the drift tubes [8]. At background rates beyond 2000 Hz/cm 2 the gas gain drop rapidly to 0 due to the large space charge density of ions created by the background hits.…”