S. Horvat

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 6–8 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:CO 2 (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.

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.

Development of Fast High-Resolution Muon Drift-Tube Detectors for High Counting Rates

Abstract 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 μ m and almost 100% tracking efficiency with 6 layers of 30xa0mm diameter drift tubes operated with an Ar:CO 2 (93:7) gas mixture at 3 bar and a gas gain of 20xa0000. The ATLAS MDT chambers are designed to cope with background counting rates due to neutrons and γ rays of up to about 300xa0kHz per tube which will be exceeded for LHC luminosities larger than the design value of 10 34 xa0cm −1 xa0s −1 . Decreasing the drift-tube diameter to 15xa0mm while keeping the other parameters, including the gas gain, unchanged reduces the maximum drift time from about 700 to 200xa0ns 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 15xa0mm diameter drift tubes of 1xa0m length has been constructed with a sense wire positioning accuracy of 20 μ m . The 15xa0mm diameter drift-tubes have been tested with cosmic rays in the Gamma Irradiation Facility at CERN at γ counting rates of up to 1.85xa0MHz.

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.

Upgrade of the ATLAS Muon Trigger for the SLHC

The outer shell of the ATLAS experiment at the LHC consists of a system of toroidal air-core magnets in order to allow for the precise measurement of the transverse momentum (pT) of muons, which in many physics channels are a signature of interesting physics processes [1,2]. For the precise determination of the muon momentum Monitored Drift Tube chambers (MDT) with high position accuracy are used, while for the fast identification of muon tracks chambers with high time resolution are used, able to select muons above a predefined pT threshold for use in the first Level of the ATLAS triggering system (Level-1 trigger). When the LHC peak luminosity of 1034xa0cm−2s−1 will be increased by a factor of 4–5 in about a decade from now (SLHC), an improvement of the selectivity of the ATLAS Level-1 triggering system will be mandatory in order to cope with the maximum allowed trigger rate of 100 kHz. For the Level-1 trigger of the ATLAS muon spectrometer this means an increase of the pT threshold for single muons. Due to the limited spatial resolution of the trigger chambers, however, the selectivity for tracks above ~ 20 GeV/c is insufficient for an effective reduction of the Level-1 rate. We describe how the track coordinates measured in the MDT precision chambers can be used to decisively improve the selectivity for high momentum tracks. The resulting increase in latency will also be discussed.

Resolution and efficiency of the ATLAS muon drift-tube chambers at high background rates

Abstract The resolution and efficiency of a precision drift-tube chamber for the ATLAS muon spectrometer with final read-out electronics was tested at the Gamma Irradiation Facility at CERN in a 100 GeV muon beam at photon irradiation rates of up to 990 Hz / cm 2 , which corresponds to twice the highest background rate expected in ATLAS. A silicon strip detector telescope served as external reference in the beam. The pulse-height measurement of the read-out electronics was used to perform time-slewing corrections, which lead to an improvement of the average drift-tube resolution from 104 to 82 μ m without irradiation, and from 128 to 108 μ m at the maximum expected rate. The measured drift-tube efficiency agrees with the expectation from the dead time of the read-out electronics up to the maximum expected rate.

Large-scale production of Monitored Drift Tube chambers for the ATLAS muon spectrometer

Precision drift tube chambers with a sense wire positioning accuracy of better than 20 μm are under construction for the ATLAS muon spectrometer. 70% of the 88 large chambers for the outermost layer of the central part of the spectrometer have been assembled. Measurements during chamber construction of the positions of the sense wires and of the sensors for the optical alignment monitoring system demonstrate that the requirements for the mechanical precision of the chambers are fulfilled.

Studies of semitransparent optoelectronic position sensors

Novel semitransparent optoelectronic position sensors, the ALMY sensors, have been developed for high-precision multi-point position and angle measurements of collimated laser beams over a large measurement range. The sensors consist of a thin film of amorphous silicon deposited on a glass substrate between two transparent layers of crossed strip electrodes. They provide a position resolution on the order of a micrometer over sensitive areas of several square centimeters. A transmittance of 80-90% has been achieved for 780 nm laser light produced by diode lasers. We report on recent optimizations of the sensor performance and tests of the long-term stability under laser illumination and of the radiation tolerance at high neutron doses. As expected, the radiation hardness of the amorphous silicon sensors exceeds the that of crystalline silicon devices. The custom designed readout electronics allows for operation at sufficiently low laser intensities in order to prevent significant degradation of the performance of the amorphous silicon sensors under illumination with laser light.

Development of precision drift tube detectors for very high background rates at the super-LHC

The muon spectrometer of the ATLAS experiment at the large hadron collider (LHC) is instrumented with three layers of precision tracking detectors each consisting of 6 or 8 layers of pressurized aluminum drift tubes of 30 mm diameter. The magnetic field of the spectrometer is generated by superconducting air-core toroid magnets. Already at the LHC design luminosity of 1034 cm2s-1, the ATLAS muon chambers have to cope with unprecedentedly high neutron and gamma ray background rates of up to 500 Hz/cm2 in the inner and middle chamber layers in the forward regions of the spectrometer. At a high-luminosity upgrade of the LHC (S-LHC), the background rates are expected to increase by an order of magnitude. The resulting high occupancies lead to a significant deterioration of the muon detection efficiency compromising the physics goals. The possibility to improve the muon detection efficiency by reducing the diameter of the drift tubes has been investigated. We report about the design and test results of prototype drift-tube detectors with thin-walled aluminum tubes of 15 mm diameter.

Status of the ALMY semitransparent, amorphous silicon sensors for optical position monitoring systems

Abstract Semitransparent optoelectronic position sensors (‘ALMY sensors’) have been developed for high-precision multipoint alignment monitoring systems. The thin-film amorphous silicon strip sensors provide up to 90% transmittance for visible laser light and a position measurement accuracy of better than 5 μm over sensitive areas of several square centimeters. We report about the results of the recent optimization of the sensor performance and of the readout electronics.