736
D. Froidevaux
conditions are harsher and the rate of muon hits is therefore larger, Cathode Strip
Chambers (CSC) are used.
The basic principle of the muon measurement in the ATLAS muon spectrometer
is to obtain three segments (or super-points) along the muon trajectory. For momenta
up to 300 GeV, the resolution is limited to a few percent by multiple scattering
and fluctuations in the energy loss in the calorimeters, and can therefore be
improved by combining the momentum measurement with that obtained in the
Inner Detector. The momentum resolution goals quoted above at higher momenta
imply a very high precision of 80 μm on the individual hits, given the three-point
measurement and the available bending power. The required precision on the muon
momentum measurement also implies excellent knowledge of the magnetic field.
The air-core toroid design leads to a magnetic field, which is modest in average
magnitude (0.5 T), but is also inhomogeneous, and must therefore be measured and
monitored with high precision (at the level of 20 G). The inhomogeneity of the field
and its rapid variations cannot be approximated by simple analytical descriptions
and have to be accounted for carefully, thereby enhancing the importance of the use
of the inner detector information to reconstruct low-momentum muon tracks with
low fake rates.
16.5.1.3 Alignment
Alignment of the muon chambers with respect to each other and with respect to
the overall detector is a critical ingredient, key to obtaining the desired performance
over the large areas involved, particularly at the highest momenta. The high accuracy
of the ATLAS stand-alone measurement necessitates a very high precision of 30 μm
on the alignment.
The chambers have however been installed with an accuracy of a few mm,
and obviously, no attempt at repositioning the chambers once their installation is
completed can realistically be made. Instead, intricate hardware systems have been
designed to measure the relative positions between chambers contributing to the
measurement of the same tracks, but also to monitor any displacements during the
detector operation. These systems are designed to provide continuous monitoring
of the positions of the chambers with or without collisions in the accelerator. The
very strict requirement of a 30 μm alignment has necessitated the design of a
complex system, in which optical sensors are mounted with very high mechanical
mounting precision (better than 20 μm in the precise coordinate). The system uses
∼5000 alignment sensors, which are either installed on the chambers or in the socalled alignment bars (long instrumented Aluminium cylinders with deformations
monitored to within 10 μm, which constitute the alignment reference system in the
end-caps). In addition, 1789 magnetic field sensors (3D Hall probes) are also being
installed on the chambers to determine with high accuracy the position and shape
of the conductors of each coil. From these accurate measurements, the field will be
determined throughout the whole volume to an accuracy of about 20 G, provided all
magnetic materials are also mapped and described accurately.
D. Froidevaux
conditions are harsher and the rate of muon hits is therefore larger, Cathode Strip
Chambers (CSC) are used.
The basic principle of the muon measurement in the ATLAS muon spectrometer
is to obtain three segments (or super-points) along the muon trajectory. For momenta
up to 300 GeV, the resolution is limited to a few percent by multiple scattering
and fluctuations in the energy loss in the calorimeters, and can therefore be
improved by combining the momentum measurement with that obtained in the
Inner Detector. The momentum resolution goals quoted above at higher momenta
imply a very high precision of 80 μm on the individual hits, given the three-point
measurement and the available bending power. The required precision on the muon
momentum measurement also implies excellent knowledge of the magnetic field.
The air-core toroid design leads to a magnetic field, which is modest in average
magnitude (0.5 T), but is also inhomogeneous, and must therefore be measured and
monitored with high precision (at the level of 20 G). The inhomogeneity of the field
and its rapid variations cannot be approximated by simple analytical descriptions
and have to be accounted for carefully, thereby enhancing the importance of the use
of the inner detector information to reconstruct low-momentum muon tracks with
low fake rates.
16.5.1.3 Alignment
Alignment of the muon chambers with respect to each other and with respect to
the overall detector is a critical ingredient, key to obtaining the desired performance
over the large areas involved, particularly at the highest momenta. The high accuracy
of the ATLAS stand-alone measurement necessitates a very high precision of 30 μm
on the alignment.
The chambers have however been installed with an accuracy of a few mm,
and obviously, no attempt at repositioning the chambers once their installation is
completed can realistically be made. Instead, intricate hardware systems have been
designed to measure the relative positions between chambers contributing to the
measurement of the same tracks, but also to monitor any displacements during the
detector operation. These systems are designed to provide continuous monitoring
of the positions of the chambers with or without collisions in the accelerator. The
very strict requirement of a 30 μm alignment has necessitated the design of a
complex system, in which optical sensors are mounted with very high mechanical
mounting precision (better than 20 μm in the precise coordinate). The system uses
∼5000 alignment sensors, which are either installed on the chambers or in the socalled alignment bars (long instrumented Aluminium cylinders with deformations
monitored to within 10 μm, which constitute the alignment reference system in the
end-caps). In addition, 1789 magnetic field sensors (3D Hall probes) are also being
installed on the chambers to determine with high accuracy the position and shape
of the conductors of each coil. From these accurate measurements, the field will be
determined throughout the whole volume to an accuracy of about 20 G, provided all
magnetic materials are also mapped and described accurately.