502
F. Bordry et al.
8.9.11 Alignment of Detector Components
Modern Experiments are complex structures made up of many individual modules,
assembled concentrically, each one being design to detect a certain physics phenomena and can be likened to a set of Russian dolls. In the working position, only
the outer skin of the experiment is visible, the position of the inner detectors has
to be reconstructed from the position of the external ones. Their impressive size
(several thousand tons, and thousands of m3) combined with their confinement
in caverns make the alignment operation difficult in such a restrictive environment.
Furthermore, in order to obtain good results in the analysis of the events
(reconstruction of particle paths), a precise knowledge of detector positions both
with respect to the beam and respect to each other is required. The geodesists have
therefore to ensure that the dimensional parameters of the experiment are fulfilled
during fabrication and assembly and to give the final position of the whole detector
with respect to the theoretical beam line.
The positioning of the experiment with respect to the beam line is done using
a geodetic experiment network. This network of points is composed of forced
centering sockets distributed in the whole cavern volume on walls and floor. It is
used during all the steps of the assembly and positioning of the detectors. The
design of the network is a very important step as its configuration influences
the final accuracy of the measurements. It is measured, once the cavern has
been delivered and is still empty, using mainly distances, angles and leveling
measurements. The use of laser tracker technology, which uses a very accurate
distance-meter, could also be envisaged. The network accuracy has to be within
a few tenths of a mm (1σ) with respect to the underground network and later to
the accelerator components. It must be periodically controlled knowing that some
points may become physically inaccessible or hidden from the others (Figs. 8.104
and 8.105).
To obtain the final co-ordinates of each detector of the experiment, a succession
of survey operations is needed from the first availability of an individual object
through its final positioning. Each individual sub-detector has to be equipped with
fiducial marks geometrically linked to the sensitive components. These references
must be included in the design of the detectors with the right specifications and
at the most strategic positions to allow their coordinate measurement in the future
with the required accuracy. The surveyors have to be involved at an early stage of
the projects.
One of the first in field operations, called the link geometry, is carried out in labs,
workshop or assembly hall by means of micro-triangulation/trilateration. Fifteen
years ago, the close range digital photogrammetry was added to these techniques
and is now extensively used. Its principle relies on the reconstruction of an object
simultaneously from several images taken from different positions and with the
best possible perspective to ensure a suitable geometry of intersecting rays. The
pictures are taken from free camera positions stationed in the object space and
F. Bordry et al.
8.9.11 Alignment of Detector Components
Modern Experiments are complex structures made up of many individual modules,
assembled concentrically, each one being design to detect a certain physics phenomena and can be likened to a set of Russian dolls. In the working position, only
the outer skin of the experiment is visible, the position of the inner detectors has
to be reconstructed from the position of the external ones. Their impressive size
(several thousand tons, and thousands of m3) combined with their confinement
in caverns make the alignment operation difficult in such a restrictive environment.
Furthermore, in order to obtain good results in the analysis of the events
(reconstruction of particle paths), a precise knowledge of detector positions both
with respect to the beam and respect to each other is required. The geodesists have
therefore to ensure that the dimensional parameters of the experiment are fulfilled
during fabrication and assembly and to give the final position of the whole detector
with respect to the theoretical beam line.
The positioning of the experiment with respect to the beam line is done using
a geodetic experiment network. This network of points is composed of forced
centering sockets distributed in the whole cavern volume on walls and floor. It is
used during all the steps of the assembly and positioning of the detectors. The
design of the network is a very important step as its configuration influences
the final accuracy of the measurements. It is measured, once the cavern has
been delivered and is still empty, using mainly distances, angles and leveling
measurements. The use of laser tracker technology, which uses a very accurate
distance-meter, could also be envisaged. The network accuracy has to be within
a few tenths of a mm (1σ) with respect to the underground network and later to
the accelerator components. It must be periodically controlled knowing that some
points may become physically inaccessible or hidden from the others (Figs. 8.104
and 8.105).
To obtain the final co-ordinates of each detector of the experiment, a succession
of survey operations is needed from the first availability of an individual object
through its final positioning. Each individual sub-detector has to be equipped with
fiducial marks geometrically linked to the sensitive components. These references
must be included in the design of the detectors with the right specifications and
at the most strategic positions to allow their coordinate measurement in the future
with the required accuracy. The surveyors have to be involved at an early stage of
the projects.
One of the first in field operations, called the link geometry, is carried out in labs,
workshop or assembly hall by means of micro-triangulation/trilateration. Fifteen
years ago, the close range digital photogrammetry was added to these techniques
and is now extensively used. Its principle relies on the reconstruction of an object
simultaneously from several images taken from different positions and with the
best possible perspective to ensure a suitable geometry of intersecting rays. The
pictures are taken from free camera positions stationed in the object space and