The surface of a silicon detector acts like a mirror. This can be used to
measure surface distorsions (=flatness) of the forward modules. The
image of a regular grid refelcted on the silicon surface is viewed using
a video camera. If the surface is not perfectly flat, the grid appears
distorted:
perfect surface
distorted surface
These distorsions can be measured and can be used to reconstruct the
surface shape:
Reconstructed surface of a silicon detector. The hight of the spherical
bow is about 18 micron
Backside of the same detector
The reconstructed surface of a complete forward module:
The maximal distorsion of this modules is 51 micron.
This simple but efficient method to measure the flattness of modules and
detectors is described
here
Distorsion as function of cooling point temperature
A module (middle ring) was mounted on two cooling blocks, cooled by a
liquid cirquit. The cooling point temperature was cycled between +28 and
-10 C and the changes of the surface dimensions were measured.
The changes are parameterized as follows:
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In x (transverse to the module) the bowing was negligable. However the
module tilted resulting in an offset of 40 microns.
In y (along the module) a bowing up to 10 micron was observed while the
relative change between the mounting blocks was about 5 microns.
Since the thermal expansion of the Al cooling blocks is about 15 microns
in this temperature range, these distorsions can be caused by changes of
the module mounts.
Conclusions:
- in y thermal distorsions are very small
- in x small distorsions of the mounting points could result to rather
large deviations at the module extremities due to the large lever arm.
ps file
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