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Hybrid approach for the alignment of the Silicon LC tracker: Integrated co-linearity monitors and offline track alignment.

The usual limiting factors in the accuracy of a position monitoring system based on optomechanical devices are: mechanical transfers, between the monitored imaging sensors and the active particle tracking elements; and non-strait propagation of the reference laser lines; quite often, extremely precise position monitoring systems suffer from poor accuracy due to the previous two factors. The approach we propose here will solve the first issue and reduce the effect of the second one.

Concept for the alignment of the silicon system.

This conceptual design is built on its successfully application to the AMS-1 tracking system [1], and on the current developments for the CMS silicon tracker alignment. The main features of the proposed concept are the following:

  • Collimated laser beam (IR spectrum) going through silicon detector modules. The laser beam would be detected directly in the Si-modules. The alignment readout is fully integrated in the silicon readout; tracks and laser beam share the same sensors removing the need of any mechanical transfer.

  • No external reference structures. All the elements of the alignment system ( laser beam collimators, steering optics, etc.) are mounted directly on the tracker elements.

  • No precise positioning of the aiming of the collimators. The number of measurements has to be redundant enough to reconstruct the detector without any knowledge of the laser beam initial parameters.

  • Optical and tracking data will be combined to optimise the alignment procedure.

  • A minimal impact of the alignment system on the layout of the tracker and its production technology.

  • Based on previous AMS-1 experience we can project that few microns resolutions would be achieved.

Gaussian laser beams and Si-sensor treatment

From the point of view of the instrumentation, the two key stones of this hybrid approach to the tracker alignment are: non-magnetic hard-rad fiber collimators, delivering a extremely pure gaussian beam; and the anti-reflecting coating of the Si-modules for increasing sensor transparency. The first issue has been already solved in the context of the CMS global alignment for visible light, custom-made titanium collimators with a fused silica optical system deliver almost pure gaussian beams; here, we need to modify the optics design for the IR range. Concerning the Si-modules, a dedicated test stand will be built for the optical characterization of the Si-modules, testing the sensor coatings and treatments. The sensor coating will reduce its reflectivity, increase the transmittance and suppress the deflection of the beam after traversing the Si-module; the testing procedure is very well understood since we have carried out it extensively on semitransparent amorphous silicon sensor developed for the CMS alignment [2].


1. Nuclear Instruments and Methods in Physics Research A 511 (2003) 76-81

2. Nuclear Instruments and Methods in Physics Research A 440 (2000) 372-387