The laser alignment system for the CMS silicon microstrip tracker

The Compact Muon Solenoid (CMS) is one of the two general purpose detectors that will be installed at the future Large Hadron Collider (LHC). For the proper operation of the CMS silicon microstrip tracker it is essential that the several sensor layers can be aligned with respect to each other with aprecision of a few hundred micron. The laser alignment system presented here has been designed for this purpose. Its goal is to measure the deformations of the mechanical support structure of the tracker with a precision better than 100 micron. The alignment system uses laser beams with a wavelength of 1000-1100 nm that can traverse several layers of silicon sensors inducing a signal in the strips. Movements of the sensors with respect to each other can then be assessed from the positions of the measured beam profiles. A key feature of this alignment system is that it uses the same sensors that also measure the particle tracks. In the first part of this work several aspects which affect the performance of the alignment system were studied in detail. First the light transmission through the sensors was assessed, which is limited by absorption losses and the reflection at the strips and the silicon-air interfaces. The total transmission through a silicon sensor was measured to be 20% at 1064 nm and 24% at 1080 nm in good agreement with the theoretical expectation. The accuracy with which the center of the laser profile can be found on a strip sensor is a function of the signal to noise ratio, the sensor pitch and the laser beam width. The contribution coming from the S/N ratio can be minimized by averaging over a few hundred laser pulses thus increasing the effective S/N ratio. In this way it is possible to pass through 4 layers of silicon and still reach a reasonable accuracy on the 5th layer. Disturbing effects like interferences due to multiple reflections and refraction at the silicon-air interfaces were studied on detector prototypes and extrapolated to the final CMS sensor design. In the second part of the work the layout of the laser alignment system for the CMS microstrip tracker is described and its performance is simulated. The system consists of 52 laser beams which pass through the tracker. 16 beams are used for the internal alignment of each endcap. 8 laser beams will align the endcaps with respect to the tracker inner and outer barrel (TIB and TOB). Here beam splitters are used to get the light onto the TIB and TOB sensors. Finally 12 more laser beams will be employed to align the entire tracker with respect to the CMS muon system. The laser sources can be laser diodes (1060-1080 nm) or Nd-YAG lasers (1064 nm) and are placed outside the CMS detector. The light is routed with fibers into the tracker volume, where collimators and beam splitters generate pairs of back-to-back oriented beams. The collimators are placed so that the beams have to pass at most four layers of silicon sensors. For this system the accuracy with which the intersection points of the laser beams with the tracker structure can be measured is better than 70 micron. This accuracy is dictated by the effects described in the first part and by the precision with which the silicon detectors can be mounted on the mechanical support structure. To simulate the performance of the system, a set of 49 parameters describing the movements of one endcap and of a set of 8 laser beams was defined. It was taken into account, that the orientation of the laser beams may change during operation. The 49 parameters can be fitted to a data set of measured laser profiles. For the expected measuring precision of 70 micron all parameters were reconstructed to better than the required 100 micron. Simulations in which the number of laser beams was changed show that this is still the case even when some of the laser beams are lost. All the results of the simulation agree very well with the analytical calculation.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2002

Author dc:creator

• Wittmer, Bruno

Contributors dc:contributor

• Schael, Stefan

• info:eu-repo/classification/ddc/530
• Physik

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-119006%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:56932