Construction and Calibration of the Laser Alignment System for the CMS Tracker

The CMS detector (Compact Muon Solenoid) is under construction at one of the four proton-proton interaction points of the LHC (Large Hadron Collider) at CERN, the European Organization for Nuclear Research (Geneva, Switzerland). The inner tracking system of the CMS experiment consisting of silicon detectors will have a diameter of 2.4 m and a length of 5.4 m representing the largest silicon tracker ever. About 15000 silicon strip modules create an active silicon area of 200 m2 to detect charged particles from proton collisions. They are placed on a rigid carbon fibre structure, providing stability within the working conditions of a 4 T solenoid magnetic field at −10oC. Knowledge of the position of the silicon detectors at the level of 100 μm is needed for an efficient pattern recognition of charged particle tracks. Metrology methods are used to survey tracker subdetectors and the integrated Laser Alignment System (LAS) provides absolute positioning of support structure elements to better than 100 μm. Relative movements of the components are resolved and monitored at the 10 μm scale. A robust and reliable optical system able to measure and control the large CMS tracker geometry with high accuracy has been developed and validated. The design and construction of such a system, fully integrated in the silicon tracker, avoiding external reference structures in order to have minimal impact on the tracker layout and consisting of radiation hard and non-magnetic components represents a new scientific challenge. The construction and integration of the LAS fulfilling the requirements, as well as its calibration and performance are described in this thesis. The working principle is based on the partial transparency of silicon for light wavelengths in the near infrared region. The absorbed part of the laser beam generates a signal in the corresponding silicon strip module serving to reconstruct its position. The transmitted part reaches the subsequent module layer generating an optical link between the two layers. Investigation of the light generation and distribution led to a definition of the optical components and their optimization for Laser Alignment purposes. Laser diodes have been qualified as light sources and singlemode optical fibres, terminated by special connectors, distribute the light to the CMS tracker detector. The beamsplitting device, a key component of the LAS light distribution inside the CMS tracker, has been studied in detail. The challenge of splitting one collimated beam into two back-to-back beams inside a small available volume has been solved by using the polarization principle. Special test setups were developed to determine the collinearity of the two outgoing beams with a precision better than 50 μrad and it has been shown that their relative orientation remains constant under working conditions. The interface between the tracker and the LAS is given by the silicon sensors which are responsible both for particle detection and for the determination of the position of the laser spot. An anti-reflex-coating has been applied on the backside of all alignment sensors to improve their optical properties without deterioration of their tracking performance. A test setup has been developed to simultaneously study the transmission and reflection properties of the alignment sensors. The results are in good agreement with calculations and confirm the high optical quality of the sensors. The working principle of the optical alignment has been verified and the resolution of the laser spot was measured in a test setup with alignment modules arranged according to the CMS tracker endcap (TEC) geometry. For almost all laser spot positions in the TEC, relative module movements at the level of 10 μm were reconstructed. In addition, it has been shown that refraction effects are negligible. Data from Laser Alignment in one TEC sector has been compared with TEC survey measurements. The reconstruction precision of better than 100 μm obtained by two laser beams was independently confirmed by the metrology data, thus validating the performance of the optical alignment. The stringent requirements imposed on the implementation and performance of the Laser Alignment System necessitated the solution of a variety of problems and led to the accumulation of considerable experience in the alignment of particle tracking detectors by optical means.