Design and high precision monitoring of detector structures at CERN

Abstract

Situated on the outskirts of Geneva, CERN is the leading center for particle physics in the world. The Large Hadron Collider (LHC) with its 27 km ring-shaped accelerator, which is currently under construction and will be operational in 2008, will begin a new era in high energy physics by revealing the basic constituents of the universe. One of the experiments is ALICE (A Large Ion - Colliding - Experiment), a detector consisting of multiple layers of sub detectors around the collision point to detect different types and properties of particles created in the collisions. Those particles are identified via their energy, momentum, track and decay products, and it is therefore important to align the various sub detectors very precisely to each other and monitor their position. The monitoring systems have to operate for an extended period of time under extreme conditions (e.g. high radiation) and must not absorb too many of the particles created in the collisions. This dissertation describes monitoring systems developed for the ALICE and CMS (Compact muon solenoid) experiments. Detector monitoring: The crucial aspect within the integration of the ALICE experiment is precise alignment of the inner detectors with respect to the central beryllium beam pipe. Based on the BCAM system (Brandeis CCD Angle Monitor), tests were carried out in order to approve the idea of mounting a BCAM on the external reference point and a reflecting mirror on the sub detector. Using a corner cube prism instead of a plane mirror eliminates the sensitivity to rotations of the mirror.<br />Results obtained from the various lab tests and final setups will show that the novel BCAM application which is now used in three out of the four LHC experiments, has several advantages over the standard two BCAM based angle monitoring.<br />Beam pipe monitoring: The fragile ALICE central beryllium beam pipe with a diameter of 59.6 mm and 0.8 mm wall thickness is supported at three points. In order to minimize the deflections and hence stresses in the beam pipe, one of the three support structures was designed with the aid of finite element analysis. The pipe will operate in an environment of 0.5 T magnetic field and is expected to absorb a dose of 10 kGy in ten years. These special constraints and the lack of access preclude most standard force monitoring systems. Previous work has shown that strain gage based systems work well under these conditions. The thesis presents an optimized strain gage based system for the ALICE beam pipe that is sensitive to changes in force of 1 N.<br />Both the BCAM - retroreflector system and the strain gage based force monitoring system provide critical information regarding the status of the beam pipe, ITS and forward detector systems. The last chapter will deal with a further BCAM - retroreflector system used in the CMS experiment in order to align the five large barrels of the super-conducting solenoid. Furthermore, it will present results from the first CMS magnet closure.