A novel straightness measurement system applied to the position monitoring of large Particle Physics Detectors

The Compact Muon Solenoid experiment, CMS, is one of the two general purpose experiments foreseen to operate at the Large Hadron Collider, LHC, at CERN, the European Laboratory for Particle Physics. The experiment aims to study very high energy collisions of proton beams. Investigation of the most fundamental properties of matter, in particular the study of the nature of the electroweak symmetry breaking and the origin of mass, is the experiment scope. The central Tracking System, a six meter long cylinder with 2.4 m diameter, will play a major role in all physics searches of the CMS experiment. Its performance depends upon the intrinsic detector performance, on the stability of the supporting structure and on the overall survey, alignment and position monitoring system. The proposed position monitoring system is based on a novel lens-less laser straightness measurement method able to detect deviations from a nominal position of all structural elements of the Central Tracking system. It is based on the reciprocal collimation of 2D light detectors allowing the definition of a virtual reference axis. Laser sources fixed on the inner tracker structural elements give a bright circular spot on the 2D detector that can be measured with an accuracy of ~ 0.2 micron . With this single spot accuracy, we are able to reconstruct the three-dimensional position of the light source with respect to the reference line and to achieve a global accuracy better then 10 micron for displacements of big objects inside the tracker volume. In this paper we describe the method and its applications, the tests and simulations performed and possible extensions of its use.