The Alignment System of the ATLAS Muon End-Cap Spectrometer

The Large Hadron Collider at CERN will offer an unparalleled opportunity to probe fundamental physics at an energy scale well beyond that reached by current experiments. The ATLAS detector is being designed to fully exploit the potential of the LHC for revealing new aspects of the fundamental structure of nature. The muon spectrometer itself must measure with a momentum resolution of s10% for muons with a transverse momentum of pT =1TeV, to fully exploit the advantages offered by the open superconducting air core muon toroid magnet system. At this level of momentum resolution the muon spectrometer relies heavily on the ability to master the alignment of the large muon chambers spaced far apart. The overall contribution of the alignment to the total sagitta error must be less than 30 μm r.m.s. In order to meet the stringent alignment requirements the positions of the muon chambers are constantly monitored with optical alignment technologies. The end-caps of this spectrometer are therefore embedded in an alignment grid that must allow for an absolute position measurement of the chambers. This alignment grid employs up to 9.6m long precision rulers (alignment bars) which have to provide the position and orientation of all alignment sensors permeating the end-caps. Simulation studies have shown that the shape of these bars must be known to 30 μm r.m.s. and the length must be known to 20 μm r.m.s. The principles of alignment and survey techniques used to do this are introduced and the current activities concerning the alignment strategy for the ATLAS muon end-cap spectrometer are presented. After consideration of the motivation and requirements, the measurement strategy and the design of the alignment bars is given. An optical and thermal in-bar instrumentation is used to provide shape information of discrete points on the bar. The strategy to calibrate the in-bar instrumentation and to measure an initial bar shape with a large coordinate measuring machine, leads to the analytic shape model that allows to interpolate between the discrete number of measured points to describe any other bar shape. Expressing these changes with virtual loads that account for all forces acting on the bar, finally leads to the required precision. The basic principles are explained and detailed design and test results are presented. As part of the process one octant, the smallest independent unit of the end-cap alignment system, has been erected in the H8 test beam area to evaluate the systems performance of the proposed alignment strategy. Measurements are presently carried out with this prototype setup and initial results will be presented. Finally, work done during the development of the alignment bars led to a proposal for the final ATLAS alignment bars and to a manual describing the process from assembling until installation.