Optical and Capacitive Alignment of ATLAS Muon Chambers for Calibration with Cosmic Rays

The intrinsic resolution of each of the RasNiK monitors installed at the Cosmic-Ray Test-Facility - whether of the reference or of the in-plane alignment system - has been determined using the so called ``multiple image'' method. For both optical systems the resolution for displacements of optical elements along the optical axis is of the order of 100~$\mu m$ and can therefore not be used to monitor deformations and displacements on the micrometer scale. The intrinsic resolution of the in-plane RasNiK monitors for optical element displacements perpendicular to the optical axis has been found to be better than 1~$\mu m$ in agreement with previous studies. For the RasNiK monitors of the reference alignment system the intrinsic resolution for these kind of displacements was determined to be about 3~$\mu m$. As individual chamber deformations and relative displacements between the reference chambers below a limit of 5~$\mu m$ over a time of 20 hours can be neglected, one can conclude that the intrinsic resolution of both systems is sufficiently good to detect deformations and displacements exceeding the stability limit. Individual chamber deformations have been reconstructed with a modified program developed by A. Ostaptchuk. In-plane alignment system data analyzed with this program have proven an excellent mechanical stability over a period of days of an individual chamber installed at the test facility. Deformations stay below the critical 5~$\mu m$ limit. Regarding the chambers as rigid bodies when reconstructing relative displacements between them is therefore a valid assumption. Furthermore, individual chamber deformations are small enough not to have an influence on the geometry of the reference alignment system directly mounted on the chambers. An analysis program to reconstruct the relative displacements between the reference chambers from the reference alignment system data has been developed for this thesis in cooperation with A. Ostaptchuk. It is based on the same algorithm as the program used for the data analysis of the in-plane system. It is extendable to a setup with a total of five chambers (3 chambers to be tested and 2 reference chambers) and allows to include the reconstruction of the relative displacements between the upper reference chamber and the test chamber using the capacitive alignment system data. Thereby it will in future be possible to reconstruct the relative chamber position of all chambers installed at the test facility with one program using one principle. The analysis of the reference alignment system data has shown that relative displacements between the reference chambers normally exceed the stability limit during the 20 hours needed to acquire cosmic-ray data. Therefore the reference alignment system is absolutely indispensable to select periods of stability. It can be assumed that the movement of the chamber support tower causes these relative displacements because of its mechanical coupling to the test-facility walls subject to thermal expansion. Further investigations like measurements with a laser interferometer could help to classify and quantify the cause of the relative displacements between the reference chambers. Tests of the program used for reconstruction of relative displacements between the reference chambers have to be conducted in future. No mechanical means allowed setting well-defined relative displacements which could then be reconstructed. A test of the program is nevertheless possible by comparing displacements reconstructed with the program and displacements calculated using cosmic-ray data. This test should be undertaken as soon as the reference chambers are fully equipped with read-out electronics. For the purpose of monitoring relative displacements between the upper reference chamber and the test chamber a capacitive alignment system is employed at the Cosmic-Ray Test-Facility. Calibration data for each of the capacitive probes has been acquired and analyzed. Two interpolation algorithms were developed and tested during the analysis of the calibration data. It has been shown that these algorithms are stable and produce reliable results, allowing to reconstruct relative vertical displacements between test and upper reference chamber with an accuracy of about 1~$\mu m$ through linear interpolation using the calibration data. It has furthermore been shown that two-dimensional linear interpolation using the calibration data allows Alignment monitoring of MDT chambers at the Munich Cosmic-Ray Test-Facility through the use of optical and capacitive alignment systems has been the focus of the studies presented in this thesis. An optical alignment system based on the RasNiK principle monitoring possible relative displacements between the reference MDT chambers has been set up by the author. The setup procedure revealed that the mechanical precision of all parts of each the optical-element support-structures is crucial for systems intended for measurements with an accuracy of a few micrometers. The reference alignment system has been used to monitor actual displacements between the reference chambers. Additionally, the in-plane alignment system of each MDT chamber, which is also based on the RasNiK principle, has been employed to monitor the individual chamber geometry of each of the installed chambers. As both systems are based on the same principle, they both showed a sensitivity to direct and stray light leading to a rapid degradation of the image quality, although an infrared light source and infrared filters are used in the standard design. Adequate light shielding allowed continuous and reliable operation of both systems. This issue will also have an impact on the alignment of the muon chambers at the ATLAS experiment, since it is foreseen to use RasNiK-based alignment systems already during the installation of the chambers. Without proper light shielding a reliable operation of these systems cannot reconstruction of relative horizontal displacements with an accuracy better than 3~$\mu m$. The capacitive alignment system is now ready and operational. This system can now also be used to align the test chambers relative to the upper reference chamber upon their installation. It can therefore be concluded that the alignment systems at the Cosmic-Ray Test-Facility have all been set up and are now fully and reliably operational. Data analysis programs have been developed which allow to include all systems. Their functionality has been tested and proven, although a final test of the results still remains to be undertaken. Finally, it can safely be said that alignment monitoring is indispensable for the success of the chamber tests with cosmic rays, even though the overall stability of the whole setup is very good.