Precision Calibration of Large Area Micro Pattern Gaseous Detectors

To cope with the increased luminosity of the Large Hadron Collider at CERN the inner- most end-cap regions of the ATLAS Muon Spectrometer, the so-called Small Wheels, have to be upgraded. Each New Small Wheel will be subdivided in eight large and eight small sectors each consisting of two trapezoidal modules. The MICRO-MEsh GAseous Structure (Micromegas) detector technology has been chosen for this upgrade. Every Micromegas module will consist of four active detection layers with a size of 2-3m2 each. Micromegas detectors in this size were never built before. To maintain the excellent momentum reso- lution of 15% at 1TeV muons of the current Muon Spectrometer the Micromegas have to reconstruct the hit position with a spatial resolution of about 100µm at high background hit rate of 15kHzcm−2. Herefore a very accurate construction and a calibration of the modules is demanded. This thesis focuses on the modules built in Germany, the so-called SM2-modules, which belong to the small modules with a size of about 2m2. Each module consists of five panels, three drift and two readout panels, with an alu- minumhoneycombcoresandwichedbytwo0.5mmthickprintedcircuitboards(PCB).The PCB surfaces are etched either as cathodes or as resistive anode readout structures. Due to production limitations in industrial PCB production the panel surface has to be con- structed out of three PCBs which have to have a strip alignment better than 30µm. This is ensured by a very accurate alignment frame during gluing of the panels. The planarity of each panel has to be better than 37µm. After completion the panels are assembled to a module with dedicated alignment pins and precise spacers to assure the same accuracy between the strips of the four surfaces of the two readout panels relative to each other as within one surface. Six so-called interconnections are used to stabilize the module against a slight overpressure of about 2-3mbar. Each assembled module will be calibrated in the LMU Cosmic Ray Facility in Garching consisting of two reference tracking detectors, i.e. Monitored Drift Tube chambers, and two trigger hodoscopes, which provide also a coarse position information on the coordinate perpendicular to the precision measurement direction. Techniques have been developed while calibrating the shift and rotation between two readout PCBs of a 1m2 prototype Micromegas with one detection plane. This measurement is very sensitive to strip pitch variations down to a few ten nm. The deformation of the drift region due to a slight over- pressure can be determined as well. This 1m2 prototype Micromegas lacks an overpressure stabilization and thus shows a deformation of about 3mm. The 1m2 large prototype Micromegas yields a spatial resolution of about 150µm for cosmic muon tracks perpendicular to the detection plane considering multiple scattering and the track prediction accuracy of the reference detectors in a measurement of about one week. A shorter measurement of about one day with the same parameters yielded a spatial resolution of about 80µm for perpendicular tracks. It seems there is a contribution of about 100µm as a function of time due to thermal movement, which is reasonable considering the size of the experimental setup. The experiment shows additionally, that largerreferenceanglesdegradetheachievablespatialresolution, whichisingoodagreement with the results of a testbeam campaign with 120GeV pions at CERN. A degraded spatial resolution is found for the 1m2 prototype when compared to small test Mircomegas with an active area of about (10 × 10) cm2 during this testbeam campaign. This is due to the high noise level of the large prototype Micromegas, which had been assembled just before the measurement. Additionally Argon and Neon based drift gases are investigated in the Cosmic Ray Facility using the large prototype Micromegas. In these measurements effects of the drift velocity, lateral and longitudinal diffusion, and the electron transparency of the mesh on pulse height, tracking efficiency and spatial resolution, are determined with the centroid and the TPC-like method, have been determined. One application of cosmic muons is muon tomography. In the Cosmic Ray Facility the point of Coulomb scattering of a traversing muon is determined and thus any area with more dense material can be identified. This demonstrates the excellent resolution of the Cosmic Ray Facility.