Construction and Quality Assurance of Large Area Resistive Strip Micromegas for the Upgrade of the ATLAS Muon Spectrometer at LHC/CERN

Large area Micromegas detectors will be employed for the first time in high-energy physics experiments. To cope with increasing background rates, associated with the steadily increasing luminosity of LHC to 10 times design luminosity, the present detector technology in the current innermost stations of the muon endcap system of the ATLAS experiment (the Small Wheel), will be replaced in 2019 by resistive strip Micromegas and small strip TGC detectors. Both technologies will provide tracking and trigger information. In the ``New Small Wheel'' the Micromegas will be arranged in eight detection layers built of trapezoidally shaped quadruplets of four different sizes covering in total about 1200\,m$^2$ of detection plane. In order to achieve 15\,\% transverse momentum resolution for 1 TeV muons, a challenging mechanical precision is required in the construction of each active plane, with an alignment of the readout strips at the level of 30\,\textmu m along the precision coordinate and 80\,\textmu m perpendicular to the plane. Each individual Micromegas plane must achieve a spatial resolution better than 100\,\textmu m at background rates up to 15\,kHz/cm$^2$ while being operated in an inhomogeneous magnetic field (B $\leq$ 0.5\,T). The required mechanical precision for the production of the components and their assembly, on such large area detectors, is a key point and must be controlled during construction and integration. Particularly the alignment of the readout strips within a quadruplet appears to be demanding. The readout strips are etched on PCB boards using photolithographic processes. Depending on the type of the module 3 or 5 PCB boards need to be joined and precisely aligned to form a full readout plane. The precision in the alignment is reached either by use of precision mechanical holes or by optical masks, both referenced to the strip patterns. Assembly procedures have been developed to build the single panels with the required mechanical precision and to assemble them in a module including the four metallic micro-meshes. Methods to confirm the precision of components and assembly are based on precise optical devices and X-ray or cosmic muon investigations. We will report on the construction procedures for the Micromegas quadruplets, on the quality control procedures and results, and on the assembly and calibration methods.