Design and construction of precision tooling for the construction of resistive strip micromegas detectors for the ATLAS Small Wheel upgrade project

Abstract

Mechanical precision is a key aspect of the 2 m2 high-rate capable Micromegas detectors for the upgrade of the Small Wheels of the ATLAS muon spectrometer. The Micromegas will be built as quadruplets with cathodes and strip-anodes made of stable honeycomb sandwiches. To achieve the required single plane resolution below 100 μm the deviation from planarity of a single detector plane must not exceed 80 μm in direction perpendicular to the precision coordinate. The global position of the readout strips has to be within 30μm for a single readout-plane of three PCBs, as well as between all four planes of a quadruplet. Precision tooling is used for the correct positioning of readout PCBs and readout sandwich planes. For quality control of the planarity of the sandwich planes a laser distance sensor combined with a coordinate measurement system has been developed. Deviation from planarity below 10 μm can be easily resolved. We will present key features of the challenging construction procedure to achieve this high level of precision as well as our alignment strategies. This includes the construction and commissioning of a 2.5 m2 lightweight rigid structure (stiffback), which has an overall planarity below 15 μm RMS and the measurement of the the blow up of outer planes of a quadruplet due to 2 mbar overpressure of the Ar:CO2 detector gas, the standard situation in ATLAS.