Precision Calibration of Large Area Micromegas Detectors Using Cosmic Rays

Abstract

Currently square-meter sized Micromegas detectors with a spatial resolution better than 100 $\mu$m are of big interest for many experiments. Due to their size the construction of these detectors is highly sophisticated as they need to fulfill strict mechanical tolerances. We developed a method to survey working detectors on potential deviations of the micro pattern readout structures from design value as well as deformations of the whole detector, using cosmic muons in a tracking facility. The LMU Cosmic Ray Facility consists of two 8\,m$^2$ Monitored Drift Tube chambers (ATLAS MDT) for precision muon tracking and two segmented trigger hodoscopes providing 10 cm position information along the wires of the MDTs with sub-ns time-resolution. It provides information on homogeneity in efficiency and pulse height of one or several Micromegas installed between the MDTs. With an angular acceptance of -30$^\circ$ to +30$^\circ$ the comparison of the MDT muon tracking with centroidal position determination or TPC-like track reconstruction in the Micromegas allows for calibration in three dimensions. We currently investigate a detector system consisting of a 1 m$^2$ and three 100 cm$^2$ resistive strip Micromegas, with emphasis on the differences in performance between large and small detectors. The small detectors behave dimensionally stable, whereas the large detector seems to show deviations from the readout strip straightness and global deformation due to the small overpressure caused by the Ar:CO$_2$ gas flux. We introduce the alignment and calibration procedure, report on homogeneity in efficiency and pulse height and present results on deformation and performance of the 1 m$^2$ Micromegas. The same measurement will be performed with a 4-layer Micromegas quadruplet of 2 m$^2$ size, in near future.