Mapping the large area straw detectors of the COMPASS experiment with X-rays

In the COMPASS experiment at CERN, large straw drift tube detectors are used for large area tracking. To minimize the total areal density, a self supporting structure of thin walled plastic tubes was chosen and, hence, a loss in mechanical precision was accepted. An X-ray CCD apparatus was built to investigate the mechanical properties and to calibrate (off-line) the wire positions. Deviations of typically 200-400 μm from the nominal positions, defined by equal spacing, are found across the detector areas of 8 m². With a calibration method based on high-resolution CCD imaging and pattern recognition algorithms, the absolute wire coordinates are determined with an accuracy better than 30 μm, which is sufficient in view of the intrinsic straw resolution of 200 μm for minimum ionizing particles. Temperature effects are clearly seen. Their inhomogeneity limits the achievable accuracy. The off-line calibration was checked with particle tracks in the experimental setup, running COMPASS with 160 GeV/c muons. The data from other detectors that cover the central straw area were used for this comparison. When a scaling factor is applied to compensate for thermal expansion, good agreement is found between the off-line X-ray and the in situ measurements.