Calibration of a high-resolution straw-tube-tracker for the COSY-TOF experiment

The Straw-Tube-Tracker (STT) is an essential detector in the COSY-TOF experiment which provides accurate position information of charged particle tracks to study strangeness physics. The STT consists of about 2700 straw tubes in 26 layers, installed in a vacuum tank about 30 cm behind the target. It is operated with 1.2 bar overpressure to provide mechanical stability despite the low material budget of 1 % X/X0 radiation length. The STT has been successfully used in the measurement of hyperon production reactions in which a spatial resolution of 150 μm and an efficiency of 98 % has been achieved. The STT performance could be improved further by optimizing the calibration. This will increase the reconstruction efficiency and resolution in order to identify delayed vertices at even shorter distances from the primary vertex. The new calibration has to include subtle effects e.g. signal propagation in wire and systematic effects of the DAQ. Hence, pp elastic scattering events were measured in Fall 2012 at 2.95 GeV/c beam momentum are analyzed for the improved calibration of the STT. The results of the improved calibration are presented.