Online calibrations and performance of the ATLAS Pixel Detector

The ATLAS Pixel Detector is the innermost detector of the ATLAS experiment at the Large Hadron Collider at CERN. It consists of 1744 silicon sensors equipped with approximately 80 M electronic channels, providing typically three measurement points with high resolution for particles emerging from the beam-interaction region, thus allowing measuring particle tracks and secondary vertices with very high precision. The readout system of the Pixel Detector is based on a bi-directional optical data transmission system between the detector and the data acquisition system with an individual link for each of the 1744 modules. Signal conversion components are located on both ends, approximately 80 m apart. The talk will give an overview of the calibration and performance of both the detector and its optical readout. The most basic parameter to be tuned and calibrated for the detector electronics is the readout threshold of the individual pixel channels. These need to be carefully tuned to optimise position resolution and particle detection efficiency. Besides the mere hit information the data for each pixel hit also contains the so-called time over threshold (ToT), which can be used as a measure for the deposited charge. The charge information is then used to improve the position resolution. The talk will describe methods for the tuning and calibrate by means of test charge injections. Furthermore, the timing of the individual pixel modu les has to be synchronised to the interaction time in order to efficiently detect and assign the particle hits to the correct interaction. We describe both the synchronisation of the modules and calibration measurements to determine basic timing properties of the pixel readout. For the optical links testing, calibration and commissioning are described as well as the performance during data-taking.