Results from the ALICE-TPC laser calibration system

A cylindrical Time Projection Chamber [1], is the main tracking device of the dedicated heavy ion LHC experiment ALICE at CERN. Each end of the cylinder holds a readout plane which is subdivided into 18 segments in azimuth and two parts in radius (s. fig. 1). Both readout planes thus consist of 18 innerand 18 outer multiwire proportional readout chambers (IROC and OROC, respectively) with cathode pad readout. A Central (high voltage) Electrode (CE) divides the TPC into two readout sides (Aand C-Side). On either readout side a system of micro-mirror bundles distributes a laser beam in four planes along the beam direction. The resulting 168 laser rays can be used to measure, the residual misalignment between adjacent readout chambers. A projection on the transverse-plane of the starlike pattern of laser rays is shown in fig. 1.