The ALICE Silicon Pixel Detector System

The European Organization for Particle Physics (CERN) in Geneva is currently constructing the Large Hadron Collider (LHC), which will allow the study of the subnuclear ranges of physics with an accuracy never achieved before. Within the LHC project, ALICE is to the study of strongly interacting matter at extreme densities and high temperatures. ALICE as many other modern High Energy Physics (HEP) experiments uses silicon pixel detectors for tracking close to the interaction point (IP). The ALICE Silicon Pixel Detector (SPD) will constitute the two innermost layers of ALICE, and will due to its high granularity provide precise tracking information. In heavy ion collisions, the track density could be as high as 80 tracks/cm2 in the first SPD layer. The SPD will provide tracking information at radii of 3.9 and 7.6 cm from the IP. It is a fundamental element for the study of the weak decays of the particles carrying heavy flavour, whose typical signature will be a secondary vertex separated from the primary vertex by a few hundred microns only. The SPD will provide a spatial resolution of around 12 µm in the rφ-direction. One of the specific challenges for the ALICE SPD will be the stringent material budget constraints (<1% per layer) in order to have as small as possible influence on the traversing particles. In the design and production process, these constraints were followed to the point, so will the sensor and the readout chip have a total thickness of only 350 µm and the signal lines from the front-end to the on-detector electronics will be deployed in complete aluminum. The results presented in this thesis illustrate the measurements performed in order to characterize the detector performance and qualify components for inclusion in the detector and depicts the work carried out needed for the production phase of the SPD. Quality assurance criteria and test procedures have been developed and fine-tuned for the different components of the SPD. The tests involved visual inspection, electrical tests as well as measurements using a radioactive source. Two beam tests were carried out in the past years. In October 2003 one pixel plane was studied in a heavy ion beam as well as in a proton/pion beam. Four other pixel planes were used as reference planes for tracking. In November 2004 a joint beam test with two planes of each subdetector of the ALICE Inner Tracking System (ITS) took place. For the first time the ALICE data acquisition system (DATE) and the ALICE trigger system were used with two planes of each subdetector. A new tracking algorithm for the proton/pion beam data taking the tilt and the azimuthal angle into account was developed. With this new tracking method it was possible also to analyze the data taken at the wide beam setting during the beam tests 2003. Further, a study of the cluster sizes as function of different operating parameters, i.e. threshold, and track incidence angle was performed giving a comparison between two sensor thicknesses (200 µm and 300 µm). A test system based on a pulsed infrared laser was established in order to test SPD assemblies and the FastOr signal generated by the pixel chip. This laser allowed for the first time a complete characterization of the working point and performance of the chip internally generated FastOr signal. This signal will contribute to the L0 trigger decision in ALICE proton-proton runs. It will be the first time a large high energy physics experiment uses tracking information from a pixel detector for low level triggering. Furthermore, the setup allowed a detailed study of the ALICE1LHCB chip characteristics with well defined timing and energy deposition in the silicon sensor. A laser calibration was performed using the internal threshold DAC and two radioactive sources (55Fe and 109Cd) providing a precise absolute calibration of the detector for the first time. The laser measurements on ALICE assemblies could directly be compared with the results of the beam test 2002 and 2003. During my doctoral thesis at CERN, I have developed and applied test criteria for the SPD components. Furthermore, I developed an infra red laser test system which allowed a detailed study of the detector performance, a special cluster size study and the functionality of the FastOr signal. The cluster size studies obtained with the laser measurements are compared with the results from the beam test data taken in high energy particle beams.