Caratterizzazione di rivelatori a pixel monolitici per l'Inner Tracking System dell'esperimento ALICE

The present thesis is focused on the assembly and characterization of the detectors developed for the upgrade of the ALICE (A Large Ion Collider Experiment) Inner Tracking System. ALICE is a general-purpose experiment at the Large Hadron Collider at CERN, principally aimed at studying high-energy heavy-ion collisions. Its Inner Tracking System (ITS) is made of cylindrical silicon detector layers surrounding the beam pipe and is immersed in a 0.5 T magnetic field produced by a large solenoidal magnet. The ITS tracks the charged particles originating from the collisions which take place in the beam pipe. Charged particles deposit a certain amount of energy in the silicon sensors of the cylindrical detector layers, thus allowing a position information to be extracted and the particle tracks reconstructed. The ALICE Inner Tracking System is undergoing a major upgrade. The old ITS is being replaced by a new one, entirely based on the Monolithic Active Pixel Sensors technology. Such devices will be introduced and described. In particular, the ALPIDE (ALice PIxel DEtector) monolithic pixel sensor was developed to equip the upgraded Inner Tracking System, which is currently in its assembly and test phase. The ALPIDE chip layout, functioning principle and working parameters are described in the present work. My thesis work is mainly devoted to the assembly and test of the upgraded ALICE ITS and the characterization of its detecting elements, both from a metrological and from an electrical point of view. In particular, the use of the ALPIDE sensor in the upgraded ITS is studied. The involvement in the detector assembly and test at the INFN Technological Laboratory in Torino constitutes the first contribution of my thesis work. The structure of the upgraded ITS and its assembly and test procedures are described in detail. The characterization of the ALPIDE-based detector elements is my second contribution to the ITS2 construction effort. The results of the metrological characterization, aiming at assessing whether the detector production met the mechanical requirements established during the design phase, are presented. An introductory study on the stability of the detector performance in terms of in-pixel signal discrimination is described. In this study, the history of a detector element is followed through its evolution in the different assembly and test stages. In order to assess the uniformity of the signal discrimination on the full detector, the distribution of the signal thresholds on all the pixels of different detection elements is also studied. I also investigated a possible defect of the pixel matrix discovered on a few chips belonging to a sub-sample of detector elements (Staves) tested during my thesis work, and I produced a mapping of the malfunctioning chips, which was needed to validate the proposed positioning of the detector elements in the final cylindrical structure of the upgraded Inner Tracking System.