Development and characterisation of Monolithic Active Pixel Sensor prototypes for the upgrade of the ALICE Inner Tracking System

ALICE (A Large Ion Collider Experiment) is dedicated to the study and characterisation of the Quark-­‐Gluon Plasma (QGP), exploiting the unique potential of ultrarelativistic heavy-­‐ion collisions at the CERN Large Hadron Collider (LHC). The increase of the LHC luminosity leading up to about 50 kHz Pb-­‐Pb interaction rate after the second long shutdown (in 2018-­‐2019) will offer the possibility to perform high precision measurements of rare probes over a wide range of momenta. These measurements are statistically limited or not even possible with the present experimental set up. For this reason, an upgrade strategy for several ALICE detectors is being pursued. In particular, it is foreseen to replace the Inner Tracking System (ITS) by a new detector which will significantly improve the tracking and vertexing capabilities of ALICE in the upgrade scenario. The new ITS will have a barrel geometry consisting of seven layers of Monolithic Active Pixel Sensors (MAPS) with high granularity, which will fulfil the material budget, readout and radiation hardness requirements for the upgrade. Intensive R&D has been carried out in the last four years on MAPS in the framework of the ALICE ITS upgrade. Various small scale sensors have been designed in the TowerJazz 0.18 um imaging sensor technology to study noise, charge collection efficiency and signal-­‐to-­‐noise ratio. This work presents the main characterization results obtained from the measurements performed on two small scale prototypes (MIMOSA-­‐32 and MIMOSA-­‐32ter) with X-­‐ray sources and beams of particles. The architecture of an innovative full scale MAPS prototype (Alice Pixel Detector, ALPIDE) is also presented that is based on an AC-­‐sensitive front end and on a hit-­‐ driven readout. The first results on the ALPIDE prototype showed that the sensor is fully functional and that it provides performance in terms of readout time, power density and noise much better than the state of the art MAPS based on the rolling shutter readout, which makes this type of sensors very attractive for employment in the new ALICE ITS.