Analysis of the drift velocity of the Silicon Drift Detector for ALICE experiment

Silicon Drift Detectors (SDD) equip the two intermediate layers of the Inner Tracking System (ITS) of the ALICE experiment. The SDD is required to provide a spatial precision of about 35 μm along the drift direction and 25 μm along the anode axis. In these detectors, the drift velocity depends on temperature (ν ∝ T^-2.4), giving a 0.8%/K variation at room temperature. To achieve the required resolution on the drift coordinate, it is necessary to assure a temperature stability within 0.1 K. The drift velocity is determined by measuring the drift time of electrons injected in the sensor volume in fixed positions by means of dedicated MOS devices (injectors). We report the current calibration strategy, the operational experience and the injector performance during the 2010-12 data taking period. Studies of drift velocity stability as a function of time were carried out in different periods and for two different configurations of the SDD cooling system. We also show the results of a study of the time needed to stabilize the temperature after ramping up the detector high voltage. This measurement is important for the detector operation because the high voltage is ramped down and up at each LHC fill during the beam injection.