The SSD commissioning phase
After a complete test of the 144 half-ladders composing the SSD and the integration with the Silicon Drift Detector, during the fall of 2007 the detector was lift down in the ALICE cavern and installed inside the TPC in the final experimental site. Each ladder was connected via approximately 40 m long twisted pair cables to the power supplies and to the FEROM modules, which provide in turn the connection with the system Data AcQuisition system (DAQ), the Detector Control System (DCS) and the Trigger System. During this preliminary phase, the corresponding functionalities of each ladder were individually verified with the locally controlled test system. Once connected and tested the cooling system, the SSD was globally powered and controlled as a part of the ALICE apparatus for the first time. The ALICE experiment is controlled by several independent "on-line systems". Each system carries out different types of operations belonging to a different domain of activities: Detector Control System (DCS), Data Acquisition (DAQ), Trigger system (TRG) and High-Level Trigger (HLT). These systems are independent, may interact with all the single particle detectors and allow groups of them to operate concurrently, within the so-called partitions of detectors. In the final setup the detectors will work all together to collect physics data. In the commissioning phase, however, detectors can run as independent objects for testing and debugging purpose. While this mode, called "standalone mode", is absolutely vital in the commissioning and testing phase, it will also be required during the data-taking phase to perform calibration procedures on individual detectors. It will therefore remain essential during the whole life of ALICE. The functionalities of the detector are mainly controlled by the Detector Control System; responsible for the control and the integration of all the ALICE sub-detectors in a single framework, the DCS provides a constant and complete monitoring of the status of the SSD ladders, for what concerns the sensor bias HV and the electronics LV power supplies, the sensor bias current and the temperature of the modules. The screenshot in fig. 1 gives an example of the on-line overview of the detector conditions, as captured during one of the commissioning tests: each triangle represents a half-ladder; the dierent colors indicate the operating status of the corresponding power supplies and of the front-end electronics.
Figure 1: The DCS software output page showing the status of HV, LV and current of the SSD ladders. The triangles indicate the ladder bias voltage status, the side circles refer to the electronics.
Stand-alone runs and global runs
The integration of the SSD detector with the mentioned control systems was tested through a large set of acquisition runs during the 2008 commissioning sessions. The SSD acquired data both as a stand-alone detector and as a part of a more complex partition of sub-detectors remotely controlled. The work was mainly devoted on one side to optimize the operating parameters, to test, to debug and to calibrate the detector, on the other side to check the compatibility of its operations with the rest of ALICE. The SSD can operate three dierent congurations, corresponding to three run categories, i.e. the stand-alone run, the pedestal run and the global run:
- The stand-alone runs are used for debug, test and calibration purposes: the sub-detector functionality and acquisition control, released by the ALICE global control, is taken by the SSD operator; a set of data is acquired by the desired SSD portion with an adjustable trigger rate. The data are corrected for the pedestal and the common mode oscillations; then they are zero-suppressed. The acquired data can be on-line monitored through the dedicated frameworks, both interactively with the MOOD (Monitor Of Online Data) and automatically with the AMORE (Automatic MOnitoRing Environment). Among the various aspects of the acquisition studied in this phase, a deep investigation was in particular carried out by taking data in stand-alone mode:
- at the default rate (40Hz) to test the synchronization between the read-out operations and the signal digitization;
- at different trigger rates to test the general behaviour of the systems and in particular the noise behaviour;
- at the default rate as a benchmark to check the output data quality of the pedestal runs.
- The pedestal run is a particular kind of stand-alone run used to acquire not filtered data and calculate for each SSD channel the pedestal, the noise and the thresholds for the zero-suppression; it also spots the not properly working channels: the offset correction and the zero suppression are in this case disabled; the acquired sample of a few hundreds of events is processed by a specific algorithm (the Detector Algorithm, DA) in order to calculate the pedestal and the noise of each single SSD channel; the algorithm produces the formatted calibration data for the FEROM system, used for the hardware zero suppression, and for the o-line reconstruction and analysis. A bad channel list including the problematic channels is generated and stored on the calibration database.
- The ALICE global runs can include the SSD in a partition with a set of other sub-detectors under the central ALICE control and are aimed at testing the global operating procedures and at acquiring particle data. In the entire commissioning phase, the SSD took part in a large set of global data acquisition runs, resulting as one of the most stable sub-detector of the ALICE apparatus: during the last 6 months of the 2008 Commissioning Run, i.e. from May '08 to October '08, it collected about 108 events during 1400 runs for a total acquisition time of almost 1000 h.