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Description of ALICE Silicon Strip Detector (SSD) Code (May 7, 2008, Enrico Fragiacomo)

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Content

1. Geometrical description of the SSD module

2. Detector response simulation

3. Cluster finding and local reconstruction

4. Calibration files (OCDB)

5. ECS Run types and DAs

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Geometrical description of the SSD module

Each of the 1698 SSD modules (748 on layer5 and 950 on layer6, module index ranging 500-2198) is a double-sided silicon strip detector.

Both P- and N-side has 768 strips, strip numbering ranging 0-767. Strip 0 on Pside is opposite to strip 0 on Nside (note that hardware numbering ranges 0-1535, with strip 1535, on Nside, opposite to strip 0 on Pside. This leads to conversions of the type strip=1535-strip for Nside, e.g. in the streamer class for rawdata decoding, see below).

P- and N-stereo angles are 7.5 milliradiant and 27.5 milliradiant, respectively, leading to a P-to-N stereo angle of 35 milliradiant. This small value eventually decreases the resolution (750 microns) along the z-coordinate parallel to the strips but reduces the number of fake intersections (ghosts). The resolution along the x-coordinate, perpendicular to the strips, is down to 20 microns.

Detector size along x (r-phi): 72960 microns

Detector size along z: 40000 microns

Detector thickness (local y): 300 microns

Strip pitch: 95 microns

Stereo angles, detector sizes and number of channels are controlled via the AliITSsegmentationSSD class, which also provides the tools for system frame conversion (local to global and viceversa).

Each SSD module is readout by 12 chips, 6 on P- and 6 on N-side, each of them reading 128 strips. Conversion from local coordinates to chip index is also provided by AliITSsegmentationSSD for dead area recovery during tracking (if the track misses the point, the tracker checks if the area is readout by a dead chip).

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Detector response simulation

The SSD response simulation and digitization proceeds via the AliITSsimulationSSD class with the method DigitiseModule(), which in turn is called for each module by the HitsToDigits method of AliITS. Calibration and segmentation information are obtained via the AliITSDetTypeSim class.

DigitiseModule calls:

1. HitsToAnalogDigits, which provides the analog strip signal from the energy release of the particle passing through the module. The step between two geant hits (tipically, one entering and one exiting the module for perpendicular tracks) is further divided into 25 microns step to allow for a better geometrical description of the charge distribution in the silicon. From the energy release in the step a certain amount of charge is produced (as electron-holes pairs generated in the middle of the step) which drifts as gaussian clouds to the two sides of the detector (electrons to the P- and holes to the N-side). The width of the cloud (which eventually determines the number of strips involved by the cloud) depends on the drift constant D and on the drift time, which in turn depends on the drift velocity and the generation point. Both D and drift velocity differ for the two types of carriers. The percentage of the charge gaussian cloud which spatially corresponds to a strip is given to that strip.

2. SdigitToDigit, which in turn:

   1. distributes the signal to the neighbouring strips according to the capacitive coupling,

   2. adds the electronic noise (modeled as a gaussian distribution) to each strip (noise values are taken from the OCDB via AliITSCalibrationSSD);

   3. kills dead strips;

   4. zero-suppresses those strips with signal below 3*sigma_noise.

   5. signal is finally converted to ADC units and uncalibrated for the gain (with values from the OCDB).

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Cluster finding and local reconstruction

Cluster finding and local reconstruction in the SSD proceeds via the AliITSClusterFinderV2SSD class. As AliReconstruction::RunLocalEventReconstruction calls AliITSReconstructor::Reconstruct for the ITS and in turn AliITSDetTypeRec::DigitsToRecPoints, two differents methods of AliITSClusterFinderV2SSD (FindRawClusters or RawdataToClusters) can be called for each SSD module depending whether the input is a digits file (essentially from simulation) or a rawdata file (from real data).

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Calibration Files (OCDB)

There are 4 directories in ITS/Calib with calibration data to be used in SSD reconstruction

1. NoiseSSD

2. PedestalSSD

3. BadChannelsSSD

4. GainSSD

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ECS Run Types amd DA

1) STANDALONE

- for test purposes

- no DA launched at EOR

2) PEDESTAL

3) PHYSICS

Nothing done for now.

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