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Description of ALICE ITS Geometry Code (Rev. 1,00 - April, 30^th 2008 - M. Sitta)

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Content

1. General description

2. Detector-specific classes

3. Initialization

4. Main methods

5. Helper classes AliITSgeom and AliITSgeomTGeo

General description

The AliITSv11Hybrid class describes the overall ITS geometry. It is a mix between the old geometry (originally coded in the AliITSvPPRasymmFMD class) and the new TGeo geometry (v11). The "old" part is described using Geant3 volumes, while the "new" one using TGeo volumes; moreover the new part is more adherent to the technical drawings and so is more realistic.

Proper flags, set in the AliITSInitGeometry class, determine whether the old or the new part is used.

Currently all the sensitive detectors and their supports, the mechanical supports and part of the services (namely the SDD cables) are described in the new geometry. The rest of the services and some other mechanics are (sometimes partly) described in the old geometry. Work is in progress to move everything in the new geometry.

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Detector-specific classes

The old geometry is fully described in the AliITSv11Hybrid class. For the new geometry on the other hand, a set of specific classes are defined

-> AliITSv11GeometrySPD describes the whole SPD geometry

-> AliITSv11GeometrySDD describes the whole SDD geometry

-> AliITSv11GeometrySSD describes the whole SSD geometry

-> AliITSv11GeometrySupport describes the geometry of some mechanical supports (thermal shields and support cylinders and cones)

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Initialization

To initialize an object of the AliITSv11Hybrid class it is enough to call its constructor in the Config.C macro

AliITSv11Hybrid *ITS = new AliITSv11Hybrid("ITS","ITS detailed version with TGeo description");

the first string is the detector name (which is actually ignored, and internally set to "ITS"), the second string is an arbitrary title. Both strings are passed to the AliITS constructor.

The class constructor initializes its private members, creates an object for each detector-specific class and sets their names, and sets the defaults for some detector parameters.

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Main Methods

The main methods of the AliITSv11Hybrid class are

• AliITSv11Hybrid::CreateGeometry first calls CreateOldGeometry to define the old (Geant3) volumes (the ones still active according to the aforementioned flags); then it calls specific methods of the detector classes to create the new geometry
• AliITSv11Hybrid::CreateOldGeometry creates the old geometry volumes; flags are used to determine which sections are to be skipped because already coded in the detector-specific classes
• AliITSv11Hybrid::CreateMaterials defines all materials and mediums used in the ITS geometry description
• AliITSv11Hybrid::AddAlignableVolumes creates entries for alignable volumes associating the symbolic volume name with the corresponding volume path
• AliITSv11Hybrid::StepManager is called at every step in the ITS to save hits according to the subdetector which the particle is in

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Helper classes AliITSgeom and AliITSgeomTGeo

AliITSgeomTGeo is a simple interface class to TGeoManager. It is used in the simulation and reconstruction in order to query the TGeo ITS geometry.

It has methods such as for example
GetModuleIndex to compute the module index number from the layer, ladder, and detector numbers
GetModuleId to compute the layer, ladder and detector number given the module index number
GetLayer to compute the layer number for a given the module index
GetMatrix to get the transformation matrix for a given module by quering the TGeoManager
GetTranslation and GetRotation to get the translation vector or the rotation matrix for a given module by quering the TGeoManager
LocalToGlobal and GlobalToLocal to make the conversion from the local sensitive reference system to the global reference system and vice-versa, for an arbitrary local or global position

Similarly the AliITSgeom class contains other methods for querying and interfacing the ITS geometry.