The TRD Geometry

Geometry Classes

The TRD geometry exists in two versions. One covers the full azimuth angle and the other leaves the regions in front of the HMPID and the PHOS empty:

The geometry is defined by the classes AliTRDgeometryHole (currently the default, but will propably become obsolete soon) and AliTRDgeometryFull. Depending on the FRAME version that is chosen in Config.C the correct TRD geometry is automatically selected:

AliFRAMEv0 -> FRAME + TRD with two holes.
AliFRAMEv1 -> FRAME without holes + TRD with optional holes. Use the options AliTRD::SetPHOShole() and AliTRD::SetRICHhole() to leave the space infront of the RICH and the HMPID empty.

The front view and the side view of the ALICE experiment show how the TRD is situated in the ALICE setup.
The geometry classes serve as a centralized container for all informations concerning the geometry and should provide all corresponding functions like e.g. transformation between local and global coordinates: AliTRDgeometry::Local2Global()

Coordinates

The global coordinate system is the general ALICE system. The local coordinate system is defined in each chamber is defined in units of pad sizes (row,column,timebin). Here the pad divisions along the z-direction are the rows and along the rphi-direction the columns. The position of the origin (pad (0,0,0)) is defined as shown here:

   Chambers seen from the top:
       +----------------------------+
   col |                            |
       |                            |     ^
       |                            | rphi|
       |                            |     |
       |0                           |     |
       +----------------------------+     +------>
                                 row           z
 
   Chambers seen from the side:           ^
       +----------------------------+ time|
  time |                            |     |
       |0                           |     |
       +----------------------------+     +------>
                                 row           z

Each individual chamber is identified by an unique detector number. For convenience this number can also be translated into another numbering scheme:

Plane number (0 - 5, starting from the innermost plane)
Sector number (0 - 17, the numbering goes clockwise and starts at y = 0)
Chamber number (0: outer left, 1: middle left, 2: inner, 3: middle right, 4: outer right)

The mapping between these numbers and the detector number is provided by the functions AliTRDgeometry::GetDetector(), AliTRDgeometry::GetPlane(), AliTRDgeometry::GetSector(), and AliTRDgeometry::GetChamber().

Volumina and Materials

The TRD geometry contains the following volumina (see also the TRD tree and AliTRDgeometry::CreateGeometry()):

TRD1-3: The TRD mother volumes for one sector. They are positioned inside the spaceframe. Material: Air
UAFI(/M/O): The aluminum frame of the inner(/middle/outer) chambers containing the readout part. Material: Aluminum
UCFI(/M/O): The carbon frame of the inner(/middle/outer) chambers containing the radiator and driftchamber. Material: Carbon
UAII(/M/O): The interior of the aluminum part of the inner(/middle/outer) chambers. Material: Air
UFII(/M/O): The interior of the carbon part of the inner(/middle/outer) chambers. Material: Air
UL01: The gas seal of the radiator. Material: G10
UL02: The gas in the radiator. Material: CO2
UL03: The foil stack. Material: PE
UL04: Entrance window to the driftvolume and HV-cathode. Material: Mylar
UL05: The driftvolume. Material: Xe (90%) + CO2 (10%) (or Xe (95%) + CH4 (5%))
UL06: The amplification region. Material: Xe (90%) + CO2 (10%) (or Xe (95%) + CH4 (5%))
UL07: The pad plane. Material: Copper
UL08: The Nomex honeycomb support structure. Material: G10
UL09: FEE and signal lines. Material: Copper
UL10: The cooling devices. Material: PE
UL11: The cooling water. Material: Water

This picture shows one TRD segment (volume TRD1, red frame), containing the inner, middle, and outer chambers in a staggered arrangement.
One single chamber consists of the frame, made of aluminum and carbon, filled with several layers to include the material inside the chambers. A cross section of one segment is shown here. The radiators, represented by a CO2 gas volumes including a PE layer (volumes UL02 and UL03), are displayed in red. The driftvolume and the amplification region (UL05 and UL06) are plotted in yellow.

The dimensions of the chambers are defined in the routines AliTRDgeometry::Init() and AliTRDgeometryFull::Init() (resp. AliTRDgeometryHole::Init()). The thicknesses and positions of the material layers can be found in the header file AliTRDconst.h. All dimensions should be identical to the ones in the technical drawings by B.Windelband.

Page maintaned by Christoph Blume. Last modification: 9/05/00.