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The geometry for the Monte Carlo is found in the repository under programs/Simulation/hdds. The description of the geometry is written in xml format. The programs hdds-geant and hdds-root translate these xml based geometry into fortran code for GEANT3 and C code for root, respectively.


  • The latest CDC geometry modification implemented a downstream gas plenum of 2.54cm thickness with an aluminized mylar window. The position of the 28 layers of straws are modeled according to the latest numbers with straws of 1.6025cm outer diameter including tolerances. The outer aluminum shell of the CDC is 2mm thick with an OD of about 4cm smaller than the end plates. 12 aluminum support rods are located outside the gas volume of the CDC between the end plates. The gas volume is an Ar/CO2 mixture. The downstream end plate is carbon fiber with 6mm thickness while the downstream gas plenum is 2.54cm thick. The upstream end plate is from Aluminum and 12.5mm thick with an upstream gas plenum of 6.99mm thickness. Some pictures with explanations about the CDC geometry as of April 09 is found HERE.
  • Some materials:
    1. Downstream end plate 6mm thick carbon fiber
    2. Downstream gas plenum 2.54cm thick with rohacell walls and aluminized mylar window
    3. Upstream end plate 12.5mm thick aluminum
    4. Upstream gas plenum 6.99cm thick with aluminum wall.
    5. CDC outer shell is 2mm aluminum
    6. CDC inner shell is 26 micro meter aluminized mylar
    7. Upstream gas window 15mm plexi glass
    8. Straw: Aluminized Kapton: 100um Kapton with 5um Aluminum


  • Relative to Geometry V4, the new (5/19/09) FDC geometry has reduced material in the active area, closer spacing between adjacent wire planes, and a refined model for the support frames. As before, the chamber gas is 40% Ar/60% CO2 and the active volumes (in a cathode-anode-cathode unit) are 1 cm thick. A list of the major changes follows.
    1. The chambers are no longer grouped in modules of three: the new unit is a single chamber, so in principle all of the rotation angles of the chambers for a package can be different or a chamber could be removed from a package.
    2. The distance between the most upstream wire plane of the first package and the most downstream wire plane of the fourth package has been increased from 170.0 cm to 182.68 cm to be consistent with the current engineering drawings but this is likely to change...
    3. The wire plane-to-wire plane separation within a package was reduced from 3cm to 2cm.
    4. The 2mm foam in the active area has been removed
    5. The thickness of each ground plane has been reduced from 50 microns to 13 microns.
    6. The package-to-package support shell has been changed from aluminum to FR-4 and the support rods are now also FR4.
    7. The compression rods are now Ultem-2300 (fiber-reinforced polyetherimide).
    8. Noryl gusset rings for the compression of the o-rings have been added.
    9. A Noryl ring for supporting the cables on the upstream side of the FDC has been added.
    10. The spacer ring between the wire plane and one of the cathode planes in a chamber is now polypropylene instead of a composite.
    11. The other support structures are now more consistent with the current engineering design.
    12. The chamber gas now fills the entire interior package volume instead of just the 1 cm active region of each chamber.
    13. The epoxy in the cathode material has been removed from the active region.
  • Some things have not been changed in this version:
    1. The cable routing is still consistent with the old geometry
    2. The mockup for the ASIC cooling system is still a very crude model and does not contain the actual ring structure we are currently planning to use.
    3. The cooling manifolds and other non-phi-symmetric elements are not yet present.
  • Some drawings of the new geometry are HERE.