major hdds update to FDC,CDC (fwd)

[Elke-Caroline Aschenauer <elke@jlab.org>]

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---------- Forwarded message ----------
Date: Tue, 15 May 2007 02:28:27 -0400
From: Richard Jones <richard.t.jones@uconn.edu>
To: Matthew Shepherd <mashephe@indiana.edu>
Cc: halld-mc@jlab.org, Alex Dzierba <dzierba@indiana.edu>
Subject: major hdds update to FDC,CDC

I have taken the new materials specifications for the CDC and FDC from gluex-doc-805 and implemented the changes
in hdds.  The modified geometry is now the latest release in subversion.  The following changes have been made.
 1. Reduce the FDC cathode plane Kapton thickness from 50 to 25 microns.
 2. Reduce the FDC cathode strip copper layer from 5 microns to 1 micron.
 3. Remove rohacell backing on cathode strip planes in the active region.  This also gets rid of the two layers
    of epoxy that bond the rohacell to the Kapton on one side, and the mylar on the other.
 4. Reduce the mylar film thickness from 25 microns to 5 microns with 0.5 microns of aluminum on one side.
 5. Change chamber gas mixture to 40/60 Ar/C02.
 6. New heterostructure for FDC support frames, as per gluex-doc-800.
 7. Replace CDC aluminum downstream plate with carbon fiber / epoxy composite.
 8. Reduce the CDC downstream plate from 6mm to 4mm thickness.
Can someone confirm that the mylar is really supposed to be 5 microns?  I inferred this value from the material
budget in gluex-doc-805 table 2 and want to make sure it was not just a mistake.

Note that the modified CDC layer structure with additional inner layers and a smaller outer radius than 60cm  is
NOT a part of the new release.  Those modifications were introduced to hdds back in March for the purpose of
exploring background rates.  However as they were originally designed the modified inner layer structure overlaps
with the target geometry and so cannot be merged into the default geometry as of yet.  Those changes have been
rolled back by commenting out the relevant sections in CentralDC_HDDS.xml.

Richard Jones