FDC frame thickness

[Simon Taylor <staylor@jlab.org>]

The nominal design of the support frame for the FDC elements implemented
in the simulation consists of rings of FR-4.  This presents a signicant
source of photon conversions that affects the performance of the FCAL.  We
have come up with an alternate solution for the cathode plane support
rings (which represent half of the thickness in centimeters of each
package) that replaces the FR4 rings with a laminate consisting of 0.25 mm
E-glass (fiberglass/epoxy), 3.75 mm high-density Rohacell, and 1 mm carbon
fiber shells.  A rough thickness calculation (in radiation lengths, X/X0)
follows for a single 5 mm thick ring:

Carbon fiber (rho= 2.265 g/cm^3, X0=42.70 g/cm^2) = 5.30 x 10^-3
HD Rohacell  (rho= 0.110 g/cm^3, X0=41.04 g/cm^2) = 1.01 x 10^-3
E-glass      (rho= 2.61 g/cm^3,  X0=25.49 g/cm^2) = 2.56 x 10^-3
Epoxy        (rho= 1.08 g/cm^3,  X0=40 g/cm^2)    = 0.27 x 10^-3
	Total                                     = 0.0091

This should be compared to X/X0 = 0.0282 for FR-4 (rho = 1.7 g/cm^3,
X0=30.17 g/cm^2).  So there is a potential factor of 3 level improvement
for a good fraction of the material in the support rings.

Simon