Coatings were applied to the Rohacell cages in order to minimize the response of the foam to changes in the environment. In particular, the time constant of the Rohacell response was desired to be on the order of several days as access to the MVD will be limited during an experimental run. This would also give the experimenters adequate time to respond to a loss of humidity or temperature control. After testing many coatings, Parylene, a vapor deposited conformal coating, was found to be the most suitable for the MVD. It is moisture resistant and because it can penetrate materials, also adds mechanical robustness . Vapor deposition assures a uniform and homogeneous coating over the entire structure . Parylene C was chosen for maximum moisture protection and to increase the mechanical robustness of the foam. The thickness of the Parylene layer was optimized to maximize the robustness and moisture resistance while minimizing the mass.
Strips of Rohacell with 0.10, 0.25, 0.50, 0.75 and 1.00 mil (thousandth of an inch) thicknesses of Parylene were tested for mechanical robustness. Using a small paint brush, each Parylened strip was brushed fifty times in order to test how much the Parylene coating increased the robustness of the Rohacell foam. During brushing, the Rohacell flakes were collected on a black surface and counted. The results, presented in figure 3, show that as the thickness of Parylene increases, the mechanical robustness of the Rohacell foam increases. In other words, the number of flakes which detach from the foam potentially causing harm to the electronics, decrease as the thickness of Parylene is increased. The increase in robustness is not linear with Parylene thickness; the gain in robustness begins to level off at 0.50mil.
Figure 4 shows that the mass of the strip increases with thickness of coating. The undesirable increase in mass was compared to the improved mechanical robustness to optimize the thickness of Parylene. Based on these results, a 0.50 mil thickness of Parylene was chosen and will be added to the cages before adhering the silicon microstrip detectors. Though the Rohacell strips showed a 60% change in mass when coated with 0.50 mil of Parylene, C-cages only showed a 45% increase. This is most likely due to the different surface area to volume ratios in the strip vs. the C-cage. The addition of a 0.50 mil Parylene coating, increases the radiation length of the Rohacell cage from 0.06% to 0.09% .