Rohacell is a suitable material for the silicon microstrip detector support, offering a light-weight and mechanically robust structure for the MVD. Parylene, the vapor deposited coating, adds mechanical robustness to the Rohacell, increases the time constant, or the time it takes moisture to enter the Rohacell and induce changes, and minimizes dramatic changes in a defined period of time in the event that the environmental controls were to fail and the humidity of the MVD environment was to increase. If the environmental controls were to fail and one assumes a worst-case scenario of 80-90% relative humidity, one would have approximately 60 hours before changes in the dimensions of the cages would begin to reach their maximum.
Environmental tests indicate that changes in the width do not exceed the -0.4% mechanical constraint. Adhering silicon wafers to the Rohacell cage reduces the expansion and contraction of the widths. Changes in the radial distances never approach the +/-1.7% physics constraint. Light-weight plastic tubes which locate the C-cages to each other minimize expansions and contractions in length in the event of environmental failure.
Rohacell is a mechanically robust and stable material for the support structure material for the silicon microstrip detectors. The Multiplicity Vertex Detector will operate under constant environmental conditions which will ensure that the Rohacell will remain stable during operation periods. It is, however, important to understand the behavior of the Rohacell under unfavorable conditions which could arise from failure of environmental controls. Such understanding also helps in defining the handling, machining and transportation procedures for the MVD.