Rohacell, a lightweight polymethacrylimide low density rigid foam, is suited for a variety of purposes. It is used as core material in aircraft construction, marine construction for hulls and decks, and radiation technology [1]. The CDF experiment at Fermi National Laboratory used Rohacell as the support structure for their silicon tracker [5]. There are six different grades of Rohacell, Rohacell 31, 51, 71, 110, 170, 190, each increasing in density. Rohacell 71 was chosen for the MVD because it best matched our needs for lowest density at highest strength. It has a density of 75 kg/m3 (4.7 lbs/ft3), a tensile strength of 270 PSI, and a low coefficient of linear thermal expansion, measuring 3.3x10-5 K-1 . Rohacell is hygroscopic and tends to expand with increasing relative humidity. The C-cages have a heightened susceptibility to changes in the relative humidity of the surrounding environment due to the thin dimensions of the cages. Actual C-cages were exposed to various environmental conditions (Section 4) in order to test the suitability of Rohacell as the MVD support structure material.
The support structure has a mechanical, as well as a physics constraint. It must be mechanically stable to ensure the silicon detectors stay properly aligned for accurate measurement of the angular distribution and the collision vertex. The maximum error in the relative position between the inner and the outer silicon detector on a "C" cage (see figure 2) must be less than 0.4mm or +/-1.7%[6], in order to maintain the MVD vertex finding performance. A re-calibration of the detector must be performed if this tolerance is exceeded in order to maintain the vertex finding efficiency. The mechanical tolerance requires that the widths (see figure 2) of the cage do not decrease by more than -0.4% or 0.2 mm, otherwise neighboring silicon wafers can touch, degrading the detector performance.
The MVD will operate under constant environmental conditions within an RF enclosure with a closed loop cooling system. Nitrogen will be flowed through the enclosed detector, keeping the MVD environment at ~ 30% relative humidity and ~ 20o C [7]. This will minimize changes in the Rohacell, improve the performance of the silicon and eliminate condensation inside the MVD enclosure. A slow controls monitoring system will maintain the humidity at the few percent level and will register any faults in the cooling system performance.
The foam cage must be radiation-resistant. It is estimated that the silicon barrels of the MVD will be exposed to ~ 20 krad of radiation over the life time of the detector. The manufacturers of Rohacell 71 do not measure any significant decrease in strength when they exposed the foam to 150 krad of radiation [1]. The Atlas project at CERN has performed tests on Rohacell 31 and found that the general characteristics of the Rohacell only begins to deteriorate after bombardment with 9.2 Mrad of radiation[8]. We do not expect to see any radiation effects in the Rohacell at our expected low doses.
The MVD's location in PHENIX makes access for maintenance difficult, if not impossible during the running periods lasting several months. Thus, the foam support must be robust enough to withstand long periods of operation without access. The support structure must also have low mass in order to minimize secondary interactions in the PHENIX electron arm acceptance.