The Relativistic Heavy-Ion Collider (RHIC) is a nucleus-nucleus collider being built at Brookhaven National Laboratory. It is capable of accelerating gold nuclei to 100 GeV/nucleon in each direction. PHENIX is one of four experiments at RHIC that will look for experimental evidence of phase transitions to and from a quark-gluon plasma [2]. The colliding beams are parallel to the axis of the MVD, which is located at the center of PHENIX, surrounding the region where the two beams collide. It is the first detector that the particles encounter after the collision.
The MVD, shown in figure 1, includes silicon strip detectors arranged in two concentric barrels, with strips running perpendicular to the beam axis. The end-caps are an arrangement of silicon pad detectors and extend the range of angular distribution measurements. The MVD measures the charged particles distribution event-by-event, provides event characterization as well as a centrality trigger to the experiment. Correlating the hits in the two barrels allows one to determine the vertex of the collision with a 2 mm resolution. Simulations indicate that the vertex finding efficiency approaches 100% in central Au+Au collisions [2]. Details on the detector can be found elsewhere [2, 3, 4].
As the MVD is the first detector in PHENIX that emitted particles from the collision encounter, it is important that it has as low of a mass as possible in order to minimize secondary interactions into the electron-sensitive acceptance of PHENIX. Obviously a zero mass silicon detector is not possible, but the MVD strives to minimize its mass whenever feasible. The mass of the MVD is approximately 11 kg and the total radiation length less than 1% .
The silicon wafers are 300 
m thick and are biased via polysilicon
resistors. The dimensions of the wafers are 53 x 50 mm2 in the
inner barrel and 53 x 74.5 mm2 in the outer barrel. The structural
unit of the barrel assemblies, the 'C-cage', is shown in figure 2.
The size of the C-cage is 53.2 x 169.05 x 71.70 mm2 . A
typical rib in the C-cage skeleton has dimensions of approximately
54 x 6 x 4 mm3 . The 'C-cage' is composed of the light-weight Rohacell
foam.
Rohacell foam was
considered as the silicon structural support because of its radiation length,
5,450 mm,
which is orders of magnitude longer than the more commonly used beryllium
and graphite-epoxy materials. Uncoated foam cages represents approximately
0.06% of the total radiation length budget.
The C-cages are half of a hexagonal barrel, segmented along the beam axis. A silicon strip detector is affixed to each of the three faces of the inner barrel. Not all faces of the outer barrels are populated with detectors (See figure 1). The two C-cages on each end of the MVD are fully populated, while the centrally located C-cages are only partially populated with silicon. Only the bottom outer face is populated on these cages in order to reduce mass in the acceptance of the electron arms of PHENIX. The entire assembly of twenty-four C-cages forms two concentric hexagonal barrels of active detecting surfaces around the collision vertex.