The MVD consists of two concentric barrels of silicon strip detectors (300um thick and 200um pitch) around the beampipe and two disk-shaped endcaps of silicon pad detectors at z=+/- 35cm. The length of the silicon strip barrels is 64cm. Each barrel layer has six azimuthal panels and the two barrel layers are mounted on a common support structure made of light-weight, rigid Rohacell foam. The inner portions of the outer barrels are only partially populated with silicon in order to reduce the amount of mass in the electron arm acceptance. Each endcap is composed of 12 wedges, with each wedge manufactured from a single 4inch wafer.
The silicon strip detectors are single-metal whereas the silicon pad detectors are probably double-metal detectors; both single and double metal designs are being considered for the pad detectors. A double metal design is preferred for the pad detectors as this would eliminate the need for a specialized kapton cable, reduce the number of necessary wirebonds, faciliate detector probing, assembly and handling, and enable a sequential readout of the detector.
The single-event coverage about midrapidity is five units in pseudorapidity (eta) for the inner barrel layer and four units in eta for the outer barrel layer; the eta coverage for event-averaged quantities is extended due to the variation of the vertex position along the direction of the colliding beams. For the disk-shaped endcaps, the eta coverage is 2.7 for a single event occurring at z=0. The vertex position can be located using hits in the barrel for z positions between -40 and +40cm, which covers the 2-sigma length of the interaction region. The multiplicity trigger is formed using the sum of silicon channels above a threshold corresponding approximately to 0.25-0.33 of a minimum ionizing particle.
Both the pad and strip detectors are read-out with identical electronics, which are optimized for minimum size, power dissipation and cost. The total silicon channel count is 34,816 (256 channels per silicon detector). The silicon strip and pad detectors are attached to the front-end electronics via custom designed kapton cables. The kapton cable is 50um thick with copper traces. The power dissipation of the front-end electronics approximates a maximum of 10mW/chan. The MCM's are housed in a Rohacell plenum and cooled with dry air, which is a less massive system than a corresponding water cooling system. The air-cooling system has a minimum of a factor of two safety margin, and cools the MCMs to less than 40oC.
The MVD resides inside of a Rohacell and aluminum RF enclosure, which opens in a "clam-shell" fashion to allow installation around the beampipe. The enclosure provides electrical and environmental isolation of the MVD, as well as mechanical protection. Dry-air will slowly flow throughout the enclosure keeping the MVD at a stable and controlled environment, at approximately 30% relative humidity and 10-25oC. The capability of lowering the ambient temperature to 10oC is desirable in order to improve the performance of the silicon detectors in case of radiation damage.
Each half cylinder consists of several aluminum endplates, with two thin wall aluminum struts to connect and support them. The endplates attach to mounting plates which reside in cavities in the magnet pole faces. The magnet nose cones include machined inserts which enable the routing of fiber optic cables from the MCM (14 per nose cone) through the nosecone and to the data collection modules. Due to the bending radius of the optical fibers, there is not adequate space to route the cables from the MVD endplate directly to the data collection module.
The silicon pad detector endcaps are mounted to a G10 circuit board which supports both the detectors and corresponding front-end electronics and also regulates DC power. Low Drop-out Regulators (LDO) are located on the top half of each motherboard to regulate the incoming DC power for the MCM's. The LDO's dissipate approximately 15W of power on each motherboard and require cooling. Due to space limitations, a leakless water cooling system capable of cooling the LDO's between 10-25 degrees Celcius was chosen over a more bulky air cooling system. The water cooling system is contained outside of the electron arm acceptance. The cooling of the motherboard will be matched to the cooling of the nearby silicon endcaps to minimize a temperature gradient. The kapton power/communications cables connect the silicon strip MCMs to the motherboard. The motherboard routes signals from the endplate connectors to connectors on which the power/communications cables attach. The daughter board routes control signals and DC power from the motherboard to the silicon pad detector MCMs.