The SVT will consist of 216 silicon drift detectors (SDD) mounted in
three concentric barrels.
Fig. 13 shows a schematic drawing of the SVT
layout.
The geometry of the detector is chosen to
be quasi-projective to minimize the required number of Si wafers
for the rapidity bin from 
= -1 to 1, to ensure a full azimuthal
coverage, and to minimize dead areas.
End views of the barrels display polygonal shapes with 8, 12 and 16
faces upon which are attached ladder structures with 4, 6 and 7 wafers
mounted on the inner, middle and outer layers, respectively.
The SDDs have an active area of 
and are glued directly
to a Be support frame.
The thickness of an individual wafer is 300 
m, which
constitutes a reasonable compromise between minimizing
the amount of material in the
STAR fiducial volume and achieving good mechanical stability as well as a
reasonable signal to noise ratio.
The support structure that will hold the ladders in their honeycomb
structured superlayer position is made of Beryllium. Each ladder will
connect on both of its long sides to an electronics carrier which houses
the first stage of front-end electronics.
To facilitate the
integration of read out electronics, each barrel is split
into two sub-layers in such a way that adjacent ladders are offset
radially. To optimize the barrel radial coverage and
to minimize dead areas, neighboring ladders will fully overlap with the
respective electronics carrier.
The SVT has an active length of 42 cm and a diameter of 30 cm.
Table 
lists some of the detector's specifications.
Figure: Schematic layout of STAR Silicon Vertex Tracker
To simplify the general design and allow possible extension of
the rapidity coverage, all ladders have the same dimensions of 44 cm x 6
cm, although the coverage is limited to at most 24 cm, 36 cm, and 42 cm of
active area respectively in the three layers.
The electronics carriers, of 54 cm 
2 cm dimension,
are mounted at an angle of 30 degrees relative to the ladder.
The ladders consist of a 0.5 mm thick and 1.5 mm wide Beryllium frame.
The electronics carriers made of solid piece of Beryllia of similar
thickness hold the FEE hybrid and also form one side of a cavity that
serves as a flow channel for the water cooling. The Beryllium has a
thickness of 2 mm and the cavity gap is 1 mm. Preliminary discussions
with precision machining companies in the U.S. show that the fabrication
of this carrier is feasible by employing laser welding.
The ladders and electronics carriers are supported at each end by two Be end caps configured as a clamp-shell to facilitate installation and removal of the whole detector assembly without breaking the vacuum in the RHIC beam pipe. The ladders and carriers extend beyond the endcaps on both ends by about 10 cm to provide space for high voltage input and control logic. Voltage divider boards, located at the water manifolds, supply a linear resistor chain for drift field control on the detector, via continuous voltage reduction from 2500 V to ground level. The carrier extensions hold the control logic for the hybrid as well as the hybrid to ADC interface.
The two endcaps are followed by
two Aluminum water manifolds which distribute the incoming water supply to the
respective carrier inlets. A fiducial volume
of 
50 cm in beam direction and 
20 cm in radial direction is
allocated to the SVT and its support.
The SVT will be mounted around the beam pipe via two support cones attached to the TPC end caps. Water cooling pipe, high voltage cables and signal cables are attached to the conical support structures.
Although the design of the SVT has matured significantly in the last year, it is possible that minor modifications may be added to maximize the tracking efficiency and resolution, adapt to wafer technology and electronics, reduce the cost, etc.
Table: Specifications of the proposed Silicon Vertex Tracker
for STAR.