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.