Ladder

Each ladder is made of two half-ladders that are electrically independent, but mechanically joined by support ribs. The support ribs are made of boron nitride (BN) sandwiched by carbon-fiber reinforced plastic (CFRP). This arrangement is necessary to avoid directly gluing CFRP to DSSD since CFRP is much stiffer but conductive, and induces noise to the detector. The dimensions of the BN and CFRP are 6.25$\sim$6.65 mm high $\times$ 0.5 mm wide and 3.7 mm high $\times$ 0.15 mm wide, respectively, giving a stiffness of $EI=3.5\sim 4.0$ Nm$^2$. The average thickness of the rib is approximately 0.15 % of the radiation length. To reduce the number of fixture types needed for ladder assembly, the ladder design is modular, with each module made as symmetrically as possible. As a result, only two basic module types are needed: a single detector module and a double detector module. Each module consists of a detector unit and a hybrid unit, as shown in Fig. . A detector unit consists of either a single detector or two detectors with an overlap joint. In order to minimize the noise of the double-detector modules, different sides of the two detectors are connected--meaning that $p$-strips on one detector are wire-bonded to $n$-strips on the other detector. This is possible because the detectors incorporate integrated coupling capacitors and the readout system can process pulses of either polarity. A hybrid unit is produced by gluing two hybrids back-to-back to read both sides of DSSD. Since the preamplifier chips are a heat source, careful attention is paid to the thermal path way through the hybrid, across glue joints, and into the heat-sink. Boron-nitride-loaded epoxy is used to join the two hybrids due to good heat conductivity of $\sim 0.8$ W/m$\cdot$K. The heat-sink is made of aluminum nitride, whose high thermal conductivity (150-200 W/m$\cdot$K) and low coefficient of thermal expansion (CTE) (2-3 ppm/K) make it an attractive option for both heat conduction and mechanical support. The thermal conductivity is further enhanced by embedding two heat-pipes in the heat-sink. Measurements indicate that the total temperature drop across the heat-sink will be 1.8$^\circ$C at normal preamplifier power levels. The attachment of the hybrid to the heat-sink is also by means of the boron-nitride loaded epoxy.