We retain the square cell and the superlayer wire configuration of the Belle CDC. There are six layers in each superlayer to make track segment finding easier; this is particularly valuable for the stereo superlayers. (There are only 3 or 4 layers in a Belle stereo superlayer.) The innermost superlayer has two additional layers that contain active guard wires. Even though the performance of these two layers is compromised by the high occupancy from beam backgrounds and the wall effect, the remaining six layers ensure that the innermost superlayer performs as well as the others. The innermost and outermost superlayers contain axial (“A”) layers, to match the shape of the inner and outer cylinders. The intervening superlayers alternate between stereo (“U” or “V”) and axial layers. In total, there are 9 superlayers (AUAVAUAVA) and 56 layers. The radial cell size is 10 mm for the innermost superlayer and ∼ 18.2 mm for the other superlayers.
From the trigger group’s simulation studies of the 3D trigger, they find a better z resolution using an alternate wire configuration (AUVAUVAUVA) in which the stereo superlayers are adjacent. If this result is confirmed and is essential for the z-trigger performance, we will re-consider the overall wire configuration. In addition, the barrel PID group will soon decide the final PID configuration, which will allow the outer radius of the CDC to be fixed. It is likely that the final wire configuration will be slightly modified as a result.
The number of cells in each layer is chosen according to the following considerations. We require multiples of 32 to match the number of electronics channels and trigger segments. For the innermost superlayer, a smaller azimuthal cell size is required to reduce the occupancy in the face of the large beam background. The lower limit is determined by the size of the feedthroughs. The optimal configuration has 160 cells are selected with the minimum azimuthal cell size of only 7 mm. To realize such a small cell size, the innermost superlayer is implemented separately as a so-called small-cell chamber that is then attached to the rest of the CDC. The overall wire configuration is shown in Table 6.2 and Fig. 6.1.
The stereo angles are listed in Table 6.2. A larger stereo angle provides better z resolution, but a large variation in the radial cell size along the z direction occurs in the boundary region between axial and stereo superlayers. To obtain a 60 mrad stereo angle, a special technique is adopted without adding insensitive regions: we string field wires in the transitions with half of the stereo angle and we adjust the radial positions at both endplates around the transitions. The same method is used in the Belle CDC [3]. The sense wire is only ∼ 1 mm closer to the field wire in this case, so that a large gain variation is avoided. The sense and field wire properties and counts are shown in Table 6.3. The properties are inherited from the Belle CDC, where there were no serious problems during more than ten years of operation. The counts are about a factor of 1.7 greater than in the Belle CDC. The 30 µmdiameter sense wires will operate at a slightly higher operating voltage so that the stronger electric field in the drift region reduces the maximum drift time. The aluminum field wires are unplated to avoid unnecessary material and to lower the cost.