Structure and wire configuration

The structure of CDC is shown in Fig. . It is asymmetric in the $z$ direction in order to provide an angular coverage of $17^o \le \theta \le 150^o$. The longest wires are 2400 mm long. The inner radius is extended down to 103.5 mm without any walls in order to obtain good tracking efficiency for low-$p_t$ tracks by minimizing the material thickness. The outer radius is 874 mm. The forward and backward small-$r$ regions have conical shapes in order to clear the accelerator components while maximizing the acceptance.
The chamber has 50 cylindrical layers, each containing between three and six either axial or small-angle-stereo layers, and three cathode strip layers. Table gives the detailed parameters of the wire configuration. CDC has a total of 8400 drift cells. We chose three layers each for the two innermost stereo super-layers and four layers each for the three outer stereo super-layers in order to provide a highly-efficient fast $z$-trigger combined with the cathode strips. We determined the stereo angles in each stereo super-layer by maximizing the $z$-measurement capability while keeping the gain variations along the wire below 10 %.

Figure: Overview of the CDC structure. The lengths in the figure are in units of mm.

Table: Configurations of the CDC sense wires and cathode strips.

Superlayer	No. of	Signal channels	Radius	Stereo angle (mrad)
type and No.	layers	per layer	(mm)	and strip pitch (mm)
Cathode	1	64 (z) $\times$ 8 ($\phi$)	83.0	(8.2)
Axial 1	2	64	88.0 $\sim$ 98.0	0.
Cathode	1	80 (z) $\times$ 8 ($\phi$)	103.0	(8.2)
Cathode	1	80 (z) $\times$ 8 ($\phi$)	103.5	(8.2)
Axial 1	4	64	108.5 $\sim$ 159.5	0.
Stereo 2	3	80	178.5 $\sim$ 209.5	71.46 $\sim$ 73.75
Axial 3	6	96	224.5 $\sim$ 304.0	0.
Stereo 4	3	128	322.5 $\sim$ 353.5	-42.28 $\sim$ -45.80
Axial 5	5	144	368.5 $\sim$ 431.5	0.
Stereo 6	4	160	450.5 $\sim$ 497.5	45.11 $\sim$ 49.36
Axial 7	5	192	512.5 $\sim$ 575.5	0.
Stereo 8	4	208	594.5 $\sim$ 641.5	-52.68 $\sim$ -57.01
Axial 9	5	240	656.5 $\sim$ 719.5	0.
Stereo 10	4	256	738.5 $\sim$ 785.5	62.10 $\sim$ 67.09
Axial 11	5	288	800.5 $\sim$ 863.0	0.

The individual drift cells are nearly square and, except for the inner three layers, have a maximum drift distance between 8 and 10 mm and a radial thickness that ranges from 15.5 to 17 mm. The drift cells in the inner layers are smaller than the others and their signals are read out by cathode strips on the cylinder walls. These cell dimensions were optimized based on the results of beam test measurements [32]. Fig. shows the cell arrangement in which the neighboring radial layers in a superlayer are staggered in $\phi$ by a half cell to resolve left-right ambiguities. The sense wires are gold plated tungsten wires of 30 $\mu$m in diameter to maximize the drift electric field. The field wires of unplated aluminum of 126 $\mu$m in diameter are arranged to produce high electric fields up to the edge of the cell and also to simplify the drift time-to-distance relation. The aluminum field wires are used to reduce the material of the chamber. The electric field strength at the surface of the aluminum field wires is always less than 20 kV/cm, a necessary condition for avoiding radiation damage [33].

Figure: Cell structure of CDC.

Three $z$-coordinate measurements at the inner-most radii are provided by cathode strips as shown in Fig.  [34]. They were glued on the inner cylinder surface of the chamber and on both sides of a 400 $\mu$m thick CFRP cylinder located between the second and third anode layers. In order to maintain the mechanical strength and accuracy, aluminum guard rings were attached on the both ends with electrically conducting glue. The deviation from a perfect cylindrical shape is less than 100 $\mu$m in radius. The cathode strips are divided into eight segments in the $\phi$ direction and have an 8.2 mm pitch in the $z$ direction. The strip width is 7.4 mm. The total number of cathode channels is 1,792. The deterioration of the momentum resolution due to multiple scattering in the cathode materials is minimized.

Figure: Cell structure and the cathode sector configuration.

The total wire tension of 3.5 tons is supported by the aluminum end-plates and the CFRP cylinder structures that extend between the end-plates. Each end plate consists of three parts; the cathode, inner and main parts as indicated in Fig. . The cathode part of a 16 mm thick flat plate corresponds to the three innermost anode layers and three cathode layers. The conically-shaped inner part covers 11 anode layers in the radial range from 103.5 to 294 mm. The end-plate thickness in the $z$ direction is 11 $\sim$ 18 mm for the backward side and 25 $\sim$ 31 mm for the forward side. The end-plate of the main part is 10 mm in thickness and has a curved profile to minimize distortions caused by the wire tension. The three end-plate parts are connected to each other by stainless steel bolts and gas sealed with silicone glue. The 754.5 mm radial space between the 3rd and 50th anode layers contains only gases and wires.
The feed-through holes were drilled by a custom designed drilling machine at the KEK machine shop. The hole position accuracy is better than 30 $\mu$m.