Central Vertex Chamber (CV)

The central vertex tracking drift chamber [16] is situated inside the central jet chamber and is 1 m long. Its main job is to measure the vertex positions of decay particles and improve the momentum resolution for charged particles. It consists of two layers of drift chambers. The inner layer containing 36 cells of axial wires and the outer layer 36 stereo cells. The outer cells contain read-out wires that are inclined at an angle of approximately $4^{\circ}$ to the inner, axial wires. The inner layer lies between radii 103 mm to 162 mm, and the outer layer between radii 188 mm and 213 mm.

The CV chamber, like all the central tracking drift chambers, is filled with 88.2% argon, 9.8% methane and 2.0% isobutane at a pressure of 4 bar. When a charged particle passes through this chamber it ionises the gas, this is detected in the sense wires. The inner axial cells provide measurements in the $r-\phi$ plane with resolution of 50 $\mu$m. Combining information from the inner and outer cells gives a resolution of about 700 $\mu$m on the position in the $z$-plane.