IWC

The dimensions of the IWC are given in Table 3.2. It consists of structural cylinders of 6.35 mm thick Hexcell that supports 768 axial anode wires made of silver-tungsten alloy 20 $\mu$m in diameter and spaced at 2.21 mm intervals. The cathode planes are made from 110 $\mu$m thick mylar with a pattern of 2 mm wide spiral aluminized strips. A gas mixture of 77.6% argon, 22.2% isobutane and 0.2% freon is bubbled through methylal. The anode wires are kept at a voltage of 3.4 kV and cathode foils are kept at ground.

The 384 strips of the outer cathode layer lie at a +45$^\circ$ angle with respect to the anode wires while those of the inner cathode layer lie at a $-$45$^\circ$ angle. Charged particles passing through the IWC induce pulses on the two cathode planes. The pulses extend locally over a cluster of the spirally wound strips. The $z$-coordinate of a track point is generated by the intersection of anode wire hits and the centroid of the corresponding strip cluster from at least one of the cathode planes.

The IWC provides $z$ coordinate information with about 97% efficiency for minimum ionizing particles. Reconstruction of charged tracks uses the $z$ coordinate information from the IWC along with that from the stereo layer of the drift chamber to provide tracking in the spectrometer. The reconstructed track point position resolution in the IWC is about 600 $\mu$m in r$-\phi$ and about 1200 $\mu$m in $z$ (26).