Drift Chamber

The drift chamber is the principal tracking element of the RMC photon-pair spectrometer (Figure 3.4). It is made up of four super-layers consisting of 272 cells, with 56, 64, 72 and 80 cells in layers one through four respectively (Table 3.2). There are 6 instrumental wires in each of these cells which are read out independently. A charged particle passing through a cell ionizes the drift chamber gas mixture (ethane/argon, 50/50).

The wires in the first, second and fourth layer run parallel to the beam axis and provide transverse $x$ and $y$ hit coordinates, while layer three, known as the stereo layer, is wound around at an angle of 7$^\circ$ to provide longitudinal $z$ information. The ambiguity in ascertaining whether the charged particle passed on the left or the right side of the wire that was hit is resolved by staggering the wires 254 $\mu$m apart alternately left and right of each cell's central plane.

Figure 3.4: A cross-sectional view of the RMC detector. The large volume drift chamber (D.C.) showing the drift chamber cells, the IWC, scintillator rings, and the position of the target.

The reconstruction efficiencies for single minimum-ionizing tracks through all layers of the drift chamber has been measured in Reference (26).