Drift chambers for the PHENIX central tracking system

The Drift Chamber (DC) is part of the central tracking system of the PHENIX detector. The DC construction consists of two independent arcs. Each of them covers an active area of ±0.35 in pseudorapidity and 90° in azimuthal angle ϕ. The DC subsystem accurately measures charged particle trajectories to determine the pt of particles and ultimately the invariant mass of pairs of particles. The DCs also participate in pattern recognition. The unique feature of the DC lies in its cell geometry. The focusing geometry eliminates the left–right ambiguity, the sensitive track sample length for each sense wire is adjusted by changing the wire’s potential due to the presence of gate (channel) wires. Back (guard) wires screen the sense wire from charged particle ionisation on the side opposite the channel wires. To reduce the count rate per readout channel, sense wires are cut in the center and attached to a light kapton support. Both ends of the sense-wire are readout. The chamber gas is a 50–50 mixture of argon–ethane. A spatial resolution of 150 μm in r−ϕ and a two-track separation of better than 1.5 mm at single-track efficiency >99% is obtainable. Small angle stereo wires provide a spatial resolution of ∼2 mm in the z-direction. There are 12 544 channels of electronics in the DC.