A multiwire proportional chamber (MWPC) is constructed with alternating planes of high voltage wires and sense wires, which are at ground. All the wires are placed in a special gas environment. Gaps between planes are usually on the order of millimeters and voltage differences are typically in the kilovolt range. When a charged particle passes through the gas in the chamber, it will ionize gas molecules. The freed electrons are accelerated towards the sense wire by the electric field, ionizing more of the gas. In this way a cascade of charge develops and is deposited on the sense wires. The smaller the diameter of the sense wires, the higher the field gradient near the wire becomes. This in turn causes a larger cascade, increasing the efficiency of the chamber.
In FOCUS, tracking downstream of the first magnet is accomplished using five multiwire proportional chambers (called PWCs in our nomenclature). Each PWC has four planes per station. The stations are labeled (from most upstream to most downstream) P0, P1, P2, P3, and P4. We naturally divide tracks into two categories: 3-chamber tracks (``outer'') which are outside the aperture of M2 and 5-chamber tracks (``inner'') which traverse the entire spectrometer.
All five chambers operate with a gas mixture of 75% argon and 25% ethane bubbled through methyl alcohol. Each chamber has four planes of sense wires. The X wires run vertically and measure the position in the horizontal direction. The Y planes measure the horizontal position. The U and Vplanes are inclined at from the horizontal. This arrangement is shown in spec:wire_orient.
The chambers P0 and P3 (called Type I stations) are identically constructed. P0 and P3 are just downstream of M1 and M2 respectively. These chambers have apertures of about 76 cm127 cm (matched to the magenet aperatures) and have 0.8 mil diameter gold plated tungsten sense wires spaced at 80 mil. The gap between high voltage planes and sense wires is 0.235''.
The Type II chambers (P1, P2, and P4) have 152 cm229 cm apertures. The sense wire spacing is 3.3 mm with 1.0 mil diameter wires. The gap between high voltage planes and sense wires is 0.240''. P1 and P2 are situated between P0 and the first magnet. P4 is situated after the last detector.
The positions and construction of the five chambers are summarized in spec:pwc_internals.
|Chamber||spacing||X||Y||U||V||(cm from target)|