The FOCUS Outer Muon (OMU) system uses Resistive Plate Chambers (RPCs) to detect high angle muons . The OE and the steel of M2 provides the filter, removing most of the hadronic contamination.
RPCs are able to operate in sizable magnetic fields, an important requirement since there are substantial fringe fields in the region from M2. The RPCs used in FOCUS are a double-gap type which provides redundancy in case one gap fails. The detectors operate in streamer mode with high voltage on either side of the ground plane and a gas mixture of 5% freon, 8% isobutane, 16% CO2, and 71% argon.
The readout strips are in the center of the two gaps. A cross section of the RPC design used by FOCUS is shown in spec:rpc.
For FOCUS, 24 RPC modules were constructed. Each module is 1.0 m by either 1.6 m or 1.8 m (depending on placement). Modules were constructed with readout strips running in three directions: along the long axis of the module, along the short axis, and at a angle. The width of each strip is 3.1 cm. Strips along the long axis are cut in half, such that each strip only covers half the height of the module. The total number of strips per module varies from 52 (along the short axis of the 1.6 m modules) to 64 (along the long axis). However, because of in M2, resolution of 12 cm is adequate. To reduce the amount of electronics and cabling required, the front end discriminators OR each four adjacent strips together, giving 13-16 output channels per module.
These modules are then arranged in views, as shown in spec:omu. Views of x, y, and u are formed, each with 8 modules, leaving an aperture (roughly matching the apertures of M2 and P3). The modules overlap slightly at the edges, which is visible in spec:omu.