Abstract
The multiwire proportional chambers of the HERMES spectrometer are described. Each chamber consists of a series anode–cathode planes which measure position along axes at −30°, 0° , and +30° relative to horizontal. The anode wire spacing is 2 mm , while the anode–cathode plane spacing is 4 mm . The chambers are located in the spectrometer magnet gap which imposed strong constraints on their design. A nonflammable gas mixture of Ar(65%), CO 2(30%) and CF 4(5%) is used and an efficiency plateau 500 V wide is obtained. The readout is performed with the LeCroy PCOS4 data acquisition system modified according to the specific requirements of the HERMES experiment. A dedicated water cooling system has been implemented to control and stabilize the surface temperature of the electronic chips mounted on the on-chamber preamplifier boards. A spatial resolution of ≃600 μm (rms) per plane has been measured. These chambers are essential for tracking low momentum particles which do not enter the drift chambers after the spectrometer magnet; these particles principally result from the decay of various unstable hadrons. The momentum and angular resolutions in tracking these low momentum particles has been studied and found to be similar to those obtained for high momentum tracks which traverse the entire tracking system of the HERMES spectrometer.
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