The heart of the HyperCP spectrometer is eight high-rate, narrow-pitch multiwire proportional chambers (MWPC). (A ninth chamber, intended to provide an additional measurement of the high momentum daughter baryon position, had an insignificant effect on the mass resolution and was eventually used as a replacement chamber.) All MWPCs have a similar construction; four anode wire planes are sandwiched by cathode foils and two outer grounded foils terminate the field region. The anode planes comprise two bend-view windings (X & X', shifted by half a wire spacing) and two windings inclined at 26.6 degrees (U & V). All together the MWPC system comprises 18,752 instrumented wires.
The wire chambers all have small anode-cathode gaps (3 mm) and narrow wire spacings in order to perform efficiently and to have good aging properties in the intense secondary beam emanating from the collimator. For the majority of the 1997 run the total secondary beam rate through the chambers was about 20 MHz, corresponding to a typical individual wire rate of 0.4 MHz and a local flux of 0.5 MHz/cm2 at the center of the most upstream chamber. The hyperon decay products inhabit the same region of the upstream wire chambers as the channeled beam while in the downstream chambers they inhabit somewhat disjoint regions. For this reason chambers C1--C4 were filled with a `fast-gas' mixture of CF4-isobutane to reduce their sensitivity to out-of-time hits. The downstream chambers C5--C8 were filled with an Argon-Ethane-isopropyl mixture, since less time resolution was needed. To accommodate the higher intensity planned for the 1999 run we intend to use the fast gas mixture in all chambers.
At the nominal intensity of 20 MHz the efficiencies across the chambers were high and relatively uniform, dipping only slightly in the beam region. Furthermore, the efficiencies were insensitive to intensity over the running range of the experiment.
In addition to its good timing properties, the CF4-isobutane mixture is also known to mitigate the effects of aging. Over the course of the 1997 run a moderate amount of charge (0.02 C/cm) was accumulated along the wires in the beam region of the upstream chambers. Periodic monitoring of Fe(55) pulse amplitude in the beam region has shown no indication of gain degradation of the anode wires due to aging. Another defense against aging is to operate the chambers at a low avalanche gain. Due to the low noise of the front-end system, the chambers achieve the efficiencies shown at an avalanche gain of only 4E4.