Performance of 0.75 mm pitch MWPCs operating at high rate

Three multiwire proportional chambers (MWPCs) with high-rate capability have been constructed for Fermi National Accelerator Laboratory experiment 705 (charmonium and direct photon production by π^±,p and p¯ at 300 GeV/c). Each chamber, with a sensitive area of 308 cm², consists of three anode planes, wound with 12.5-μm-diameter gold-plated tungsten/rhenium wire, facing 25-μm graphite-coated kapton cathode planes at a distance of 3 mm. Wire spacing is 0.75 mm in two of the MWPCs and 1.00 mm in the third one. After a few weeks of running on `magic gas´ with a beam flux of up to ~10⁷ particles/cm² s, severe damage occurred to the graphite coating. All cathode planes were replaced by aluminized kapton, and the chambers worked successfully for the next three months until the end of the run, when a 10% efficiency drop was observed after a total accumulated charge of ~1 C/cm of sense wire. After the end of the run the chambers were disassembled and inspected. A white deposit with a characteristic wire pattern etched away from the Al coating was observed in the region of the beam spot