Wire chamber ageing with CF/sub 4//isobutane and argon/ethane mixtures

The ageing characteristics of CF/sub 4//isobutane (80:20) and argon/ethane (50:50) in identical test cells have been investigated. The gas gain and gas flow rate were varied and the measurements extended beyond 6 C/cm of wire. Study of a third gas mixture Ar/eth/CF/sub 4/ (48:48:4) is at an early stage. The Ar/eth mixture has shown a variety of problems including cathode foil etching, anode deposits, dark currents, and pulse-height degradation. In contrast the CF/sub 4//iso mixture produced some cathode etching for low flow velocities and only very minor anode deposits. With very little pulse-height degradation or dark current this mixture is a considered excellent candidate for a high-rate chamber. A small-area multiwire proportional chamber using this mixture has been evaluated in a pion beam with particle fluxes up to 3*10/sup 7/ particles/cm/sup 2/-s.<>