The CF 4 /isobutane (80:20) gas mixture and high rate proportional chambers

In several laboratories there is considerable interest in developing proportional chambers that are able to operate at high rate and suffer minimal degradation over an extended period of operation. However, the causes of damage and degradation in these devices are still poorly understood. At TRIUMF we have a continuing program to study ageing effects. Using several identical single wire test cells, we have investigated the ageing characteristics of CF4/Isobutane (80:20), Argon/Ethane (80:20), and Argon/Ethane/CF4 (48:48:4) [1–3]. With our early results [1] showing a superior ageing performance of the CF4/Iso mixture, we decided to test a small admixture (4%) of CF4 to Ar/Et to see if it would inhibit damage. Parameters such as flow rate, gas gain, anode wire current density, and materials in contact with the gas stream have been varied. Some tests have been extended beyond 8 Coulomb/cm of wire. The Ar/Et chambers have shown a high incidence of pulse height degradation, dark currents, cathode foil etching, and deposits on the electrodes. A strong correlation between anode wire current density and rate of damage (%/C/cm) is indicated for Ar/Et chambers. The CF4/Iso chambers have shown effectively zero pulse height degradation and few other problems to accumulated charges exceeding 5 C/cm. The addition of 4% CF4 to the Ar/Et mixture has dramatically improved the ageing performance of the Ar/Et/CF4 cells.