Tests of wire chamber ageing with CF4/isobutane (80.20), argon/ethane (50:50), and argon/ethane/CF4 (48:48:4)

In a series of parallel tests the ageing characteristics of CF₄ isobutane (80:20), argon ethane (50:50), and argon ethane/CF₄ (48:48:4), have been investigated. 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, except for one low-flow chamber with some cathode etching and cathode related pulse height degradation. The addition of 4% CF₄ to the Ar/Et mixture has dramatically improved the ageing performance of the Ar/Et/CF₄ cells