Etching of anode wire deposits with CF4/isobutane (80:20) avalanches

An ionization exposure of 0.5 C per cm of wire in a gas mixture of CF4/isobutane (80:20) is shown to reverse anode wire damage in single-wire chambers. Several chambers aged in argon/ethane (50:50) and argon/ethane/ethanol (50:50:0.2) and having pulse height reduction of 25-30% have recovered pulse heights and currents to greater than 98% of their initial values. Inspection of the anode wires indicates that the thick deposits caused by the exposure in argon/ethane have been removed. Auger electron spectroscopy reveals only a thin residual layer containing primarily carbon and oxygen. This etching ability of CF4/isobutane (80:20) avalanches may explain the extremely good ageing characteristics previously reported for this mixture. (orig.)