A study of ageing effects in wire chambers

Abstract

Following the observation of deposits on an anode wire from the AFS central detector at the ISR some two years ago, systematic tests were carried out to establish the origin of the silicon, which was detected as a major component, and to possibly find techniques which could reduce this deposition process. In irradiated test chambers, filled with Ar/C₂H₆, with and without methylal, and Ar/CO₂, gain reductions were observed, initially at ∼ 4%/10¹⁶ e/mm but increasing finally to ∼ 20%/10¹⁶ e/mm. High voltage instabilities occurred after about 2.3 × 10¹⁶ e/mm. The introduction of a cold trap, which should absorb heavier silanes, did not halt the ageing process, nor did prechambers with HV. Tests with a perspex chamber also showed ageing, but without silicon as a major component. Flat cathode chambers did not appear to age, at least to ∼ 15–20 × 10¹⁶ e/mm.