Chemical model for wire chamber aging in CF4/iC4H10 gases

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Abstract
Aging of proportional counters in CF4/iC4H10 mixtures is studied as a function of gas composition. Anode surfaces are analyzed by Auger electron spectroscopy. Anode‐wire deposits are formed from 95/5 and 90/10 mixtures of CF4/iC4H10; etching of deposits occurs in 50/50 and 80/20 mixtures of CF4/iC4H10 and in pure CF4. Gold‐plated wires are resistant to aging resulting from chemical attack by CF4, but non‐gold‐plated wires are too reactive for use in CF4‐containing gases. An apparent cathode aging process resulting in loss of gain rather than in a self‐sustained discharge current is observed in CF4 and CF4‐rich gases. Principles of low‐pressure rf plasma chemistry are used to interpret the plasma chemistry in avalanches (≥1 atm, dc). To understand anode aging in CF4/iC4H10 gases, a four‐part model is developed considering: (i) plasma polymerization of iC4H10; (ii) etching of wire deposits by CF4; (iii) deposition that occurs as a result of radical scavenging in strongly etching environments; and (iv) reactivity of the wire surface. Practical guidelines suggested by the model and application of the model to other fluorine‐containing gases are discussed.

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