Ageing studies with argon/methane based gas mixtures

Ageing studies on a single wire proportional counter with ArCH4 (90 : 10) (P10) and Ar/CH4/H2 (89.5 : 10 : 0.5) (P10H2) mixtures irradiated with 6.4 keV X-ray photons from an X-ray generator, under well controlled conditions, are reported. For both mixtures, results are presented for the variation of the gain as a function of the accumulated charge per centimetre of the wire. The intensities of the main emissions (CI line at 193.1 nm and of the CH (A-X, B-X, C-X) molecular bands centred, respectively, at 431, 389 and 314 nm) are also measured. No ageing effects were observed up to an accumulated charge of 350 mC/cm with the P10H2 mixture. The tests performed with P10 following the P10H2 run, both for low and high irradiation rates, showed also negligible ageing, in contradiction with previous results. This may indicate that there is still some hydrogen adsorbed in the system that prevents the chamber from ageing.