Space distribution of streamers in straw drift tubes

The appearance of a limited streamer mode in straw drift tubes operating with a Xe/CO2/CF4 gas mixture has been investigated. The space distribution of streamers across the straw has been measured for different gas gains and anode wire offsets. Streamer mode discharges have been found to be generated when ionization from the particles occurs within a narrow zone (FWHM≈400 μm) surrounding the anode wire. It was shown that the streamer contribution sharply increases with high voltage reaching almost 100% at a gas gain of about 1.2×105. It was found that the streamer intensity is very sensitive to the wire offset. Consequently, at a gas gain of 7×104 the number of streamers doubles when a wire is offset by 300 μm. This may greatly boost the aging of anode wires because streamers are an additional source of active radicals generated in the gas discharge since the total charge produced by streamer may be a hundred times larger than that from a proportional mode discharge.