Streamer suppression with SF6 in RPCs operated in avalanche mode

We show that the addition of small amounts of sulphur hexafluoride, SF6, to the present ATLAS baseline gas mixture C2H2F4/C4H10 has a very strong effect in suppressing the avalanche-to-streamer transition. An RPC of 50×50 cm2 area, 2 mm gas gap was operated in pure avalanche, streamer-free mode in a voltage range of about 1 kV. The results of the test suggest that the observed fast signal could be interpreted as the sum of contributions of a number of primary saturated avalanches.