Observation of transition between proportional and Geiger–Müller modes in helium–isobutane gas mixtures

Pulse-shape analysis is used for counting secondary avalanches in a proportional tube filled with a Helium-based gas mixture. It is shown that, at a high electric field, transition from proportional mode to Geiger–Müller (G–M) mode takes place if the concentration of the quencher is small. This fact is particularly relevant when such gas mixtures are to be used for particle identification purposes or in high rate environment.