After-pulse from helium-filled proportional counter at low temperatures near 4.2 K

After-pulses, which appear in output signals from the helium-filled proportional counter (HFPC) cooled at low temperature (5–10 K), have been analyzed with a technique of non-linear least-squares fit; the drift time Ti of positive helium ions from anode to cathode, the spread in the drift time ΔTi and the second Townsend ionization coefficient Γi of the positive ions have been determined. It is known that the carrier of positive charge in pure helium gas is the dimer ion He2+ at room temperature. We have found that Ti and ΔTi are larger than the values estimated with the mobility of He2+ and Γi is much smaller than the value for He2+. results indicate that the carrier at low temperatures near 4.2 K is not He2+, but heavier molecular ions Hen+; the size n is more than 10 in the present HFPC. It has been concluded that the cluster ion of large molecular helium plays an essential role in the proportional operation of HFPC, especially to suppress the continuous electric discharge. This self-suppression of discharge by the formation of cluster ions is discussed with some data of atomic and molecular heliums. The same quenching mechanism by cluster ions in other rare gases is also suggested.