Primary ionization cluster counting with low-pressure multistep detectors

The authors describe a novel method of primary ionization cluster counting (PCC), using a low-pressure multistep counter coupled to a conversion volume. Due to the small density of the primary clusters in the low-pressure gas and to the high gain of the multistep amplification structure, the clusters are efficiently counted. Taking advantage of the enhanced relativistic rise at low gas pressures and the reduced interference from δ-electrons, the PCC can be efficiently applied to dE/dx relativistic charged particle identification. The authors present preliminary results obtained with relativistic electrons and a UV laser. For minimum ionizing electrons, they counted about two primary clusters/cm, at 20 torr of isobutane. The distribution is Poissonian, with δ~n¯^0.5. The PCC is shown to be an efficient tool for the study of the primary ionization process in gas media