A single-electron counter for nanodosimetry

A detector has been devised able to measure with high resolution the primary ionisation yield in tissue-equivalent gas volumes of a few nanometers equivalent length. The sensitive ionisation volume is a wall-less millimetric region defined by a properly shaped electric field. Free electrons created by the radiation inside the sensitive volume are collected into an electron multiplier, capable of efficiently counting single electrons at low gas pressure. The single-electron detection system consists of a long drift column attached to a multistep proportional counter. The electron cloud created by the radiation inside the sensitive volume, diffuses along the drift column. Single electrons, successively arriving at the multiplier are amplified, giving rise to a pulse trail from which the original number of ionisation electrons is counted. The experimental set-up, the electron counting principle, and first data are presented and discussed. (author)