A SINGLE-ELECTRON COUNTER FOR NANODOSIMETRY

A detector has been devised able to measure with high resolution the p rimary ionisation yield in tissue-equivalent gas volumes gf a few nano metres equivalent length. The sensitive ionisation volume is a wall-le ss millimetric region defined by a properly shaped electric field. Fre e electrons treated by the radiation inside the sensitive volume are c ollected into an electron multiplier, capable of efficiently counting single electrons at low gas pressure. The single-electron detection sy stem consists of a long drift column attached to a multistep proportio nal counter. The electron cloud created by the radiation inside the se nsitive volume, diffuses along the drift column. Single electrons, suc cessively arriving at the multiplier are amplified, giving rise to a p ulse trail from which the original number of ionisation electrons is c ounted. The experimental set-up, the electron counting principle, and first data are presented and discussed.