Wall-less ion-counting nanodosimetry applied to protons

A wall-less ion-counting nanodosemeter, conceived for precise ionisation-cluster measurements in an accelerator environment, is described. The technique provides an accurate means for counting single radiation-induced ions, in dilute gas models of condensed matter. The sensitive volume dimensions, a few tissue-equivalent nm in diameter by a few tens of nm, are tunable by a proper choice of the gas pressure and electric fields; nanometric sub-sections can be electronically selected. Detailed ion-cluster distributions are presented for protons of 7.15, 13.6 and 19.3 MeV, in biologically relevant DNA-like sensitive volumes of low-pressure propane. Experimental results are compared to model simulations.