Experimental and Monte Carlo Simulation Studies to Investigate the Working Principle of Compact Nanodosimeters

TL;DR

The study addresses whether ion-impact ionization of gas inside a compact nanodosimeter cell hole is the primary mechanism for signal generation. It combines a 1.5 mm cell-hole prototype operated in propane at 1–2 mbar with comprehensive Geant4-DNA, Garfield++, and Elmer FEM simulations to model track structure, ion drift, and electric fields, and it derives the signal-creation probability $p_s$ from measured signal yields $\eta$ using $1 - \eta = \frac{1}{N} \sum_{i=1}^{N} (1-p_s)^{n_i}$. Results show a pronounced ICSD peak near cluster size $\approx 20$, central ion drift within the hole, and signal yields that rise with cathode voltage but remain in the low single-digit to tens of percent range, while $p_s$ per collected ion stays in the single-digit percent regime (max ~3.5% at 1 mbar). The findings support the prevailing assumption that gas-phase ionization dominates signal formation, with wall-emission and IISEE contributions deemed negligible under the tested conditions, and highlight the need for accurate ionization cross sections to enable robust predictive modeling and detector optimization.

Abstract

In recent years, compact nanodosimetric detectors based on ion multiplication in low-pressure gas have been developed and gained attention in the scientific community. These detectors use strong electric fields to collect and multiply positive ions produced by the incident radiation in mm-sized cell holes in dielectric materials, achieving a nm-equivalent spatial resolution of the localization of ionization events, when scaled to liquid water with unit density. Their design assumes that ion impact ionizations of gas molecules within the cell holes dominate signal formation, yet this assumption has lacked direct physical verification. Electron emission from the cell hole walls due to ion impact could also contribute, requiring alternative designs to optimize efficiency. To investigate this, a nanodosimetric detector featuring a single cell hole with a diameter of 1.5 mm in a dielectric plate was developed. Ion collection and multiplication were achieved by applying a negative high voltage to the glass cathode 0.5 mm below the cell hole, assisted by a low drift field above the plate. A grounded readout electrode with a 0.8 mm hole covers the cell hole to prevent interactions of collected ions with the hole walls. High signal yields in 1 mbar and 2 mbar propane gas were observed and indicated that ion impact ionizations of the gas molecules are indeed the primary mechanism for signal induction. The compact nanodosimeter setup was further modeled with Geant4-DNA and Garfield++ for deeper insight. The results of these studies are important for understanding and developing a new class of nanodosimeters with potential applications in particle therapy, radiation protection, space dosimetry, and particle physics.

Figures (10)

• Figure 1: Illustration of the working principle of state-of-the-art compact nanodosimeters based on ion multiplication in low-pressure gas.
• Figure 2: Top view of the detector interior attached to the lid of the low-pressure chamber.
• Figure 3: Schematic of the detector in the $x$–$z$ plane, where the $z$-axis is normal to the dielectric plate and the $x$-axis is aligned with the alpha-particle beam axis. The sketch highlights the central axis of the circular hole in the dielectric plate and is superimposed with electric field streamlines generated by the cathode at –800 V and the anode, positioned at $z$ = 23.5 mm (not shown), with a potential of 30 V.
• Figure 4: Simplified Geant4 model of the experimental setup. The collimator is simplified, only showing the central gas-filled hole and a thin layer of brass surrounding it. The walls of the low-pressure chamber are not shown for better visibility.
• Figure 5: Geant4-DNA simulated ICSD produced in the SV with a diameter of 3.09 mm for a cathode voltage of -800 V and a propane gas pressure of 1 mbar.