Simulations Pin Down How Efficiently a Nanodosimeter Counts Single Ions

When a charged particle such as an alpha particle tears through matter, it strips electrons from atoms along its path, leaving behind a microscopic trail of ionizations separated by just a few nanometers. Reading that trail directly inside a volume the size of a virus is impossible with any existing instrument, so physicists have learned to cheat: they let the particle pass through a puff of rarefied gas that contains the same number of atoms per unit area as a nanometer-thick slab of water, and then count the individual ions it produces.

The device in question is the Jet Counter, developed at the National Centre for Nuclear Research in Poland. It works by pulse-expanding nitrogen gas into an interaction chamber, forming a gas jet that simulates a nanometric target volume at unit density. As a charged particle crosses the sensitive volume, it ionizes nitrogen molecules, and an electric field sweeps those ions out of the chamber and toward an electron multiplier detector. The number of ions registered in a single event, called the ionization cluster size, encodes the fine structure of the particle track, which is precisely the information needed to understand radiation damage at the cellular level.

Knowing the exact detection efficiency of such a device is not a technical nicety but a scientific necessity. Because no ion acquisition system can register every ionization event, the measured cluster sizes are systematically biased unless the losses are quantified and corrected. The team, led by Mateusz Filipek with colleagues including Marcin Pietrzak, set out to determine the total efficiency of the Jet Counter as the product of three separate factors: the efficiency of extracting ions from the sensitive volume, the efficiency of guiding them to the detector, and the efficiency with which the detector itself registers each arriving ion.

To model the extraction stage, the researchers turned to SIMION, version 8.1, a software package that calculates electric fields and the trajectories of charged particles within them. They simulated the scenario of a 3.8 MeV alpha particle passing through the sensitive volume, generating nitrogen molecular ions whose positions, energies and directions were randomized according to the physical properties of the gas. The expected number of primary ions was set at 50,000, and after accounting for secondary ionization from delta electrons, the total rose to 51,670 ions tracked through a realistic geometry of the extraction electrodes.

The simulations revealed a clear pattern: the more negative the voltage applied to the extraction electrode behind the chamber, the better the ions were pulled out, and the faster they traveled. At minus 30 volts, extraction efficiency reached 93.9 percent with an average flight time of 16.5 microseconds. Doubling the voltage to minus 60 volts pushed efficiency to about 95.6 percent, but at that point most ions were captured by the next electrode instead of continuing their journey. The team ultimately settled on minus 12 volts as the lowest voltage at which a problematic gap effect disappears while still achieving extraction efficiency above 91 percent. Interestingly, the field analysis showed that the extraction electrode mainly collimates the ion beam rather than pulling ions from the chamber; the flowing gas itself does much of the uniform extraction work.

Temperature emerged as a surprisingly powerful lever. Running simulations across a range from 200 to 350 kelvin, the researchers found that extraction efficiency falls linearly as the initial gas temperature rises, with a coefficient of determination of 0.990. The fitted slope corresponds to a change of about 1.7 ten-thousandths per kelvin. The physical explanation lies in momentum transfer: hotter gas molecules knock the ions farther afield, pushing some of them outside the volume where the electric fields of the grids can still grab them. In principle, efficiency would be maximized by cooling the gas toward its solidification point, which at a pressure of 25 pascals would occur near 40 kelvin, but the Jet Counter cannot reach such conditions, so experiments proceed at ambient temperature, accepting this as one more factor limiting efficiency.

The guiding stage, in which ions travel from the extraction electrode to the detector, was estimated at about 90 percent using SIMION in vacuum conditions. The losses there come mainly from ions colliding with electrodes, particularly the final conical electrode, when strong focusing is needed to thread them into the narrow entrance of the ion detector. Notably, after passing the extraction electrode, fewer than 1 percent of ions are lost to collisions with gas molecules, because the ions overtake the gas jet and travel through vacuum that has not yet been spoiled by the expanding gas.

The detector itself, a discrete dynode electron multiplier, was characterized using a procedure adapted for the Jet Counter. Its efficiency depends on the voltage across the multiplier terminals, the energy of the impacting ion, and the discrimination level set against electrical noise, which sits at roughly 10 millivolts root-mean-square under typical conditions. Higher bias voltage increases the multiplication factor but also raises the risk of spontaneous discharge and shortens the detector’s lifespan, while ion impact energy shows an optimum near 8 kiloelectronvolts above which fewer electrons are knocked out. As a working compromise, the team identified a bias of 3.1 kilovolts and a discrimination level of 20 millivolts, yielding a detection efficiency of 72.8 percent with an uncertainty of 3.8 percent. Raising the accelerating voltage to 5 kilovolts, which the Jet Counter system can accommodate, improves detection to between 75 and 80 percent.

Combining the three stages, the overall detection efficiency of the Jet Counter under optimal conditions comes to approximately 60 percent, calculated as roughly 0.9 times 0.9 times 0.75. To validate this estimate, the researchers embedded it in a Monte Carlo simulation of the entire experimental setup using the Geant4-DNA toolkit, version 11.0.4, with improvements including a brass holder for the Mylar chamber wall and a physical collimator replacing a previously used virtual source. Alpha particles of 5.478 megaelectronvolts were started inside a 3.7-micrometer gold layer to reproduce the energy spectrum of the americium source used in real measurements. When the simulated ionization cluster size distributions were compared with experimental data for an equivalent target size corresponding to 0.45 micrograms per square centimeter in liquid water, the agreement was satisfactory, and a direct comparison of mean cluster sizes pointed to an exact efficiency value of 57 percent.

The study’s authors conclude that the Jet Counter’s total detection efficiency lies between 55 and 60 percent, and they emphasize a practical lesson for the field: the efficiency parameter should be reported alongside every measurement obtained with the device. Some disagreement remained at the tails of the distributions, likely because the simulation assumed that each ion is detected independently and with uniform spatial efficiency, an assumption that real variations between gas injections may violate. Modeling that interplay between track structure and ion drift is the logical next step, but the current results confirm that neither the experimental approach nor the simulation setup contains any fundamental misjudgment.

Subject of Research: Ion detection efficiency of the Jet Counter nanodosimeter

Article Title: Study of ion detection efficiency in Jet Counter nanodosimeter

Article References: Filipek, M., Bancer, A., Brzozowska, B., Szeflinski, Z., & Pietrzak, M. (2026). Study of ion detection efficiency in Jet Counter nanodosimeter. EPJ Techniques and Instrumentation, 13(1), Article 2. https://doi.org/10.1140/epjti/s40485-025-00119-1

Image Credits: AI Generated

DOI: 10.1140/epjti/s40485-025-00119-1

Keywords: nanodosimetry, Jet Counter, ion detection efficiency, SIMION simulation, Geant4-DNA, Monte Carlo, ion extraction, nitrogen gas, alpha particles, radiation physics, cluster size distribution, instrumentation