Secondary ionisations in a wall-less ion-counting nanodosimeter: quantitative analysis and the effect on the comparison of measured and simulated track structure parameters in nanometric volumes

Abstract

Abstract. The object of investigation in nanodosimetry is the physical characteristics of the microscopic structure of ionising particle tracks, i.e. the sequence of the interaction types and interaction sites of a primary particle and all its secondaries, which reflects the stochastic nature of the radiation interaction. In view of the upcoming radiation therapy with protons and carbon ions, the ionisation structure of the ion track is of particular interest. Owing to limitations in current detector technology, the only way to determine the ionisation cluster size distribution in a DNA segment is to simulate the particle track structure in condensed matter. This is done using dedicated computer programs based on Monte Carlo procedures simulating the interaction of the primary ions with the target. Hence, there is a need to benchmark these computer codes using suitable experimental data. Ionisation cluster size distributions produced in the nanodosimeter's sensitive volume by monoenergetic protons and alpha particles (with energies between 0.1 MeV and 20 MeV) were measured at the PTB ion accelerator facilities. C3H8 and N2 were alternately used as the working gas. (2013)]. Measured and simulated characteristics of the particle track structure are generally in good agreement for protons over the entire energy range investigated. For alpha particles with energies higher than the Bragg peak energy, a good agreement can also be seen, whereas for energies lower than the Bragg peak energy differences of as much as 25% occur. Significant deviations are only observed for large ionisation cluster sizes. These deviations can be explained by a background consisting of secondary ions. These ions are produced in the region downstream of the extraction aperture by electrons with a kinetic energy of about 2.5 keV, which are themselves released by ions of the "primary" ionisation cluster hitting an electrode in the ion transport system. Including this background of secondary ions in the simulated cluster size distributions leads to a significantly better agreement between measured and simulated data, especially for large ionisation clusters.

Measurement of track structure parameters of low and medium energy helium and carbon ions in nanometric volumes

“…The original setup of the experiment is described in detail in [10]. Later improvements regarding the data acquisition system and the data evaluation procedure as well as an improved characterisation of the device are described in detail in [11]. The nanodosimeter shown in Figure 2 consists of an interaction region filled with a rarefied target gas, an electrode system to extract ions from the interaction region, an evacuated acceleration stage with an ion-counting detector at its end and a primary particle detector.…”

“…Additionally, the figure shows, for the same target gas and pressure, the comparison of the spatial distribution of the extraction efficiency for the complete drift time distribution together with the corresponding simulation results. In the measured data, the mean ionisation cluster size M1(d,h), and in the simulated data, the extraction efficiency η(d,h) [11] integrated along the primary particle track, are plotted as a function of the impact parameter d and the height h of the primary particle track above the nominal beam height, which is the height in which the particle track is parallel to the plane of the lower electrode of the interaction region. M1(d,h) and η(d,h) are normalised relative to the respective maximum.…”

“…These simulations were carried out for the radiation qualities investigated and, after correcting the simulations with respect to the background of secondary ionisations [11], they were compared to the measured ionisation cluster size distributions for the impact parameter d = 0.…”

“…The uncertainties encountered in the experiment are described in detail in [11]. In order to obtain the overall position resolution for the intensity profile over the whole length of the detector surface, the number of pixels in which the number of counts were between 10% and 90% of the mean value of the number of counts obtained in the pixels in the corresponding plateau was determined and averaged over the whole intensity profile.…”

Nano-scale processes behind ion-beam cancer therapy

“…The paper by Boscolo et al [22] is devoted to the calculation of the efficiency of thermoluminescent dosimeters. The paper of Hilgers et al [23] presents the results of a simulation of an ion-counting nanodosimeter. Theoretical studies of ionization of biomolecules with protons are reported by Paredes et al [24].…”