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

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\u27s 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. The measured data were compared with the simulation results obtained with the PTB Monte-Carlo code PTra [B. Grosswendt, Radiat. Environ. Biophys. 41, 103 (2002); M.U. Bug, E. Gargioni, H. Nettelbeck, W.Y. Baek, G. Hilgers, A.B. Rosenfeld, H. Rabus, Phys. Rev. E 88, 043308 (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. Graphical abstract: [Figure not available: see fulltext.

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

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\u27s 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. The measured data were compared with the simulation results obtained with the PTB Monte-Carlo code PTra [B. Grosswendt, Radiat. Environ. Biophys. 41, 103 (2002); M.U. Bug, E. Gargioni, H. Nettelbeck, W.Y. Baek, G. Hilgers, A.B. Rosenfeld, H. Rabus, Phys. Rev. E 88, 043308 (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. Graphical abstract: [Figure not available: see fulltext.

Nanodosimetric particle track simulations in water and DNA media

This work provides the first set of electron-impact interaction cross section data of DNA constituents based on experiments. These data permit to investigate the accuracy by which water cross sections can be used to represent DNA media in track structure simulations of electrons with energies between 7 eV and 1 keV. Liquid water or water vapour is conventionally used in track structure simulation codes to estimate radiobiological effects, however, the interaction cross sections of liquid water have not been experimentally verified. Initially, electron track structure simulations in liquid water with the codes PTra and Geant4 were benchmarked with respective experimental literature data. For this purpose, PTra was augmented with reviewed water cross section data for electrons and protons. The evaluated cross section data for electron impact on the DNA constituents tetrahydrofuran, trimethylphosphate, pyrimidine and purine were implemented in PTra for simulations of monoenergetic electrons in DNA media. The DNA media consisted of a composition of DNA constituents with different water content. Due to substantial differences in the cross section data of water and DNA constituents, a significant enhancement of calculated clustered ionisation and excitation events in DNA media relative to water was observed for electrons with energies below 150 eV. In consequence, the probability to produce biologically relevant ionisation clusters in the vicinity of a 1 MeV proton track is higher for DNA media compared to water. As a first step towards modelling the transport of ions in DNA medium, simulations of protons (0.1–10 MeV) and alpha particles (0.1–20 MeV) in nitrogen and propane were benchmarked by comparing simulated and measured nanodosimetric quantities

Total electron-scattering cross sections of pyrimidine

Total electron-scattering cross sections of pyrimidine, the basic component for the nucleic bases cytosine and thymine, were measured for electron energies from 5 eV to 1 keV using the linear transmission method. The measured results were compared to semiempirical data obtained by means of the additivity rule and to experimental data for benzene since it has a similar ring structure and the same number of valence electrons as pyrimidine. Furthermore, integral elastic and inelastic electron-scattering cross sections of pyrimidine were calculated by applying the spherical complex optical potential model. The sum of both cross sections agrees reasonably well with the experimental total electron-scattering cross sections of pyrimidine in the energy range from 20 eV to 1 keV. The experimental data are, however, significantly lower than the theoretical cross sections when including the contribution of rotational excitations to the electron scattering

Simulation of ionisation clusters formed in nanometric volumes of the deoxyribose-substitute tetrahydrofuran

Purpose: To investigate the implications of using interaction cross sections of liquid water for the target volume when studying radiation action at the DNA level by particle track structure simulations. Materials and methods: Absolute interaction cross sections for low energy electrons between 20 eV and 1 keV were measured for tetrahydrofuran (THF), which is a substitute for deoxyribose. From these data a complete interaction cross section data set was derived and integrated in our PTB Track structure Monte Carlo code \u27PTra\u27. Simulations of electron track structure in THF and water were performed and ionisation cluster size distributions in nanometric target volumes were determined. From these a nanodosimetric estimate for the probability to produce a double strand break was derived. Results: The probability distribution of ionisation cluster sizes was found to be shifted towards smaller values for a THF-filled target as compared to a water-filled one. For all electron energies investigated, the nanodosimetric estimates for double-strand break probability in the THF-filled target have lower values than for a target of liquid water. Conclusion: The preliminary results indicate that simulations based on cross sections of water would overestimate the initial direct radiation damage to the DNA

Ionization cross section data of nitrogen, methane, and propane for light ions and electrons and their suitability for use in track structure simulations

Track structure Monte Carlo simulations are frequently applied in micro- and nanodosimetry to calculate the radiation transport in detail. The use of a well-validated set of cross section data in such simulation codes ensures accurate calculations of transport parameters, such as ionization yields. These cross section data are, however, scarce and often discrepant when measured by different groups. This work surveys literature data on ionization and charge-transfer cross sections of nitrogen, methane, and propane for electrons, protons, and helium particles, focusing on the energy range between 100 keV and 20 MeV. Based on the evaluated data, different models for the parametrization of the cross section data are implemented in the code PTRA, developed for simulating proton and alpha particle transport in an ion-counting nanodosimeter. The suitability of the cross section data is investigated by comparing the calculated mean ionization cluster size and energy loss with experimental results in either nitrogen or propane. For protons, generally good agreement between measured and simulated data is found when the Rudd model is used in PTRA. For alpha particles, however, a considerable influence of different parametrizations of cross sections for ionization and charge transfer is observed. The PTRA code using the charge-transfer data is, nevertheless, successfully benchmarked by the experimental data for the calculation of nanodosimetric quantities, but remaining discrepancies still have to be further investigated (up to 13% lower energy loss and 19% lower mean ionization cluster size than in the experiment). A continuation of this work should investigate data for the energy loss per interaction as well as differential cross section data of nitrogen and propane. Interpolation models for ionization and charge-transfer data are proposed. The Barkas model, frequently used for a determination of the effective charge in the ionization cross section, significantly underestimates both the energy loss (by up to 19%) and the mean ionization cluster size (up to 65%) for alpha particles. It is, therefore, not recommended for particle-track simulations

Nanodosimetric characterization of ion beams

The characterization of particle track structure is essential for an estimation of radiobiological effects, particularly in the case of densely ionizing radiation. The particle track structure can be characterized by nanodosimetric quantities which are measurable by means of a nanodosimeter. Results obtained from experiments with the nanodosimeter can be used to validate track structure simulations, which are essential for estimating track structure parameters in biological material. For this purpose, the dedicated Monte Carlo code PTra has been developed to simulate the nanodosimeter setup as well as nanometric targets consisting of water. Recently, electron-impact cross section data of DNA constituents measured at PTB were implemented into PTra. A calculation of nanodosimetric quantities in DNA-analog media shows considerable differences to results obtained in water medium, particularly for electron energies lower than 200 eV. These discrepancies become more considerable when nanodosimetric quantities are used to estimate biological effects. This paper aims to provide an overview of the present status of nanodosimetry, focusing on the experimental and simulation work at PTB. Furthermore, the suitability of simple models directly linking nanodosimetric track structure characteristics and radiobiological effectiveness is discussed

Effect of a static magnetic field on nanodosimetric quantities in a DNA volume

Abstract Purpose: With the advent of magnetic resonance imaging (MRI)-guided radiation therapy it is becoming increasingly important to consider the potential influence of a magnetic field on ionising radiation. This paper aims to study the effect of a magnetic field on the track structure of radiation to determine if the biological effectiveness may be altered. Methods: Using the Geant4-DNA (GEometry ANd Tracking 4) Monte Carlo simulation toolkit, nanodosimetric track structure parameters were calculated for electrons, protons and alpha particles moving in transverse magnetic fields up to 10 Tesla. Applying the model proposed by Garty et al., the track structure parameters were used to derive the probability of producing a double-strand break (DSB). Results: For simulated primary particles of electrons (200 eV-10 keV), protons (300 keV-30 MeV) and alpha particles (1-9 MeV) the application of a magnetic field was shown to have no significant effect (within statistical uncertainty limits) on the parameters characterizing radiation track structure or the probability of producing a DSB. Conclusions: The null result found here implies that if the presence of a magnetic field were to induce a change in the biological effectiveness of radiation, the effect would likely not be due to a change in the track structure of the radiation

Doubly differential cross sections for electron-impact ionization of propane in the energy range from 30 eV to 1 keV

Doubly differential electron-impact ionization cross sections of propane were comprehensively measured for electron energies between 30 eV and 1 keV as a function of secondary electron energies and emission angles. The measurements were carried out for secondary electron energies from 3 eV to about half of the primary energy and for emission angles between 10° and 135°. To facilitate practical application and implementation of the data into numerical codes used for radiation transport calculations, a semi-empirical formula was constructed on the basis of existing models. The semi-empirical formula is capable of reproducing the measured data well over a wide energy and angular range. Singly differential ionization cross sections were obtained by the integration of the experimental data over the emission angles and total ionization cross sections (TICSs) were determined by the integration of the data both over the emission angles and secondary electron energies. They were compared to the theoretical results calculated using the binary-encounter-Bethe (BEB) model. The calculated TICSs mostly agree with the data published by other groups within the experimental uncertainties