Michael Dingfelder

Electron inelastic-scattering cross sections in liquid water

Electron inelastic-scattering cross-section data for use as input in electron track-structure calculations in liquid water are re-examined and improved. The dielectric-response function used in such cross-sections is estimated on the basis of optical data and other experimental and theoretical information. The mean excitation energy for stopping power is obtained to be 81.8 eV. which is close to the recent experimental value. 79.75 + 0.5 eV, of Bichsel and Hiraoka. Inelastic-scattering cross sections are evaluated within the first Born approximation. Electron-exchange effects and semi-empirical corrections to account for non-Born effects at low energies are also incorporated.

Track structures, DNA targets and radiation effects in the biophysical Monte Carlo simulation code PARTRAC.

This review describes the PARTRAC suite of comprehensive Monte Carlo simulation tools for calculations of track structures of a variety of ionizing radiation qualities and their biological effects. A multi-scale target model characterizes essential structures of the whole genomic DNA within human fibroblasts and lymphocytes in atomic resolution. Calculation methods and essential results are recapitulated regarding the physical, physico-chemical and chemical stage of track structure development of radiation damage induction. Recent model extension towards DNA repair processes extends the time dimension by about 12 orders of magnitude and paves the way for superior predictions of radiation risks.

Simulation of DNA Damage after Proton Irradiation

Abstract Friedland, W., Jacob, P., Bernhardt, P., Paretzke, H. G. and Dingfelder, M. Simulation of DNA Damage after Proton Irradiation. Radiat. Res. 159, 401–410 (2003). The biophysical radiation track simulation model PARTRAC was improved by implementing new interaction cross sections for protons in water. Computer-simulated tracks of energy deposition events from protons and their secondary electrons were superimposed on a higher-order DNA target model describing the spatial coordinates of the whole genome inside a human cell. Induction of DNA double-strand breaks was simulated for proton irradiation with LET values between 1.6 and 70 keV/μm and various reference radiation qualities. The yield of DSBs after proton irradiation was found to rise continuously with increasing LET up to about 20 DSBs per Gbp and Gy, corresponding to an RBE up to 2.2. About half of this increase resulted from a higher yield of DSB clusters associated with small fragments below 10 kbp. Exclusion of experimentally unresolved multiple DSBs reduced the maximum DSB yield by 30% and shifted it to an LET of about 40 keV/μm. Simulated fragment size distributions deviated significantly from random breakage distributions over the whole size range after irradiation with protons with an LET above 10 keV/μm. Determination of DSB yields using equations derived for random breakage resulted in an underestimation by up to 20%. The inclusion of background fragments had only a minor influence on the distribution of the DNA fragments induced by radiation. Despite limited numerical agreement, the simulations reproduced the trends in proton-induced DNA DSBs and fragment induction found in recent experiments.

Inelastic-collision cross sections of liquid water for interactions of energetic protons ☆

Abstract Cross-section data for inelastic interactions of energetic protons with liquid water, for use, e.g. as input in track structure analysis, are derived for an energy range from 0.1 keV to 10 GeV. At proton kinetic energies above about 500 keV, the first Born approximation and the dielectric-response function determined earlier are used. At proton energies above several hundred MeV in particular, the Fermi-density effect is also incorporated. At energies below about 500 keV, which corresponds to a residual range of about 8.9 × 10 −6 m, cross-section values are derived semi-empirically by an extensive and critical analysis of experimental and theoretical information concerning not only cross sections for individual processes such as ionisation, excitation, and charge transfer but also stopping power and other relevant quantities. Spectra of secondary electrons resulting from ionising collisions are also presented. The analysis also includes considerations of phase effects on cross sections.

Comparisons of Calculations with PARTRAC and NOREC: Transport of Electrons in Liquid Water

Abstract Dingfelder, M., Ritchie, R. H., Turner, J. E., Friedland, W., Paretzke, H. G. and Hamm, R. N. Comparisons of Calculations with PARTRAC and NOREC: Transport of Electrons in Liquid Water. Radiat. Res. 169, 584–594 (2008). Monte Carlo computer models that simulate the detailed, event-by-event transport of electrons in liquid water are valuable for the interpretation and understanding of findings in radiation chemistry and radiation biology. Because of the paucity of experimental data, such efforts must rely on theoretical principles and considerable judgment in their development. Experimental verification of numerical input is possible to only a limited extent. Indirect support for model validity can be gained from a comparison of details between two independently developed computer codes as well as the observable results calculated with them. In this study, we compare the transport properties of electrons in liquid water using two such models, PARTRAC and NOREC. Both use interaction cross sections based on plane-wave Born approximations and a numerical parameterization of the complex dielectric response function for the liquid. The models are described and compared, and their similarities and differences are highlighted. Recent developments in the field are discussed and taken into account. The calculated stopping powers, W values, and slab penetration characteristics are in good agreement with one another and with other independent sources.

Induction and Processing of Oxidative Clustered DNA Lesions in 56Fe-Ion-Irradiated Human Monocytes

Abstract Tsao, D., Kalogerinis, P., Tabrizi, I., Dingfelder, M., Stewart, R. D. and Georgakilas, A. G. Induction and Processing of Oxidative Clustered DNA Lesions in 56Fe-Ion-Irradiated Human Monocytes. Radiat. Res. 168, 87–97 (2007). Space and cosmic radiation is characterized by energetic heavy ions of high linear energy transfer (LET). Although both low- and high-LET radiations can create oxidative clustered DNA lesions and double-strand breaks (DSBs), the local complexity of oxidative clustered DNA lesions tends to increase with increasing LET. We irradiated 28SC human monocytes with doses from 0–10 Gy of 56Fe ions (1.046 GeV/ nucleon, LET = 148 keV/μm) and determined the induction and processing of prompt DSBs and oxidative clustered DNA lesions using pulsed-field gel electrophoresis (PFGE) and Number Average Length Analysis (NALA). The 56Fe ions produced decreased yields of DSBs (10.9 DSB Gy−1 Gbp−1) and clusters (1 DSB:∼0.8 Fpg clusters:∼0.7 Endo III clusters: ∼0.5 Endo IV clusters) compared to previous results with 137Cs γ rays. The difference in the relative biological effectiveness (RBE) of the measured and predicted DSB yields may be due to the formation of spatially correlated DSBs (regionally multiply damaged sites) which result in small DNA fragments that are difficult to detect with the PFGE assay. The processing data suggest enhanced difficulty compared with γ rays in the processing of DSBs but not clusters. At the same time, apoptosis is increased compared to that seen with γ rays. The enhanced levels of apoptosis observed after exposure to 56Fe ions may be due to the elimination of cells carrying high levels of persistent DNA clusters that are removed only by cell death and/or “splitting” during DNA replication.

Comprehensive track-structure based evaluation of DNA damage by light ions from radiotherapy-relevant energies down to stopping

Track structures and resulting DNA damage in human cells have been simulated for hydrogen, helium, carbon, nitrogen, oxygen and neon ions with 0.25–256 MeV/u energy. The needed ion interaction cross sections have been scaled from those of hydrogen; Barkas scaling formula has been refined, extending its applicability down to about 10 keV/u, and validated against established stopping power data. Linear energy transfer (LET) has been scored from energy deposits in a cell nucleus; for very low-energy ions, it has been defined locally within thin slabs. The simulations show that protons and helium ions induce more DNA damage than heavier ions do at the same LET. With increasing LET, less DNA strand breaks are formed per unit dose, but due to their clustering the yields of double-strand breaks (DSB) increase, up to saturation around 300 keV/μm. Also individual DSB tend to cluster; DSB clusters peak around 500 keV/μm, while DSB multiplicities per cluster steadily increase with LET. Remarkably similar to patterns known from cell survival studies, LET-dependencies with pronounced maxima around 100–200 keV/μm occur on nanometre scale for sites that contain one or more DSB, and on micrometre scale for megabasepair-sized DNA fragments.

Updated model for dielectric response function of liquid water

A modified and updated version of the model of the dielectric response function of liquid water as currently implemented in the PARTRAC code is presented. The updated version takes advantage of the newer experimental information from the Sendai group and implements some improvements in modeling and usability.

Electron emission from amorphous solid water induced by passage of energetic protons and fluorine ions.

Abstract Absolute doubly differential electron emission yields were measured from thin films of amorphous solid water (ASW) after the transmission of 6 MeV protons and 19 MeV (1 MeV/nucleon) fluorine ions. The ASW films were frozen on thin (1-µm) copper foils cooled to approximately 50 K. Electrons emitted from the films were detected as a function of angle in both the forward and backward direction and as a function of the film thickness. Electron energies were determined by measuring the ejected electron time of flight, a technique that optimizes the accuracy of measuring low-energy electron yields, where the effects of molecular environment on electron transport are expected to be most evident. Relative electron emission yields were normalized to an absolute scale by comparison of the integrated total yields for proton-induced electron emission from the copper substrate to values published previously. The absolute doubly differential yields from ASW are presented along with integrated values, providing single differential and total electron emission yields. These data may provide benchmark tests of Monte Carlo track structure codes commonly used for assessing the effects of radiation quality on biological effectiveness.

Galactic cosmic ray simulation at the NASA Space Radiation Laboratory

Most accelerator-based space radiation experiments have been performed with single ion beams at fixed energies. However, the space radiation environment consists of a wide variety of ion species with a continuous range of energies. Due to recent developments in beam switching technology implemented at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory (BNL), it is now possible to rapidly switch ion species and energies, allowing for the possibility to more realistically simulate the actual radiation environment found in space. The present paper discusses a variety of issues related to implementation of galactic cosmic ray (GCR) simulation at NSRL, especially for experiments in radiobiology. Advantages and disadvantages of different approaches to developing a GCR simulator are presented. In addition, issues common to both GCR simulation and single beam experiments are compared to issues unique to GCR simulation studies. A set of conclusions is presented as well as a discussion of the technical implementation of GCR simulation.