M E Galassi

Limitations (and merits) of PENELOPE as a track-structure code

Abstract Purpose: To outline the limitations of PENELOPE (acronym of PENetration and Energy LOss of Positrons and Electrons) as a track-structure code, and to comment on modifications that enable its fruitful use in certain microdosimetry and nanodosimetry applications. Methods: Attention is paid to the way in which inelastic collisions of electrons are modelled and to the ensuing implications for microdosimetry analysis. Results: Inelastic mean free paths and collision stopping powers calculated with PENELOPE and two well-known optical-data models are compared. An ad hoc modification of PENELOPE is summarized where ionization and excitation of liquid water by electron impact is simulated using tables of realistic differential and total cross sections. Conclusions: PENELOPE can be employed advantageously in some track-structure applications provided that the default model for inelastic interactions of electrons is replaced by suitable tables of differential and total cross sections.

A multi-scale ab initio theoretical study of the production of free radicals in swift ion tracks in liquid water

Using a multi-scale theoretical approach from first principles, we show that the production of HO2 radicals in liquid water can be understood from the initial Coulomb explosion of doubly ionized water molecules. Based on the separation of time scales, we used three different theoretical models, each one associated with a specific time scale. The initial ~1 fs of water radiolysis is taken care of with a Monte Carlo code whose basic ingredients are cross-sections. These have been calculated in the present work using the continuum distorted wave eikonal initial state (CDW-EIS) model framework. Our calculated cross-sections nicely demonstrate that double ionization of water molecules is one major event compatible with the experimental HO2 molecular rate production. The subsequent tens of fs following the double ionization of one water molecule of the liquid medium have been described with microscopic ab initio Car–Parrinello molecular dynamics simulations. Dynamics shows that the water Coulomb explosion leads to the formation of two H3O+ ions and an atomic oxygen atom. The final stage of the Coulomb explosion (up to the ms timescale) has been modelled with a chemical Monte Carlo code, assessing that the production of HO2 results from the O + OH → HO2 reaction in the liquid phase.

Quantum-mechanical predictions of DNA and RNA ionization by energetic proton beams

Among the numerous constituents of eukaryotic cells, the DNA macromolecule is considered as the most important critical target for radiation-induced damages. However, up to now ion-induced collisions on DNA components remain scarcely approached and theoretical support is still lacking for describing the main ionizing processes. In this context, we here report a theoretical description of the proton-induced ionization of the DNA and RNA bases as well as the sugar-phosphate backbone. Two different quantum-mechanical models are proposed: the first one based on a continuum distorted wave-eikonal initial state treatment and the second perturbative one developed within the first Born approximation with correct boundary conditions (CB1). Besides, the molecular structure information of the biological targets studied here was determined by ab initio calculations with the Gaussian 09 software at the restricted Hartree-Fock level of theory with geometry optimization. Doubly, singly differential and total ionization cross sections also provided by the two models were compared for a large range of incident and ejection energies and a very good agreement was observed for all the configurations investigated. Finally, in comparison with the rare experiment, we have noted a large underestimation of the total ionization cross sections of uracil impacted by 80 keV protons,whereas a very good agreement was shown with the recently reported ionization cross sections for protons on adenine, at both the differential and the total scale.

Theoretical predictions for ionization cross sections of DNA nucleobases impacted by light ions.

Induction of DNA double strand breaks after irradiation is considered of prime importance for producing radio-induced cellular death or injury. However, up to now ion-induced collisions on DNA bases remain essentially experimentally approached and a theoretical model for cross section calculation is still lacking. Under these conditions, we here propose a quantum mechanical description of the ionization process induced by light bare ions on DNA bases. Theoretical predictions in terms of differential and total cross sections for proton, α-particle and bare ion carbon beams impacting on adenine, cytosine, thymine and guanine bases are then reported in the 10 keV amu(-1)-10 MeV amu(-1) energy range. The calculations are performed within the first-order Born approximation (FBA) with biological targets described at the restricted Hartree-Fock level with geometry optimization. Comparisons to recent theoretical data for collisions between protons and cytosine point out huge discrepancies in terms of differential as well as total cross sections whereas very good agreement is shown with our previous classical predictions, especially at high impact energies (E(i) ≥ 100 keV amu(-1)). Finally, in comparison to the rare existing experimental data a systematic underestimation is observed in particular for adenine and thymine whereas a good agreement is reported for cytosine. Thus, further improvements appear as necessary, in particular by using higher order theories like the continuum-distorted-wave one in order to obtain a better understanding of the underlying physics involved in such ion-DNA reactions.

Proton-induced single electron capture on DNA/RNA bases.

In this work, we report total cross sections for the single electron capture process induced on DNA/RNA bases by high-energy protons. The calculations are performed within both the continuum distorted wave and the continuum distorted wave-eikonal initial state approximations. The biological targets are described within the framework of self-consistent methods based on the complete neglect of differential overlap model whose accuracy has first been checked for simpler bio-molecules such as water vapour. Furthermore, the multi-electronic problem investigated here is reduced to a mono-electronic one using a version of the independent electron approximation. Finally, the obtained theoretical predictions are confronted with the scarcely available experimental results.

Water versus DNA: new insights into proton track-structure modelling in radiobiology and radiotherapy.

Water is a common surrogate of DNA for modelling the charged particle-induced ionizing processes in living tissue exposed to radiations. The present study aims at scrutinizing the validity of this approximation and then revealing new insights into proton-induced energy transfers by a comparative analysis between water and realistic biological medium. In this context, a self-consistent quantum mechanical modelling of the ionization and electron capture processes is reported within the continuum distorted wave-eikonal initial state framework for both isolated water molecules and DNA components impacted by proton beams. Their respective probability of occurrence-expressed in terms of total cross sections-as well as their energetic signature (potential and kinetic) are assessed in order to clearly emphasize the differences existing between realistic building blocks of living matter and the controverted water-medium surrogate. Consequences in radiobiology and radiotherapy will be discussed in particular in view of treatment planning refinement aiming at better radiotherapy strategies.

The continuum distorted wave eikonal initial state model for transfer ionization in H+, He2+ + He collisions

We employ the independent-event model to study the transfer-ionization (TI) process in H+, He2+ + He collisions. We consider both the ionization-capture and capture-ionization paths. The single-particle probabilities for all channels are calculated with the continuum distorted wave eikonal initial state model. With the probabilities for TI we calculate doubly differential and total cross sections which are compared with the available experimental data. Our present results are in better agreement with the experiments than present calculations using the independent-electron model and previous calculations using the independent-event model, but with single-particle probabilities calculated with the continuum distorted-wave model.

Absolute total ionization cross sections of uracil (C4H4N2O2) in collisions with MeV energy highly charged carbon, oxygen and fluorine ions

Ionization and fragmentation of uracil molecules (C4H4N2O2, m = 112 amu) in collisions with fast highly charged C, O and F ions have been investigated using a time-of-flight mass spectrometer. The measurement of total ionization cross sections (TCS) is reported for different charge states (q), such as Fq + with q = 5?8; Oq + with q = 5,7; Cq + with q = 5 and 6. These studies reveal a (q/v)?1.5 dependence of TCS, in contrast, to the well-known q2-dependence in ion?atom collisions. Scaling properties of the TCS with projectile energy and charge states are obtained. The experimental results for TCS measurements are compared with the theoretical calculations performed within classical and quantum mechanical frameworks. The trends in energy dependence of the TCSs is qualitatively well reproduced by the different models and more specifically by the classical description, which provides the best agreement with measurements.

Coherence in collisionally induced electron emission from diatomic heteronuclear molecules

Electron emission from diatomic heteronuclear targets by bare ion impact is analysed. Interference patterns are shown to appear in electron angular distributions, which are explained in terms of coherent emission from the neighbourhood of the atomic centres composing the target. The relationship between electron ionization by ion and circularly polarized photon impact is investigated.

Orientation and interference effects in single ionization of H2 by fast ions

The single ionization of H2 by fast ions is theoretically studied using a version of the continuum distorted wave eikonal initial state model which takes into account the two-centre geometry of the molecule. Five-fold differential cross sections as a function of the electron momenta in the final state and the orientation of the molecule are calculated for the impact of 13.7 MeV/u-C6+ ions. The angular distribution of emitted electrons shows distinctive structures which arise from the coherent emission from the two nuclei of the molecule which produces well defined interference patterns for all possible orientations of the molecule with respect to the incident beam. When the molecule is aligned in the same direction as the incident beam it is possible to obtain a closed formula which gives the position of the maxima and minima in the interference pattern.