Dr
Ziad Francis
(IRSN FRANCE)
Track structure simulation using the Monte-Carlo method has become increasingly important not only on the macroscopic scale but also on the nanometric and the molecular simulation level. Many studies (mostly done by Nikjoo et al., and Goodhead et al.) showed that damage clustering in cell nucleus is related to the energy deposits distribution inside the cell. However, experimentally it is impossible to guess the exact position of each energy deposit point inside irradiated cells. Monte-Carlo simulations can reproduce the ionizing particle track taking into account all the interactions according to calculated cross sections. Many nano-level codes were developed so far, but generally these codes are dedicated for private use and for a very specific application (Partrac, PITS, OREC…) which makes their access practically difficult and sometimes even impossible due to lack of information, user-support or a simple users manual.
The Geant4-DNA project comes as a free open source set of physics processes that can handle nanometric level simulations for electrons (8.22 eV – 1 MeV), protons (100 eV – 100 MeV) and alpha particles (1 keV – 10 MeV). Users can benefit from dedicated documentation, forums and necessary amount of support for new users through workshops and tutorials.
This study shows the recent developments in Geant4 concerning the inelastic and the elastic interactions of these particles in liquid water, as water is considered to be the dominant component in a living cell. Models of inelastic scattering cross sections are based on the First Born Approximation (FBA) theory. For low incident energies the FBA becomes inapplicable, correction terms are then used for electrons and semi empirical models (Rudd model for ionisation) are used for Alpha particles and protons. Charge transfer process is also taken into account according to Dingfelder’s model. Relativistic dynamics were recently added for high energy (20 keV – 1 MeV) electrons cross sections.
Validation results are still in progress and are not shown in this work. However, a comparison shows a good agreement between microdosimetric calculations (proximity functions and clustering results) obtained using Geant4-DNA and results obtained by other nano-level codes (Nikjoo et al., Chen and Kellerer).
Summary
This work describes the Geant4-DNA physics processes set. Processes concern electrons, protons and alpha particles interactions in water. The First Born Approximation (FBA) was used for inelastic cross sections. At low energies, in the case of electrons the FBA becomes inaccurate and should be corrected by several terms (exchange cross section and Coulomb field correction). For protons and alpha particles no corrections were used the theory was replaced by the Rudd semi-empirical model below a certain energy threshold. Electron elastic scattering was calculated using two different models; the Champion model and the screened Rutherford model. Validations showed good agreement with experimental data. Relativistic kinematics were added for electrons cross sections extending the possibility to simulate electrons up to 1 MeV of incident energy. Clustering algorithms showed good agreement with published microdosimetric calculations in the literature
Keymords
Geant4-DNA, Monte-Carlo, Microdosimetry, Inelastic Cross-sections, Elastic Cross-sections, First Born Approximation, energy deposits clustering, DNA level.
| Are you a Memeber of the Geant4 Collaboration (yes/no) |
yes
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