Track structure simulations of proximity functions in liquid water using the Geant4-DNA toolkit

The mechanistic Monte Carlo modeling of biological effects of ionising radiation at sub-cellular and DNA scale requires the accurate simulation of track structures in the biological medium, commonly approximated as liquid water. The formalism of microdosimetry allows one to describe quantitatively the spatial distribution of energy deposition in the irradiated medium, which is known to relate to the deleterious effects in the irradiated cellular targets. The Geant4-DNA extension of the Geant4 open-source and general-purpose Monte Carlo simulation toolkit has been recently evaluated for the simulation of microdosimetry spectra, allowing, in particular, the calculation of lineal energy distributions. In this work, we extend the microdosimetric functionalities of Geant4-DNA by the development of a new Geant4-DNA example dedicated to the simulation of differential proximity functions. Simulation results are presented for the proximity function of electrons, protons, and alpha particles over a wide energy range using the different physical models of electron interactions available in Geant4-DNA. The influence of sub-excitation processes and electron tracking cut is discussed. Results are compared to literature data when available. As an example, a simple calculation of the relative biological effectiveness (RBE) in the context of the Theory of Dual Radiation Action using the present proximity functions yields up to a factor of 2 variation of the electron RBE in the energy range from 100 eV to 100 keV

A model for Geant4-DNA to simulate ionization and excitation of liquid water by protons travelling above 100 MeV

International audienceBiological damage induced by ionizing radiation plays a major role in many application fields as radiotherapy and microdosimetry. Geant4-DNA Monte Carlo track-structure code has the capability to simulate the passage of radiation through liquid water, containing physical, physicochemical and chemical processes that lead the early DNA damage. For proton projectile, current models reach up to an incident energy of 100 MeV. In order to cover the entire energy regime involved in proton radiotherapy, this work presents a new model that extends proton ionization and excitation of liquid water up to 300 MeV. Calculation of cross section dataset is made for ionization of five ionization shells and five excitation levels of liquid water using the Relativistic Plane Wave Born Approximation (RPWBA). Implementation is validated through the spower and range examples of the official release, obtaining an agreement within 1% with respect to reference data published in ICRU90 report. (Elsevier

The influence of the surrounding atmosphere on plasmas sustained by the "torch a injection axiale"

The microwave torch named "torche a injection axiale" (TIA), i.e. torch with axial gas injection, was developed by the group of Moisan in 1993. This plasma torch can, depending on the geometry of the nozzle, excite many kinds of gases or mixtures, such as air, CO2 and noble gases. Therefore, it is a very promising plasma source for spectrochemical purposes. The plasmas sustained by the TIA normally expand in the open air and are typically 10 cm long and 2 mm in diameter. Microwave powers up to 2 kW can be used without the need for external cooling. In a previous paper we presented the electron density and temperature determined by Thomson scattering. In a plasma with helium as main gas ne ranges between 0.64 and 5.110 20 m-3 and Te is around 25000 K. In an argon plasma the electron temperature is lower and the electron density is higher: 17000 K and around 1021 m-3 respectively. In both cases the original nozzle is used (6 References)

Optical emission spectroscopy on Ar/N/sub 2/ and Ar/N/sub 2//C/sub 2/H/sub 2/ expanding thermal plasmas

This work has been carried out in connection with the possibilities to deposit carbon nitride materials by expansion thermal plasma assisted chemical vapour deposition (ETP-A-CVD). With the same technique high deposition rates and good quality a-Si:H and a-C:H materials have been obtained. A study of the intensity of atomic lines and molecular bands in a Ar/N/sub 2/ and Ar/N/sub 2//C/sub 2/H/sub 2/ expanding thermal plasma has been performed. In the case of the Ar/N/sub 2//C/sub 2/H/sub 2/ mixture rotational and vibrational temperatures were obtained by comparing computer simulated spectra of the CN(B/sup 2/ Sigma -X/sup 2/ Sigma , Delta v=0) spectral system bands with the experimental spectra. The CN ground state density is determined by taking into account the self-absorption of the CN band

Towards a general collisional radiative model

Collisional Radiative Models (CRMs) are a widely used tool in the modelling of plasmas. The results of such models appear as source terms in the particle and energy balances of plasma transport models. In case the change of internal energy of the heavy particle is due to electron collisions and radiative processes the Atomic State Distribution Function (ASDF) can be calculated from plasma parameters, such as the electron density and temperature, solving a set of continuity equations. If the electron density is low only a few excited levels have to be taken into account. This holds for, for example, the modelling of conventional fluorescent lamps where, due to an electron density of around 51017 m-3, a model with only four to six excited atomic mercury levels already gives an accurate description of the discharge (Waymouth, 1971). If the electron density rises more levels must be taken into account. This is necessary because of the onset of the stepwise excitation and ionization processes (van Mullen, 1990). If one would use a purely numerical technique all the levels which are not in partial Local Saha Equilibrium (pLSE) (van Mullen, 1990)] have to be taken into account. Fortunately it is possible to reduce the number of levels that need to be treated numerically by using a so-called Analytical Top Model (ATM) for the densities of the higher excited state

Geant4-DNA example applications for track structure simulations in liquid water: A report from the Geant4-DNA Project

This Special Report presents a description of Geant4‐DNA user applications dedicated to the simulation of track structures (TS) in liquid water and associated physical quantities (e.g., range, stopping power, mean free path…). These example applications are included in the Geant4 Monte Carlo toolkit and are available in open access. Each application is described and comparisons to recent international recommendations are shown (e.g., ICRU, MIRD), when available. The influence of physics models available in Geant4‐DNA for the simulation of electron interactions in liquid water is discussed. Thanks to these applications, the authors show that the most recent sets of physics models available in Geant4‐DNA (the so‐called “option4” and “option 6” sets) enable more accurate simulation of stopping powers, dose point kernels, and W‐values in liquid water, than the default set of models (“option 2”) initially provided in Geant4‐DNA. They also serve as reference applications for Geant4‐DNA users interested in TS simulations