Significant role of secondary electrons in the formation of a multi-body chemical species spur produced by water radiolysis.

Scientific insights into water photolysis and radiolysis are essential for estimating the direct and indirect effects of deoxyribonucleic acid (DNA) damage. Secondary electrons from radiolysis intricately associated with both effects. In our previous paper, we simulated the femtosecond (1 × 10 s) dynamics of secondary electrons ejected by energy depositions of 11-19 eV into water via high-energy electron transport using a time-dependent simulation code.

Liquid water radiolysis induced by secondary electrons generated from MeV-energy carbon ions.

Fast ion beams induce damage to deoxyribonucleic acid (DNA) by chemical products, including secondary electrons, produced from interaction with liquid water in living cells. However, the production process of these chemical products in the Bragg peak region used in particle therapy is not fully understood. To investigate this process, we conducted experiments to evaluate the radiolytic yields produced when a liquid water jet in vacuum is irradiated with MeV-energy carbon beams.

Consideration of the dielectric response for radiation chemistry simulations.

The spur reaction, a spatially nonhomogeneous chemical reaction following ionization, is crucial in radiolysis or photolysis in liquids, but the spur expansion process has yet to be elucidated. One reason is the need to understand the role of the dielectric response of the solvating molecules surrounding the charged species generated by ionization. The dielectric response corresponds to the time evolution of the permittivity and might affect the chemical reaction-diffusion of the species in a spur expansion process.

A step-by-step simulation code for estimating yields of water radiolysis species based on electron track-structure mode in the PHITS code.

. Time-dependent yields of chemical products resulting from water radiolysis play a great role in evaluating DNA damage response after exposure to ionizing radiation. Particle and Heavy Ion Transport code System (PHITS) is a general-purpose Monte Carlo simulation code for radiation transport, which simulates atomic interactions originating from discrete energy levels of ionizations and electronic excitations as well as molecular excitations as physical stages.

First-principles simulation of an ejected electron produced by monochromatic deposition energy to water at the femtosecond order.

This study uses a time-dependent first-principles simulation code to investigate the transient dynamics of an ejected electron produced in the monochromatic deposition energy from 11 to 19 eV in water. The energy deposition forms a three-body single spur comprising a hydroxyl radical (OH˙), hydronium ion (HO), and hydrated electron (e). The earliest formation involves electron thermalization and delocalization dominated by the molecular excitation of water.

Improvement of the hybrid approach between Monte Carlo simulation and analytical function for calculating microdosimetric probability densities in macroscopic matter.

. Estimation of the probability density of the microdosimetric quantities in macroscopic matter is indispensable for applying the concept of microdosimetry to medical physics and radiological protection. The Particle and Heavy Ion Transport code System (PHITS) enables estimating the microdosimetric probability densities due to its unique hybrid modality between the Monte Carlo and analytical approaches called the microdosimetric function.

Nature of the physicochemical process in water photolysis uncovered by a computer simulation.

In this work, we investigate the physicochemical process of water photolysis to bridge physical and chemical processes by a newly developed first-principles calculation code. The deceleration, thermalization, delocalization, and initial hydration of the extremely low-energy electrons ejected by water photolysis are sequentially tracked in the condensed phase. We show herein the calculated results for these sequential phenomena during 300 fs.

Initial yield of hydrated electron production from water radiolysis based on first-principles calculation.

Many scientific insights into water radiolysis have been applied for developing life science, including radiation-induced phenomena, such as DNA damage and mutation induction or carcinogenesis. However, the generation mechanism of free radicals due to radiolysis remains to be fully understood. Consequently, we have encountered a crucial problem in that the initial yields connecting radiation physics to chemistry must be parameterized.

An Analytical Method for Quantifying the Yields of DNA Double-Strand Breaks Coupled with Strand Breaks by γ-H2AX Focus Formation Assay Based on Track-Structure Simulation.

Complex DNA double-strand break (DSB), which is defined as a DSB coupled with additional strand breaks within 10 bp in this study, induced after ionizing radiation or X-rays, is recognized as fatal damage which can induce cell death with a certain probability. In general, a DSB site inside the nucleus of live cells can be experimentally detected using the γ-H2AX focus formation assay. DSB complexity is believed to be detected by analyzing the focus size using such an assay.

Application of a simple DNA damage model developed for electrons to proton irradiation.

Proton beam therapy allows irradiating tumor volumes with reduced side effects on normal tissues with respect to conventional x-ray radiotherapy. Biological effects such as cell killing after proton beam irradiations depend on the proton kinetic energy, which is intrinsically related to early DNA damage induction. As such, DNA damage estimation based on Monte Carlo simulations is a research topic of worldwide interest.

Impact of the Lorentz force on electron track structure and early DNA damage yields in magnetic resonance-guided radiotherapy.

Magnetic resonance-guided radiotherapy (MRgRT) has been developed and installed in recent decades for external radiotherapy in several clinical facilities. Lorentz forces modulate dose distribution by charged particles in MRgRT; however, the impact of Lorentz forces on low-energy electron track structure and early DNA damage induction remain unclear. In this study, we estimated features of electron track structure and biological effects in a static magnetic field (SMF) using a general-purpose Monte Carlo code, particle and heavy ion transport code system (PHITS) that enables us to simulate low-energy electrons down to 1 meV by track-structure mode.

Development and validation of proton track-structure model applicable to arbitrary materials.

A novel transport algorithm performing proton track-structure calculations in arbitrary materials was developed. Unlike conventional algorithms, which are based on the dielectric function of the target material, our algorithm uses a total stopping power formula and single-differential cross sections of secondary electron production. The former was used to simulate energy dissipation of incident protons and the latter was used to consider secondary electron production.

Track-structure modes in particle and heavy ion transport code system (PHITS): application to radiobiological research.

Purpose: In radiation physics, Monte Carlo radiation transport simulations are powerful tools to evaluate the cellular responses after irradiation. When investigating such radiation-induced biological effects, it is essential to perform track structure simulations by explicitly considering each atomic interaction in liquid water at the sub-cellular and DNA scales. The Particle and Heavy-Ion Transport code System (PHITS) is a Monte Carlo code which enables to calculate track structure at DNA scale by employing the track-structure modes for electrons, protons and carbon ions.

Verification of KURBUC-based ion track structure mode for proton and carbon ions in the PHITS code.

The particle and heavy ion transport code system (PHITS) is a general-purpose Monte Carlo radiation transport simulation code. It has the ability to handle diverse particle types over a wide range of energy. The latest PHITS development enables the generation of track structure for proton and carbon ions (H, C) based on the algorithms in the KURBUC code, which is considered as one of the most verified track-structure codes worldwide.

A Simplified Cluster Analysis of Electron Track Structure for Estimating Complex DNA Damage Yields.

Complex DNA damage, defined as at least two vicinal lesions within 10-20 base pairs (bp), induced after exposure to ionizing radiation, is recognized as fatal damage to human tissue. Due to the difficulty of directly measuring the aggregation of DNA damage at the nano-meter scale, many cluster analyses of inelastic interactions based on Monte Carlo simulation for radiation track structure in liquid water have been conducted to evaluate DNA damage. Meanwhile, the experimental technique to detect complex DNA damage has evolved in recent decades, so both approaches with simulation and experiment get used for investigating complex DNA damage.

A significant role of non-thermal equilibrated electrons in the formation of deleterious complex DNA damage.

Although most of the radiation damage to genomic DNA could be rendered harmless using repair enzymes in a living cell, a certain fraction of the damage is persistent resulting in serious genetic effects, such as mutation induction. In order to understand the mechanisms of the deleterious DNA damage formation in terms of its earliest physical stage at the radiation track end, dynamics of low energy electrons and their thermalization processes around DNA molecules were investigated using a dynamic Monte Carlo code. The primary incident (1 keV) electrons multiply collide within 1 nm (equivalent to three DNA-base-pairs, 3bp) and generate secondary electrons which show non-Gaussian and non-thermal equilibrium distributions within 300 fs.

Dynamic Behavior of Secondary Electrons in Liquid Water at the Earliest Stage upon Irradiation: Implications for DNA Damage Localization Mechanism.

To clarify the formation of radiation damage in DNA, the dynamic behavior of low-energy secondary electrons produced by ionizing radiation in water was studied by using a dynamic Monte Carlo code that considers the Coulombic force between electrons and their parent cations. The calculated time evolution of the mean energy, total track length, and mean traveling distance of the electrons indicated that the prehydration of the electrons occurs competitively with thermalization on a time scale of hundreds of femtoseconds. The decelerating electrons are gradually attracted to their parent cations by Coulombic force within hundreds of femtoseconds, and finally about 12.

Deceleration processes of secondary electrons produced by a high-energy Auger electron in a biological context.

Purpose: To simulate the deceleration processes of secondary electrons produced by a high-energy Auger electron in water, and particularly to focus on the spatial and temporal distributions of the secondary electron and the collision events (e.g. ionization, electronic excitation, and dissociative electron attachment) that are involved in the multiplication of lesions at sites of DNA damage.

Evaluation of DNA damage induced by Auger electrons from Cs.

Purpose: To understand the biological effect of external and internal exposure from Cs, DNA damage spectrum induced by directly emitted electrons (γ-rays, internal conversion electrons, Auger electrons) from Cs was compared with that induced by Cs γ-rays.

Methods: Monte Carlo track simulation method was used to calculate the microscopic energy deposition pattern in liquid water. Simulation was performed for the two simple target systems in microscale.

Development of a simulation method for dynamics of electrons ejected from DNA molecules irradiated with X-rays.

Purpose: To develop a method for simulating the dynamics of the photoelectrons and Auger electrons ejected from DNA molecules irradiated with pulsed monochromatic X-rays.

Materials And Methods: A 30-base-pair (bp) DNA molecule was used as the target model, and the X-rays were assumed to have a Gaussian-shaped time distribution. Photoionization and Auger decay were considered as the atomic processes.

Polarization of Healpha radiation due to anisotropy of fast-electron transport in ultraintense-laser-produced plasmas.

An atomic kinetics code is developed to gain insight into the generation of polarized Healpha by fast electron transport relevant to fast ignition. The calculation predicts a very small polarization in the dense region (>or=100 times the critical density) due to frequent elastic transitions between magnetic sublevels, while high polarization is observable in the low density region ( View Article and Find Full Text PDF