Water radiolysis with heavy ions of energies up to 28GeV

Increase of OH radical yields due to the decomposition of hydrogen peroxide by gold nanoparticles under X-ray irradiation

We elucidate the decomposition mechanism of hydrogen peroxide, which is formed by water radiolysis, by gold nanoparticles (GNPs) under X-ray irradiation. The variations in yields of hydrogen peroxide generated in the presence of GNPs are evaluated using the Ghormley technique. The increase of yields of OH radicals has been quantified using Ampliflu® Red solutions. Almost all hydrogen peroxide generated by irradiation of <25 Gy is decomposed by GNPs, while the yield of OH radicals increases by 1.6 times. The amount of OH radicals thus obtained is almost equivalent to that of the decomposed hydrogen peroxide. The decomposition of hydrogen peroxide is an essential reaction to produce additional OH radicals efficiently in the vicinity of GNPs.

Intermediate LET-like effect in distal part of proton Bragg peak revealed by track-ends imaging during super-Fricke radiolysis

Upstream of the efficiency of proton or carbon ion beams in cancer therapy, and to optimize hadrontherapy results, we analysed the chemistry of Fricke solutions in track-end of 64-MeV protons and 1.14-GeV carbon ions. An original optical setup is designed to determine the primary track-segment yields along the last millimetres of the ion track with a sub-millimetre resolution. The Fe3+-yield falls in the Bragg peak to (4.9 ± 0.4) × 10-7 mol/J and 1.9 × 10-7 mol/J, under protons and carbon ions respectively. Beyond the Bragg peak, a yield recovery is observed over 1 mm for proton beams. It is attributed to the intermediate-LET of protons in this region where their energy decreases and energy distribution becomes broader, in relation with the longitudinal straggling of the beam. Consequently to this LET decrease in the distal part of the Bragg peak, Fe3+-yield increases. For the first time, this signature is highlighted at the chemical level under proton irradiation. Nevertheless, this phenomenon is not identified for carbon ion beams since their straggling is lower. It would need a greater spatial resolution to be observed.

GEANT4-DNA simulation of temperature-dependent and pH-dependent yields of chemical radiolytic species

Objective.GEANT4-DNA can simulate radiation chemical yield (G-value) for radiolytic species such as the hydrated electron (eaq-) with the independent reaction times (IRT) method, however, only at room temperature and neutral pH. This work aims to modify the GEANT4-DNA source code to enable the calculation ofG-values for radiolytic species at different temperatures and pH values.Approach.In the GEANT4-DNA source code, values of chemical parameters such as reaction rate constant, diffusion coefficient, Onsager radius, and water density were replaced by corresponding temperature-dependent polynomials. The initial concentration of hydrogen ion (H+)/hydronium ion (H3O+) was scaled for a desired pH using the relationship pH = -log10[H+]. To validate our modifications, two sets of simulations were performed. (A) A water cube with 1.0 km sides and a pH of 7 was irradiated with an isotropic electron source of 1 MeV. The end time was 1μs. The temperatures varied from 25 °C to 150 °C. (B) The same setup as (A) was used, however, the temperature was set to 25 °C while the pH varied from 5 to 9. The results were compared with published experimental and simulated work.Main results.The IRT method in GEANT4-DNA was successfully modified to simulateG-values for radiolytic species at different temperatures and pH values. Our temperature-dependent results agreed with experimental data within 0.64%-9.79%, and with simulated data within 3.52%-12.47%. The pH-dependent results agreed well with experimental data within 0.52% to 3.19% except at a pH of 5 (15.99%) and with simulated data within 4.40%-5.53%. The uncertainties were below ±0.20%. Overall our results agreed better with experimental than simulation data.Significance.Modifications in the GEANT4-DNA code enabled the calculation ofG-values for radiolytic species at different temperatures and pH values.

Quantitative estimation of track segment yields of water radiolysis species under heavy ions around Bragg peak energies using Geant4-DNA

We evaluate the track segment yield G' of typical water radiolysis products (e_aq^-, ^·OH and H₂O₂) under heavy ions (He, C and Fe ions) using a Monte Carlo simulation code in the Geant4-DNA. Furthermore, we reproduce experimental results of ^·OH of He and C ions around the Bragg peak energies (< 6 MeV/u). In the relatively high energy region (e.g., > 10 MeV/u), the simulation results using Geant4-DNA have agreed with experimental results. However, the G-values of water radiolysis species have not been properly evaluated around the Bragg peak energies, at which high ionizing density can be expected. Around the Bragg peak energy, dense continuous secondary products are generated, so that it is necessary to simulate the radical-radical reaction more accurately. To do so, we added the role of secondary products formed by irradiation. Consequently, our simulation results are in good agreement with experimental results and previous simulations not only in the high-energy region but also around the Bragg peak. Several future issues are also discussed regarding the roles of fragmentation and multi-ionization to realize more realistic simulations.

Chemical Dosimetry in the "Water Window": Ferric Ions and Hydroxyl Radicals Produced by Intense Soft X Rays

A standard Fricke dosimeter was used to measure the absorbed dose via the oxidation yields of Fe³⁺ ions in an aqueous environment induced by soft X rays within the "water window" spectral range. We also exploited the property of a neutral solution containing terephthalic acid as a tool for selective detection of OH radicals. Both dosimetric systems were irradiated using the experimental pulsed laser-plasma soft X-ray source as well as conventional 1.25-MeV gamma rays. Radiation chemical yields of Fe³⁺ ions and OH radicals were determined to be (5.13 ± 0.94) × 10^-1 µmol·J^-1 (4.95 ± 0.91 100eV^-1) and (2.33 ± 0.35) × 10^-2 µmol·J^-1 (0.23 ± 0.03 100eV^-1), respectively. Measurements were supported by Monte Carlo simulations to estimate the linear energy transfer of the water window radiation. The simulation results are in good agreement with expected linear energy transfer of ions inducing the same Fe³⁺ ion and OH radical radiation chemical yield.

Radiation Chemical Yield of Hydroxyl Radicals for Accelerator-based Boron Neutron Capture Therapy: Dose Assessment of 10B(n,α)7Li Reaction Using Coumarin-3-Carboxilic Solution

Evaluation of the characteristics of accelerator-based thermal neutron fields is recognized as an important issue when discussing the effectiveness of boron neutron capture therapy (BNCT). In this study, we propose that the radiation chemical yield (G value) of hydroxyl radicals (G_oh•) can be considered a universal parameter for the description of the accelerator-based thermal neutron field. The G_oh• of the ¹⁰B(n,α)⁷Li reaction was quantitatively evaluated using an aqueous coumarin-3-carboxylic acid (3CCA) solution, and was discriminated from that of contaminations (i.e., γ rays and fast neutrons). The G_oh• of the ¹⁰B(n,α)⁷Li reaction was 0.107 ± 0.004 OH•/100 eV, which is almost equivalent to that exposed to α particles with an energy of 6.0 MeV. Since the G_oh• of γ rays from a ⁶⁰Co source is 2.03 ± 0.05 OH•/100 eV, this lower value suggests that indirect action by the ¹⁰B(n,α)⁷Li reaction is not dominant in BNCT. However, our results indicate that one can assess the ⁶⁰Co equivalent dose of the ¹⁰B(n,α)⁷Li reaction in water from the G_oh• derived using aqueous 3CCA solution in the accelerator-based thermal neutron field.

Dosimetry as a Catch in Radiobiology Experiments

Experimental radiobiological studies in which the effects of ionizing radiation on a biological model are examined often highlight the biological aspects while missing detailed descriptions of the geometry, sample and dosimetric methods used. Such omissions can hinder the reproducibility and comparability of the experimental data. An application based on the Geant4 simulation toolkit was developed to design experiments using a biological solution placed in a microtube. The application was used to demonstrate the influence of the type of microtube, sample volume and energy of a proton source on the dose distribution across the sample, and on the mean dose in the whole sample. The results shown here are for samples represented by liquid water in the 0.4-, 1.5- and 2.0-ml microtubes irradiated with 20, 30 and 100 MeV proton beams. The results of this work demonstrate that the mean dose and homogeneity of the dose distribution within the sample strongly depend on all three parameters. Furthermore, this work shows how the dose uncertainty propagates into the scored primary DNA damages in plasmid DNA studies using agarose gel electrophoresis. This application is provided freely to assist users in verifying their experimental setup prior to the experiment.

Analysis of redox states of protic and aprotic solutions irradiated by low linear energy transfer carbon-ion beams using a 2,2-diphenyl-1-picrylhydrazyl radical

The redox states of protic and aprotic solutions were evaluated after carbon-ion irradiation.

Sequential radiation chemical reactions in aqueous bromide solutions: pulse radiolysis experiment and spur model simulation

Pulse radiolysis experiments were carried out to observe transient absorptions of reaction intermediates produced in N₂O- and Ar-saturated aqueous solutions containing 0.9–900 mM NaBr.

Hydroxyl radical-induced degradation of fenuron in pulse and gamma radiolysis: kinetics and product analysis

Radiolytic reactions of phenylureas were studied in detail with fenuron model compound in dilute aqueous solutions using pulse radiolysis for detection of the intermediates, gamma radiolysis with UV-Vis and HPLC-MS techniques for analysis of the final products. The kinetics of oxidation was followed by COD, TOC and toxicity measurements. During radiolysis of aerated solutions hydroxyl radical ((•)OH), eaq (-), H(•) and O2 (•-)/HO2 (•) reactive intermediates are produced, the degradation of solute takes place practically entirely through (•)OH reactions. Therefore, the product distribution is similar to the distributions reported in other advanced oxidation processes with (•)OH as main reactant. (•)OH mainly reacts with the aromatic ring, forming cyclohexadienyl radical as an intermediate. This radical in pulse radiolysis has a wide absorption band in the 310-390 nm wavelength range with a maximum at 350 nm. Cyclohexadienyl radical reacts with dissolved O2 with a rate coefficient of ∼ 4 × 10(8) mol(-1) dm(3) s(-1) forming peroxy radical. The latter may eliminate HO2 (•) giving phenols or undergoes fragmentation. The one-electron oxidant (•)OH on average induces more than two-electron oxidations. The toxicity first increases with absorbed dose, then decreases. This increase is partly due to phenols formed during the first degradation period.