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 (e_aq^-) 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 of G-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 1 µs. The temperatures varied from 25 °C to 150 °C. B) 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 simulate G-values for radiolytic species at different temperatures and pH values. Our temperature-dependent results agreed with experimental data within 0.64% to 9.79%, and with simulated data within 3.52% to 12.47%. The pH-dependent results agreed well with experimental data within 0.52% to 3.19% except at pH of 5 (15.99%) and with simulated data within 4.40% to 5.53%. The uncertainties were below ± 0.20%. Overall our results agreed better with experimental than simulation data.
Significance: Modifications in GEANT4-DNA code enabled calculation of G-values for radiolytic species at different temperature and pH values.

An integrated Monte Carlo track-structure simulation framework for modeling inter and intra-track effects on homogenous chemistry

OBJECTIVE

The TOPAS-nBio Monte Carlo track structure simulation code, a wrapper of Geant4-DNA, was extended for its use in pulsed and longtime homogeneous chemistry simulations using the Gillespie algorithm.
Approach: Three different tests were used to assess the reliability of the implementation and its ability to accurately reproduce published experimental results: (1) A simple model with a known analytical solution, (2) the temporal evolution of chemical yields during the homogeneous chemistry stage, and (3) radiolysis simulations conducted in pure water with dissolved oxygen at concentrations ranging from 10 µM to 1 mM with [H2O2] yields calculated for 100 MeV protons at conventional and FLASH dose rates of 0.286 Gy/s and 500 Gy/s, respectively. Simulated chemical yield results were compared closely with data calculated using the Kinetiscope software which also employs the Gillespie algorithm.
Main results: Validation results in the third test agreed with experimental data of similar dose rates and oxygen concentrations within one standard deviation, with a maximum of 1% difference for both conventional and FLASH dose rates. In conclusion, the new implementation of TOPAS-nBio for the homogeneous long time chemistry simulation was capable of recreating the chemical evolution of the reactive intermediates that follow water radiolysis. 
Significance: Thus, TOPAS-nBio provides a reliable all-in-one chemistry simulation of the physical, physico-chemical, non-homogeneous, and homogeneous chemistry and could be of use for the study of FLASH dose rate effects on radiation chemistry.

The general-purpose Geant4 Monte Carlo toolkit and its Geant4-DNA extension to investigate mechanisms underlying the FLASH effect in radiotherapy: Current status and challenges

FLASH radiotherapy is a promising approach to cancer treatment that offers several advantages over conventional radiotherapy. With this novel technique, high doses of radiation are delivered in a short period of time, inducing the so-called FLASH effect - a phenomenon characterized by healthy tissue sparing without alteration of tumor control. The mechanisms behind the FLASH effect remain unknown. One way to approach this problem is to gain insight into the initial parameters that can distinguish FLASH from conventional irradiation by simulating particle transport in aqueous media using the general-purpose Geant4 Monte Carlo toolkit and its Geant4-DNA extension. This review article discusses the current status of Geant4 and Geant4-DNA simulations to investigate mechanisms underlying the FLASH effect, as well as the challenges faced in this research field. One of the primary challenges is to accurately simulate the experimental irradiation parameters. Another challenge is the temporal extension of the simulations. This review also focuses on two hypotheses to explain the FLASH effect - namely the oxygen depletion hypothesis and the inter-track interactions hypothesis - and discusses how the Geant4 toolkit can be used to investigate them. The aim of this review is to provide an overview of Geant4 and Geant4-DNA simulations for FLASH radiotherapy and to highlight the challenges that need to be overcome in order to better study the FLASH effect.

Modeling of scavenging systems in water radiolysis with Geant4-DNA

PURPOSE

This paper presents the capabilities of the Geant4-DNA Monte Carlo toolkit to simulate water radiolysis with scavengers using the step-by-step (SBS) or the independent reaction times (IRT) methods. It features two examples of application areas: (1) computing the escape yield of H₂O₂ following a ⁶⁰Co γ-irradiation and (2) computing the oxygen depletion in water irradiated with 1 MeV electrons.

METHODS

To ease the implementation of the chemical stage in Geant4-DNA, we developed a user interface that helps define the chemical reactions and set the concentration of scavengers. The first application area example required two computational steps to perform water radiolysis using NO₂^- and NO₃^- as scavengers and a ⁶⁰Co irradiation. The oxygen depletion computation technique for the second application area example consisted of simulating track segments of 1 MeV electrons and determining the radio-induced loss and gain of oxygen molecules.

RESULTS

The production of H₂O₂ under variable scavenging levels is consistent with the literature; the mean relative difference between the SBS and IRT methods is 7.2 % ± 0.5 %. For the oxygen depletion 1 µs post-irradiation, the mean relative difference between both methods is equal to 9.8 % ± 0.3 %. The results in the microsecond scale depend on the initial partial pressure of oxygen in water. In addition, the computed oxygen depletions agree well with the literature.

CONCLUSIONS

The Geant4-DNA toolkit makes it possible to simulate water radiolysis in the presence of scavengers. This feature offers perspectives in radiobiology, with the possibility of simulating cell-relevant scavenging mechanisms.

Application of electron beam water radiolysis for sewage sludge treatment-a review

A review of the applicability of electron beam water radiolysis for sewage sludge treatment is presented. Electron beam treatment has been proven to be a successful approach to the disinfection of both wastewater and sewage sludge. Nevertheless, before 2000, there were concerns about the perceived high capital costs of the accelerator and with public acceptance of the usage of radiation for water treatment purposes. Nowadays, with increased knowledge and technological development, it may be not only possible but also desirable to use electron beam technology for risk-free sewage sludge treatment, disposal and bio-friendly fertiliser production. Despite the developing interest in this method, there has been no attempt to perform a review of the pertinent literature relating to this technology. It appears that understanding of the mechanism and primary parameters of disinfection is key to optimising the process. This paper aims to reliably characterise the sewage sludge electron beam treatment process to elucidate its major issues and make recommendations for further development and research. Graphical abstract.

Advanced oxidation process based on water radiolysis to degrade and mineralize diclofenac in aqueous solutions

Residual pharmaceutical compounds (PCs) are among the emerging organic contaminants detected in our water cycle. Diclofenac (Dic) is one of the commonly detected pharmaceutical contaminant in aquatic systems. This study was designed to investigate the degradation and mineralization of Dic in aqueous solutions by ionizing radiation emitted from radioactive Co60 under several conditions. Ultra-performance liquid chromatography, ion chromatography and TOC measurements confirmed the radiolytic degradation of Dic. The absorbed doses needed to degrade 99% Dic at 25, 50, 100, 190, 280, and 480 μM were 0.560, 0.950, 1.950, 4.000, 5.400, and 7.400 kGy, respectively. This process follows pseudo-first-order kinetics. The γ-ray/N₂O system decreased the dose required to degrade 99% to 1.47 kGy. The presence of bromide anions inhibits degradation. Remarkably, adding H₂O₂, S₂O₈^2-, or N₂O promotes mineralization. Conversely, the absence of dissolved oxygen hinders mineralization. This study provides a viable finding that ionizing radiation are useful tolls to remedy water containing pharmaceutical organic compounds.

Gallic acid degradation by electron beam irradiation under various conditions

In this study, aqueous solutions of gallic acid (GA) were irradiated in an electron beam (EB) accelerator under different experimental conditions (various initial GA concentrations, presence or absence of oxidant and oxygen). For an initial GA concentration of 50 μM, complete GA degradation was achieved with an absorbed dose of 850 Gy in the presence of dissolved oxygen. Both GA removal and mineralization are favored when oxygen is present. The addition of persulfate anions (S₂O₈^2-) or hydrogen peroxide (H₂O₂) also increased the efficiency of GA degradation and mineralization. For an absorbed dose of 14 kGy, GA mineralization reached approximately 45%, 55%, and 72% for the EB, EB/H₂O₂, and EB/S₂O₈^2-systems, respectively. Three transformation products were tentatively identified in the presence of oxygen, these are the result of hydroxylation and ring opening reactions. No specific transformation product was found for the sulfate radical anion (SO₄^-●) reaction. Four additional compounds, including a dimer, were identified in oxygen-free solutions. These findings demonstrate that water radiolysis based on EB irradiation is an efficient process to activate H₂O₂ and S₂O₈^2- anions and is an advanced oxidation process (AOP).

Radiolytic H2 Production in Martian Environments

Hydrogen, produced by water radiolysis, has been suggested to support microbial communities on Mars. We quantitatively assess the potential magnitude of radiolytic H2 production in wet martian environments (the ancient surface and the present subsurface) based on the radionuclide compositions of (1) eight proposed Mars 2020 landing sites, and (2) three sites that individually yield the highest or lowest calculated radiolytic H2 production rates on Mars. For the proposed landing sites, calculated H2 production rates vary by a factor of ∼1.6, while the three comparison sites differ by a factor of ∼6. Rates in wet martian sediment and microfractured rock are comparable with rates in terrestrial environments that harbor low concentrations of microbial life (e.g., subseafloor basalt). Calculated H2 production rates for low-porosity (<35%), fine-grained martian sediment (0.12-1.2 nM/year) are mostly higher than rates for South Pacific subseafloor basalt (∼0.02-0.6 nM/year). Production rates in martian high-porosity sediment (>35%) and microfractured (1 μm) hard rock (0.03 to <0.71 nM/year) are generally similar to rates in South Pacific basalt, while yields for larger martian fractures (1 and 10 cm) are one to two orders of magnitude lower (<0.01 nM/year). If minerals or brine that amplify radiolytic H2 production rates are present, H2 yields exceed the calculated rates.

Modeling the pressure rise of a liquid target on a medical cyclotron: Steady-state analysis

The steady-state behaviour of a liquid target used to produce medical isotopes by low-energy cyclotrons is studied. A model based on the conservation of mass and energy is proposed to describe the pressure rise of the target assuming equilibrium between liquid and vapour phases during irradiation. The effects of water radiolysis are taken into account. Excellent agreement is achieved between the model, and both constant-temperature bath tests and experiments conducted on a 13MeV cyclotron at TRIUMF.

A quantitative model of water radiolysis and chemical production rates near radionuclide-containing solids

We present a mathematical model that quantifies the rate of water radiolysis near radionuclide-containing solids. Our model incorporates the radioactivity of the solid along with the energies and attenuation properties for alpha (α), beta (β), and gamma (γ) radiation to calculate volume normalized dose rate profiles. In the model, these dose rate profiles are then used to calculate radiolytic hydrogen (H2) and hydrogen peroxide (H2O2) production rates as a function of distance from the solid-water interface. It expands on previous water radiolysis models by incorporating planar or cylindrical solid-water interfaces and by explicitly including γ radiation in dose rate calculations. To illustrate our model's utility, we quantify radiolytic H2 and H2O2 production rates surrounding spent nuclear fuel under different conditions (at 20 years and 1000 years of storage, as well as before and after barrier failure). These examples demonstrate the extent to which α, β and γ radiation contributes to total absorbed dose rate and radiolytic production rates. The different cases also illustrate how H2 and H2O2 yields depend on initial composition, shielding and age of the solid. In this way, the examples demonstrate the importance of including all three types of radiation in a general model of total radiolytic production rates.

A laboratory study of the oxidation of non toxic Cr(III) to toxic Cr(VI) by OH(•) free radicals in simulated atmospheric water droplets conditions: potential environmental impact

In atmospheric waters, oxidation of Cr(III) to Cr(VI) by OH(•) free radicals is a major environmental hazard since non-toxic species is transformed into toxic one. It is important to obtain some details concerning this oxidation reaction. In this study we simulated this oxidation by steady state radiolysis using (60)Co radioactive source and pulse radiolysis technique using a 2.5MeV van de Graaff electron accelerator and investigated its kinetics in the pH range 1 to 9. Our findings showed that the reaction was highly pH dependant with a maximum yield at pH 4. The electron transfer proceeds via an inner sphere mechanism with (i) formation of the [OH(•)-Cr(III)] adduct with an equilibrium constant of 2.34×10(4)mol(-1)dm(-3) then (ii) followed by an electron transfer from Cr(III) to OH(•) within the adduct with a rate constant of 2.51×10(4)s(-1). The implication of this oxidation to atmospheric chromium contamination is discussed.