[Show abstract][Hide abstract] ABSTRACT: The effect of temperature on the yields of H2 and hydrated electrons (eaq-) in the low linear energy transfer (LET) radiolysis of liquid water has been modeled by Monte Carlo track chemistry simulations using phenol/N2O aqueous solutions from 25 up to 350 °C. N2O was used to scavenge eaq- and H atoms formed in spurs giving N2 as a product. The primary aim of this work is to elucidate the main factors that account for the anomalous increase in the H2 yield with temperature. Comparing our calculated H2 and N2 yields with experiments led us to re-evaluate certain parameters involved in radiolysis, such as the H-/H2O dissociative electron attachment (DEA) cross section and its variation with temperature. Most importantly, we found that the prompt DEA process largely dominates the temperature dependence of the primary yield of H2 over most of the temperature range considered. Unlike what has been proposed by some authors in the literature, our simulations showed that the oxidation of water by H atoms contributes only ∼12% of the total g(H2) at 350 °C and is thus insufficient to quantitatively explain, by itself, the increase in g(H2) with temperature that is observed experimentally above 200 °C.
[Show abstract][Hide abstract] ABSTRACT: Monte Carlo track chemistry simulations have been used to calculate the yields of hydronium ions (H3O+) that are formed within spurs/tracks of the low/high linear energy transfer (LET) radiolysis of pure, deaerated water during and shortly after irradiation. The in situ formation of H3O+ renders the spur/track regions temporarily more acid than the surrounding medium. Although experimental evidence for an acidic spur has already been reported, there is only fragmentary information on its magnitude and time dependence. Here, spur/track H3O+ concentrations and the corresponding pH values are obtained from our calculated yields of H3O+ as a function of time (in the interval of ~1 ps to 1 ms). We selected four impacting ions and we used two different spur/track models: 1) an isolated “spherical” spur model characteristic of low-LET radiation (such as 300-MeV protons, which mimic 60Co Υ/fast electron irradiation, LET ~ 0.3 keV/μm) and 2) an axially homogeneous “cylindrical” track model for high-LET radiation (such as 150-keV protons, LET ~ 70 keV/μm; 1.75-MeV/nucleon helium ions, LET ~ 70 keV/μm; and 0.6-MeV/nucleon helium ions, LET ~ 146 keV/μm). Very good agreement is found between our calculated time evolution of G(H3O+) in the radiolysis of pure, deaerated water by 300-MeV incident protons and the available experimental data at 25 °C. For all cases studied, an abrupt transient acid pH effect is observed at times immediately after the initial energy release. This effect, which we call an “acid spike”, is found to be greatest for times shorter than ~1 ns in isolated spurs. In this time range, the pH remains nearly constant at ~3.3. For cylindrical tracks, the acid spike response to ionizing radiation is far more intense than that for the spherical spur geometry. For the three high-LET irradiating ions considered, the pH is around 0.5 on a time scale of ~100 ps. At longer times, the pH increases gradually for all cases, ultimately reaching a value of 7 (neutral pH) at ~1 μs for the spherical geometry and ~0.1 ms for the cylindrical geometry. It does not appear that the transient acid spike effect described here has been explored in water or in a cellular environment subject to the action of ionizing radiation, especially high-LET radiation. In this regard, this work raises a number of questions about the potential implications of this effect in radiobiology, some of which are briefly evoked.
[Show abstract][Hide abstract] ABSTRACT: A reliable understanding of radiolysis processes in supercritical water (SCW)-cooled reactors is crucial to developing chemistry control strategies that minimize the corrosion and degradation of materials. However, directly measuring the chemistry in reactor cores is difficult due to the extreme conditions of high temperature and pressure and mixed neutron and gamma-radiation fields, which are incompatible with normal chemical instrumentation. Thus, chemical models and computer simulations are an important route of investigation for predicting the detailed radiation chemistry of the coolant in a SCW reactor and the consequences for materials. Surprisingly, information on the fast neutron radiolysis of water at high temperatures is limited, and even more so for fast neutron irradiation of SCW. In this work, Monte Carlo simulations were used to predict the G values for the primary species e(-)aq, H(•), H2, (•)OH and H2O2 formed from the radiolysis of pure, deaerated SCW (H2O) by 2 MeV monoenergetic neutrons at 400°C as a function of water density in the range of ∼0.15-0.6 g/cm(3). The 2 MeV neutron was taken as representative of a fast neutron flux in a reactor. For light water, the moderation of these neutrons after knock-on collisions with water molecules generated mostly recoil protons of 1.264, 0.465, 0.171 and 0.063 MeV. Neglecting oxygen ion recoils and assuming that the most significant contribution to the radiolysis came from these first four recoil protons, the fast neutron yields were estimated as the sum of the G values for these protons after appropriate weightings were applied according to their energy. Calculated yields were compared with available experimental data and with data obtained for low-LET radiation. Most interestingly, the reaction of H(•) atoms with water was found to play a critical role in the formation yields of H2 and (•)OH at 400°C. Recent work has underscored the potential importance of this reaction above 200°C, but its rate constant is still controversial.
Full-text · Article · Nov 2014 · Radiation Research
[Show abstract][Hide abstract] ABSTRACT: Monte Carlo track chemistry simulations were used to determine the yields (or G-values) of hydrogen peroxide in the radiolysis of neutral water and dilute aqueous bromide solutions by low linear energy transfer (LET similar to 0.3 keV mu m(-1)) radiation (e.g., gamma-rays from Co-60, fast electrons or high-energy protons) and tritium beta-particles (mean LET similar to 6 keV mu m(-1)) at 25 degrees C. We investigated the influence of Br- ions, as selective scavengers of (OH)-O-center dot radical precursors of H2O2, on the inhibition of G(H2O2) for these two types of radiation. Studying this system under a wide range of Br- concentrations (5 x 10(-7) to 0.2 M) and using a well-accepted mechanism for radiolysis in the presence or absence of air, we examined the chemical changes in the scavengeability of H2O2 produced by 300 MeV irradiating protons (used in this work to reproduce the effects of Co-60 gamma/fast electron radiolysis) and tritium beta-electron radiolysis. We found that these changes could be related to differences in the initial spatial distributions of radiolytic species (i.e., the structure of the electron tracks, the low-energy beta-electrons of tritium depositing their energy almost entirely as cylindrical "short tracks" and the energetic Compton electrons produced by gamma-radiolysis forming mainly spherical "spurs"), in full agreement with previous experimental and theoretical work. Simulations showed that the short track geometry of higher LET tritium beta-electrons in both water and aqueous bromide solutions favored a clear increase in G(H2O2) compared to Co-60 gamma-rays. Moreover, the presence of oxygen was seen to scavenge hydrated electrons (e(aq)(-)) and H-center dot atoms on the similar to 10(-7) s time scale, thereby protecting H2O2 from further reactions with these species in the homogeneous stage of radiolysis. This protection against e(aq)(-) and H-center dot atoms therefore led to an increase in the long time H2O2 yields, as seen experimentally. Finally, for both deaerated and aerated solutions, the H2O2 yield in tritium beta-radiolysis was found to be more easily suppressed than in the case of cobalt gamma-radiolysis, and interpreted by the quantitatively different chemistry between spurs and short tracks. These differences in the scavengeability of H2O2 precursors in passing from 300 MeV irradiating protons to tritium beta-electron irradiation were in good agreement with experimental data, thereby lending strong support to the picture of tritium beta-radiolysis in terms of short tracks of high local LET.
[Show abstract][Hide abstract] ABSTRACT: Monte Carlo simulations were used to calculate the yields for the primary species (e(-)aq, H(•), H2, (•)OH and H2O2) formed from the radiolysis of neutral liquid water by mono-energetic 2 MeV neutrons at temperatures between 25-350°C. The 2 MeV neutron was taken as representative of a fast neutron flux in a reactor. For light water, the moderation of these neutrons generated elastically scattered recoil protons of ∼1.264, 0.465, 0.171 and 0.063 MeV, which at 25°C, had linear energy transfers (LETs) of ∼22, 43, 69 and 76 keV/μm, respectively. Neglecting the radiation effects due to oxygen ion recoils and assuming that the most significant contribution to the radiolysis came from these first four recoil protons, the fast neutron yields could be estimated as the sum of the yields for these protons after allowance was made for the appropriate weightings according to their energy. Yields were calculated at 10(-7), 10(-6) and 10(-5) s after the ionization event at all temperatures, in accordance with the time range associated with the scavenging capacities generally used for fast neutron radiolysis experiments. The results of the simulations agreed reasonably well with the experimental data, taking into account the relatively large uncertainties in the experimental measurements, the relatively small number of reported radiolysis yields, and the simplifications included in the model. Compared with data obtained for low-LET radiation ((60)Co γ rays or fast electrons), our computed yields for fast neutron radiation showed essentially similar temperature dependences over the range of temperatures studied, but with lower values for yields of free radicals and higher values for molecular yields. This general trend is a reflection of the high-LET character of fast neutrons. Although the results of the simulations were consistent with the experiment, more experimental data are required to better describe the dependence of radiolytic yields on temperature and to test more thoroughly our modeling calculations.
Full-text · Article · May 2014 · Radiation Research
[Show abstract][Hide abstract] ABSTRACT: The stochastic modeling of the (60)Co γ/fast-electron radiolysis of the ceric-cerous chemical dosimeter has been performed as a function of temperature from 25-350°C. The system used is a dilute solution of ceric sulfate and cerous sulfate in aqueous 0.4 M sulfuric acid. In this system, H(•) (or HO2(•) in the presence of dissolved oxygen) and H2O2 produced by the radiolytic decomposition of water both reduce Ce(4+) ions to Ce(3+) ions, while (•)OH radicals oxidize the Ce(3+) present in the solution back to Ce(4+). The net Ce(3+) yield is given by G(Ce(3+)) = g(H(•)) + 2 g(H2O2) - g((•)OH), where the primary (or "escape") yields of H(•), H2O2 and (•)OH are represented by lower case g's. At room temperature, G(Ce(3+)) has been established to be 2.44 ± 0.8 molecules/100 eV. In this work, we investigated the effect of temperature on the yield of Ce(3+) and on the underlying chemical reaction kinetics using Monte Carlo track chemistry simulations. The simulations showed that G(Ce(3+)) is time dependent, a result of the differences in the lifetimes of the reactions that make up the radiolysis mechanism. Calculated G(Ce(3+)) values were found to decrease almost linearly with increasing temperature up to about 250°C, and are in excellent agreement with available experimental data. In particular, our calculations confirmed previous estimated values by Katsumura et al. (Radiat Phys Chem 1988; 32:259-63) showing that G(Ce(3+)) at ∼250°C is about one third of its value at room temperature. Above ∼250°C, our model predicted that G(Ce(3+)) would drop markedly with temperature until, instead of Ce(4+) reduction, Ce(3+) oxidation is observed. This drop is shown to occur as a result the reaction of hydrogen atoms with water in the homogeneous chemical stage.
No preview · Article · Apr 2014 · Radiation Research
[Show abstract][Hide abstract] ABSTRACT: Supercritical water (SCW) has attracted increasing attention after the Generation IV International Forum selected the supercritical water-cooled reactor (SCWR) as one of six concepts for further investigation. The reference design for the SCWR calls for an operating pressure of 25 MPa and a core outlet temperature as high as 625 °C. Tritium is of special interest in these proposed systems, because of the appreciable quantities that would be produced. Regarding the water chemistry in SCWR systems, there is however a complete lack of information on the radiolysis of SCW by tritium β-particles. Because direct measurement of the chemistry under such extreme conditions of high temperature, pressure, and mixed neutron and β/γ radiation fields is difficult, chemical models and computer simulations are important for predicting the detailed radiation chemistry of the cooling water in a SCWR core and the impact on materials. In this study, Monte Carlo simulations were used to predict the yields (or G-values) for the primary species e−aq, H˙, H2, ˙OH, and H2O2 formed from the radiolysis of deaerated SCW (H2O) by the low-energy β-electrons (18.6 keV maximum) of tritium at 400 °C as a function of water density in the range of 0.15-0.6 g cm−3 (24-56 MPa). The objective was to elucidate the (time-dependent) mechanisms involved in the self-radiolysis of tritiated water under supercritical conditions. Calculated yields were compared with data obtained for low-“linear energy transfer” (LET) radiation (such as 60Co γ-rays or high-energy electrons) and fast neutrons. Our simulations revealed that there was a strong resemblance between the density dependences of the different yields for the radiolysis of SCW with tritium β− particles and fast neutrons, corroborating very well with a model of tritium β radiolysis mainly driven by the chemical action of “short tracks” of high local LET. As for the effect of density on the various yields, there was an increased “cage” escape of free radicals at low-density SCW. In contrast, these density effects acted in the opposite sense in the high-density liquid-like region where the caged free radical products were forced to remain as colliding neighbors and recombine, thereby increasing the molecular yields. Finally, the occurrence of the reaction of H˙ atoms with water in the homogeneous chemical stage was found to play a critical role in the formation yields of H2 and ˙OH at 400 °C. Recent work has recognized the potential importance of this reaction above 200 °C, but its rate constant is still not well known.
[Show abstract][Hide abstract] ABSTRACT: Monte Carlo simulations were used to investigate the chemistry of pure water and aqueous solutions after irradiation with different kinds of radiation: tritium β-rays and high-energy electrons or 60Co γ-rays. The objective of this work was to elucidate the mechanisms involved in the self-radiolysis of tritiated water, and to examine the importance of the effects of higher “linear energy transfer” (LET) by comparing 3H β-electrons (mean initial energy of 5.7 keV) with 60Co γ-rays (1-MeV electrons). We considered several chemical systems for which experimental data were available. These included pure water, aqueous solutions of sulfuric acid, and aqueous ferrous sulfate solutions in aerated 0.4 M H2SO4 (Fricke dosimeter). Simulations clearly showed quantitatively different yields of radical and molecular products produced by the radiolysis of water with tritium β− particles compared with corresponding yields from γ or energetic electron radiolysis. As a rule, lower radical and higher molecular yields were observed for 3H β-rays. These differences in yields are completely consistent with differences in the nonhomogeneous distribution of primary transient species (i.e., the structure of electron tracks) in the two cases. In the “short-track” (columnar) geometry of tritium β-electron radiolysis, radicals were formed in much closer initial proximity than in the “spur” (spherical) geometry of γ radiolysis. The “short-track” geometry favors radical-radical reactions in the diffusing tracks, which increases the proportion of molecular products at the expense of the radical products. The same trend in yields of radical and molecular products was also found under acidic conditions as well as in the aerated Fricke dosimeter. Unfortunately, comparison with experimental data was rather limited due to the paucity of experimental information for the radiolysis of water by 3H β-particles. Despite this deficiency, our simulations reproduced very well the significant increase observed in the yield of H2 at the microsecond time scale for 3H β-electrons (0.6 molecule/100 eV) compared to 60Co γ-rays (0.45 molecule/100 eV). Furthermore, our predicted yield of Fe3+ ions for tritium β-electron radiolysis of Fricke (acidic ferrous sulfate) solutions compared well with the literature values (11.9-12.9 molecules/100 eV). In particular, it was shown that the measured yield of the Fricke dosimeter was best reproduced if a single, “mean” or “equivalent” electron energy of 7.8 keV was used to mimic the energy deposition by the tritium β-particles (rather than the commonly used mean of 5.7 keV that mimics the tritium beta energy spectrum), in full accordance with a recommendation of ICRU Report 17. This decrease in G(Fe3+) compared to the value observed for 60Co γ-rays (15.5 ± 0.2 molecules/100 eV) was mostly due to the decrease in the yield of the escape radical products. Such results, even if fragmentary, corroborate very well with previous experimental and theoretical work, and support a model of tritium β radiolysis mainly driven by the chemical action of short tracks of high local LET.
[Show abstract][Hide abstract] ABSTRACT: Monte Carlo simulations were used to calculate time-dependent yields of
OH radicals in the low-LET radiolysis of water from 25 to 350
°C. The excellent agreement found, at 25 °C, with
bothOH yields directly measured in picosecond pulse
radiolysis and those inferred from scavenger experiments, resolves a
long-standing problem in models of the radiation chemistry of water
concerning short-time OH decay kinetics. Above ˜200
°C, the OH yields markedly increased at long times due to
the reaction H + H2O → H2 +
OH. Our results suggest a way to assess this reaction's rate
constant, which is still controversial.
No preview · Article · Nov 2013 · Chemical Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Monte Carlo simulations were used to calculate the yield of hydrated electrons (eaq(-)) in the low-linear energy transfer radiolysis of supercritical water at 400 °C as a function of water density over the range of ∼0.15 to 0.6 g cm(-3). Very good agreement was found between our calculations and picosecond pulse radiolysis experimental data at ∼60 ps and 1 ns at high density (>0.35 g cm(-3)). At densities lower than ∼0.35 g cm(-3), our eaq(-) yields were lower than the experimental data, especially at ∼60 ps. However, if we incorporated into the simulations a prompt geminate electron-cation (H2O˙(+)) recombination (prior thermalization of the electron) that decreased as the density decreased, our computed eaq(-) yields at ∼60 ps and 1 ns compared fairly well with the experimental data for the entire density range studied.
No preview · Article · Sep 2013 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: In the spirit of the radiation chemical "spur model", the lifetime of a spur (τ(s)) is an important indicator of overlapping spurs and the establishment of homogeneity in the distribution of reactive species created by the action of low linear energy transfer (LET) radiation (such as fast electrons or γ irradiation). In fact, τ(s) gives the time required for the changeover from nonhomogeneous spur kinetics to homogeneous kinetics in the bulk solution, thus defining the so-called primary (or "escape") radical and molecular yields of radiolysis, which are obviously basic to the quantitative understanding of any irradiated chemical system. In this work, τ(s) and its temperature dependence have been determined for the low-LET radiolysis of deaerated 0.4 M aqueous solutions of H(2)SO(4) and pure liquid water up to 350 °C using a simple model of energy deposition initially in spurs, followed by random diffusion of the species of the spur during track expansion until spur overlap is complete. Unlike our previous τ(s) calculations, based on irradiated Fricke dosimeter simulations, the current model is free from any effects due to the presence of oxygen or the use of scavengers. In acidic solutions, the spur lifetime values thus obtained are in very good agreement with our previous calculations (after making appropriate corrections, however, to account for the possibility of competition between oxygen and Fe(2+) ions for H˙ atoms in the Fricke dosimeter, an effect which was not included in our original simulations). In this way, we confirm the validity of our previous approach. As expected, in the case of pure, oxygen-free water, our calculated times required to reach complete spur overlap are essentially the same (within uncertainty limits) as those found in acidic solutions. This explicitly reflects the fact that the diffusion coefficients for the hydrated electron and the H˙ atom that are involved in the overall calculation of the lifetime of spurs in neutral or acidic media, respectively, are of similar magnitude over the 25-350 °C temperature range studied.
Full-text · Article · Nov 2012 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: In multicellular organisms, intercellular communication is essential for homeostatic functions and has a major role in tissue responses to stress. Here, we describe the effects of expression of different connexins, which form gap junction channels with different permeabilities, on the responses of human cells to ionizing radiation. Exposure of confluent HeLa cell cultures to 137 Cs γ rays, 3.7 MeV α particles, 1000 MeV protons or 1000 MeV/u iron ions resulted in distinct effects when the cells expressed gap junction channels com-posed of either connexin26 (Cx26) or connexin32 (Cx32). Irradiated HeLa cells expressing Cx26 generally showed decreased clonogenic survival and reduced metabolic activity relative to parental cells lacking gap junction communication. In contrast, irradiated HeLa cells expressing Cx32 generally showed enhanced sur-vival and greater metabolic activity relative to the control cells. The effects on clonogenic survival corre-lated more strongly with effects on metabolic activity than with DNA damage as assessed by micronucleus formation. The data also showed that the ability of a connexin to affect clonogenic survival following ioniz-ing radiation can depend on the specific type of radiation. Together, these findings show that specific types of connexin channels are targets that may be exploited to enhance radiotherapeutic efficacy and to formulate countermeasures to the harmful effects of specific types of ionizing radiation.
Full-text · Article · Nov 2012 · Journal of Radiation Research
[Show abstract][Hide abstract] ABSTRACT: Fast kinetics and time-dependent yields of the hydrated electron (e À aq) in pure water under conditions of high temperature and pressure up to the supercritical region were investigated by picosecond and nanosecond pulse radiolysis experiments. More significant decays at short times followed by plateau components at longer times were observed with increasing temperature, suggesting faster spur reaction processes. In supercritical water, it was also found that the e À aq yields strongly depend on the pressure (density). Comparison of these measurements with Monte-Carlo computer simulations allowed us to identify spur reactions of e À aq that occur predominantly at high temperatures and also to provide new key information on certain spur model parameters. In particular, the experimental time-dependent e À aq yields were best reproduced if the electron thermalization distance decreases with increasing temperature. This ''shrinkage'' of spur sizes at high temperatures was attributed to an increase in the scattering cross sections of subexcitation electrons, likely originating from a decrease in the degree of structural order of water molecules as the temperature is increased.
Full-text · Article · Oct 2012 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: Since the invention of cancer radiotherapy, its primary goal has been to maximize lethal radiation doses to the tumor volume
while keeping the dose to surrounding healthy tissues at zero. Sadly, conventional radiation sources (γ or X rays, electrons)
used for decades, including multiple or modulated beams, inevitably deposit the majority of their dose in front or behind
the tumor, thus damaging healthy tissue and causing secondary cancers years after treatment. Even the most recent pioneering
advances in costly proton or carbon ion therapies can not completely avoid dose buildup in front of the tumor volume. Here
we show that this ultimate goal of radiotherapy is yet within our reach: Using intense ultra-short infrared laser pulses we
can now deposit a very large energy dose at unprecedented microscopic dose rates (up to 1011 Gy/s) deep inside an adjustable, well-controlled macroscopic volume, without any dose deposit in front or behind the target
volume. Our infrared laser pulses produce high density avalanches of low energy electrons via laser filamentation, a phenomenon
that results in a spatial energy density and temporal dose rate that both exceed by orders of magnitude any values previously
reported even for the most intense clinical radiotherapy systems. Moreover, we show that (i) the type of final damage and its mechanisms in aqueous media, at the molecular and biomolecular level, is comparable to
that of conventional ionizing radiation, and (ii) at the tumor tissue level in an animal cancer model, the laser irradiation method shows clear therapeutic benefits.
Full-text · Article · Sep 2012 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: The ceric sulfate dosimeter is based on the radio-induced reduction of Ce 4+ in acidic medium. For low linear en-ergy transfer (LET) radiation, the yield of Ce 3+ is 2.4 molecules / 100 eV, regardless of the presence of oxygen. To investi-gate the reaction mechanisms of the ceric sulfate dosimeter, we simulated the chemical reaction kinetics curves and the evolution of G(Ce 3+), G(O2), and G(H2) in the ceric sulfate solution with and without oxygen. Studies of G(Ce 3+) as func-tion of the initial concentration of Ce 3+ and of the LET were also done. One important finding of this study is that • OH rad-icals are scavenged by the reaction • OH + HSO4 – → SO4 • – + H2O, rather than by the reaction • OH + Ce 3+ → Ce 4+ + OH – . Key words: ceric sulfate dosimeter, radiolysis, free-radical and molecular yields, linear energy transfer (LET), Monte Carlo simulations. Résumé : Le dosimètre cérique est basé sur la réduction radio-induite de Ce 4+ en milieu acide. Pour les radiations de faible transfert d'énergie linéaire (TEL), le rendement de Ce 3+ est 2,4 molécules / 100 eV, indépendamment de la présence d'oxy-gène. Pour étudier les mécanismes du dosimètre cérique, nous avons simulé l'évolution temporelle de G(Ce 3+), G(O2) et G (H2) dans une solution de sulfate cérique avec et sans oxygène. Des études des rendements en fonction de la concentration initiale de Ce 3+ et du TEL ont également été réalisées. Une découverte importante de cette étude est que les radicaux • OH sont captés par la réaction • OH + HSO4 – → SO4 • – + H2O, plutôt que par la réaction • OH + Ce 3+ → Ce 4+ + OH – . Mots‐clés : dosimètre cérique, radiolyse, rendements radicalaires et moléculaires, transfert d'énergie linéaire (TEL), simula-tions Monte Carlo.
Full-text · Article · Aug 2012 · Canadian Journal of Chemistry
[Show abstract][Hide abstract] ABSTRACT: The "spur lifetime" (τ(s)) in the low-linear energy transfer (LET) radiolysis of supercritical water (SCW) at 400 °C has been determined as a function of water density by using a simple model of energy deposition initially in spurs, followed by the random diffusion (Brownian motion) of the species formed until spur expansion is complete. The values of τ(s) are found to decrease from ∼5.0 × 10(-6) to 5.0 × 10(-8) s over the density range from 0.15 to 0.6 g cm(-3). Using Monte-Carlo simulations, our calculated density dependence of the "escape" hydrated electron (e(aq)(-)) yield (i.e., at time τ(s)) reproduces fairly well Bartels and co-workers' scavenged e(aq)(-) yield data, suggesting that these data may have been measured at times close to τ(s).
No preview · Article · Jul 2012 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: Cystamine, an organic disulfide (RSSR), is among the best of the known radiation-protective compounds and has been used to protect normal tissues in clinical radiation therapy. Recently, it has also proved to be beneficial in the treatment of disorders of the central nervous system in animal models. However, the underlying mechanism of its action at the chemical level is not yet well understood. The present study aims at using the ferrous sulfate (Fricke) dosimeter to quantitatively evaluate, both experimentally and theoretically, the radioprotective potential of this compound. The well-known radiolysis of the Fricke dosimeter by (60)Co γ rays or fast electrons, based on the oxidation of ferrous ions to ferric ions by the oxidizing species (•)OH, HO(2)(•), and H(2)O(2) produced in the radiolytic decomposition of water, forms the basis for our method. The presence of cystamine in Fricke dosimeter solutions during irradiation prevents the radiolytic oxidation of Fe(2+) and leads to decreased ferric yields (or G values). The observed decrease in G(Fe(3+)) increases upon increasing the concentration of the disulfide compound over the range 0-0.1 M under both aerated and deaerated conditions. To help assess the basic radiation-protective mechanism of this compound, a full Monte Carlo computer code is developed to simulate in complete detail the radiation-induced chemistry of the studied Fricke/cystamine solutions. Benefiting from the fact that cystamine is reasonably well characterized in terms of radiation chemistry, this computer model proposes reaction mechanisms and incorporates specific reactions describing the radiolysis of cystamine in aerated and deaerated Fricke solutions that lead to the observable quantitative chemical yields. Results clearly indicate that the protective effect of cystamine originates from its radical-capturing ability, which allows this compound to act by competing with the ferrous ions for the various free radicals--especially (•)OH radicals and H(•) atoms--formed during irradiation of the surrounding water. Most interestingly, our simulation modeling also shows that the predominant pathway in the oxidation of cystamine by (•)OH radicals involves an electron-transfer mechanism, yielding RSSR(•+) and OH(-). A very good agreement is found between calculated G(Fe(3+)) values and experiment. This study concludes that Monte Carlo simulations represent a very efficient method for understanding indirect radiation damage at the molecular level.
No preview · Article · Apr 2012 · Radiation Research
[Show abstract][Hide abstract] ABSTRACT: Cellular exposure to ionizing radiation leads to oxidizing events that alter atomic structure through direct interactions of radiation with target macromolecules or via products of water radiolysis. Further, the oxidative damage may spread from the targeted to neighboring, non-targeted bystander cells through redox-modulated intercellular communication mechanisms. To cope with the induced stress and the changes in the redox environment, organisms elicit transient responses at the molecular, cellular and tissue levels to counteract toxic effects of radiation. Metabolic pathways are induced during and shortly after the exposure. Depending on radiation dose, dose-rate and quality, these protective mechanisms may or may not be sufficient to cope with the stress. When the harmful effects exceed those of homeostatic biochemical processes, induced biological changes persist and may be propagated to progeny cells. Physiological levels of reactive oxygen and nitrogen species play critical roles in many cellular functions. In irradiated cells, levels of these reactive species may be increased due to perturbations in oxidative metabolism and chronic inflammatory responses, thereby contributing to the long-term effects of exposure to ionizing radiation on genomic stability. Here, in addition to immediate biological effects of water radiolysis on DNA damage, we also discuss the role of mitochondria in the delayed outcomes of ionization radiation. Defects in mitochondrial functions lead to accelerated aging and numerous pathological conditions. Different types of radiation vary in their linear energy transfer (LET) properties, and we discuss their effects on various aspects of mitochondrial physiology. These include short and long-term in vitro and in vivo effects on mitochondrial DNA, mitochondrial protein import and metabolic and antioxidant enzymes.