Jean-Paul Jay-Gerin

Université de Sherbrooke, Sherbrooke, Quebec, Canada

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Publications (74)178.67 Total impact

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    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.
    Radiation Research 11/2014; · 2.45 Impact Factor
  • RSC Advances 09/2014; · 3.71 Impact Factor
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    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.
    Radiation Research 05/2014; · 2.70 Impact Factor
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    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.
    Radiation Research 04/2014; · 2.70 Impact Factor
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    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.
    RSC Advances 01/2014; 4(44):22980. · 3.71 Impact Factor
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    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.
    Chemical Physics Letters 11/2013; · 1.99 Impact Factor
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    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.
    Physical Chemistry Chemical Physics 09/2013; · 4.20 Impact Factor
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    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.
    RSC Advances 01/2013; 3(42):19282. · 3.71 Impact Factor
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    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.
    Physical Chemistry Chemical Physics 11/2012; · 4.20 Impact Factor
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    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.
    Journal of Radiation Research 11/2012; 54(2). · 1.69 Impact Factor
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    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.
    Proceedings of the National Academy of Sciences 09/2012; 109(38):E2508-E2513. · 9.81 Impact Factor
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    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.
    Canadian Journal of Chemistry 08/2012; · 1.01 Impact Factor
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    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).
    Physical Chemistry Chemical Physics 07/2012; 14(32):11277-80. · 4.20 Impact Factor
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    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.
    Radiation Research 04/2012; 177(6):813-26. · 2.70 Impact Factor
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    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.
    Physical Chemistry Chemical Physics 01/2012; 14(14):14325-14333. · 4.20 Impact Factor
  • Edouard I Azzam, Jean-Paul Jay-Gerin, Debkumar Pain
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    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.
    Cancer letters 12/2011; 327(1-2):48-60. · 5.02 Impact Factor
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    ABSTRACT: Monte-Carlo simulations of the radiolysis of the ferrous sulfate (Fricke) dosimeter with low-linear energy transfer (LET) radiation (such as (60)Co γ-rays or fast electrons) have been performed as a function of temperature from 25 to 350 °C. The predicted yields of Fe(2+) oxidation are found to increase with increasing temperature up to ∼100-150 °C, and then tend to remain essentially constant at higher temperatures, in very good agreement with experiment. By using a simple method based on the direct application of the stoichiometric relationship that exists between the ferric ion yields so obtained G(Fe(3+)) and the sum {3 [g(e(-)(aq) + H˙) + g(HO(2)˙)] + g(˙OH) + 2 g(H(2)O(2))}, where g(e(-)(aq) + H˙), g(HO(2)˙), g(˙OH), and g(H(2)O(2)) are the primary radical and molecular yields of the radiolysis of deaerated 0.4 M H(2)SO(4) aqueous solutions, the lifetime (τ(s)) of the spur and its temperature dependence have been determined. In the spirit of the spur model, τ(s) is an important indicator for overlapping spurs, giving the time required for the changeover from nonhomogeneous spur kinetics to homogeneous kinetics in the bulk solution. The calculations show that τ(s) decreases by about an order of magnitude over the 25-350 °C temperature range, going from ∼4.2 × 10(-7) s at 25 °C to ∼5.7 × 10(-8) s at 350 °C. This decrease in τ(s) with increasing temperature mainly originates from the quicker diffusion of the individual species involved. Moreover, the observed dependence of G(Fe(3+)) on temperature largely reflects the influence of temperature upon the primary free-radical product yields of the radiolysis, especially the yield of H˙ atoms. Above ∼200-250 °C, the more and more pronounced intervention of the reaction of H˙ atoms with water also contributes to the variation of G(Fe(3+)), which may decrease or increase slightly, depending on the choice made for the rate constant of this reaction. All calculations reported herein use the radiolysis database of Elliot (Atomic Energy of Canada Limited) and Bartels (University of Notre Dame) that contains all the best currently available information on the rate constants, reaction mechanisms, and g-values in the range 20 to 350 °C.
    Physical Chemistry Chemical Physics 06/2011; 13(22):10690-8. · 4.20 Impact Factor
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    ABSTRACT: Here, we report that intense ultra-short laser pulses produce a plasma of low energy electrons (LEEs) by the inverse Bremsstrahlung effect and multiphoton ionization process. The phenomena show five striking characteristics. First, the self-focusing of ultra-short laser pulses creates a plasma of LEEs (6.5 eV), which is concentrated in filaments through an avalanche process. Second, kinetically hot 6.5 eV electrons interact with surrounding molecules resulting in reactive radical species. Third, the dose rate reaches an enormous level of ~2.8 × 1011 Gy/s as determined by a cericcerous sulfate dosimetry and this leads to an ultra-high deposition of energy of between 4.6 × 107 to 8.16 × 107 keV/μm. Fourth, filaments of variable length are produced by femtosecond pulses depending on the pulse duration as determined by a tissue-equivalent radiation polymer gel dosimeter and imaged by magnetic resonance imaging (MRI). These results reveal that one of the very interesting novelty of filamentation is the very low entrance dose, similar to proton irradiation. Lastly, filamentary irradiation results in the decomposition of thymidine in the absence and the presence of oxygen similar to the radiolysis of water.
    Proceedings of SPIE - The International Society for Optical Engineering 06/2011; · 0.20 Impact Factor
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    ABSTRACT: We investigated the roles of gap junction communication and oxidative stress in modulating potentially lethal damage repair in human fibroblast cultures exposed to doses of α particles or γ rays that targeted all cells in the cultures. As expected, α particles were more effective than γ rays at inducing cell killing; further, holding γ-irradiated cells in the confluent state for several hours after irradiation promoted increased survival and decreased chromosomal damage. However, maintaining α-particle-irradiated cells in the confluent state for various times prior to subculture resulted in increased rather than decreased lethality and was associated with persistent DNA damage and increased protein oxidation and lipid peroxidation. Inhibiting gap junction communication with 18-α-glycyrrhetinic acid or by knockdown of connexin43, a constitutive protein of junctional channels in these cells, protected against the toxic effects in α-particle-irradiated cell cultures during confluent holding. Upregulation of antioxidant defense by ectopic overexpression of glutathione peroxidase protected against cell killing by α particles when cells were analyzed shortly after exposure. However, it did not attenuate the decrease in survival during confluent holding. Together, these findings indicate that the damaging effect of α particles results in oxidative stress, and the toxic effects in the hours after irradiation are amplified by intercellular communication, but the communicated molecule(s) is unlikely to be a substrate of glutathione peroxidase.
    Radiation Research 03/2011; 175(3):347-57. · 2.70 Impact Factor
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    ABSTRACT: The influence of the energetic gap on the effective distance-decay rate of electronic coupling (eff) in DNA is investigated in the context of the superexchange mechanism. The DNA double helix is described by a tight-binding electronic Hamiltonian model, in which all orbitals have the same energy and interact with one another through an exponentially decaying function of distance. Our numerical results concerning the eff values obtained for two different DNA molecules are analyzed within the theoretical framework of the "continuous-medium approximation," previously developed by Lopez-Castillo et al. (J.-M. Lopez-Castillo, A. Filali-Mouhim, I.L. Plante, and J.-P. Jay-Gerin. J. Phys. Chem. 99 : 6864-6875, 1995). We find that the intervening DNA bridge between the donor and acceptor sites is defined by a unique dimensionless control parameter Γ/E, where E is the energy of the orbitals of this medium with respect to those of the redox site orbitals (energetic gap) and Γ is the electronic band width of the bridge considered as a continuous medium. In the narrow-band regime, our "through-space" coupling model predicts eff values that are in good order of magnitude agreement with those calculated by other theoretical approaches as well as with those obtained from experiment. Moreover, under equivalent energetic conditions, the DNA-mediated transfers of holes and electrons differ considerably. This difference depends upon the sign of the parameter Γ/E.Key words: DNA, electronic coupling, effective distance-decay rate, energetic gap, superexchange mechanism, continuous-medium approximation, long-distance electron and hole transfers.Nous étudions, dans le cadre du mécanisme de superéchange, l'influence du gap énergétique sur le taux de décroissance effectif du couplage électronique (eff) dans l'ADN. L'ADN est décrit par un Hamiltonien électronique modèle, basé sur l'approximation des liaisons fortes, dans lequel les diverses orbitales ont toutes la même énergie et interagissent entre elles selon une fonction exponentielle de la distance. Nos résultats numériques concernant les valeurs de eff obtenues pour deux molécules différentes d'ADN sont analysés en s'appuyant sur la théorie de l'« approximation du milieu continu », développée précédemment par Lopez-Castillo et al. (J.-M. Lopez-Castillo, A. Filali-Mouhim, I.L. Plante et J.-P. Jay-Gerin. J. Phys. Chem. 99 : 6864-6875, 1995). Nous mettons en évidence que le pont d'ADN intervenant entre le donneur et l'accepteur est défini par un paramètre de contrôle unique, sans dimension, Γ/E, où E est l'énergie séparant les orbitales de ce milieu de celles des sites rédox (gap énergétique) et Γ est la largeur de bande électronique du pont considéré comme milieu continu. Dans la limite des larges gaps, notre modèle de couplage « à travers l'espace » reproduit les bons ordres de grandeur des valeurs de eff trouvées tant expérimentalement que par d'autres approches théoriques. Nous trouvons de plus que, pour des conditions énergétiques équivalentes, les transferts d'électron et de trou au sein de l'ADN diffèrent considérablement. Cette différence est attribuée au signe du paramètre Γ/E.Mots clés : ADN, couplage électronique, coefficient de décroissance effectif, gap énergétique, mécanisme de superéchange, approximation du milieu continu, transferts d'électron et de trou à longue distance.
    Canadian Journal of Physiology and Pharmacology 02/2011; 79(2):122-129. · 1.55 Impact Factor

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684 Citations
178.67 Total Impact Points

Institutions

  • 1997–2013
    • Université de Sherbrooke
      • Department of Nuclear Medicine and Radiobiology
      Sherbrooke, Quebec, Canada
  • 2012
    • The University of Tokyo
      • Nuclear Professional School
      Edo, Tōkyō, Japan
  • 2011
    • Rutgers New Jersey Medical School
      • Department of Radiology (RWJ Medical School)
      Newark, NJ, United States
  • 2002
    • Burapha University
      • Faculty of Science
      Muang Jol Buri, Chon Buri, Thailand
  • 1998
    • Université du Québec
      Québec, Quebec, Canada