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Monte Carlo multitrack chemistry simulations were used in combination with a cylindrical, “instantaneous pulse” irradiation model to study the effect of high dose rates on the early, transient yields ( G values) of the “primary products” ([Formula: see text], H•, H2, •OH, H2O2, H3O+, OH−, …) of the radiolysis of supercritical water (SCW) at 400 °C and 25 MPa pressure. Our simulation model consisted of randomly irradiating SCW with single pulses of N incident 300-MeV protons, which mimic the low linear energy transfer of 60Co γ/fast electron irradiations. The effect of dose rate was studied by varying N. Generally, high dose rates were found to favor radical–radical reactions, which increases the proportion of the molecular products at the expense of the radical products. However, as an exception, G(H•) increases with increasing dose rate in the track stage of radiolysis, predominantly due to the reaction of hydrated electrons with hydronium ions (H3O+). In addition, the generation of acidic spikes due to proton transfer reactions in the physicochemical stage was also examined. Interestingly, an early, transient, very acidic (pH ∼ 3.5) response was observed at high radiation dose rates across the entire irradiated volume. The present work raises the question of whether the potential oxidizing species •OH and H2O2 and these highly acidic pH spikes at high dose rates could promote a corrosive environment under proposed Generation IV SCW-cooled reactor or small modular reactor operating conditions that would lead to progressive degradation of materials.

Monte Carlo multi-track chemistry simulations were carried out to study, from a radiation chemistry perspective, the effect of “linear energy transfer” (LET) on the transient yields and concentrations of radiolytic oxygen consumption in the high-dose-rate (∼107 Gy/s) radiolysis of both pure air-saturated (0.25 mmol/L O2) and oxygenated (30 µmol/L O2) cell water, in the interval ∼1 ps–10 µs. Our simulation model consisted of randomly irradiating water with single pulses of 5 MeV (LET ∼ 8 keV/µm), 1.5 MeV (LET ∼ 19.5 keV/µm), and 0.7 MeV (LET ∼ 33 keV/µm) protons at 25 °C. Similar to what is observed with low-LET irradiation (∼300 MeV protons, LET ∼ 0.3 keV/µm), our calculations showed that, in pure, aerated water, the concentration of depleted oxygen, [−O2], exhibits a pronounced maximum around ∼0.1–0.2 µs for all three high-LET irradiating protons studied. This maximum increased markedly with increasing LET. As expected, the effect of adding competing scavengers of both hydrated electrons and •OH radicals on the radiolytic O2 depletion in oxygenated cell water (a more bio-mimetic model of cells) irradiated by 5 MeV protons delivered at the same dose rate led to a marked decrease in the maximum of [−O2] around 1 µs. However, contrary to what is observed for low-LET irradiation, we found that the transient O2 consumption is quite substantial under high-LET irradiation conditions. This is explained by the fact that, even though their underlying mechanism of action differs, high-LET particles affect radiolysis yields in a similar way to high dose rates.

In this study, we use Monte Carlo track chemistry simulations to show that “dry” secondary electrons, precursors of the “hydrated” electron (e−aq), can be scavenged on the sub-picosecond time scale prior to hydration, by a high concentration (>0.1–1 M) of azide ions (N3−) in water irradiated with 60Co γ-rays and tritium β-electrons at 25 °C. This is a striking result, as N3− is known to react very slowly with e−aq. These processes tend to significantly reduce the yields of H2 as observed experimentally. For both energetic Compton electrons (“linear energy transfer”, LET ∼ 0.3 keV/µm), which are generated by the cobalt-60 γ-rays, and 3H β-electrons (LET ∼ 6 keV/µm), our H2 yield results confirm previous Monte Carlo simulations, which indicated the necessity of including the capture of the precursors to e−aq. Interestingly, our calculations show no significant changes in the scavenging of “dry” electrons at high azide concentrations in passing from γ-radiolysis to tritium β-radiolysis (i.e., with LET). This led us to the conclusion that the higher H2 yield observed experimentally for 3H β-electrons compared with 60Co γ-rays is mainly explained by the difference in the radiation track structures during the chemical stage (>1 ps). The higher LET of tritium β-electrons leads to more molecular products (H2 in this case) in tritium radiolysis than in γ-radiolysis. Finally, a value of ∼0.5 nm was derived for the reaction distance between N3− and the “dry” electron from the H2 yields observed in 60Co γ-radiolysis at high N3− concentrations.

Large doses of ionizing radiation delivered to tumors at ultra-high dose rates (i.e., in a few milliseconds) paradoxically spare the surrounding healthy tissue while preserving anti-tumor activity (compared with conventional radiotherapy delivered at much lower dose rates). This new modality is known as “FLASH radiotherapy” (FLASH-RT). Although the molecular mechanisms underlying FLASH-RT are not yet fully understood, it has been suggested that radiation delivered at high dose rates spares normal tissue via oxygen depletion followed by subsequent radioresistance of the irradiated tissue. To date, FLASH-RT has been studied using electrons, photons, and protons in various basic biological experiments, pre-clinical studies, and recently in a human patient. However, the efficacy of heavy ions, such as energetic carbon ions, under FLASH-RT conditions remains unclear. Given that living cells and tissues consist mainly of water, we set out to study, from a pure radiation chemistry perspective, the effects of ultra-high dose rates on the transient yields and concentrations of radiolytic species formed in water irradiated by 300-MeV per nucleon carbon ions (LET ∼ 11.6 keV/µm). This mimics irradiation in the “plateau” region of the depth–dose distribution of ions, i.e., in the “normal” tissue region in which the LET is rather low. We used Monte Carlo simulations of multiple, simultaneously interacting radiation tracks together with an “instantaneous pulse” irradiation model. Our calculations show a pronounced oxygen depletion around 0.2 μs, strongly suggesting, as with electrons, photons, and protons, that irradiation with energetic carbon ions at ultra-high dose rates is suitable for FLASH-RT.

Monte Carlo track chemistry simulations were used to investigate the effects of multiple ionization (MI) of water on the yields (G values) of the ferrous sulfate (Fricke) dosimeter, which was irradiated with low-energy α and lithium ion recoils from the 10B(n,α)7Li nuclear reaction as a function of temperature from 25 to 350 °C. Calculations were performed individually for 1.47 MeV α-particles and 0.84 MeV lithium nuclei with dose-average linear energy transfer (LET) values of ∼196 and 225 keV/µm at 25 °C, respectively. The total yields were obtained by summing the G values for each recoil α and Li ion weighted with its fraction of the total energy absorbed. At room temperature, our G(Fe3+) values calculated under aerated and deaerated conditions only agreed well with the experimental results, provided the MI of water was incorporated in the simulations. This strongly supports the importance of the role of MI of water in the high-LET radiolysis of water. We also simulated the effects of MI of water on G-values for the primary species of the radiolysis of deaerated 0.4 M H2SO4 aqueous solutions by 10B(n,α)7Li recoils. As with the Fricke dosimeter, the best agreement between experiment and simulation was found at 25 °C when the MI of water was included in the simulations. It was also shown that G(Fe3+) decreases slightly as a function of temperature over the range of 25–350 °C. However, at elevated temperatures, no experimental data were available with which to compare our results.

FLASH radiotherapy is a new irradiation method in which large doses of ionizing radiation are delivered to tumors almost instantly (a few milliseconds), paradoxically sparing healthy tissue while preserving anti-tumor activity. Although this technique is primarily studied in the context of electron and photon therapies, proton delivery at high dose rates can also reduce the adverse side effects on normal cells. So far, no definitive mechanism has been proposed to explain the differences in the responses to radiation between tumor and normal tissues. Given that living cells and tissues consist mainly of water, we set out to study the effects of high dose rates on the radiolysis of water by protons in the energy range of 150 keV – 500 MeV (i.e., for linear energy transfer (LET) values between ∼72.2 and 0.23 keV/μm, respectively) using Monte Carlo simulations. To validate our methodology, however, we, first, report here the results of our calculations of the yields (G values) of the radiolytically produced species, namely the hydrated electron ([Formula: see text]), •OH, H•, H2, and H2O2, for low dose rates. Overall, our simulations agree very well with the experiment. In the presence of oxygen, [Formula: see text] and H• atoms are rapidly converted into superoxide anion or hydroperoxyl radicals, with a well-defined maximum of [Formula: see text] at ∼1 μs. This maximum decreases substantially when going from low-LET 500 MeV to high-LET 150 keV irradiating protons. Differences in the geometry of the proton track structure with increasing LET readily explain this diminution in [Formula: see text] radicals.

Monte Carlo track chemistry simulations were used to calculate the time-dependent yields (G values) for the radical (eaq-, H˙, ˙OH) and molecular (H2, H2O2) "primary species" formed in the low-linear energy transfer (LET) radiolysis of deaerated, pure water (H2O) in the range of ∼1 ps to 1 ms between 25 and 700 °C, at 25 MPa pressure. Beyond the critical point, we used in the calculations the new supercritical water (SCW) radiolysis database of Liu et al., in particular their reported reaction rate constants. A striking conclusion of these simulations is the sharp increase in G(˙OH) and G(H2), and the corresponding decrease in G(H˙), which are observed above 200 °C, due to the oxidation of water by the H˙ atom (H˙ + H2O → ˙OH + H2) in the homogeneous chemical stage of radiolysis. These results may have important implications for proposed Generation-IV SCW-cooled reactors for the control and management of water chemistry and for the maintenance of the structural integrity of materials.

A reliable understanding of radiolysis processes in supercritical water (SCW) cooled reactors is required to ensure optimal water chemistry control. In this perspective, Monte Carlo track chemistry simulations of the radiolysis of pure, deaerated SCW at 400 °C by 2 MeV mono-energetic neutrons were carried out as a function of water density between 0.15 and 0.6 g/cm3. The yields of hydronium ions (H3O+) formed at early time were obtained based on the G values calculated for the first three generated recoil protons. Combining our calculated G(H3O+) values with a cylindrical track model allowed us to estimate the concentrations of H3O+ and the corresponding pH values. An abrupt, transient, and highly acidic pH response (“acid spikes”) was observed at early times around the “native” fast neutron and recoil proton trajectories. This intra-track acidity was found to be strongest at times of less than a few tens to a hundred of picoseconds, depending on the value of the density considered (pH ∼ 1). At longer times, the pH gradually increased for all densities, finally reaching a constant value corresponding to the non-radiolytic, pre-irradiation concentration of H3O+, due to the autoprotolysis of water. Interestingly, the lower the density of the water, the longer the time required to reach this constant value. Because many in-core processes in nuclear reactors critically depend on the pH, the present work raises the question whether such highly acidic pH fluctuations, though local and transitory, could promote or contribute to corrosion and degradation of materials under proposed SCW-cooled reactor operating conditions.

Cystamine, an organic diamino-disulfide, is among the best of the known radiation-protective compounds, although the underlying molecular mechanisms by which it operates remain poorly understood. This study aims to use the aqueous ferrous sulfate (Fricke) dosimeter to evaluate the protective properties of this compound when present during irradiation by fast incident protons in the energy range of 150 keV – 500 MeV, that is, for “linear energy transfer” (LET) values ranging from ∼72.3 to 0.23 keV/μm. The presence of cystamine in irradiated Fricke solutions prevents the oxidation of Fe2+ ions by the oxidizing species produced in the radiolysis of acidic water, resulting in reduced Fe3+ ion yields. A Monte Carlo computer code is used to simulate the radiation-induced chemistry of the studied Fricke–cystamine solutions under aerated conditions while covering a wide range of cystamine concentrations from 5 × 10−7 to 1 mol/L. Results indicate that the protective activity of cystamine is due to its radical-capturing ability, a clear signature of the strong antioxidant profile of this compound. In addition, our simulations show that at low and intermediate concentrations of cystamine, its protective efficiency decreases with increasing LET, which is consistent with previous work. This finding stems from differences in the geometry of the track structures that change from low-LET isolated spherical “spurs” to high-LET dense continuous cylindrical tracks as LET increases. This study concludes that Monte Carlo simulations represent a powerful method for understanding, at the molecular level, indirect radiation damage to complex molecules such as cystamine.

The rate constant, k(eaq- + H3O+), for the reaction of hydronium ions with hydrated electrons in supercritical water (SCW) at 400 °C has been evaluated as a function of water density using Monte Carlo track chemistry simulations of the radiolysis of SCW over the range of 0.15-0.6 g cm-3. Results are consistent with recent predictions using the so-called "cage effect" model.

There is growing interest in the radiation chemistry of supercritical water (SCW), as its use as a coolant in a nuclear reactor (Generation IV) is the logical evolution of the current (Generation III or less) water-cooled reactors. However, current knowledge about the potential effects of water radiolysis in a Gen-IV supercritical water-cooled reactor (SCWR) is incomplete. In this work, Monte Carlo track chemistry simulations of the low linear energy transfer (LET) radiolysis of SCW (H2O) at 400 °C are used in combination with a spherical "spur" model to study the effect of water density on the in situ radiolytic formation of H3O+ ions and the corresponding abrupt, transient, highly acidic pH response ("acid spikes") that is observed immediately after irradiation. The magnitude and duration of this acidic pH effect depend on the water density in the considered range of 0.15-0.6 g cm-3. It is strongest at times less than a few tens of picoseconds with the pH remaining nearly constant at ∼1.6 and 1.9 for the highest ("liquid-like") and lowest ("gas-like") density, respectively. At longer times, the pH gradually increases for all densities and finally reaches a constant value corresponding to the non-radiolytic, pre-irradiation concentration of H3O+, due to the autoprotolysis of water. Our results show that the lower the density of the water, the longer the time required to reach this constant value, ranging from ∼50 ns at 0.6 g cm-3 (pH ∼ 5.6) to ∼1 μs at 0.15 g cm-3 (pH ∼ 8.5). The generation of these highly acidic pH fluctuations around the "native" radiation tracks, though local and transient, raises questions about the potential implications of this effect in proposed Gen-IV SCW-cooled reactors regarding corrosion and degradation of materials.

"FLASH radiotherapy" is a new method of radiation treatment by which large doses of radiation are delivered at high dose rates to tumors almost instantaneously (a few milliseconds), paradoxically sparing healthy tissue while preserving anti-tumor activity. To date, no definitive mechanism has been proposed to explain the different responses of the tumor and normal tissue to radiation. As a first step, and given that living cells and tissues consist mainly of water, we studied the effects of high dose rates on the transient yields (G values) of the radical and molecular species formed in the radiolysis of deaerated/aerated water by irradiating protons, using Monte Carlo simulations. Our simulation model consisted of two steps: 1. The random irradiation of a right circular cylindrical volume of water, embedded in nonirradiated bulk water, with single and instantaneous pulses of N 300-MeV incident protons ("linear energy transfer" or LET ∼ 0.3 keV/µm) traveling along the axis of the cylinder; and 2. The development of these N proton tracks, which were initially contained in the irradiated cylinder, throughout the solution over time. The effect of dose rate was studied by varying N, which was calibrated in terms of dose rate. For this, experimental data on the yield G(Fe3+) of the super-Fricke dosimeter as a function of dose rate up to ∼1010 Gy/s were used. Confirming previous experimental and theoretical studies, significant changes in product yields were found to occur with increasing dose rate, with lower radical and higher molecular yields, which result from an increase in the radical density in the bulk of the solution. Using the kinetics of the decay of hydrated electrons, a critical time (τc), which corresponds to the "onset" of dose-rate effects, was determined for each value of N. For the cylindrical irradiation model, τc was inversely proportional to the dose rate. Moreover, the comparison with experiments with pulsed electrons underlined the importance of the geometry of the irradiation volume for the estimation of τc. Finally, in the case of aerated water radiolysis, we calculated the yield of oxygen consumption and estimated the corresponding concentration of consumed (depleted) oxygen as a function of time and dose rate. It was shown that this concentration increases substantially with increasing dose rate in the time window ∼1 ns-10 µs, with a very pronounced maximum around 0.2 µs. For high-dose-rate irradiations (>109 Gy/s), a large part of the available oxygen (∼0.25 mM for an air-saturated solution) was found to be consumed. This result, which was obtained on a purely water radiation chemistry basis, strongly supports the hypothesis that the normal tissue-sparing effect of FLASH stems from temporary hypoxia due to oxygen depletion induced by high-dose-rate irradiation.

The use of supercritical water (SCW) in GEN IV reactors is a logical approach to the ongoing development of nuclear energy. A proper understanding of the radiation chemistry and reactivities of transients in a reactor core under SCW conditions is required to achieve optimal water chemistry control and safety. A Monte Carlo simulation study of the radiolysis of SCW at 400 °C by incident 2 MeV monoenergetic neutrons (taken as representative of a fast neutron flux in a reactor) was carried out as a function of water density between ∼150 and 600 kg/m3. The in situ formation of H3O+ by the generated recoil protons was shown to render the “native” track regions temporarily very acidic (pH ∼ 1). This acidity, though local and transitory (“acid spikes”), raises the question whether it may promote a corrosive environment under proposed SCW-cooled reactor operating conditions that would lead to progressive degradation of reactor components.

It is well known that molecular oxygen is a product of the radiolysis of water with high-linear energy transfer (LET) radiation, which is distinct from low-LET radiation wherein O(2) radiolytic yield is negligible. Since O(2) is a powerful radiosensitizer, this fact is of practical relevance in cancer therapy with energetic heavy ions, such as carbon ions. It has recently been discovered that large doses of ionizing radiation delivered to tumors at very high dose rates (i.e., in a few milliseconds) have remarkable benefits in sparing healthy tissue while preserving anti-tumor activity compared to radiotherapy delivered at conventional, lower dose rates. This new method is called “FLASH radiotherapy” and has been tested using low-LET radiation (i.e., electrons and photons) in various pre-clinical studies and recently in a human patient. Although the exact mechanism(s) underlying FLASH are still unclear, it has been suggested that radiation delivered at high dose rates spares normal tissue via oxygen depletion. In addition, heavy-ion radiation achieves tumor control with reduced normal tissue toxicity due to its favorable physical depth-dose profile and increased radiobiological effectiveness in the Bragg peak region. To date, however, biological research with energetic heavy ions delivered at ultra-high dose rates has not been performed and it is not known whether heavy ions are suitable for FLASH radiotherapy. Here we present the additive or even synergistic advantages of integrating the FLASH dose rates into carbon-ion therapy. These benefits result from the ability of heavy ions at high LET to generate an oxygenated microenvironment around their track due to the occurrence of multiple (mainly double) ionization of water. This oxygen is abundant immediately in the tumor region where the LET of the carbon ions is very high, near the end of the carbon-ion path (i.e., in the Bragg peak region). In contrast, in the “plateau” region of the depth-dose distribution of ions (i.e., in the normal tissue region), in which the LET is significantly lower, this generation of molecular oxygen is insignificant. Under FLASH irradiation, it is shown that this early generation of O(2) extends evenly over the entire irradiated tumor volume, with concentrations estimated to be several orders of magnitude higher than the oxygen levels present in hypoxic tumor cells. Theoretically, these results indicate that FLASH radiotherapy using carbon ions would have a markedly improved therapeutic ratio with greater toxicity in the tumor due to the generation of oxygen at the spread-out Bragg peak.

The effect of the azide ion N3− on the yield of molecular hydrogen in water irradiated with 60Co γ-rays (∼1 MeV Compton electrons) and tritium β-electrons (mean electron energy of ∼7.8 keV) at 25 °C is investigated using Monte Carlo track chemistry simulations in conjunction with available experimental data. N3− is shown to interfere with the formation of H2 through its high reactivity towards hydrogen atoms and, but to a lesser extent, hydrated electrons, the two major radiolytic precursors of the H2 yield in the diffusing radiation tracks. Chemical changes are observed in the H2 scavengeability depending on the particular type of radiation considered. These changes can readily be explained on the basis of differences in the initial spatial distribution of primary radiolytic species (i.e., the structure of the electron tracks). In the “short-track” geometry of the higher “linear energy transfer” (LET) tritium β-electrons (mean LET ∼5.9 eV nm−1), radicals are formed locally in much higher initial concentration than in the isolated “spurs” of the energetic Compton electrons (LET ∼0.3 eV nm−1) generated by the cobalt-60 γ-rays. As a result, the short-track geometry favors radical–radical reactions involving hydrated electrons and hydrogen atoms, leading to a clear increase in the yield of H2 for tritium β-electrons compared to 60Co γ-rays. These changes in the scavengeability of H2 in passing from tritium β-radiolysis to γ-radiolysis are in good agreement with experimental data, lending strong support to the picture of tritium β-radiolysis mainly driven by the chemical action of short tracks of high local LET. At high N3− concentrations (>1 M), our H2 yield results for 60Co γ-radiolysis are also consistent with previous Monte Carlo simulations that suggested the necessity of including the capture of the precursors to the hydrated electrons (i.e., the short-lived “dry” electrons prior to hydration) by N3−. These processes tend to reduce significantly the yields of H2, as is observed experimentally. However, this dry electron scavenging at high azide concentrations is not seen in the higher-LET 3H β-radiolysis, leading us to conclude that the increased amount of intra-track chemistry intervening at early time under these conditions favors the recombination of these electrons with their parent water cations at the expense of their scavenging by N3−.

Monte Carlo track chemistry simulations were carried out to predict the yields (G-values) of all primary radical and molecular species produced in the radiolysis of pure, neutral water and 0.4 M sulfuric acid aqueous solutions by the recoil ions of the 10B(n,α)7Li nuclear reaction as a function of temperature from 25 to 350 °C. The calculations were performed individually for 1.47 MeV α-particles and 0.84 MeV lithium nuclei with “dose-average” linear energy transfer (LET) values of ∼196 and 225 eV nm−1 at 25 °C, respectively. The overall yields were calculated by summing the G-values for each recoil ion weighted by its fraction of the total energy absorbed. In the calculations, the actual effective charges carried by the two helium and lithium ions (due to charge exchange effects) were taken into account and the (small) contribution of the 0.478 MeV γ-ray, also released from the 10B(n,α)7Li reaction, was neglected. Compared with data obtained for low-LET radiation (60Co γ-rays or fast electrons), our computed yields for the 10B(n,α)7Li radiolysis of neutral deaerated water showed essentially similar temperature dependence 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 the 10B(n,α)7Li recoil ions. Overall, the simulation results agreed well with existing estimates at 20 and 289 °C. For deaerated 0.4 M H2SO4 solutions, reasonable agreement between experiment and simulation was also found at room temperature. Nevertheless, more experimental data for both neutral and acidic solutions would be needed to better describe the dependence of radiolytic yields on temperature and to test our modeling calculations more thoroughly. Moreover, measurements of the (eaq− + eaq−) reaction rate constant in near-neutral water would help us to determine whether the predicted non-monotonic inflections above ∼150 °C in G(H2) and G(H2O2) are confirmed.

Maintaining the structural integrity of materials in nuclear power plants is an essential issue associated with safe operation. Hydrogen (H2) addition or injection to coolants is a powerful technique that has been widely applied such that the reducing conditions in the coolant water avoid corrosion and stress corrosion cracking (SCC). Because the radiation-induced reaction of ˙OH + H2 → H˙ + H2O plays a crucial role in these systems, the rate constant has been measured at operation temperatures of the reactors (285-300 °C) by pulse radiolysis, generating sufficient data for analysis. The reverse reaction H˙ + H2O → ˙OH + H2 is negligibly slow at ambient temperature; however, it accelerates considerably quickly at elevated temperatures. Although the reverse reaction reduces the effectiveness of H2 addition, reliable rate constants have not yet been measured. In this study, the rate constants have been determined in a temperature range of 250-350 °C by pulse radiolysis in an aqueous I- solution.

Monte Carlo track chemistry simulations of the low linear energy transfer radiolysis of pure, deaerated liquid water have been used in combination with a spherical spur model to examine the effect of temperature on the in situ formation of H3O+ ions and the corresponding abrupt transient “acid-spike” response that is observed after irradiation. The magnitude and duration of this acid-spike effect were found to be a very sensitive function of temperature. At 25 °C, it is most intense at times less than ∼1 ns, with a pH of 3.3 remaining nearly constant. In contrast, at higher temperatures, there is an increasingly acidic but much shorter pH response. At 350 °C, the pH is around 1.7 on a time scale of ∼10 ps. At longer times, the pH gradually increases for all temperatures, ultimately reaching a constant value corresponding to the non-radiolytic, pre-irradiation concentration of H3O+ arising through water’s autoprotolysis at ∼1–10 μs following irradiation. It does not appear that this transient acid-spike ef...

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 acidic 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 γ/fast electron irradiation, LET ∼ 0.3 keV μm−1) and (2) an axially homogeneous “cylindrical” track model for high-LET radiation (such as 150 keV protons, LET ∼ 70 keV μm−1; 1.75 MeV per nucleon helium ions, LET ∼ 70 keV μm−1; and 0.6 MeV per nucleon helium ions, LET ∼ 146 keV μm−1). 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.

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.

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 ∼ 0.3 keV μm−1) radiation (e.g., γ-rays from 60Co, fast electrons or high-energy protons) and tritium β-particles (mean LET ∼ 6 keV μm−1) at 25 °C. We investigated the influence of Br− ions, as selective scavengers of ˙OH 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 × 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 60Co γ/fast electron radiolysis) and tritium β-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 β-electrons of tritium depositing their energy almost entirely as cylindrical “short tracks” and the energetic Compton electrons produced by γ-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 β-electrons in both water and aqueous bromide solutions favored a clear increase in G(H2O2) compared to 60Co γ-rays. Moreover, the presence of oxygen was seen to scavenge hydrated electrons (eaq−) and H˙ atoms on the ∼10−7 s time scale, thereby protecting H2O2 from further reactions with these species in the homogeneous stage of radiolysis. This protection against eaq− and H˙ 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 β-radiolysis was found to be more easily suppressed than in the case of cobalt γ-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 β-electron irradiation were in good agreement with experimental data, thereby lending strong support to the picture of tritium β-radiolysis in terms of short tracks of high local LET.

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.

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 both OH 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 + H 2 O → H 2 + OH. Our results suggest a way to assess this reaction’s rate constant, which is still controversial.

It has been a longstanding issue in the radiation chemistry of water that, even though H 2 is a molecular product, its “escape” yield g (H 2 ) increases with increasing temperature. A main source of H 2 is the bimolecular reaction of two hydrated electrons ( e - aq ). The temperature dependence of the rate constant of this reaction ( k 1 ), measured under alkaline conditions, reveals that the rate constant drops abruptly above ~150°C. Recently, it has been suggested that this temperature dependence should be regarded as being independent of pH and used in high-temperature modeling of near-neutral water radiolysis. However, when this drop in the e - aq self-reaction rate constant is included in low (isolated spurs) and high (cylindrical tracks) linear energy transfer (LET) modeling calculations, g (H 2 ) shows a marked downward discontinuity at ~150°C which is not observed experimentally. The consequences of the presence of this discontinuity in g (H 2 ) for both low and high LET radiation are briefly discussed in this communication. It is concluded that the applicability of the sudden drop in k 1 observed at ~150°C in alkaline water to near-neutral water is questionable and that further measurements of the rate constant in pure water are highly desirable. Received:13 June 2013; Revised: 27 August 2013; Accepted: 28 August 2013

Background: The temperature dependence of the rate constant (k) of the bimolecular reaction of two hydrated electrons (e-aq) measured in alkaline water exhibits an abrupt drop between 150 and 200 oC; above 250 oC, it is too small to be measured reliably. Although this result is well established, the applicability of this sudden drop in k(e-aq + e-aq) above 150 oC, as recommended by Bartels and co-workers, to neutral or slightly acidic solution still remains uncertain. Recent work by Hatomoto et al. combined ultrashort pulse radiolysis experiments with spur diffusion kinetic model simulations; this work suggested that in near-neutral water the abrupt change in k above 150 oC does not occur and that k should increase, rather than decrease, at temperatures greater than 150 oC with roughly the same Arrhenius dependence of the data below 150 oC.Method of approach: In view of this uncertainty of k, Monte Carlo simulations were used to examine the sensitivity of the density dependence of the yield of e-aq in the low-LET radiolysis of SCW (H2O) at 400 oC on variations in the temperature dependence of k. Two different values of the e-aq self-reaction rate constant at 400 oC were used: one based on the temperature dependence of k above 150 oC as measured in alkaline water (4.2 x 108 M-1 s-1) and the other based on an Arrhenius extrapolation of the values below 150 oC as initially proposed by Elliot (2.5 x 1011 M-1 s-1).Results: In both cases, the density dependences of our calculated e-aq yields at 60 ps and 1 ns were found to compare fairly well with the available picosecond pulse radiolysis experimental data (for D2O) for the entire water density range studied (0.15-0.6 g/cm3). Only a small effect of k on the variation of G(e-aq) as a function of density at 60 ps and 1 ns could be observed.Conclusions: Our calculations did not allow us to unambiguously confirm (or deny) the applicability of the predicted sudden drop of k(e-aq + e-aq) at 150 oC in near-neutral water.Keywords: Radiolysis; Linear energy transfer (LET); Supercritical water at 400 oC; Water density; Hydrated electron; Radiolytic yield; Temperature dependence of the e-aq self-reaction rate constant; Monte Carlo simulations.

The long-term viability of a supercritical water-cooled reactor (SCWR) will depend on the ability of designers and operators to control and maintain water chemistry conditions that will minimize co...

Our Monte-Carlo modeling of the high-temperature radiolysis of water by low-LET radiation was re-examined in an attempt to reconcile our computed g-values of the various radiolytic products ( e aq - , OH, H , H2, and H2O2) with recently re-assessed experimental data up to 350 °C. The inclusion in our simulations of the abrupt drop in the rate constant for the self-reaction of e aq - above 150 °C led us to re-evaluate the temperature dependence of certain parameters intervening in the physicochemical stage of the radiolysis. A very good agreement was found between model and experiment. The importance of the reaction of H atoms with water in the unexplained yield of H2 above 200 °C was also briefly discussed.

The optical absorption spectra of the solvated electron ([Formula: see text]) in sub- and super-critical methanol are measured by both electron pulse radiolysis and laser photolysis techniques, at temperatures in the range 220–270 °C. Over the density range studied (~0.45–0.59 g/cm3), the position of the absorption maximum ([Formula: see text]) of [Formula: see text] is found to shift only slightly to the red with decreasing density. In agreement with our previous work in water, at a fixed pressure, [Formula: see text] decreases monotonically with increasing temperature in passing through the phase transition at Tc (239.5 °C). By contrast, at a fixed density, [Formula: see text] exhibits a minimum as the solvent passes above the critical point into the supercritical state. These behaviors are discussed in terms of microscopic arguments based on the changes that occur in the methanol properties and methanol structure in the sub- and super-critical regimes. The effect of the addition of a small amount of water to the alcohol on the optical absorption energy of [Formula: see text] is also investigated.

Monte Carlo simulations are used to calculate the primary radical yields [Formula: see text], g(•OH), the sum [[Formula: see text] + g(•OH) + g(H•)], and the ratio g(H•)/[Formula: see text] in the low linear energy transfer (LET) radiolysis of supercritical water (SCW) at 400 °C in the high-density, liquid-like region near ∼0.5 g/cm3. Using all the currently available information on the reactivities and diffusion coefficients of the radiation-induced species under these conditions, and assuming the aqueous medium to be a “continuum”, a good accord is found between our calculations and the available experimental data. In particular, our computed [Formula: see text] yields at 60 ps and 1 ns compare very well with recently reported direct time-dependent [Formula: see text] yield measurements in SCW (D2O) at 400 °C and 0.570 g/cm3 using picosecond pulse radiolysis experiments.

Formation yields of methyl viologen cation radicals G(MV*+) (100 eV)(-1) have been measured in deaerated aqueous solutions of 0.25 mM methyl viologen (MV(2+)) containing various concentrations of formate anion (0.01-2 M) after irradiation with six different ion beams (4He(2+), 12C(6+), 20Ne(10+), 28Si(14+), 40Ar(18+) and 56Fe(26+) with incident energies varying from 0.6 to 28 GeV) provided by the Heavy Ion Medical Accelerator in Chiba (HIMAC) at the National Institute of Radiological Science (NIRS) in Japan. The sample solutions are irradiated at the incident energy of the ions using 1-cm irradiation cells. Corresponding LET values cover the range from 2.2 to 185 eV/nm. G(MV*+) increases with increasing formate concentration. In 4He(2+) radiolysis, it increases from 5.7 to 7.1 as the concentration of formate is increased from 0.01 to 2 M, while in 56Fe(26+) radiolysis, the MV*+ yield value changes from 2.2 to 4.1. The other values lie between the yields for 4He(2+) and 56Fe(26+). In addition, G(MV*+) decreases with increasing LET. In the case of 12C(6+) radiolysis, G(MV*+) increases with increasing energy of the carbon ions from 135 to 400 MeV/nucleon, i.e., with decreasing LET from 21 to 11 eV/nm. In parallel to the above measurements, Monte Carlo simulations of the radiolysis of the MV(2+)/formate solutions have been performed. Ionic strength effects on reactions between charged species are taken into account. To reproduce the experimental results, previously unreported reactions such as e(aq)(-) + MV*+, MV*(+) + *OH and *COO- + *OH have been introduced in the reaction scheme. After optimization, the rate constants of these latter two reactions are determined to be (3 +/- 0.5) x 10(10) and (5 +/- 0.5) x 10(10) M(-1) s(-1), respectively. By contrast, the reaction between e(aq)- and MV*+ is too slow to affect G(MV*+). On the basis of these calculations, characteristics of intratrack reactions induced by heavy-ion beams are discussed in reference not only to the scavenger method used for measurement of water decomposition product yields but also to the differences in the relative spatial distribution of the reactants as well as the places where their intratrack reaction occurs within the geometry of the ion track structure.

Monte Carlo simulations are used to investigate the effects of acidity (pH) on the primary yields of various chemical species produced in the radiolysis of de-aerated aqueous sulfuric acid solutions over the range from neutral solution to 0.4 mol/L H2SO4. The effects of the quality of radiation, measured in terms of linear energy transfer (LET), have also been studied for LET varying from ~0.3 to 15 keV/µm at ambient temperature. Our results show that an increase in acidity (1 < pH < 4) leads to an increase in the yield [Formula: see text] of the "reducing" free radicals (hydrated electron and H• atom) and a slight increase in G·OH and [Formula: see text], while there is a slight decrease in [Formula: see text] At pH < 1, •OH radicals react with HSO4- anions to form SO4·– radicals, resulting in a steep decrease in G.OH. By contrast, in the range of pH from ~4 to 7, the calculated yield values are independent of sulfuric acid concentration. In both neutral water and 0.4 mol/L H2SO4 (pH 0.46) solutions, the primary molecular yields increase upon increasing LET to ~15 keV/µm with a concomitant decrease in those of free radicals. As an exception, GH. at first increases with LET, reaching a maximum near 6.5 keV/µm before decreasing steeply at higher LET. The results obtained are generally in good agreement with available experimental data over the whole acidity and LET ranges studied. Finally, as an application, we have simulated the radiation-induced oxidation of ferrous sulfate solutions in aerated aq. 0.4 mol/L H2SO4 (Fricke dosimeter) as a function of time up to ~50 s and addressed the effects of LET on the resulting ferric ion yield at 25 °C. The production of Fe3+ ions is highly sensitive to free-radical yields, especially H• atoms (via formation of HO2•), resulting in a marked decline of G(Fe3+) with increasing LET. The general trend of the observed variation of G(Fe3+) with radiation quality is well reproduced by our computed Fe3+ ion yield values.Key words: liquid water, acidic (H2SO4) aqueous solutions, radiolysis, free-radical and molecular yields, linear energy transfer (LET), Fricke dosimeter, Monte Carlo simulations.

Monte Carlo simulations are used to investigate the effects of multiple ionization of water molecules on the yields of formation of free radical and molecular species, including molecular oxygen, in the radiolysis of pure, deaerated liquid water by using different types of radiation (1H+, 4He2+, 12C6+, and 20Ne9+ ions) up to approximately 900 keV/microm, at neutral pH and 25 degrees C. Taking into account the double, triple, and quadruple ionizations of water, the primary (or "escape") yields (at 10(-6) s) of the various radiolytic species (G(e(aq)-), G(H*), G(H2), G(*OH), G(HO2*/O2*-), and G(H2O2) are calculated as a function of the linear energy transfer (LET) of the radiation. Our results quantitatively reproduce the large increase observed in G(HO2*/O2*-) at high LET. Under the conditions of this study, the mechanisms of triple and quadruple ionizations contribute only weakly to the production of HO2*/O2*-. With the exception of protons, our calculations also simultaneously predict a maximum in G(H2O2) corresponding to the LET of approximately 4.5-MeV helium ions (approximately 100 keV/microm) and approximately 110-MeV carbon ions (approximately 180 keV/microm). This maximum occurs where G(HO2*/O2*-) begins to rise sharply, suggesting, in agreement with previous experimental data, that the yields of HO2*/O2*- and H2O2 are closely linked. Moreover, our results show a steep increase in the initial and primary yields of molecular oxygen with increasing LET, giving support to the "oxygen in heavy-ion tracks" hypothesis. By contrast, it is found that, in the whole LET range considered, the incorporation of multiple ionization in the simulations has only little effect on the variation of our computed G(e(aq)-), G(H*), G(H2), and G(*OH) values as a function of LET. As expected, G(e(aq)-) and G(*OH) decrease continuously with increasing LET. G(H*) at first increases and then decreases at high LET. Finally, G(H2) monotonically rises with increasing LET. Our calculated yield values compare generally very well with experiment.

This article examines a non-invasive spectrofluorometric method using the 2′,7′-dichlorofluorescein diacetate (DCHF-DA) assay for quantifying the intracellular reactive oxygen species (ROSi) produced in four cultured cancer cell lines: drug-sensitive (K562) and drug-resistant (K562/adr) human erythromyelogenous leukemia cell lines, and drug-sensitive (GLC4) and drug-resistant (GLC4/adr) human small cell lung carcinoma cell lines. The oxidation of the probe to the fluorescent dichlorofluorescein (DCF) was continuously monitored by following the DCF fluorescence intensity as a function of time using a standard spectrofluorometer in the presence of an extracellular DCF fluorescence quencher (Co2+). By fitting the spectrofluorometric data to a kinetic model based on the following two reactions: (i) deacetylation of DCHF-DA to the oxidant-sensitive compound 2′,7′-dichlorofluorescein (DCHF) by cellular esterase enzymes (pseudo-first-order rate constant: ke) and (ii) oxidation of DCHF by ROSi (second-order rate constant: k2), the parameters intervening in DCF formation, ke and the product of k2 by the ROSi concentration, were quantitatively determined for the different cell lines studied. The results revealed that the intracellular esterase content or activity is similar in K562, K562/adr, and GLC4 cells, but 5-fold higher in GLC4/adr cells. The product k2[ROSi] was found to be similar in the four cell lines considered, with a mean value of (5.3±0.9)×10−7 cell−1 s−1. Assuming that H2O2 (in combination with peroxidases) is the primary responsible species for DCHF oxidation in intact cells, and using the rate constant value k 2 = 790 ± 62 M - 1 s - 1 established in our laboratory for the reaction of DCHF with H2O2 in the presence of horseradish peroxidase, the mean value of the intracellular levels of ROSi in those cells was estimated to be 0.67±0.16 nM per cell. Such a value compares favorably to H2O2 intracellular steady-state concentrations that have been estimated in the literature for a few other cell types.

Monte-Carlo simulations are used to investigate the effects of multiple ionization on the formation of HO 2 /O 2 − and H 2 O 2 in the radiolysis of water by 12 C 6+ ions up to ∼430 keV/μm, at neutral pH and 25 °C. Taking into account the double and triple ionizations of water molecules, the primary yields G HO 2 /O 2 − and G H 2 O 2 are calculated as a function of linear energy transfer (LET). Our results quantitatively reproduce the large increase observed in G HO 2 /O 2 − at high LET. They also simultaneously predict a slight maximum in G H 2 O 2 at ∼180–200 keV/μm, in excellent agreement with experiment. Under the conditions of this study, triple ionization is found to contribute only negligibly to G HO 2 /O 2 − .

In this work, we sought to investigate the relation existing between MDR1 mRNA levels, the resistance factor (RF), and the efficiency of efflux of pirarubicin (THP) mediated by P-glycoprotein (P-gp) in multidrug-resistant (MDR) K562 sublines. The MDR K562 sublines were selected from K562/adr cells by exposure to different adriamycin concentrations: 300 nM (K562/300), 1000 nM (K562/1000), and 10 000 nM (K562/10000), yielding RF values of 23.2, 26.5, and 39.6, respectively. The analysis of the P-gp encoding MDR1 gene overexpression by reverse transcriptase – polymerase chain reaction provided evidence of increased MDR1 mRNA levels when the adriamycin concentration used for the MDR cell selection increased. We used spectrofluorometric methods to determine the kinetics of the uptake and P-gp-mediated efflux of THP in the different selected MDR K562 sublines. Our data showed that (i) the maximal rate of P-gp-mediated efflux of THP, Vmax, increased with increasing RF; (ii) the observed Michaelis constant, Km, had the same value for all selected sublines, thus leading to an overall increase in the ratio Vmax/Km(5.1 × 10–3, 6.2 × 10–3, 6.8 × 10–3, and 9.3 × 10–3s–1for K562/adr, K562/300, K562/1000, and K562/10000 cells, respectively), and (iii) the determination of the Hill coefficient (nH) gave values close to 2, which suggested a positive cooperative transport of THP with the expelling of two molecules of THP per turnover of P-gp. This study demonstrated that, in the K562/adr sublines used in our experiments, P-gp played a major role in conferring the MDR phenotype. Moreover, under our experimental conditions, intracellular acidic organelles were shown to contribute to decreased drug–target interaction and, thereby, decreased cytotoxicity. The variation of the concentrations of THP accumulated in the acidic organelles as a function of the total THP concentration added to the cells was the same, within the limits of experimental errors, whatever the degree of resistance of the studied MDR K562 sublines. Finally, this study suggested that, in the selected MDR K562 sublines, the K+/H+antiporter exchanger could be activated by the pirarubicin transport, leading to a probable acidification of intracellular pH. The P-gp-mediated efflux of THP and an accumulation of THP in acidic organelles confer an advantage for MDR cells in surviving prolonged exposure to cytotoxic agents and giving rise to high degrees of resistance. Key words: multidrug resistance, P-glycoprotein, pirarubicin, acidic organelles, MDR1 mRNA levels, fluorescence spectroscopy, kinetic parameters.

A re-examination of our Monte-Carlo modeling of the radiolysis of liquid water by low linear-energy-transfer (LET ~ 0.3 keV µm–1) radiation is undertaken herein in an attempt to reconcile the results of our simulation code with recently revised experimental hydrated electron (e–aq) yield data at early times. The thermalization distance of subexcitation electrons, the recombination cross section of the electrons with their water parent cations prior to thermalization, and the branching ratios of the different competing mechanisms in the dissociative decay of vibrationally excited states of water molecules were taken as adjustable parameters in our simulations. Using a global-fit procedure, we have been unable to find a set of values for those parameters to simultaneously reproduce (i) the revised e–aq yield of 4.0 ± 0.2 molecules per 100 eV at "time zero" (that is, a reduction of ~20% over the hitherto accepted value of 4.8 molecules per 100 eV), (ii) the newly measured e–aq decay kinetic profile from 100 ps to 10 ns, and (iii) the time-dependent yields of the other radiolytic species H•, •OH, H2, and H2O2 (up to ~1 µs). The lowest possible limiting "time-zero" yield of e–aq that we could in fact obtain, while ensuring an acceptable agreement between all computed and experimental yields, was ~4.4 to 4.5 molecules per 100 eV. Under these conditions, the mean values of the electron thermalization distance and of the geminate electron–cation recombination probability, averaged over the subexcitation electron "entry spectrum," are found to be equal to ~139 Å and ~18%, respectively. These values are to be compared with those obtained in our previous simulations of liquid water radiolysis, namely ~88 Å and ~5.5%, respectively. Our average electron thermalization distance is also to be compared with the typical size (~64–80 Å) of the initial hydrated electron distributions estimated in current deterministic models of "spur" chemistry. Finally, our average probability for geminate electron–cation recombination agrees well with an estimated value of ~15% recently reported in the literature. In conclusion, this work shows that an adaptation of our calculations to a lower hydrated electron yield at early times is possible, but also suggests that the topic is not closed. Further measurements of the e–aq yields at very short times are needed. Key words: liquid water, radiolysis, electron–cation geminate recombination, electron thermalization distance, hydrated electron (e–aq), e–aq decay kinetics, time-dependent molecular and radical yields, Monte-Carlo simulations.

Monte-Carlo simulations are performed to calculate the temperature dependence of the primary hydrated electron yield (Geaq-) for liquid water irradiated by low linear-energy-transfer radiation (LET ~ 0.3 keV µm–1) in the range 25–325°C. Calculations are carried out by taking properly into account the effect of the time and temperature dependencies of the water dielectric constant on the electron–cation geminate recombination. Our computed Geaq- values slightly increase with increasing temperature, in good agreement with experiment. The product Geaq- εmax(eaq-), estimated by using existing experimental data of the maximum molar extinction coefficient εmax(eaq-), remains nearly constant or slightly increases, depending on the temperature dependence chosen for εmax. Our Geaq-εmax(eaq-) values compare generally well with most experimental data, as well as with the predictions of deterministic diffusion-kinetic model calculations. Moreover, our results indicate that the static dielectric constant of water (εs) does not play any significant role on the electron–cation recombination at early times. Such a finding is inconsistent with the interpretation, proposed by certain authors in the literature, that Geaq- should in fact decrease as temperature is increased because of an increased electron–cation geminate recombination due to a lowering of εs. Finally, the temperature dependence of the hydrated electron yields, calculated at various times between 10 ps and 1 µs, shows that at low LET, the time required to establish homogeneous chemistry in the bulk of the solution is ~10–6 s in the range ~25–100°C, and that this time diminishes to ~10–7 s at higher temperatures. Key words: liquid water, radiolysis, temperature, hydrated electron (eaq-), radiolytic yields, electron–cation geminate recombination, dielectric constant, molar extinction coefficient of eaq-, homogenization time.

Monte-Carlo simulations are used to calculate the primary yield of hydrogen peroxide (GH2O2) of the radiolysis of pure, deaerated liquid water as a function of linear energy transfer (LET) of the incident radiation over the range ~0.3–100 keV µm–1, at 25 and 300°C. The radiations include 1H+, 2H+, 4He2+, 7Li3+, and 12C6+ ions with energies from 0.17 MeV to 3.6 GeV. At 25°C, it is found that our GH2O2 values, calculated with protons of different initial energies, show a monotonic increase as a function of LET, in agreement with the commonly assumed expectation of an increase in molecular yields with increasing LET. Our calculated H2O2 yields at 300°C increase significantly faster with LET than do their corresponding 25°C values, showing that the temperature dependence of GH2O2 at higher LET is less than for low-LET radiation. We also report our results on the temporal variations of the H2O2 yields, in the interval ~1 × 10–13 – 1 × 10–6 s, at 25 and 300°C and for the different types of radiation considered. Finally, we find that for incident ions of equal LET > 10 keV µm–1, GH2O2 decreases as the ion velocity increases, from protons (or deuterons) to carbon ions. These differences produced in GH2O2 by changing the type of radiation are explained by the greater mean energy of secondary electrons from the higher velocity ions, which penetrate to a greater average distance from the actual particle track, with a corresponding decrease in molecular yields. Our calculated GH2O2 values compare generally well with the experimental data available from the literature and are also in good accord with the predictions of deterministic diffusion-kinetic model calculations reported earlier.Key words: liquid water, radiolysis, primary yields, hydrogen peroxide (H2O2), linear energy transfer (LET), accelerated protons and heavy ions, temperature, Monte-Carlo simulations.

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 of the reaction of hydrogen atoms with water in the homogeneous chemical stage.

Monte-Carlo simulations are used to calculate the primary radical and molecular yields (g-values) of water radiolysis versus temperature in the range 25–300°C, for different high linear energy transfer radiations (protons, 2 H + , and 7 Li 3+ ) up to ∼ 65 keV / μm . The results are compared, at ∼10−7 s, to experimental data at 25°C, 95°C, and 180°C. Excellent agreement is obtained for g(e−aq) and g( OH ) over the whole temperature range, but g(H2), [g(H )+g(H2)], and g(H2O2) are substantially larger than the measured values at 180°C. Our g-values are also compared with fast-neutron radiolysis yields and with deterministic modeling calculations. The agreement is generally satisfactory.

Monte-Carlo simulations are used to calculate the primary yield of hydrogen atoms (GH ) of water radiolysis versus linear energy transfer (LET) up to ∼100 keV/μm, at 25°C and 300°C. The GH values calculated at 25°C using protons show a maximum near 6.5 keV/μm before decreasing steeply at higher LET. At 300°C, GH remains constant below ∼7 keV/μm, while at higher LET it decreases faster than do its corresponding 25°C values. For different radiation types (protons, 2 H + , 4 He 2+ , 7 Li 3+ , and 12 C 6+ ) of equal LET>10 keV/μm, GH increases as the incident ion velocity increases. The results compare generally well with experiment.

Meesungnoen, J., Benrahmoune, M., Filali-Mouhim, A., Mankhetkorn, S. and Jay-Gerin, J-P. Monte Carlo Calculation of the Primary Radical and Molecular Yields of Liquid Water Radiolysis in the Linear Energy Transfer Range 0.3–6.5 keV/μm: Application to 137Cs Gamma Rays. Monte Carlo simulations of the radiolysis of neutral liquid water and 0.4 M H2SO4 aqueous solutions at ambient temperature are used to calculate the variations of the primary radical and molecular yields (at 10–6 s) as a function of linear energy transfer (LET) in the range ∼0.3 to 6.5 keV/μm. The early energy deposition is approximated by considering short (∼20–100 μm) high-energy (∼300–6.6 MeV) proton track segments, over which the LET remains essentially constant. The subsequent nonhomogeneous chemical evolution of the reactive species formed in these tracks is simulated by using the independent reaction times approximation, which has previously been used successfully to model the radiolysis of water under various conditions. The...

The yields of species such as e-aq, H • , • OH, H 2 and H 2 O 2 , formed from the radiolysis of neutral liquid water by the incidence of 0.8-MeV neutrons at temperatures between 25 and 350°C, were calculated by using Monte Carlo simulations. The slowing down of these neutrons through elastic scattering produced recoil protons elastically of ~0.5057, 0.186, and 0.0684 MeV which had linear energy transfers (LETs) of ~40, 67 and 76 keV/µm, respectively, at 25°C. The effects of neutron radiation can be predicted based on the contribution of those first three recoil protons by neglecting the radiation effects due to oxygen ion recoils. Then, the fast neutron yields could be estimated by summing the yields of contributing protons after corresponding weightings were used according to their energy. In this work, yields were calculated at 10 -7 and 10 -6 s after incidence of neutron radiation in water at the aforementioned temperature range. Overall, there is a reasonably good agreement between our calculated and existing experimental G -values for the entire temperature range. However, we proposed an hypothesis that the not very significant difference between experimental data and our calculated data is due to the different measuring time used in obtaining the experimental data as compared to the ones used in our calculation. Our computed yields for 0.8-MeV fast neutron radiation show an essentially similar temperature dependences over the range of temperature studied with 2-MeV fast neutron and low-LET radiation, but with lower values for yields of free radicals and higher values for molecular yields. Received: 04 October 2014 ; Revised: 23 March 2016 ; Accepted: 23 March 2016