Sunuchakan Sanguanmith

Utilization of the Ferrous Sulfate (Fricke) Dosimeter for Evaluating the Radioprotective Potential of Cystamine: Experiment and Monte Carlo Simulation

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 60Co γ rays or fast electrons, based on the oxidation of ferrous ions to ferric ions by the oxidizing species •OH, HO2•, and H2O2 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 Fe2+ and leads to decreased ferric yields (or G values). The observed decrease in G(Fe3+) 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(Fe3+) values and experiment. This study concludes that Monte Carlo simulations represent a very efficient method for understanding indirect radiation damage at the molecular level.

Self-radiolysis of tritiated water. 1. A comparison of the effects of 60Co γ-rays and tritium β-particles on water and aqueous solutions at room temperature

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.

Yields of H2 and hydrated electrons in low-LET radiolysis of water determined by Monte Carlo track chemistry simulations using phenol/N2O aqueous solutions up to 350 °C

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.

Calculation of the Yields for the Primary Species Formed from the Radiolysis of Liquid Water by Fast Neutrons at Temperatures between 25–350°C

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 (60Co 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.

Self-radiolysis of tritiated water. 2. Density dependence of the yields of primary species formed in the radiolysis of supercritical water by tritium β-particles at 400 °C

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.

Radiolysis of Supercritical Water at 400°C: A Sensitivity Study of the Density Dependence of the Yield of Hydrated Electrons on the (eaq−+eaq−) Reaction Rate Constant

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.

Effect of Temperature on the Low-Linear Energy Transfer Radiolysis of the Ceric-Cerous Sulfate Dosimeter: A Monte Carlo Simulation Study

The stochastic modeling of the 60Co 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 Ce4+ ions to Ce3+ ions, while •OH radicals oxidize the Ce3+ present in the solution back to Ce4+. The net Ce3+ yield is given by G(Ce3+) = g(H•) + 2 g(H2O2) – g(•OH), where the primary (or “escape”) yields of H•, H2O2 and •OH are represented by lower case gs. At room temperature, G(Ce3+) 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 Ce3+ and on the underlying chemical reaction kinetics using Monte Carlo track chemistry simulations. The simulations showed that G(Ce3+) is time dependent, a result of the differences in the lifetimes of the reactions that make up the radiolysis mechanism. Calculated G(Ce3+) 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(Ce3+) at ∼250°C is about one third of its value at room temperature. Above ∼250°C, our model predicted that G(Ce3+) would drop markedly with temperature until, instead of Ce4+ reduction, Ce3+ 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.

Self-radiolysis of tritiated water. 4. The scavenging effect of azide ions (N3−) on the molecular hydrogen yield in the radiolysis of water by 60Co γ-rays and tritium β-particles at room temperature

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 of the radiolysis of water induced by the recoil ions of the 10B(n,α)7Li nuclear reaction. 1. Calculation of the yields of primary species up to 350 °C

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.