Effect of multiple ionization on the radiolysis of liquid water irradiated with heavy ions a theoretical study using Monte-Carlo simulations

L'eau est un constituant majeur de la plupart des organismes vivants. Elle joue, à ce titre, un rôle central en radiobiologie, en introduisant une part d'effet indirect et/ou quasi direct résultant de sa radiolyse. Une large majorité d'études de chimie sous rayonnement des solutions aqueuses concernent les rayonnements à faible transfert d'énergie linéique (TEL) tels que les rayons y de 6°Co ou les électrons accélérés. Bien qu'elle soit encore incomplète à certains points de vue, un panorama de la radiolyse de l'eau pure ou contenant divers solutés à faible TEL peut être obtenu dans la plupart des cas. Il n'en est cependant pas ainsi pour les rayonnements à grand TEL où plusieurs aspects de cette radiolyse n'ont pas jusqu'ici été pleinement résolus. Parmi les points d'interrogation qui demeurent, citons : (1) le rendement primaire des radicaux hydroperoxyle/anion superoxyde (pKa = 4,8) augmente en fonction du TEL, et ceci même en l'absence d'oxygène. Un tel comportement est contraire à ce que l'on observe pour les autres rendements radicalaires. L'origine de ces radicaux n'est pas clairement établie, même s'ils constituent l'espèce radicalaire principale à haut TEL; (2) le rendement primaire en H20 2 croît avec le TEL jusqu'à un maximum, après lequel il décroît; aucune explication satisfaisante n'a été offerte quant à la présence d'une telle décroissance de ce produit moléculaire à haut TEL; (3) les détails précis du mécanisme par lequel les rayonnements à grand TEL sont très efficaces pour inactiver les cellules tumorales hypoxiques sont encore mal connus. Une interprétation possible de la réduction du facteur d'amplification de l'oxygène (FAO) à haut TEL est donnée par la génération in situ d'un microenvironnement oxygéné autour des trajectoires à grande densité d'ionisation (hypothèse de l'"oxygène à l'intérieur même des trajectoires").Abstract: Water makes up a predominant part of the milieu of living tissue, and, not surprisingly, plays a central role for understanding the interaction of ionizing radiation with biological systems. Most aqueous radiation chemistry studies have involved low-linear energy transfer (LET) radiation, such as [superscript 60]Co [gamma]-rays or fast electrons. A survey of the literature shows that the radiolysis of liquid water at low LET is generally well understood. However, at high LET, several reported data have not hitherto been quantitatively explained : (1) The primary yield of hydroperoxyl/superoxide anion (HO[subscript 2][superscript ¨]/O[subscript 2][superscript ¨-], p[kappa][subscript a] = 4.8) radicals increases with increasing LET, a behavior that is contrary to the other radical yields. As yet, the origin of these HO[subscript 2][superscript ¨]/O[subscript 2][superscript ¨-] radicals is not clearly established, even though they are the major radical species produced at high LET; (2) The primary yield of hydrogen peroxide rises with increasing LET to a maximum, after which it falls. No suitable explanation for the presence of such a decrease in H[subscript 2]O[subscript 2] yields at high LET has been offered; (3) The exact details of the mechanism by which high-LET radiations are very efficient for the inactivation of tumoral hypoxic cells, are still not well known. One possible explanation for the decreased radiobiological oxygen enhancement ratio (OER) at high LET is offered by the generation in situ of an oxygenated microenvironment around the tracks of more densely ionizing radiations (the so-called"oxygen-in-the-track" hypothesis). This work has been originally motivated by the hypothesis proposed by FERRADINI and JAY-GERIN (1998) that multiple ionization (MI) of water would be responsible for the large HO[subscript 2][superscript ¨]/O[subscript 2][superscript ¨-] yield produced in liquid water subject to heavy-ion irradiation. The purpose of this study is to test the validity of this hypothesis. To this aim, Monte Carlo track structure simulations are used to calculate the G-values of the various radiolytic species, including O[subscript 2], generated in the radiolysis of deaerated liquid water by several different types of radiation ([superscript 1]H[superscript +], [superscript 4]He[superscript 2+], [superscript 12]C[superscript 6+], and [superscript 20]Ne[superscript 9+] ions) over a wide range of LET up to [tilde] 900 keV/[micro]m, at neutral pH and in 0.4 M H[subscript 2]SO[subscript 4] (pH 0.46) solutions at 25ÀC. It is found that, upon incorporating the mechanisms of double, triple, and quadruple ionizations of water in the calculations, a quantitative agreement between theory and experiment can be obtained. In particular, in neutral (pH 7) solutions, our results reproduce very well the large increase observed in [Special characters omitted.] at high LET. Under the conditions of this study, the mechanisms of triple and quadruple ionizations make only a minor contribution to the yield of HO[subscript 2][superscript ¨]/O[subscript 2][superscript ¨-]. With the exception of protons, our calculations also simultaneously predict a maximum in [Special characters omitted.] around 100-200 keV/[micro]m in accord with experiment. For each irradiating ion considered, this maximum occurs precisely at the point where [Special characters omitted.] begins to rise sharply, suggesting, in agreement with experiments, that the yields of HO[subscript 2][superscript ¨]/O[subscript 2][superscript ¨-] and H[subscript 2]O[subscript 2] are closely linked. Moreover, the incorporation of MI in our simulations has only little effect on the variation of the computed [Special characters omitted.] and G[subscript ¨OH] values as a function of LET. In the case of acidic solutions irradiated by [superscript 12]C[superscript 6+] and [superscript 20]Ne[superscript 9+] ions, our results also predict a well-defined maximum in the curve of [Special characters omitted.] as a function of LET of [tilde] 1.4 molec./100 eV ([tilde] 45% greater in magnitude than that found in neutral water) around 180-200 keV/[micro]m, in good agreement with experiment. Finally, our simulation results show a steep increase in the initial and primary yields of O[subscript 2] with increasing LET. For example, for 24-MeV [superscript 12]C[superscript 6+] ions (LET [tilde] 500 keV/[micro]m), the initial in situ track concentration of oxygen is estimated to be about 3 orders of magnitude higher than the concentration of O[subscript 2] found in typical human cells. Such results, which largely plead in favor of the"oxygen in the heavy-ion track" hypothesis, could have profound consequences in radiobiology and in particular explain the observed reduction in the oxygen enhancement ratio (OER) with increasing LET. In conclusion, our results strongly support the importance of the role of MI in the heavy-ion radiolysis of water at high LET. They suggest that MI, although infrequent relative to single ionization events, is very efficient chemically

Effect of multiple ionization on the radiolysis of liquid water irradiated with heavy ions a theoretical study using Monte-Carlo simulations

Water makes up a predominant part of the milieu of living tissue, and, not surprisingly, plays a central role for understanding the interaction of ionizing radiation with biological systems. Most aqueous radiation chemistry studies have involved low-linear energy transfer (LET) radiation, such as [superscript 60]Co [gamma]-rays or fast electrons. A survey of the literature shows that the radiolysis of liquid water at low LET is generally well understood. However, at high LET, several reported data have not hitherto been quantitatively explained : (1) The primary yield of hydroperoxyl/superoxide anion (HO[subscript 2][superscript ¨]/O[subscript 2][superscript ¨-], p[kappa][subscript a] = 4.8) radicals increases with increasing LET, a behavior that is contrary to the other radical yields. As yet, the origin of these HO[subscript 2][superscript ¨]/O[subscript 2][superscript ¨-] radicals is not clearly established, even though they are the major radical species produced at high LET; (2) The primary yield of hydrogen peroxide rises with increasing LET to a maximum, after which it falls. No suitable explanation for the presence of such a decrease in H[subscript 2]O[subscript 2] yields at high LET has been offered; (3) The exact details of the mechanism by which high-LET radiations are very efficient for the inactivation of tumoral hypoxic cells, are still not well known. One possible explanation for the decreased radiobiological oxygen enhancement ratio (OER) at high LET is offered by the generation in situ of an oxygenated microenvironment around the tracks of more densely ionizing radiations (the so-called"oxygen-in-the-track" hypothesis). This work has been originally motivated by the hypothesis proposed by FERRADINI and JAY-GERIN (1998) that multiple ionization (MI) of water would be responsible for the large HO[subscript 2][superscript ¨]/O[subscript 2][superscript ¨-] yield produced in liquid water subject to heavy-ion irradiation. The purpose of this study is to test the validity of this hypothesis. To this aim, Monte Carlo track structure simulations are used to calculate the G-values of the various radiolytic species, including O[subscript 2], generated in the radiolysis of deaerated liquid water by several different types of radiation ([superscript 1]H[superscript +], [superscript 4]He[superscript 2+], [superscript 12]C[superscript 6+], and [superscript 20]Ne[superscript 9+] ions) over a wide range of LET up to [tilde] 900 keV/[micro]m, at neutral pH and in 0.4 M H[subscript 2]SO[subscript 4] (pH 0.46) solutions at 25ÀC. It is found that, upon incorporating the mechanisms of double, triple, and quadruple ionizations of water in the calculations, a quantitative agreement between theory and experiment can be obtained. In particular, in neutral (pH 7) solutions, our results reproduce very well the large increase observed in [Special characters omitted.] at high LET. Under the conditions of this study, the mechanisms of triple and quadruple ionizations make only a minor contribution to the yield of HO[subscript 2][superscript ¨]/O[subscript 2][superscript ¨-]. With the exception of protons, our calculations also simultaneously predict a maximum in [Special characters omitted.] around 100-200 keV/[micro]m in accord with experiment. For each irradiating ion considered, this maximum occurs precisely at the point where [Special characters omitted.] begins to rise sharply, suggesting, in agreement with experiments, that the yields of HO[subscript 2][superscript ¨]/O[subscript 2][superscript ¨-] and H[subscript 2]O[subscript 2] are closely linked. Moreover, the incorporation of MI in our simulations has only little effect on the variation of the computed [Special characters omitted.] and G[subscript ¨OH] values as a function of LET. In the case of acidic solutions irradiated by [superscript 12]C[superscript 6+] and [superscript 20]Ne[superscript 9+] ions, our results also predict a well-defined maximum in the curve of [Special characters omitted.] as a function of LET of [tilde] 1.4 molec./100 eV ([tilde] 45% greater in magnitude than that found in neutral water) around 180-200 keV/[micro]m, in good agreement with experiment. Finally, our simulation results show a steep increase in the initial and primary yields of O[subscript 2] with increasing LET. For example, for 24-MeV [superscript 12]C[superscript 6+] ions (LET [tilde] 500 keV/[micro]m), the initial in situ track concentration of oxygen is estimated to be about 3 orders of magnitude higher than the concentration of O[subscript 2] found in typical human cells. Such results, which largely plead in favor of the"oxygen in the heavy-ion track" hypothesis, could have profound consequences in radiobiology and in particular explain the observed reduction in the oxygen enhancement ratio (OER) with increasing LET. In conclusion, our results strongly support the importance of the role of MI in the heavy-ion radiolysis of water at high LET. They suggest that MI, although infrequent relative to single ionization events, is very efficient chemically

High-dose-rate effects in the radiolysis of water at elevated temperatures.

Monte Carlo track chemistry simulations were used to study the effects of high dose rates on the radical (e-aq, H•, and •OH) and molecular (H2 and H2O2) yields in the low linear energy transfer (LET) radiolysis of liquid water at elevated temperatures between 25–350 C. Our simulation model consisted of randomly irradiating water by single pulses of N incident protons of 300 MeV (LET ~ 0.3 keV/μm), which penetrate at the same time perpendicular to this water within the surface of a circle. The effect of dose rate was studied by varying N. Our simulations showed that, at any given temperature, the radical products decrease with increasing dose rate and, at the same time, the molecular products increase, resulting from an increase in the inter-track, radical-radical reactions. Using the kinetics of the decay of hydrated electrons at 25 and 350 C, we determined a critical time (τc) for each value of N, which corresponds to the “onset” of dose-rate effects. For our irradiation model, τc was inversely proportional to N for the two temperatures considered, with τc at 350 C being shifted by an order of magnitude to shorter times compared to its values at 25 C. Finally, the data obtained from the simulations for N = 2,000 generally agreed with the observation that during the track stage of the radiolysis, free radical yields increase, while molecular products decrease with increasing temperature from 25 to 350 C. The exceptions of e-aq and H2 to this general pattern are briefly discussed.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Linear energy transfer dependence of transient yields in water irradiated by 150 keV – 500 MeV protons in the limit of low dose rates

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 (e-aq ), •OH, H•, H2, and H2O2, for low dose rates. Overall, our simulations agree very well with the experiment. In the presence of oxygen, e-aq and H• atoms are rapidly converted into superoxide anion or hydroperoxyl radicals, with a well-defined maximum of G(HO2/O2-) 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 HO2/O2- radicals.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Characterizing the Early Acidic Response in Advanced Small Modular Reactors Cooled with High-Temperature, High-Pressure Water

Utilizing Monte Carlo multi-track chemistry simulations along with a cylindrical instantaneous pulse (Dirac) irradiation model, we assessed the initial acidic response in both subcritical and supercritical water under high radiation dose rates. This investigation spans a temperature range of 300 to 500 °C at a nominal pressure of 25 MPa, aligning with the operational conditions anticipated in proposed supercritical water (SCW)-cooled small modular reactors (SCW-SMRs). A pivotal finding from our study is the observation of a significant ‘acid spike’ effect, which shows a notable intensification in response to increasing radiation dose rates. Our results bring to light the potential risks posed by this acidity, which could potentially foster a corrosive environment and thereby increase the risk of accelerated material degradation in reactor components

Generation of ultrafast, transient, highly acidic pH spikes in the radiolysis of water at very high dose rates. Relevance for FLASH radiotherapy.

Monte Carlo multi-track chemistry simulations were carried out to study the effects of high dose rates on the transient yields of hydronium ions (H3OThe accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Early and Transient Formation of Highly Acidic pH Spikes in Water Radiolysis under the Combined Effect of High Dose Rate and High Linear Energy Transfer

(1) Background: Water radiolysis leads to the formation of hydronium ions H3O+ in less than 50 fs, resulting in the formation of transient acidic pH spikes in the irradiated water. The purpose of this study is to examine the time evolution of these spikes of acidity under irradiation conditions combining both high absorbed dose rate and high-LET radiation. (2) Methods: The early space–time history of the distributions of the various reactive species was obtained using our Monte Carlo multitrack chemistry simulation code IONLYS-IRT. To simulate different LETs, we used incident protons of varying energies as radiation sources. The “instantaneous pulse” (or Dirac) model was used to investigate the effect of dose rate. (3) Results: One major finding is that the combination of high dose rates and high LETs is clearly additive, with a very significant impact on the pH of the solution. For example, at 1 ns and for a dose rate of ~107 Gy/s, the pH drops from ~4.7 to 2.7 as the LET increases from ~0.3 to 60 keV/μm. (4) Conclusions: Confirming previous work, this purely radiation chemical study raises the question of the possible importance and role of these spikes of acidity in underpinning the physical chemistry and biology of the “FLASH effect”

Scavenging of "dry" electrons prior to hydration by azide ions: Effect on the formation of H2 in the radiolysis of water by 60Co γ-rays and tritium β-electrons.

In this study, we use Monte Carlo track chemistry simulations to show that "dry" secondary electrons, precursors of the "hydrated" electron (eThe accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author