Subexcitation Electrons Research Articles

Track structure simulations of proximity functions in liquid water using the Geant4-DNA toolkit

The mechanistic Monte Carlo modeling of biological effects of ionising radiation at sub-cellular and DNA scale requires the accurate simulation of track structures in the biological medium, commonly approximated as liquid water. The formalism of microdosimetry allows one to describe quantitatively the spatial distribution of energy deposition in the irradiated medium, which is known to relate to the deleterious effects in the irradiated cellular targets. The Geant4-DNA extension of the Geant4 open-source and general-purpose Monte Carlo simulation toolkit has been recently evaluated for the simulation of microdosimetry spectra, allowing, in particular, the calculation of lineal energy distributions. In this work, we extend the microdosimetric functionalities of Geant4-DNA by the development of a new Geant4-DNA example dedicated to the simulation of differential proximity functions. Simulation results are presented for the proximity function of electrons, protons, and alpha particles over a wide energy range using the different physical models of electron interactions available in Geant4-DNA. The influence of sub-excitation processes and electron tracking cut is discussed. Results are compared to literature data when available. As an example, a simple calculation of the relative biological effectiveness (RBE) in the context of the Theory of Dual Radiation Action using the present proximity functions yields up to a factor of 2 variation of the electron RBE in the energy range from 100 eV to 100 keV.

Methods for the Simulation of the Slowing of Low-Energy Electrons in Water.

A computational Monte Carlo simulation approach for modeling the thermalization of low-energy electrons is presented. The simulation methods rely on, and use, experimentally based cross sections for elastic and inelastic collisions. To demonstrate the different simulation options, average numbers of interactions and the range of low-energy electrons with initial energies ranging from 1 to 20 eV are calculated for density normalized gaseous water. Experimental gas-phase cross sections for (subexcitation) electrons of energies in the range of 1-20 eV were taken from the compilation of Hayashi. The ballistic collision-by-collision simulations provide information on the intricacies of the thermalization processes not available experimentally. © 2018 Wiley Periodicals, Inc.

Theoretical study of Jesse effect in tritium measurements using ionization chambers

Jesse effect caused by impurities in helium might enhance the output signal significantly in tritium measurements with ionization chamber, which will lead to overestimation of tritium concentration in experiments. A theoretical method was proposed to evaluate Jesse effect quantitatively. Results indicate that besides Penning ionization, sub-excitation electrons also place very important influence on ionization enhancement by Jesse effect. An experiential expression about the relationship between enhancement factor and impurity concentration was established, in which second order of it fits experimental results very well. Theoretical calculation method in this paper is also applicable to evaluate Jesse effect in other kinds of mixtures besides hydrogen as impurities in helium. In addition, Jesse effects about tritium molecules as impurities have also been investigated.

Processes and time-scales of energy loss of low-energy electrons (

The various processes responsible for the energy degradation and eventual thermalization of low-energy subexcitation electrons (<5eV) in liquid water are completely delineated for the first time, while previous work assumed the formation of thermalized electrons in the 10−11–10−12s for liquid water at room temperature. Chief among these are intramolecular vibrational and rotational excitation, dielectric interaction and excitation of intermolecular vibration through H-bond stretching and bending. Cross-sections for these processes are taken from experiments or derived theoretically. Individual time scales are obtained for subexcitational and subvibrational regimes in water at room and higher temperature.

Time-dependent yield of the hydrated electron in subcritical and supercritical water studied by ultrafast pulse radiolysis and Monte-Carlo simulation

Fast kinetics and time-dependent yields of the hydrated electron (e(-)(aq)) in pure water under conditions of high temperature and pressure up to the supercritical region were investigated by picosecond and nanosecond pulse radiolysis experiments. More significant decays at short times followed by plateau components at longer times were observed with increasing temperature, suggesting faster spur reaction processes. In supercritical water, it was also found that the e(-)(aq) yields strongly depend on the pressure (density). Comparison of these measurements with Monte-Carlo computer simulations allowed us to identify spur reactions of e(-)(aq) that occur predominantly at high temperatures and also to provide new key information on certain spur model parameters. In particular, the experimental time-dependent e(-)(aq) yields were best reproduced if the electron thermalization distance decreases with increasing temperature. This "shrinkage" of spur sizes at high temperatures was attributed to an increase in the scattering cross sections of subexcitation electrons, likely originating from a decrease in the degree of structural order of water molecules as the temperature is increased.

Low energy (0–10 eV) electron driven reactions in the halogenated organic acids CCl3COOH, CClF2COOH, and CF3CHNH2COOH (trifluoroalanine)

Negative ion formation following resonant electron attachment to the three title molecules is studied by means of a beam experiment with mass spectrometric detection of the anions. All three molecules exhibit a pronounced resonance in the energy range around 1 eV which decomposes by the loss of a neutral hydrogen atom thereby generating the closed shell anion (M-H)(-) (or RCOO(-)), a reaction which is also a common feature in the non-substituted organic acids. The two chlorine containing molecules CCl(3)COOH and CClF(2)COOH exhibit an additional strong and narrow resonance at very low energy (close to 0 eV) which decomposes by the cleavage of the C-Cl bond with the excess charge finally localised on either of the two fragments Cl(-) and (M-Cl)(-). This reaction is by two to three orders of magnitude more effective than hydrogen loss. Apart from these direct bond cleavages (C-Cl, O-H) resonant attachment of subexcitation electrons trigger additional remarkably complex unimolecular decompositions leading, e.g., to the formation of the bihalide ions ClHCl(-) and ClHF(-) from CCl(3)COOH and CClF(2)COOH, respectively, or the loss of a neutral CF(2) unit from trifluoroalanine thereby generating the fluoroglycine radical anion. These reactions require substantial rearrangement in the transitory negative ion, i.e., the cleavage of different bonds and formation of new bonds. F(-) from both chlorodifluoroacetic acid and trifluoroalanine is formed at comparatively low intensity (more than three orders of magnitude less than Cl(-) from the chlorine containing molecules) and predominantly within a broad resonant feature around 7-8 eV characterised as core excited resonance.

Time- and space-resolved Monte Carlo study of water radiolysis for photon, electron and ion irradiation

Time-dependent yields of the most important products of water radiolysis E(aq)(-), (*)OH, H(*), H(3)O(+), H(2), OH(-) and H(2)O(2) have been calculated for (60)Co-photons, electrons, protons, helium- and carbon-ions incident onto water. G values have been evaluated for the interval from 1 ps to 1 mus after initial energy deposition as a function of time, as well as after 1 ns and at the end of the chemical stage as a function of linear energy transfer (LET), covering an interval from approximately 0.2 up to 750 keV/microm by means of different particle types. In this work, the modules of the biophysical Monte Carlo track structure code PARTRAC dealing with the simulation of prechemical and chemical stages have been improved to extend interaction data sets for heavier ions. Among other newly selected parameter values, the thermalisation distance between the point of generation and hydration of subexcitation electrons has been adopted from recent data in the literature. As far as data from the literature are available, good agreement has been found with the calculated time- and LET-dependent yields in this work, supporting the selection of the revised parameter values.

Production of low-energy electrons by ionizing radiation

Low-energy electrons are involved in almost all of the physical, physico-chemical and chemical phenomena underlying radiation chemistry and play a central role in determining the effects of ionizing radiation chemistry. A detail quantitative description of their production is desirable, but not available in even the simplest liquids. The distribution defining the cascade of low-energy electrons produced in the radiolysis of liquid water is characterized. The most probable energy and mean energy for secondary electron produced by a primary ion are ∼ 9 – 10 and 50–60 eV, respectively, depending on the specific energy of the primary ion. The energy spectrum of the sub-excitation electrons produced by attenuation of all electrons to < 25 eV is determined, and its mean is also 9 eV.

Reactions in nanofilms of trifluoroacetic acid (CF 3COOH) driven by low energy electrons

Electron stimulated desorption (ESD) of fragment anions and thermal desorption spectroscopy (TDS) is applied to study reactions in thin films of a few CF 3COOH/CF 3COOD monolayers following low energy electron impact (0–20 eV). In ESD, the fragment ions H −/D − and F − are observed via resonant features in the energy range 5–12 eV similar to the dissociative electron attachment (DEA) resonances present in gas phase CF 3COOH. Extended electron irradiation of the film at energies above 9 eV generates CO 2 and CHF 3/CF 4 as revealed by a combination of TDS and ESD experiments. The yield for these reactions continuously increases with energy indicating that they are initiated via electronically excited neutrals rather than negative ion resonances. No reactions induced by sub-excitation electrons (≈1 eV) could be observed using the present experimental techniques. In a very recent study on electron attachment to clusters of CF 3COOH [J. Langer, I. Martin, G. Karwasz, E. Illenberger, Int. J. Mass Spectrom. Ion Processes 249/250 (2006), 477], the ion–molecule complex CF 3COO −·H 2O appeared as strongest signal via a resonance located at 0.4 eV.

Monte Carlo Simulation of Water Radiolysis for Low-energy Charged Particles

The paper describes the development of chemical modules simulating the prechemical and chemical stages of charged particle tracks in pure liquid water. These calculations are based on our physical track structure codes for electrons and ions (KURBUC, LEPHIST and LEAHIST) which provide the initial spatial distribution of H2O+, H2O* and subexcitation electrons at approximately 10(-15) s. We considered 11 species and 26 chemical reactions. A step-by-step Monte Carlo approach was adopted for the chemical stage between 10(-12) s and 10(-6) s. The chemistry codes enabled to simulate the non-homogeneous chemistry that pertains to electron, proton and alpha-particle tracks of various linear energy transfers (LET). Time-dependent yields of chemical species produced by electrons and ions of different energies were calculated. The calculated primary yields (G values at 10(-6) s) of 2.80 for OH and 2.59 for e(aq)- for 1 MeV electrons are in good agreement with the published values. The calculated G values at 10(-6) s for a wide range LETs from of 0.2 to 235 keV microm(-1) were obtained. The calculations show the LET dependence for OH and H2O2. The electron penetration ranges were calculated in order to discuss the role of low energy electrons.

Fragmentation of propanoic acid by subexcitation electrons

Low energy electron attachment to propanoic acid (CH 3CH 2COOH) leads to a variety of negative fragment ions formed by dissociative electron attachment. The most intense reactions are operative at subexcitation energies (<3 eV) at cross-sections on the 10 −20 m 2 scale. They are due to dehydrogenation from the O site (direct cleavage of the O–H bond), but also more complicated reactions associated with hydrogen transfer and substantial geometric and electronic rearrangement. The present results demonstrate the potential of slow electrons to efficiently decompose organic acids.

Thymine excision from DNA by subexcitation electrons

We show the ability of slow electrons to effectively decompose gas phase thymidine into the sugar and thyminyl moiety, at comparable intensities. The reaction proceeds via dissociative electron attachment at energies below the threshold for electronic excitation (<3 eV), with the excess charge localized on the respective fragments. Within nucleic acid, this reaction may excise thymine from the DNA strands. Since ballistic low energy electrons are the predominant radiolytic species, the present findings represent an essential reaction in the understanding of the initial events in radiation damage of DNA.

Complete chemical transformation of a molecular film by subexcitation electrons (

The potential of slow electrons to act as a soft tool to control a chemical reaction in the condensed phase is demonstrated. By setting the energy of a well defined electron beam to values below 3 eV, the surface of a thin film of 1,2-C(2)F(4)C(l2) molecules can completely be transformed into molecular chlorine (and by-products, possibly perfluorinated polymers). At higher energies (>6 eV) some equilibrium state between product and educt composition can be achieved, however, accompanied by a gradual overall degradation of the film. The effect of complete transformation is based on both the selectivity and particular energy dependence of the initial step of the reaction which is dissociative electron attachment to C(2)F(4)C(l2), but also the fact that the initial molecule is efficiently decomposed by subexcitation electrons while the product C(l2) is virtually unaffected.

Low-energy electron penetration range in liquid water.

Monte Carlo simulations of electron tracks in liquid water are performed to calculate the energy dependence of the electron penetration range at initial electron energies between 0.2 eV and 150 keV, including the subexcitation electron region (<7.3 eV). Our calculated electron penetration distances are compared with available experimental data and earlier calculations as well as with the results of simulations using newly reported amorphous ice electron scattering cross sections in the range approximately 1-100 eV.

Radiolysis of liquid water: An attempt to reconcile Monte-Carlo calculations with new experimental hydrated electron yield data at early times

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.

Modification of an NF 3 film by sub-excitation electrons

By applying the IRAS technique (infrared-absorption-reflection spectroscopy) and electron stimulated desorption (ESD) of negative ions we demonstrate that a 10 monolayer (ML) film of NF 3 is degraded in the course of low-energy electron irradiation in the range 0–5 eV which is far below the electronic excitation of NF 3. Degradation is accompanied by the desorption of F − fragment ions from the film and formation of NF 2 radicals and N 2F 4 molecules in the film. The energy dependence of the degradation cross-section follows that for resonant (dissociative) electron attachment in the low-energy region (≈0–5 eV) and increases above 6 eV. We therefore identify (dissociative) electron capture at low energy as the only initial reaction responsible for the chemical changes in the NF 3 film.

Radiolysis of Liquid Water at Temperatures up to 300 °C: A Monte Carlo Simulation Study

Monte Carlo simulations were performed to calculate the temperature dependence of the primary yields (g-values) of the radical and molecular products of the radiolysis of pure, deaerated liquid water by low linear-energy-transfer (LET) radiation. The early energy deposition was approximated by considering short segments (∼100 μm) of 300-MeV proton tracks (corresponding to an average LET of ∼0.3 keV/μm). The subsequent nonhomogeneous chemical evolution of the reactive species formed in these tracks was simulated by using the independent reaction times approximation, which has previously been used successfully to model the radiolysis of liquid water at ambient temperature under various conditions. Our calculated g-values for the radiolytic species: , OH, H, H2, and H2O2, are presented as a function of temperature over the range 25−300 °C. They show an increase in g( ), g(OH), and [g(H) + g(H2)] and a decrease in g(H2O2) with increasing temperature, in agreement with existing experimental data. The sensitivity of the results to the values of reaction rate constants and to the temperature dependence of electron thermalization distances (rth) was also investigated. It was found that the best agreement with experiment occurs when the distances of electron thermalization decrease with increasing temperature, a result that is at variance with the predictions of previous modeling studies. Such a decrease in rth as the temperature increases could be linked to an increase in the scattering cross sections of subexcitation electrons that would account for the corresponding decrease in the degree of structural order of water molecules. Our simulations also suggest that the variations of the g-values with temperature, and especially that of g(H2), are better described if we account for the screening of the Coulomb forces between the two in the bimolecular self-reaction of the hydrated electron. Finally, the time-dependent yields of and OH are presented as functions of temperature, in the range 10-12−10-6 s. It was found that the temporal variation of g( ) at elevated temperatures is sensitive to the temperature dependence of rth, suggesting that measurements of the decay of hydrated electrons as functions of time and temperature could, in turn, provide information on the thermalization of subexcitation electrons. The good overall accord of our calculated results with the experimental data available from the literature demonstrates that Monte Carlo simulation methods offer a most promising avenue at present to further develop our understanding of temperature effects in the radiolysis of liquid water.

Computer-Aided Stochastic Modeling of the Radiolysis of Liquid Water

A computer-aided stochastic model of the radiolysis of liquid water has been developed. It is based on Monte Carlo simulation of charged-particle tracks, a random-flight method to simulate diffusion of species and the Debye-Smoluchowski theory of reactions between radicals. The model takes into account the formation of ionizations, excitations and subexcitation electrons at the physical stage. The corresponding initial yields at approximately 1 fs were found to be G(ion) = 4.37, G(exc) = 2.81 and G(e-sub) = 4.38. The energy spectrum of subexcitation electrons has been calculated. Autoionizations and dissociations of excited molecules, hole migration, electron thermalization, geminate recombination and the cage effect are considered at the physico-chemical stage. The mean thermalization distance of subexcitation electrons is 24.5 nm. The initial yields of e(aq)-, H+, OH, H, H2 and O are 4.89, 4.86, 5.96, 1.10, 0.15 and 0.15, respectively. The reactions between radicals and products as well as their diffusion are simulated at the chemical stage. The decay kinetics of the most important radicals is reported together with the time evolution of the most important reactions. The yields of the reactive radicals, e(aq)-, H+, OH, H and OH-, at 1 ps are 4.84, 4.85, 5.87, 1.09 and 0.0, respectively. The respective steady-state yields at 10 micros are 2.70, 3.58, 2.89, 1.17 and 0.79. The yields of molecular products, H2O2 and H2, are 0.73 and 0.47 at 10 micros. The concentration-dependent yields of e(aq)-, OH and H2O2 are calculated in three different aqueous solutions. The predictions of the model agree fairly well with experimental data.

Discussion

Monte Carlo simulation techniques have been used to model the sequence of events that are generated by the interaction of ionising radiations with pure liquid water. Those events include (i) the energy depositions that occur through the ionisation and the excitation of water molecules, and (ii) the relaxation pathways and the ultrafast reactions of the subexcitation electrons, of the transient water anions and cations, and of the excited water molecules. The subsequent homogeneous and nonhomogeneous chemical reactions that take place along the radiation tracks when the radiolytic species diffuse into the medium are treated in the following paper (Frongillo et al., 1998a). The principles and the physical laws on which our simulation codes are based are explained in some detail to identify their strengths and weaknesses and to point out the main approximations that are made. The incident particles that are considered are electrons with energies up to 150 keV and protons of less than 300 MeV. The specificity of the liquid phase for the interaction of those particles with water is accounted for in several ways, through appropriate choices of cross sections as well as ionisation and excitation potentials. Our simulation codes, called TRACPRO and TRACELE, are used to provide information on intermediate steps of the action of the radiation which are not readily observable. For example, the yields of the initial radiolytic species on the femtosecond time scale are determined and the relative contributions of their various formation channels are given. The energy distribution of the secondary electrons is also shown.

Stochastic Simulation of the Electron Radiolysis of Water and Aqueous Solutions

Stochastic modeling of the radiolysis of water and of aqueous solutions employing simulated track structures and the independent reaction times methodology is used to investigate the physical and chemical processes underlying observed radiation chemical kinetics. The calculations accurately reproduce both the time dependent yields of eaq- and the scavenging capacity dependence of the (scavenged) yields of eaq-, OH, H2, and H2O2 measured experimentally. The local spatial distribution of eaq- is described by a Gaussian of standard deviation 4.0 nm. This distribution reflects the “thermalization” of the subexcitation electron. The value matches recent experimental estimates but is somewhat wider than predicted in earlier (deterministic) studies. The Gaussian distribution used for H3O+, OH, H, and O has a standard deviation of 0.75 nm, which is of the same order obtained previously using deterministic methods. This distribution is due to the distance traveled between electronic collisions of low-energy (<25 e...