Diffusion-kinetic Calculations Research Articles

Hydroxyl radical yields in the heavy ion radiolysis of water

The yields of hydroxyl radicals in the radiolysis of water with protons and carbon ions have been examined using experimental scavenging techniques coupled with Monte Carlo track simulations. Combined with previous results using helium ions, this set of data gives valuable information on the potential for radiation induced damage to biological systems for light ions with very different track structures. Carbon dioxide production from aerated formic acid solutions in concentrations ranging from 10−3 to 1 M was used as a probe of hydroxyl radical yields from about 6 ns to 818 ns. Numerical interpolation of the results at slightly different ion energies in combination with data from gamma radiolysis allows for a systematic analysis of both track average and track segment yields. As expected, considerable track chemistry is found to occur on the nanosecond to microsecond time scales. Monte Carlo track simulations employing stochastic diffusion-kinetic calculations of product yields are found to reproduce experimental observations satisfactorily. The track simulations are used to extract hydroxyl radical kinetics in pure water at neutral conditions.

Modelling spur chemistry for alkaline and acidic water at high temperatures

The effect of pH and associated ionic strength on the primary yields in the radiolysis of pressurised water has been assessed by diffusion-kinetic calculations for temperatures in the range 100–300°C. Account has been taken for ionic strength I up to 0.1 mol kg−1, assuming that the counter ions of H+ in acid solutions and of OH− in base solutions have unit charge. In acid solutions, the H+ ions react with e−aq. The decrease in G(e−aq) and the increase in G(H) with decreasing pH becomes substantial for [H+] ≥ 1 × 10−4 m, but the primary yields of oxidising species are almost constant. In alkaline solutions, the OH− anions affect the spur chemistry of radiation-generated protons and hydroxyl radicals for [OH−] ≥ 1 × 10−4 m. The scavenging of H atoms and hydrogen peroxide becomes significant for [OH−] ≥ 1 × 10−2 m. The total yields G(OH) + G(O−) and G(H2O2) + G(HO2−) are independent of base concentration below 0.01 m. In more alkaline solutions, G(OH) + G(O−) increases, whereas G(H2O2) + G(HO2−) decreases with increasing [OH−]. Calculations showed the substantial yield of the reaction O− + e−aq in 0.1 m base solution. Spur chemistry in alkaline hydrogenated water is not affected by the presence of H2 if less than 0.001 m of hydrogen is added.

Hydrated Electron Yields in the Heavy Ion Radiolysis of Water

Experimental measurements coupled with Monte Carlo track simulations have been used to examine the yields of hydrated electrons in the radiolysis of water with protons, helium ions, and carbon ions. Glycylglycine, in concentrations ranging from 10(-4) to 1 M, was employed as a scavenger and the production of the ammonium cation used as a probe of hydrated electron yields from about 2 ns to 20 mus. Monte Carlo track simulations employing diffusion-kinetic calculations of product yields are found to reproduce experimental observations satisfactorily. Model details are used to elucidate the heavy ion track physics and chemistry. Comparison of the heavy ion results with those found in gamma radiolysis shows intratrack reactions are significant on the nanosecond to microsecond time scale as the ion track relaxes, and that a constant (escape) yield is never attained on this time scale. Numerical interpolation techniques are used to obtain both track average and track segment yields for use in practical applications or comparison with other models. The model results give the first hints that initial ( approximately 5 ps) hydrated electron yields, and possibly other water decomposition products, are dependent on the type and energy of the incident radiation.

Isotopic effect in the radiolysis of water. Diffusion-kinetic modelling up to 300°C

Diffusion-kinetic calculations [1-3] have been analysed to determine the isotopic effect in the radiolysis of water with ionising radiation of linear energy transfer characteristics (LET) from 0.2 to 60 eV/nm and at temperatures up to 300°C. This analysis shows that, for low LET radiation, the spur decay of e-aq is slower in D2O and results in a higher yield of e-aq, g(e-aq), at 10-7 -10-6s after the ionisation event. In low LET radiolysis, g(OD) ≈ g(OH) over the whole range of temperature but in high LET radiolysis g(OD) is clearly lower than g(OH). The isotopic effect on the yields of the radical products is enhanced by increasing LET but diminished by increasing temperature. The yields of the molecular products show the opposite isotopic effect to their radical precursors, namely g(D2) is 10-20% lower than g(H2) and g(D2O2) > g(H2O2). A particularly significant difference between g(D2O2) and g(H2O2) has been found at LET = 20 eV/nm. The isotopic dependence of the g-values estimated for fast neutron radiolysis is also presented.

On the Radiation Chemical Kinetics of the Precursor to the Hydrated Electron

Comparisons of the predictions of stochastic diffusion kinetic calculations using electron track structures with experimental scavenger data show that the precursor to the hydrated electron, epre-, plays a role in determining the total yield of electrons scavenged in a number of systems. The significance of epre- scavenging on yields depends on the scavenger of interest and on the rate coefficient of the scavenger's reaction with the hydrated electron, eaq-. For Cd2+, where the reaction (eaq- + Cd2+) is fast, the consequences of epre- scavenging are experimentally not apparent, for NO3-, a less efficient eaq- scavenger, the effects of epre- reaction are apparent in concentrated solutions, and for SeO42-, an inefficient eaq- scavenger, epre- scavenging is obvious even in dilute solution.

Diffusion-Kinetic Theories for LET Effects on the Radiolysis of Water

Diffusion-kinetic methods are used to investigate the effects of incident particle linear energy transfer (LET) on the radiolysis of water and aqueous solutions. Chemically realistic deterministic diffusion-kinetic calculations examining the scavenging capacity dependences of the scavenged yield of e[sub aq][sup [minus]] and of OH demonstrate that the scavenged yields are related to the underlying time-dependent kinetics in the absence of the scavenger by a simple Laplace transform relationship. This relationship is also shown to link the effect of an e[sub eq][sup [minus]] scavenger on the formation of H[sub 2] with the time dependence of H[sub 2] production in the absence of the scavenger. The simple Laplace relationship does not work well when applied to H[sub 2]O[sub 2] formation in high-LET particle tracks even though such a relationship is valid with low-LET particles. It is found that while the secondary reaction of H[sub 2]O[sub 2] with e[sub aq][sup [minus]] can be neglected in low-LET particle radiolysis, it is of considerable significance in the tracks produced by high-LET particles. The increased importance of this reaction with increasing LET is the major reason for the failure of the Laplace relationship for H[sub 2]O[sub 2]. 55 refs., 9 figs., 2 tabs.

Diffusion-kinetic calculations of the effect of nitrous oxide on the yields of ionic species in the radiation chemistry of water

Deterministic diffusion-kinetic calculations show that the yields of the cationic and the anionic species, H 3 O + and e aq - +OH - , produced in the fast electron radiolysis of neutral water are significantly reduced by the addition of nitrous oxide as an electron scavenger. This reduction is demonstrated to be a direct consequence of the conversion of the hydrated electron to the hydroxide anion which undergoes more rapid intraspur reactions with H 3 O + than does e aq -

Radiation chemical diffusion kinetic calculations with prescribed and non-prescribed diffusion—I: Spherical and cylindrical cases

Comparisons are made of species concentration profiles, and G values, both as functions of time, as calculated by the Schwarz modified prescribed diffusion method (1) and by the FACSIMILE numerical method. (2) This is done for spherically symmetrical (spur) cases and cylindrical (track) cases. It is shown that when the initial spatial distribution of the aquated electron is wider than that of the hydroxyl radical and the hydrogen ion, a depression is formed at early times at the centre of the electron concentration profile. The prescribed diffusion method cannot simulate this central minimum concentration and, therefore, tends to overestimate the amount of early inter-species reaction between e - aq and OH, and between e - aq and H +. This results in a faster predicted decay of G(e - aq) when prescribed diffusion is used. The FACSIMILE method is also used to calculate the effect of an initial distribution with a central minimum in the e - aq concentration profile. Comparisons are made with experiment and with other recent modelling calculations.

Self-trapping of Radiation Produced Electrons

THE problem treated here is the trapping of free electrons produced by ionizing radiation in dielectric systems. Detailed theoretical descriptions of the equilibrium properties of the solvated electron are available1–3, but its formation is still under discussion. Electrons are formed and thermalized in the coulomb field of the positive ions, but not fully recaptured by the ions. Magee4 considered the auto-ionization of the H3O radicals as a possible secondary source of electrons; Freeman5–7 assumed the electrons to be trapped in the defect sites of the medium at the instant of thermalization, while we thought8,9 that in polar dielectrics the escape of the electrons from the ions can be a free-energy decreasing, and therefore spontaneous, process. In our approach, the electrons which escape are treated as quasi-free, and the question of the kinetics and mechanism of electron trapping arises. Diffusion kinetic calculations on the radiolysis of water10,11 suggest very high initial hydrated electron densities, which require fast trapping processes.