Water radiation chemical yields

Notwithstanding Platzman s theory, most calculations of radiation-chemical yields in water and aqueous solutions were performed using the free-radical model (see Magee, 1953 Samuel and Magee, 1953 Ganguly and Magee, 1956). The hypothesis was that the recapture time of the electron would be shorter than the dielectric relaxation time. Therefore, recombination would outcompete solvation. 

In neutral water the radiation chemical yields G are 2.7 x 10 7 mol J-1 for the hydrated electron, 2.8 x ] 0 7 mol, 1 1 for the "OH radical and 6 x 10 x mol J-1 for the H atom. These values vary slightly with the solute concentration, due to increased reaction with the solute in the radiation spurs. In order to study the reaction of one radical without interference of the others, scavengers have to be added to the system. The best scavengers are those which will convert the unwanted radical to the studied one. This can be done with eaq, which can be converted to "OH or to H by the addition of N2O or H+, respectively (equations 3 and 4). 

In this chapter, the various radiation chemical yields, known as G-values, are defined as follows g(X) is the yield of the species in reactions (6) and (7) G°(X) is the yield of the initial products of water radiolysis at the end of the physicochemical stage and G(X) is an experimentally measured yield. In some publications Gx, rather than g(X), is used to represent the primary yield of the species X. The yields in reactions (6) and (7) are numerically very similar, but one must be sure of the units that are being used to express them. In the older literature, it was common practice to quote G-values without units when the units were in fact molecules (100 eV) ... 

Table 2.1. Radiation-chemical yields (G values units 10-7 mol J-1) of water radicals, ions and molecular products at a scavenger capacity of ca. 2 x 10s s-1 under the conditions of sparsely ionizing radiation (60Co-y-rays, high-energy electrons) in the presence of different saturating gases (von Sonntag 1987)...

Given radiation chemical yields expressed as G values, it is possible to calculate the concentrations of oxidative and reductive species in pure water at a known absorbed dose. The SI unit of dose is the gray (Gy), which equals an energy deposition of 1 J kg-1. For example, the concentration of OH-produced in pure, neutral water by absorbing 1 kGy is ... 

In aqueous solutions, the water is ionized and excited by the ionizing radiation, and the reactive species (e ", H and OH) (Chapter 1) formed may diffuse and react with the solute. The amount of reactive species formed by water radiolysis is proportional to the absorbed dose multiplied by their respective radiation chemical yields. 

The production of H2 in the radiolysis of water has been extensively re-examined in recent years [8], Previous studies had assumed that the main mechanism for H2 production was due to radical reactions of the hydrated electron and H atoms. Selected scavenger studies have shown that the precursor to the hydrated electron is also the precursor to H2. The majority of H2 production in the track of heavy ions is due to dissociative combination reactions between the precursor to the hydrated electron and the molecular water cation. Dissociative electron attachment reactions may also play some role in y-ray and fast electron radiolysis. The radiation chemical yield, G-value, of H2 is 0.45 molecule/100 eV at about 1 microsecond in the radiolysis of water with y-rays. This value may be different in the radiolysis of adsorbed water because of its dissociation at the surface, steric effects, or transport of energy through the interface. 

The yield of reactive. species, from water can be calculated from the energy equivalent (1 kGy is equal to 6.242 x 10 eV/g) and the radiation chemical yield G (the mean number of elementary entities produced, destroyed, or changed per 100 eV) for W uer (equal to 3). For 25 kGy the yield of reactive species from water is equal to 4.5 x 10 per g. 

This chapter begins with a brief sununary of the scheme for water radiolysis, followed by a description of the chemical systems used to obtain radiation chemical yields, or G-values, and rate constants at elevated temperatures that are pertinent to this scheme for both high and low LET, in H2O and DjO. Next, there is a section showing how the data can be accommodated in a simple spur difiusion model, and finally Arrhenius parameters for a number of reactions of more general interest are presented and discussed. 

In the CM-chitosan solutions that contained 0.02mol/L H2O2 (condition c) or 2.5 x 10" mol/L N2O (condition b), were 280% and 150% of that in condition a (Nj-saturated). In condition d, because 0.76 mol/L isopropanol was added into the CM-chitosan solution, G decreased to 5% of that in condition a. Similar to the degradation mechanism of chitosan in aqueous solution, the radiation energy of y-ray is absorbed mainly by water in dilute CM-chitosan aqueous solutions, and the direct effect of radiation on CM-chitosan can be neglected. The radiation chemical yield of reactive species released in the radiolysis of water are constant in the wide range of pH. In condition a, CM-chitosan aqueous solution was radiated with saturated N2. The active species that resulted in... 

In conditions a, e, and f, the reactive transients were the products of water radiolysis. Their radiation chemical yields may be diverse in a neutral or alkaline solution, but the difference is often neglected. Therefore, the radiation chemical yields of these transients such as H, OH, and e q" are constant in the wide range of pH. However, e q" can also convert to "H quantitatively in acid solution such as in condition f (reaction (31.22)). 

The chemical effect of radiolysis is controlled by a combination of absorbed dose and the radiation chemical yield value (G). Absorbed dose is measured in grays (1 Gy = 1 J/kg) and the G value indicates how many molecules of a species are produced per 100 ev (1 ev = 1.602 x 10J) of absorbed energy. Ershov and Gordeev (2008) provide a model for the yield of Hj, H2O2, and Oj from the radiolysis of water and aqueous solutions and Spinks and Woods (1990) give details about the connection between reaction kinetics and radiolysis processes. 

E. Edwards, D. Bartels, et al., Radiation Chemical Yields of Water in Neutron and Gamma... 

In contrast to liquid water, a detailed mechanistic understanding of the physical and chemical processes occurring in the evolution of the radiation chemical track in hydrocarbons is not available except on the most empirical level. Stochastic diffusion-kinetic calculations for low permittivity media have been limited to simple studies of cation-electron recombination in aliphatic hydrocarbons employing idealized track structures [56-58], and simplistic deterministic calculations have been used to model the radical and excited state chemistry [102]. While these calculations have been able to reproduce measured free ion yields and end product yields, respectively, the lack of a detailed mechanistic model makes it very difficult... 

These reactions are probably responsible for the low G values of hydrogen and hydrogen peroxide in liquid water. The G value in radiation chemistry refers to the chemical yield in units of molecules formed or disappeared per 100 electron volts of energy input. 

For the most part, doses quoted in this review are given in electron volts absorbed per gram of material, ev/gm. The electron volt is a suitable unit if one wishes to fix his attention on individual atomic or molecular events, and it has become generally used in radiation chemistry. The yields of radiation-chemical processes are conventionally given in molecules affected per 100 ev (symbolized by G). Thus 6r —H2O) = 4.6 means that 4.6 molecules of water are decomposed by the absorption of 100 ev of the specified radiation under the other given conditions. 

Although the radiation yield of hydrogen from irradiated polymers became no concern from the point of view of safety, it remained the object of interest from the point of view of mechanisms of radiation induced chemical reactions. In analogy to water radiolysis, the yield of hydrogen become of interest if radiations of higher LET values were used for irradiation of polymers [19], These are proton beams and alpha radiation, which can suggest different yields of hydrogen, and they really do. 

The yield of radiation chemical decomposition of water molecules is lower in ice than in liquid or gaseous state. For instance in neutral water the G-value of water decomposition is... 

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