Spur processes

The numbers in reaction (6) are the so-called primary yields (expressed here in units of 10 mol J ) of the radical and molecular products that are present when the spur processes are complete at ca. 10 sec. The original units for these yields, which are still in use, are molecules (100 eV) and the relationship between the two sets is 1 mol J =9.65x10 molecules (100 eV) Thus, reaction (6) may also be written as reaction (7) ... 

Shortly after the discovery of the hydrated electron. Hart and Boag [7] developed the method of pulse radiolysis, which enabled them to make the first direct observation of this species by optical spectroscopy. In the 1960s, pulse radiolysis facilities became quite widely available and attention was focussed on the measurement of the rate constants of reactions that were expected to take place in the spurs. Armed with this information, Schwarz [8] reported in 1969 the first detailed spur-diffusion model for water to make the link between the yields of the products in reaction (7) at ca. 10 sec and those present initially in the spurs at ca. 10 sec. This time scale was then only partially accessible experimentally, down to ca. 10 ° sec, by using high concentrations of scavengers (up to ca. 1 mol dm ) to capture the radicals in the spurs. From then on, advancements were made in the time resolution of pulse radiolysis equipment from microseconds (10 sec) to picoseconds (10 sec), which permitted spur processes to be measured by direct observation. Simultaneously, the increase in computational power has enabled more sophisticated models of the radiation chemistry of water to be developed and tested against the experimental data. 

Although reaction (30) is too slow to interfere significantly with spur processes up to pH 14, it may compete with the reaction of H with other solutes in the bulk solution under these conditions and thus increase... 

Swiatla-Wojcik D, Buxton GV (1995) Modeling of radiation spur processes in water at temperatures up to 300°C. J Phys Chem 99 11464-11471... 

The work of Jacobsen (1984, 1986) and Mogensen (1982), described in subsection 4.8.1 above, pointed to the potential importance of positronium formation as a consequence of spur processes in dense molecular species. Table 4.1 drew attention to the fact that the positronium fractions for many molecular gases have been found to be both density and temperature dependent. We will not attempt a detailed compilation of these data here, but examples of the density and temperature variations observed are shown in Figure 4.31 for SF6 and CO2 gases. The lines are fits to equation (4.41), and the reader is referred to the work of Jacobsen (1986) for a full discussion of the fitting procedures and assumptions. The fact that a reasonable fit to the data can be produced is strong supporting evidence for positronium formation in spurs. 

In spite of the wealth of data accumulated in the past decades, Ps formation in nonpolar solvents is less understood than in polar solvents. A reason for this is that data from pulse radiolysis that could be used for comparison are less numerous than for polar solvents. Globally, the spur processes described above are also present the main differences between the polar and nonpolar solvents arise from the absence of solvation in the latter case. 

Three levels of quantification of the spur processes are possible, from the... 

Eq.(7) thus identifies with eq. (1), with k = aVn/(a [ + am + aiV). All empirical equations given before can be obtained in this way and extension to more complex cases (e.g., successive and competing spur reactions) is easily done (19). Developments of this approach are possible, such as by taking into account the statistical distribution of the solute molecules in the spurs. Although their physical grounds are of course questionable, the equations derived provide a quantitative approach to the data and the parameters (k, K, f, etc) prove to be very useful for comparisons (between solutes, as a function of temperature, of solvent to correlate with data from other fields, etc). The usefulness of such simple treatments may well reflect a genuinely simple situation for the actors of the spur processes where probabilistic factors may have a more important impact than detailed dynamics. 

The spur processes described for the liquid state should apply to solids. As compared to the scheme of reactions (I)—(IX), solvation is to be excluded. Localization in the sense that the particle would preserve some non negligible mobility as such, is also to be discarded in most cases. 

The theoretical base of the spur process is Onsager s theory of the geminate pair recombination. Contrary to this, the blob model is most appropriate for consideration of early radiation-chemical processes in multiparticle track entities, such as blobs and ionization columns. The main distinction between the spur and blob comes from the large difference in the initial number of ion-electron pairs they contain. 

The above discussion stresses the key role of solvent polarity and structure in determining the subsequent behaviour of the ionic species generated in the primary processes. Thus, in water with its high dielectric constant the bulk of the e and escape the Coulombic field and the spur processes depend only on their random diffusion. 

Electrons are stabilized by the surrounding water molecules to form hydrated electrons in less than 1 ps. The yields at this stage are defined as initial yields. The resulting transient species such as e, H, OH are distributed locally along the track where the energy deposits, called spur. The spatial distribution gready depends on the LET of the incident beam. Then these products diffuse randomly and either react together or escape into the bulk solution. After the completion of spur processes, which take place within s, the products... 

The simplest model to interpret our results is based on the assumption that the effects of nitrate ion are caused by spur scavenging reactions. We propose that two spur processes are necessary to denote the formation of earliest detectable intermediates in the radiolysis of concentrated sodium nitrate solutions in 0.8N sulfuric acid ... 

According to the model, ionisation and excitation events and the resulting products Caq , H, H2, OH, H2O2 and occur in clusters called spurs. For low LET radiation the spurs are separated by large distances relative to their diameter for high LET radiation they overlap to form a continuous cylinder. The decomposition products then diffuse randomly and either react together or escape into the bulk solution. It is the competition between reaction and escape which determines the yields of the radical and molecular species extant when the spur processes are complete. The time for this completion is generally taken to be 10 -10" s and the yields at this time are known as the primary yields. The spur reactions are listed in Table 1. 

The next possible way of Ps formation is the recombination of e with one of the excess electrons produced in the short track (Ps formation by the spur process). 

This process is quite akin to geminate recombination in radiation chemistry and indeed many experimental results show excellent parallelism between the data of Ps formation and radiation chemistry (7,8). The main part of Ps we observe in polymers appears to come from reaction (3), but if the epithermal Ps via reaction (1) survive the oxidation reaction (2) it will also show up. The energetics of Ps formation via the spur process are given by. 

These reactions partly occur in the spur and there is a strong competition between spnr reactions and diffusion out of the spur. This process, the so-called spur expansion, is finished in about 10 s. Scavengers being present in 10 -10 mol dm concentration react only with those intermediates that escape spur reactions. The yields of the eaq, OH, and H intermediates in neutral and slightly alkaline solutions are usually taken as 0.28, 0.28, and 0.057 pmol J . In order to efficiently interfere with the spur processes scavenger concentrations in the mol dm range should be applied. 

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