Spurs average

When averaged over the distribution of energy loss for a low-LET radiation (e.g., a 1-MeV electron), the most probable event in liquid water radiolysis generates one ionization, two ionizations, or one ionization and excitation, whereas in water vapor it would generate either one ionization or an excitation. In liquid water, the most probable outcomes for most probable spur energy (22 eV) are one ionization and either zero (6%) or one excitation (94%) for the mean energy loss (38 eV), the most probable outcomes are two ionizations and one excitation (78%), or one ionization and three excitations (19%). Thus, it is clear that a typical spur in water radiolysis contains only a few ionizations and/or excitations. 

It is unfortunate that in many later applications of the model the spur size distribution was ignored and an average spur size was used, inconsistent with the statistics of energy loss events. 

Further refinements of the model (Ganguly and Magee, 1956) include interspur overlap in the same track. The track is seen as a string of beads. The beads are spurs, the average spacing of which corresponds to the local LET of the particle. The free path of the incident particle between spur centers is distributed exponentially with a mean value given by... 

In recent years, two different approaches, deterministic [9,19] and stochastic [10,20], have been used with a good level of success to model the radiation chemistry of water. Each approach leads to reasonable agreement between calculated results and experimental data obtained for a wide range of LET from room temperature up toca. 300°C [9,10]. There are, however, fundamental differences between the two models. The deterministic model is based on the concept of an average spur [8,9,19,23] at the end of the physicochemical stage (ca. 10 sec), which contains the products of processes (I), (II), (III), (IV), and (V) in certain yields and spatial distributions, and in thermal equilibrium with the liquid. For low LET... 

The average energy loss per unit track length of a 1-MeV electron is about 0.2 eV/nm [64]. With an average energy loss per collision event of 60 eV the mean separation of spurs is 300 nm, which is much too far apart for interspur reactions. (It is assumed throughout... 

Usually, the average concentration of a reactive species such as OH or H is not directly observed in pulse radiolysis experiments, partly because the timescale of spur reactions is so short (< 10 ns) that most spur reactions have occurred during the radiation pulse which produces these species and partly because these species are very difficult to monitor on such a timescale. Instead, solutes are often added to water prior to radiolysis and the quantity of products formed by reaction of the solute... 

In the independent pair model, the average or expectation value of N is the survival probability of the radical pairs when each is distributed as a Gaussian. It can be calculated by convolution and will not be unity even at time t = 0 because some pairs are formed with r0 pair survival probability was called IIa(f). Assuming that the distances of separation of the reactant pairs is independent of all other distances (this is not strictly true) and that only one pair reacts at any one time, Clifford et al. developed the probability that a spur initially containing N0 reactants [and hence M0 = (N0/2) (N0 — 1) pairs] and IV at time t [and hence M — (AT/2) (IV —1) pairs] does not react further for a time r and this was shown to be... 

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