Isolated spurs

In liquefied rare gases (LRG) the ejected electron has a long thermalization distance, because the subexcitation electrons can only be thermalized by elastic collisions, a very inefficient process predicated by the small mass ratio of the electron to that of the rare gas atom. Thus, even at a minimum of LET (for a -1-MeV electron), the thermalization distance exceeds the interionization distance on the track, determined by the LET and the W value, by an order of magnitude or more (Mozumder, 1995). Therefore, isolated spurs are never seen in LRG, and even at the minimum LET the track model is better described with a cylindrical symmetry. This matter is of great consequence to the theoretical understanding of free-ion yields in LRG (see Sect. 9.6). 

Electron Energy Isolated Spurs Short Tracks Blobs... 

The IRT method was applied initially to the kinetics of isolated spurs. Such calculations were used to test the model and the validity of the independent pairs approximation upon which the technique is based. When applied to real radiation chemical systems, isolated spur calculations were found to predict physically unrealistic radii for the spurs, demonstrating that the concept of a distribution of isolated spurs is physically inappropriate [59]. Application of the IRT methodology to realistic electron radiation track structures has now been reported by several research groups [60-64], and the excellent agreement found between experimental data for scavenger and time-dependent yields and the predictions of IRT simulation shows that the important input parameter in determining the chemical kinetics is the initial configuration of the reactants, i.e., the use of a realistic radiation track structure. 

Many of the expected track effects discussed above are observable with this system. For instance, high-energy protons give about the same HO2 yield as fast electrons because they both deposit energy in isolated spurs. One can readily observe that LET is not a unique parameter for describing yields. 

The ionization of a molecule and the rupture of a chemical bond by ionizing radiation necessarily result in the pairwise formation of radical species. The pairwise correlation of radical species will be more or less retained in solid polymers where the radical migration is restricted. This heterogeneity of spatial distribution of radical species affects the radiation chemistry of polymers. Another source of spatial heterogeneity is the heterogeneous deposition of radiation energy [6, 7]. Low LET radiations such as y-rays produce an ensemble of isolated spurs. Each spur is composed of a few ion-pairs and/or radical... 

Radiation chemistry and Ps chemistry differ in the objects and processes they study. Being a probe of Ps chemistry, the positron delivers information about processes near and inside its terminal blob, while radiation chemistry primarily investigates the processes in isolated spurs. 

Since the optical approximation is valid for all fast electrons, and since the latter are all able to induce the entire spectrum of optical transitions, the energy distribution of spurs is the same for isolated spurs and for the spurs in blobs or short tracks. The average energy per spur is thus equal in all entities and the partition of energy between spurs, blobs, and short tracks is approximately given by the above ratios, too. This is one of the reasons why the yield gs caused by slow electrons is uniformly added to the whole area of the yield g° of spurs in Figure 1. 

Dole and co-workers have reported yields of alkyl free radicals in polyethylene irradiated at 77 K ranging from 2.7 to 3.7 (141,145,149). Furthermore, Cracco, Arvia, and Dole (49) reported that on warming, alkyl radicals decay by a first-order process, and they attributed this to reactions between alkyl radicals within isolated spurs. The persistent free radicals on warming to room temperature are the allyl radicals II. The impact of long-term stability of radical species on the stability of polyethylene has been underlined by studies of Jahan and co-workers (150-157) of ultrahigh molecular weight polymer used in medical implants. 

In diffusion controlled kinetics, two different types of reactions can take place, namely geminate recombination and bulk reactions. Geminate recombination arises in isolated spurs, before any significant diffusion has taken place and entails the reaction between isolated pairs of A and B particles. In this case it becomes meaningless to define their concentration. For geminate recombination, the survival probability (r, t) (or its complement W[r, t)), which is the probability of surviving reaction to a time t, given an initial separation r, is one of the most important physical quantities in radiation chemistry. 

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