Track structure

The safety valves of other cars operated, thereby releasing more LPG. At 7 33 a m., the twenty-seventh car ruptured with explosive force. Four fragments were hurled in different directions (Figure 2.21). The east end of the car dug a crater in the track structure, and was then hurled about 180 m (600 ft) eastward. The west end of the car was hurled in a southwesterly direction for a total distance of about 90 m (300 ft). This section struck and collapsed the roof of a gasoline service station. Two other sizable portions of the tank were hurled in a southwesterly direction and came to rest at points 180 m (600 ft) and 230 m (750 ft) from the tank. 

The foregoing analysis was for gas-phase water radiolysis. Similar estimates may be made for the condensed phases and for other media when the relevant yields and energetics become progressively available. On the whole, these species will gradually thermalize and become available for track reactions. Such reactions are greatly influenced by track structure, which is taken up in the following section. 

By track structure is meant the distribution of energy loss events and their geometrical dispositions. Naturally, track structure becomes rather important for second-order reactions in the condensed phase. Track structure, coupled with a reaction scheme and yields of primary species, forms the basis of radiation-chemical theory. 

It is clear from the present discussion that LET alone does not completely determine track structure. Track length (or particle velocity) is also a contributing factor. For example, at the same intermediate LET, high-energy heavy-ion tracks would be cylindrical while those for the KeV electrons would be better described as either spheroidal or as partially overlapped spurs (Samuel and Magee, 1953). 

Magee and Chatterjee (1980) give a different criterion for the size of a chemical core depending on a scavenger reaction in competition with radical recombination. Its nature, however, is extraneous to the physical track structure. 

Bartczak et al. s entire track calculation is contingent on the specific detail of track structure. They argue that branch tracks of energy <50 or 100 KeV should be treated as a single entity whereas those of higher energy could be broken into their constituent spurs and tracks. In this manner, Bartczak and Hummel (1993) found that... 

The importance of track structure, the migration of species, the role of oxygen, the study of model compounds and the use of pulse radiolysis techniques are discussed. 

The organization of this chapter is as follows. In the following section, Sec. 4.2, the elastic and inelastic interaction cross sections necessary for simulating track structure (geometry) will be discussed. In the next section, ionization and excitation phenomena and some related processes will be taken up. The concept of track structure, from historical idea to modern track simulation methods, will be considered in Sec. 4.4, and Sec. 4.5 deals with nonhomogeneous kinetics and its application to radiation chemistry. The next section (Sec. 4.7) describes some application to high temperature nuclear reactors, followed by special applications in low permittivity systems in Sec. 4.8. This chapter ends with a personal perspective. For reasons of convenience and interconnection, it is recommended that appropriate sections of this chapter be read along with Chapters 1 (Mozumder and Hatano), 2 (Mozumder), 3 (Toburen), 9 (Bass and Sanche), 12 (Buxton), 14 (LaVerne), 17 (Nikjoo), and 23 (Katsumura). 

Track structure simulation has found application in many areas of radiation research since the pioneering studies of Mozumder and Magee [35]. These studies all employ essentially the same type of approach, a collision-to-collision modeling of the trajectory of the primary radiation particle and of its daughter secondary electrons, with the most significant difference between different calculations being the interaction cross sections used to describe the... 

The simulation of a heavy ion track structure employs essentially the same methodology as described for energetic electrons except that... 

The first of these factors reduces the complexity of the simulation, but the second has entirely the opposite effect as charge cycling events affect both the energy of the primary ion and its inelastic collision cross section. While the proximity of energy loss events does not affect the details of track structure simulation (at reasonable LET), it may cause significant complications in subsequent diffusion-kinetic calculations due to the (potentially unphysi-cally) high local concentration of radiation-induced reactants. 

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. 

Figure 11 Effect of temperature on the yield of OH radicals from gamma radiolysis. Experiments (A) Kent and Sims [100], ( ) Elliot et al. [101] Calculation (solid line) IRT modeling using track structure simulation.

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... 

For a given medium such as water, its density and the velocity of the incident heavy ion contribute to the formation of the columnar track structure. The spacing of the primary energy loss events must be within the delocalization of the secondary electrons for elfects attributed to high LET to occur. For this reason, typical LET effects will not be... 

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