Track fast electron

Such a presentation of a fast electron track as a set of spurs, blobs, and short tracks is widely used in radiation chemistry for describing the processes that occur in a condensed medium exposed to electron or gamma radiation.7 However, this presentation is not the only one there is. Other possible approaches are discussed in Ref. 305, where, in particular, the authors note that the most general description of track structures is the one using correlation functions. 

Zaider and Brenner (1984) have developed computer code for fast chemical reactions on electron tracks Zaider et ah (1983) have performed MC simulation of... 

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 discussion in the previous section suggests that the track of a heavy ion becomes more like that of a fast electron with increasing velocity. Therefore one expects that in the high velocity limit the yields of water products with heavy ions are the same as with fast electrons or y-rays. The yields for the major products of water radiolysis in fast electron or y-radiolysis are given in Table 1. These values were taken from a number of different sources in conjunction with the results predicted by model calculations [73,116,119-123]. Material balance shows that almost four molecules of water are decomposed for every 100 eV of energy absorbed by fast electrons or y-rays. Because only about six water molecules are initially decomposed, most of the water products escape intraspur reactions in fast electron or y-radiolysis. 

From the preceding discussion it may be inferred that in frozen aqueous solutions there are basically two different mechanisms by which H atoms can be formed (1) by reaction of the electron with acid anions according to Equation 4, and (2) by direct radiolysis of water molecules according to Equation 1. Earlier investigations of aqueous solutions at room temperature have also lead to the same conclusion (70). Although there are no experimental data from either the room temperature studies or from those on the frozen solutions, as to the actual nature of the second process, it is believed (70) that it is the dissociation of excited water molecules formed in the tracks of the fast electrons. 

Papers328,329 by Polyansky and one of the authors contain calculations of the spectral and energy yields of VCR in tracks of electrons with energies attained in accelerators used in radiation chemistry. It is emphasized that the VCR can induce photoradiation processes in tracks of fast electrons, that is, the inner photoradiation effect. 

As in the case of fast electrons, the main contribution to energy losses of heavy charged particles (ions) comes from ionization losses (see Section V), which are responsible for most of the radiation-chemical effects. Therefore, we consider here only the structure of that part of an ion s track where the ionization losses are dominating. The role of elastic interaction between ions and atoms of the medium, which becomes essential only at the end of the ion s track, is not considered in this section. 

At high velocities the track of an ion can be considered as consisting of separate track structures similar to those of a fast electron. However, the ions used in radiation chemistry usually have initial energies not exceeding 10 MeV per nucleon. At such energies, even in the case of protons,... 

The radiation effect produced by heavy ions can be simulated with electrons if the ions have high velocities. It is in this case that the spatial distribution of active particles in the track is close to the one in tracks of fast electrons. At small velocities of heavy ions the tracks of delta electrons overlap each other to a considerable extent, which results in the concentration of charged particles in a microvolume of the ion s track being very high. With development of powerful pulsed electron accelerators it became possible to create high concentrations of active particles in a medium. According to Ref. 372, with such accelerators one is able to reproduce and study the processes occurring in tracks of heavy ions. 

In this chapter we will briefly discuss mechanisms of the positron slowing down, the spatial structure of the end part of the fast positron track, and Ps formation in a liquid phase. Our discussion of the energetics of Ps formation will lead us to conclude that (1) the Ore mechanism is inefficient in the condensed phase, and (2) intratrack electrons created in ionization acts are precursors of Ps. This model, known as the recombination mechanism of Ps formation, is formulated in the framework of the blob model. Finally, as a particular example we consider Ps formation in aqueous solutions containing different types of scavengers. 

Particle Tracks. Thus, irrespective of the particulate or photon nature of the primary radiation, the net effect is the formation of tracks consisting of ionized and excited molecules. These tracks, and their detailed structure can be revealed by the Cloud Chamber invented by Wilson in 1911(11). For fast electrons (low LET) the tracks mainly consist of spherical regions called spurs which contain from one to four ion-pairs which are separated in condensed phases by about 10 A. For more highly ionizing particles such as a-particles the tracks are essentially cylindrical columns of ionized and excited molecules. 

Electrons tracks are less dense than the tracks of heavy charged particles and the spurs are more widely spaced (Chapter 4). The induced physical processes are comparable in many regards to the effect of exposure to heavy particles or ions. X- and gamma rays actually are indirect methods of producing fast electrons in matter. As a consequence, chemical effects can be considered very similar in nature. Some specific features may nevertheless arise from the significantly different dose rates i.e. amount of absorbed energy per unit time) as a consequence of different LET values. 

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