Track of an ionizing particle

The Optical Approximation, Primary Radiation Chemical Yields, and Structure of the Track of an Ionizing Particle... 

Except for the work of Platzman (5, 12, 13) and Magee (6, 7), the theoretical considerations in radiation chemistry have been limited to the first two items only. Recently, the most general procedure (item 3) has been applied to both the theory of primary radiation chemical yield (1, 14, 16,17, 19) and to the theory of initial structure of the track of an ionizing particle (10,19). The purpose of this paper is to compare these two aspects of absorption of ionizing radiation. 

At usual intensities of irradiation, the tracks of individual primary electrons are well isolated. On the other hand, the average distance between subsequent collisions along the track of a particular electron is much reduced in the condensed medium. This enhances the probability of mutual interaction of the intermediates within the track. This fact has led to the idea of an isolated track of an ionizing particle as a web along which the physicochemical and chemical events develop in space and time. 

An important difference between the radiation chemistry of water and of organic liquids is that the concept of the spur (a reasonably well-defined volume in which the formation of the reactive species occurs along the track of the ionizing particle) becomes hazy. The radicals formed in water tend to recombine rather than react with the environment immediately after formation. The volume in which recombination is likely defines the spur. The radical products of irradiated organic liquids, however, are more likely to interact with their immediate environment than to undergo recombination. This is evidenced by the low molecular yields of hydrogen from irradiated organic systems. 

In liquids with sufficiently high electron mobilities, the ionization electrons produced in the track of an individual particle or quantum can be detected separately. An advantage of this method is the fact that once the ionization electrons have escaped into the bulk of the liquid, no losses due to volume recombination with positive charge carriers occur. A disadvantage is, however, that electron attachment to electronegative impurities influences the electron signal. This is the foimdation of the application of liquids in electron pulse chambers (see Section 9.2). 

Ionization Path (Track)—The trail of ion pairs produced by an ionizing particle in its passage through matter. 

The density of excitation and ionization is not necessarily the same for all radiation qualities. For example, it is greater along the track of an a-par-ticle than for an electron track. For a primary-recoil electron produced by Co 7-rays in water, the distance between successive ionizations is about 1000 A. TTie ionized track is, therefore, sparse. At each point of ionization, secondary electrons give rise to further ionizations, forming a group of ion-pairs. In contrast, a-particles form a continuous track as a result of overlapping between the spheres of ionization. 

As a high-energy particle passes through matter, it creates an ionization track that contains positive ions. These ions are chemically reactive because their bonds are weakened by the loss of bonding electrons. Even though each cation eventually recaptures an electron to return to electrical neutrality, many ions first undergo chemical reactions that are the source of the damage done by nuclear radiation. 

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