Charged Particle Tracks in Liquids

Charged particle tracks in liquids are formally similar to cloud chamber or bubble chamber tracks. In detail, there are great differences in track lifetime and observability. Tracks in the radiation chemistry of condensed media are extremely short-lived and are not amenable to direct observation. Also, it must be remembered that in the cloud or bubble chamber, the track is actually seen at a time that is many orders of magnitude longer than the formation time of the track. The manifestation occurs through processes extraneous to track formation, such as condensation, formation of bubbles, and so forth. In a real sense, therefore, charged particle tracks in radiation chemistry are metaphysical constructs. 

While we have a fair degree of understanding of the radiation chemistry with low LET radiation in liquid saturated hydrocarbons, the situation is rather unclear for the chemical processes occurring in high LET charged particle tracks. 

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). 

The most popular method of measuring electrophoretic mobility is microelectrophoresis. In microelectrophoresis, particles are placed in a closed capillary with electrodes at either end. When an electric field is applied, the particles migrate towards the electrode and their velocities are measured. Because the capillary walls are charged, the applied electric field will also induce an electroosmotic flow. However, since the capillary is closed, a back pressure creates a net zero flow in the mbe (see Fig. 3). The particle velocity is a combination of the electrophoretic motion and the fluid flow. To obtain the true electrophoretic mobility, the particles must be tracked along the stationary layer where the fluid velocity is zero. For a circular capillary the liquid velocity across the tube is given by the electroosmotic flow and the back pressure flow ... 

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