Charged particles, VIII

VIII. Tracks of Charged Particles and Their Structure... 

A charged particle emits Vavilov-Cerenkov radiation (VCR) when its velocity becomes greater than the speed of light in the medium, that is, when v > dn where nr is the refractive index of the medium. This is so when the energy of the particle is sufficiently high—for electrons in water it must be >270 keV. And though the contribution of VCR to the energy losses is small, it can nevertheless be essential in photosensitive media (see Section VIII.C). 

The passage of a charged particle through a medium results in the formation of disturbance areas along the particle s trajectory that contain excited molecules, positive ions, and knocked out electrons and atoms. These disturbances areas make up the track of the particle (see Section VIII). An important role in the process of formation of the track and in the following radiation-chemical transformations is played by the degree of delocalization of the initially absorbed energy. 

VIII. TRACKS OF CHARGED PARTICLES AND THEIR STRUCTURE... 

Now, as we know, a vibrating charged particle acts like an antenna —it sends out electromagnetic vibrations of its own in the form of spherical waves and, in fact, the energy drawn per unit time from the primary ray and converted into scattered radiation by a (free or loosely bound) electron is given (Appendix VIII, p. 275) by... 

VIII. Time-Dependent Perturbations Radiation Theory Time-Dependent Perturbations, 107. The Wave Equation for a System of Charged Particles under the Influence of an External Electric or Magnetic Field, 108. Induced Emission and Absorption of Radiation, 110. The Einstein Transition Probabilities, 114. Selection Rules for the Hydrogen Atom, 116. Selection Rules for the Harmonic Oscillator, 117. Polarizability Rayleigh and Raman Scattering, 118. 

At v < v0Z213 the effective charge zeff is proportional to v/v0. So the dependence of se on velocity in this case is solely due to the logarithmic term in formula (5.2), which decreases as the ion s velocity falls. Consequently, as the ion slows down, se must also decrease, that is, its behavior is exactly opposite to the case of protons and alpha particles, where se increases as the particle s velocity falls, until it approaches Bragg s peak. In Section VIII.D we will show how this particuliarity affects the structure of the track of a multicharged ion. 

The above relationship can be represented by a curve with a maximum (Fig. VIII-1) for o0 70 mJ m 2 and the charge, q, equal to elementary charge, the position of maximum corresponds to particle radius of 1 - 2 nm. 

Fig. VIII-1. The equilibrium pressure, p, as a function of radius of charged aerosol particles...

In the case of electrolytes that are capable of causing the reversal of surface charge of dispersed particles, coagulation occurs only at electrolyte concentrations corresponding to rather small values of the -potential. As electrolyte concentration becomes higher, the C, -potential decreases coagulation starts as soon as the C, -potential drops below the critical value. This corresponds to the first critical coagulation concentration, C.C.C. -1, (c, point in the curves shown in Fig. VIII-19). Further increase in electrolyte... 

VIII.l that for conducting particles the charge received in the field of a corona discharge is smaller than the theoretical value. For insulators the reverse is the case [144]. 

Fig. VIII.l. Charge received in a corona discharge as a function of the diameter of the metal particles. 1) Calculated 2) experimental.

The Debye parameter /c, on the other hand, is mainly affected by the charge and concentration of ions in solution that screen the particles Stern potential [Eq. (4)]. The ions that can contribute to increase the Debye parameter may be supplied by different sources, as, for example impure raw materials purposely added salts in shotcrete applications (Section VIII) partially or totally soluble particles (e.g., cement, dissolving powders) and dispersants that display low adsorption ability on the particle surface and remain dissolved in the... 

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