Stopping power heavy ions

The striking discrepancy between the g-values used in the present nonlinear approach and the frequently used ZBL values (Fig. 7) may be explained as follows. The fitting expression for q given in Ref. [9] was obtained from a very extensive analysis of stopping power data performed within the framework of a linear approach [15]. But, as we have shown, the linear models overestimate the stopping of heavy ions. Hence, the lower -values obtained by this procedure compensate the excessive stopping of the linear approach. 

The details of the stopping power equations have been very well summarized elsewhere [22,34-38]. Most applications involve ions of low charge and high velocity in which the Bethe formalism is valid. The nonrelativistic stopping power equation of Bethe for heavy ions is given approximately by. 

The dotted lines in Fig. 1 show the stopping powers for the different ions at a constant velocity in units of MeV/amu. This unit of energy is very often used in heavy ion radiolysis and it is based on the classical formula for kinetic energy, E = V2 MV, where M is the heavy ion mass. As seen in Eq. (1), the ion velocity is a dominant parameter in energy loss processes and the MeV/amu energy unit is more convenient to use than converting to absolute velocity units. Remember that MeV/amu is actually proportional to the square of the velocity. 

Figure 1 Stopping power, —dEjdx, of some heavy ions [42] and electrons [44] in water as a function of energy. The dotted lines show the stopping power for heavy ions of equal velocity.

The electronic stopping power involves a collision between the heavy ion nucleus and an electron of the medium. The reduced mass of the encounter can always be assumed to... 

The lack of nuclear collisions for channeled ions is not the only phenomenon that affects the pulse height. It is known that the electron density is much reduced along the channel. As a result, the electronic stopping power is lower and, consequently, so is the charge density produced by the heavy ion. Thus, not only the nuelear but also the recombination defect is reduced for the channeled ions. 

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