Stopping electronic

A comparison of the nuclear and electronic stopping cross-sections expressed in reduced notation is shown in Fig. 5.3. Recall that s is proportional to ion energy and that (s)m is proportional to ion velocity. 

The total energy loss is the sum of the three fractions, and may be written 

Since charge exchange represents only a very small fraction of the total energy loss, it will be neglected in the following considerations. 

At even higher energy another mechanism starts to play a role, namely Rutherford scattering at the atomic nuclei. But for the energy region we deal with it has not to be taken into account. 

In the theory of Bohr the energy loss of an ion interacting with an atom is 

This is only valid for a fully ionized atom moving with a higher velocity than the K-shell electrons. B is an interaction parameter for which Bethe gave the expression 

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Ema data can be quantitated to provide elemental concentrations, but several corrections are necessary to account for matrix effects adequately. One weU-known method for matrix correction is the 2af method (7,31). This approach is based on calculated corrections for major matrix-dependent effects which alter the intensity of x-rays observed at a particular energy after being emitted from the corresponding atoms. The 2af method corrects for differences between elements in electron stopping power and backscattering (the correction), self-absorption of x-rays by the matrix (the a correction), and the excitation of x-rays from one element by x-rays emitted from a different element, or in other words, secondary fluorescence (the f correction). 

The electronic stopping power of the 2 MeV Ne+ ions in the palladium acetate films is much larger than that of 2 MeV He ions. The most obvious difference between the effects of the two ions is in the appearance of the films at the high dose limit. A 0.90 nm thick palladium acetate film exposed to 2 MeV Ne+ ion irradiation until no further spectroscopic changes occur looks black, compared with the metallic silvery films produced in the He ion irradiation. However... 

At moderate energies, the electron can acquire relativistic speeds. Including this effect as well as corrections due to shell and density effects, the electron stopping number may be written as... 

The total electronic stopping cross sections for the three independent directions in which the water molecule has been oriented with respect to the beam axis are presented in Fig. 2, along with the experimental results of Reynolds et al. [28] for comparison. 

An interesting recent development is the application of an electron-nuclear-dynamics code [68] to penetration phenomena [69]. The scheme is capable of treating multi-electron systems and may he particularly useful for low-velocity stopping in insulating media, where alternative treatments are essentially unavailable. However, conceptional problems in the data analysis need attention, such as separation of nuclear from electronic stopping and, in particular, the very definition of stopping force as discussed in Section 5.2. 

Calculation of proton electronic stopping within the OLPA-TFD(l/8)W scheme 4.3. The approach to pressure effects on stopping... 

Fig. 1. Velocity dependence of the electronic stopping cross section for protons in atomic...

Fig. 2. Velocity dependence of the electronic stopping cross section for protons in propane. (—) prediction by the OLPA-FSGO treatment [42]. ( ) Oddershede and Sabin calculations [29]. (----) best fit curve to experimental data from Refs. [44,45].

Fig. 4. Comparison of theoretical predictions for the electronic stopping cross section of...

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