Knock-on collisions

For knock-on collisions, one uses the Rutherford cross section for free electrons, and the number of free electrons is taken equal to the integral of the oscillator strength up to the energy loss e (dispersion approximation). Thus,... 

The contribution of knock-on collisions to the stopping power is now given by... 

To calculate the energy partition between the core and the envelope, Mozumder et al. (1968) considered the equipartition of deposited energy between glancing and knock-on collisions (Sect. 2.3.4). Of the ejected electrons... 

If a surface reaction is to involve more than monolayer-chemisorption, then the species adsorbed on the surface must be able to migrate into the second and deeper layers forming new chemical bonds and often new molecular species. This is step 3, product formation, and it often requires an activation mechanism to proceed, i.e., a monolayer is formed and the reaction stops unless the substrate is held at elevated temperature or there is ion or electron bombardment. Damage-enhanced diffsusion, knock-on collisions, and bond breaking may promote the reaction in the presence of ion bombardment. Although the precise mechanisms are unclear, it is certain that electron and ion bombardment cause step 3 to occur in some instances where the chemical reaction does not proceed in the absence of radiation. 

Further evidence for the importance of electronic excitation as a primary means of defect creation comes from studies of "sub-threshold" damage in which the energy of the incoming particle is less than that required for a "knock-on" collision. 

Glancing and knock-on collisions have to be considered in the case of fast electrons [11]. A glancing collision corresponds to the dissipation of less than 100 eV while larger energy loss occurs during head-on collisions. 

For knock-on collisions, the classical cross-section K(de/e2) can be used. It can be shown, in this case, that... 

Thus, with non-relativistic electrons, equal contributions to the stopping power result from glancing and knock-on collisions. Relativistic particles and the special case of electrons have been treated by Mozumder and Magee [11]. 

A more refined model was then presented by Mozumder and Magee [11]. It takes into account glancing and knock-on collisions and gives a complete picture of the track. Many new entitites called blobs, short tracks and branch tracks were defined in order to obtain this picture. 

Radiation Damage. It has been known for many years that bombardment of a crystal with eneigetic (keV to MeV) heavy ions produces regions of lattice disorder. An implanted ion entering a solid with an initial kinetic energy of 100 keV comes to rest in the time scale of about 10 13 due to both electronic and nuclear collisions. As an ion slows down and comes to rest in a crystal, it makes a number of collisions with the lattice atoms. In these collisions, sufficient eneigy may be transferred from the ion to displace an atom from its lattice site. Lattice atoms which are displaced by an incident ion are called primary knock-on atoms (PKA). A PKA can in turn displace other atoms, secondary knock-ons, etc. This process creates a cascade of atomic collisions and is collectively referred to as the collision, or displacement, cascade. The disorder can be directly observed by techniques sensitive to lattice structure, such as electron-transmission microscopy, MeV-partide channeling, and electron diffraction. 

The second subregion corresponds to large to and q and is related to close collisions (the knock-on). At very large transferred momenta (qa0> 1) the inelastic scattering by a molecule (an atom) is actually the elastic scattering by a free electron with the cross section given by Rutherford formula. In this case the function /(to, q) can be presented analytically as a delta function ... 

A heavy charged particle can knock out an electron from a molecule with maximum energy Emax —2/ra>2 (at v>v0), whereas for a fast electron with the same velocity the knocked out electron has max — mu2/2. Consequently, while an electron can knock out electrons with velocity no greater than its own, a heavy particle, in head-on collisions, produces delta electrons with velocities which can be twice as high as that of the ion. As a result, the energy of such delta electrons can be distributed to the regions of the medium far more remote from the point of initial ionization than in the case of electron irradiation. 

With carbonization, coloration of the polymers occurs [21]. Mechanisms of coloration or blackening of polymers induced by ion beams have been studied [22, 23] and two different models of blackening processes have been proposed direct knock-on of atoms from polymer chains by nuclear collision [22] and high density electronic excitation effects by an electronic excitation process [23]. 

Two types of direct interactions arc distinguished, knock-on reactions and transfer reactions. Knock-on reactions may proceed in various ways the incident particle may transfer a part of its energy to a nucleon and continue on its way (inelastic scattering), it may induce collective motion of the nucleus (vibration or rotation) or it may be captured by the nucleus and transfer its energy to one or several nucleons which leave the nucleus. The number of nucleon-nucleon collisions increases with increasing energy of the incident particles. 

Head-on collisions, real knock-outs -break bonds in the hydrogen and oxygen molecules, allowing a new product to be formed. 

Fig. 7.1. Schematic of the formation of collision cascade by a primary knock-on atom (after Thompson 1969)...

A lattice atom receiving less than a critical energy Ed is not displaced. Similarly, if a knock-on atom emerges from a collision with E < Ed, it does not contribute further to the cascade. Also, atoms receiving energy between Ed and 2Ed are displaced but cannot themselves further increase the total number of displacements. 

These energy-loss values include the nuclear-collision contributions from the whole cascade, taking into account the electronic losses suffered by the knock-on atoms. These values of for Group IV elements can be used for the adjacent Group III of V elements without introducing significant errors. 

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