Primary knock atom

The first case, that of the so-called thermal spike, is one in which energy received by the atom is smaller than 2 Ei. The energy of the primary knocked atom is progressively transmitted to its neighbors this process... 

The second case, that of the so-called displacement spike is one in which the knocked atom receives an energy greater than 2Ei. The primary knocked atom can travel a certain length until its energy becomes smaller than 2Ei. Along its path, secondary knocked atoms may be produced. This process may be repeated, till the energy of these atoms becomes lower than 2Ed. Each of these is then the center of a thermal spike. ... 

If, on the other hand, the target is made of heavy elements Z > 30), the energy received by the primary knocked atom is completely transmitted to the lattice as displacement spikes or as thermal spikes. 

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

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

This effect is especially important when the knock-on atom (or nucleus) is produced as the result of an elastic collision with a fast neutron (or other energetic heavy particle). The energy of the primary knock-on can then be quite high, and the cascade may be extensive. A single fast neutron in the greater than or equal to 1 MeV range can displace a few thousand atoms. Most... 

Extended defects can be produced in all classes of HTS by radiation techniques, if the interaction of the particles with the constituents of the superconductor is sufficiently strong, i.e., the primary knock-on atom has energies above 1 keV in the case of elastic nuclear interactions, or the interaction occurs electronically and leads to amorphisation of the material over extended areas. I will discuss in the following the most important types of radiation suitable for the production of extended defects and the nature of these defects, which was established in most cases by TEM. 

The atoms knocked out of the lattice positions by impinging particles - which are called primary knock-on atoms (PKAs) - can have various energies from zero to Tm, even for monochromatic incident radiation. In addition, the PKA energy spectrum depends on the incident particle type (mass, charge) and energy. Differences in the PKA energy spectrum lead to differences in the damage caused. The formation of a PKA is equivalent to the formation of a vacancy-interstitial defect pair (or Frenkel pair). 

Figure 4.24 Distribution of radiation defects in clusters (a, c) and energy transfer to the lattice in an idealised metallic alloy (b, d) for (a, b) 15 keV and (c, d) 150keV primary knock-on atoms...

Interactions between fast neutrons and lattice atoms can transfer energies ranging from a few eV to tens of keV. The primary knock-on atoms (PKAs) lose energy though interactions with both the bound electrons and the atoms of the solid. If the energy transferred to a lattice atom is greater that some threshold value (typically... 

PKA primary knock-on atom SCH separate confinement heterostructures... 

Molecular-dynamics calculations provide valuable insight into the evolution with time of defect structures created in the collision caiscade. Consider, for example, the molecular-dynamics simulations of low-energy displacement cascades in the Bll-ordered compound CuTi (Figure 7) by Zhu et al, (1992). Figure 8 shows the number of Frenkel pairs produced by a Cu primary knock-on atom (PKA) as a function of recoil energy at the end of the collisional phase (0.2 p ) and at the end of the cooling phase (2.5 ps). The number of Frenkd... 

Radiation, Secondary—A particle or ray that is produced when the primary radiation interacts with a material, and which has sufficient energy to produce its own ionization, such as bremsstrahlung or electrons knocked from atomic orbitals with enough energy to then produce ionization (see Delta Rays). 

Compared with the momentum of impinging atoms or ions, we may safely neglect the momentum transferred by the absorbed photons and thus we can neglect direct knock-on effects in photochemistry. The strong interaction between photons and the electronic system of the crystal leads to an excitation of the electrons by photon absorption as the primary effect. This excitation causes either the formation of a localized exciton or an (e +h ) defect pair. Non-localized electron defects can be described by planar waves which may be scattered, trapped, etc. Their behavior has been explained with the electron theory of solids [A.H. Wilson (1953)]. Electrons which are trapped by their interaction with impurities or which are self-trapped by interaction with phonons may be localized for a long time (in terms of the reciprocal Debye frequency) before they leave their potential minimum in a hopping type of process activated by thermal fluctuations. 

It is of interest to review ideas as to the point of hydrogen abstraction. Through 1939 most investigators believed that attack of paraffins was at the primary C—H bonds at the end of a chain (75, 109, 167, 168, 217, 220, 224), Attack at the a-carbon atom of substituted benzenes (217) and at the end methyl of olefins (109) was proposed. Preferential attack at 1° C—H bonds fitted in with the comparative ease of oxidation of n-paraffins and their low knock ratings. 

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