Primary knock-on atoms

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)...

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... 

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