Spike displacement

Fig. 12. Displacement spike arising from an energetic ion in a solid ...

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. ... 

The minimum energy required for the production of displacements may be calculated by means of the same approximate methods used in the case of beta rays this minimum value, however, is much smaller in the present case, because the particles under consideration are 10 to 10" times heavier 52, 54). Although quantitative determination remains impossible, an estimation made by Seitz 54) considers the energy fraction dissipated as displacement to be lower than 0.1%. Let us also point out that certain displaced atoms receive an energy sufficient for the creation of displacement spikes. Some experimental information about atom displacements in metals and semiconductors is available at the present time. Table IX shows values obtained in the case of germanium irradiation. The number of displaced atoms, for dissipated dose of 10 e.v., is seen to be between 10 and 10 (54, 55)... 

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. 

Fig. 7.5. Schematic of a highly damaged volume of material, formed when the mean free path between collisions, Ad, approaches the atomic spacing of the target atoms. The dense cascade is referred to as a displacement spike (after Brinkman, 1956)...

STATE the effect a large number of displacement spikes has on the properties of a metal. 

Thermal and displacement spikes can cause distortion that is frozen as stress in the microscopic area. These spikes can cause a change in the material s properties. 

A displacement spike occurs when many atoms in a small area are displaced by a knock-on (or cascade of knock-ons). A 1 MeV neutron may affect approximately 5000 atoms, making up one of these spikes. The presence of many displacement spikes will change the properties of the material being irradiated. A displacement spike contains large numbers of interstitials and lattice vacancies (referred to as Frenkel pairs or Frenkel defects when considered in pairs). The presence of large numbers of vacancies and interstitials in the lattice of a metal will generally increase hardness and decrease ductility. In many materials (for example, graphite, uranium metal) bulk volume increases occur. 

Displacement spikes occur when many atoms in a small area are displaced by a knock-on. 

The presence of many displacement spikes changes the properties of the metal being irradiated, such as increasing hardness and decreasing ductility. 

Except for chemicals produced in the environment by radiation, such as HNO3 and H2O2, which have a secondary effect on corrosion, or formation of localized displacement spikes during radiation, the effect of radiation may be expected to parallel that of cold work. That is, metals for which the corrosion rate is controlled by oxygen diffusion should suffer no marked change in rate after irradiation. In acids, on the other hand, irradiated steel (but not pure iron) would presumably have a greater increase in rate than would irradiated nickel, which is less sensitive to cold working. 

In the first phase, collective dynamic atomic motion occurs over several tens of picoseconds, and atomic relocation occurs at random. This transitional phase in the development of the cascade is named the displacement spike [49]. 

The atomic configuration during the displacement spike phase is apparently unstable and heads back to equilibrium after this phase. The first stage involves the instantaneous recombination (collapse) of closed Frenkel pairs. 

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