Damage primary knock

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. 

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). 

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. 

Gamma rays are completely different from neutrons for their action on the matter. Electromagnetic radiation as gamma rays knock electrons off atoms or molecules causing primary ionisation. This damage process was assessed in all tested organisms (Table 4). 

---