DOE-HDBK-1017/2-93Plant MaterialsATOMIC DISPLACEMENT DUE TO IRRADIATIONAtomicDisplacementsIf a target or struck nucleus gains about 25 eV of kinetic energy (25 eV to 30 eV for mostmetals) in a collision with a radiation particle (usually a fast neutron), the nucleus will bedisplaced from its equilibrium position in the crystal lattice, as shown in Figure 3.The target nucleus (or recoiling atom) that is displaced is called a knocked-on nucleus or just aFigure 3Thermal and Fast Neutrons Interactions with a Solidknock-on(or primary knock-on). When a metal atom is ejected from its crystal lattice thevacated site is called a vacancy. The amount of energy required to displace an atom is calleddisplacement energy. The ejected atom will travel through the lattice causing ionization andheating. If the energy of the knock-on atom is large enough, it may in turn produce additionalcollisions and knock-ons. These knock-ons are referred to as secondary knock-ons. The processwill continue until the displaced atom does not have sufficient energy to eject another atom fromthe crystal lattice. Therefore, a cascade of knock-on atoms will develop from the initialinteraction of a high energy radiation particle with an atom in a solid. This effect is especially important when the knock-on atom (or nucleus) is produced as the resultof an elastic collision with a fast neutron (or other energetic heavy particle). The energy of theprimary knock-on can then be quite high, and the cascade may be extensive. A single fastneutron in the greater than or equal to 1 MeV range can displace a few thousand atoms. Most Rev. 0Page 33MS-05
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