If a target or struck nucleus gains about 25 eV of kinetic energy (25 eV to 30 eV for most metals) in a collision with a radiation particle (usually a fast neutron), the nucleus will be displaced from its equilibrium position in the crystal lattice, as shown in Figure 3.

FAST NEUTRO

Figure 3 Thermal and Fast Neutrons Interactions with a Solid

The target nucleus (or recoiling atom) that is displaced is called a knocked-on nucleus or just a knock-on (or primary knock-on). When a metal atom is ejected from its crystal lattice the vacated site is called a vacancy. The amount of energy required to displace an atom is called displacement energy. The ejected atom will travel through the lattice causing ionization and heating. If the energy of the knock-on atom is large enough, it may in turn produce additional collisions and knock-ons. These knock-ons are referred to as secondary knock-ons. The process will continue until the displaced atom does not have sufficient energy to eject another atom from the crystal lattice. Therefore, a cascade of knock-on atoms will develop from the initial interaction 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 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 of these displacements are temporary. At high temperatures, the number of permanently displaced atoms is smaller than the initial displacement.

During a lengthy irradiation (for large values of the neutron fluence), many of the displaced atoms will return to normal (stable) lattice sites (that is, partial annealing occurs spontaneously). The permanently displaced atoms may lose their energy and occupy positions other than normal crystal lattice sites (or nonequilibrium sites), thus becoming interstitials. The presence of interstitials and vacancies makes it more difficult for dislocations to move through the lattice. This increases the strength and reduces the ductility of a material.

At high energies, the primary knock-on (ion) will lose energy primarily by ionization and excitation interactions as it passes through the lattice, as shown in Figure 3. As the knock-on loses energy, it tends to pick up free electrons which effectively reduces its charge. As a result, the principle mechanism for energy losses progressively changes from one of ionization and excitation at high energies to one of elastic collisions that produce secondary knock-ons or displacements. Generally, most elastic collisions between a knock-on and a nucleus occur at low kinetic energies below A keV, where A is the mass number of the knock-on. If the kinetic energy is greater than A keV, the probability is that the knock-on will lose much of its energy in causing ionization.

Summary

The important information in this chapter is summarized below.

Atomic Displacement Due To Irradiation Summary

Beta and gamma radiation produce ionization and excitation of electrons, which does very little damage.

Heavier particles, such as protons, (x-particles, fast neutrons, and fission fragments, usually transfer energy through elastic or inelastic collisions to cause radiation damage. These particles in organic material break the chemical bonds, which will change the material's properties.

Knock-on is a target nucleus (or recoiling atom) that is displaced.

Vacancy is the vacated site when a metal atom is ejected from its crystal lattice.

Interstitial is a permanently displaced atom that has lost its energy and is occupying a position other than its normal crystal lattice site.