ATOMIC DISPLACEMENT DUE TO IRRADIATION
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.
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, a-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.