THERMAL AND DISPLACEMENT SPIKES DUE TO IRRADIATION
THERMAL AND DISPLACEMENT SPIKES
DUE TO IRRADIATION
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.
DEFINE the following terms:
STATE the effect a large number of displacement spikes has on the
properties of a metal.
As mentioned previously, the knock-ons lose energy most readily when they have lower energies,
because they are in the vicinity longer and therefore interact more strongly. A thermal spike
occurs when radiation deposits energy in the form of a knock-on, which in turn, transfers its
excess energy to the surrounding atoms in the form of vibrational energy (heat). Some of the
distortion from the heating can be frozen as a stress in this microscopic area.
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.