Radiation-Induced Defects in Crystals

structural defects formed upon the irradiation of crystals by beams of nuclear particles and intense electromagnetic radiation, namely, γ- and X-radiation. The structural defects cause changes in the mechanical and other physical properties of crystals. The restoration of these properties, that is, the elimination of the radiation defects in crystals, is achieved through heating. The study of radiation defects in crystals began in the mid-1940’s with the development of reactor technology. E. Wigner, in 1942, was the first to suggest the possibility that a crystal lattice is destroyed as a consequence of the displacement of atoms from their equilibrium positions upon interaction with fast neutrons and nuclear fission fragments. It was then also proposed that the displacement of atoms should affect the properties of materials.

A distinction is made between simple and complex radiation defects in crystals. Interstitials and vacancies are simple defects. One of each of these defects is formed when a nuclear particle imparts an energy greater than some threshold energy ℰ₀ to an atom at the lattice point. The value of ℰ₀ depends on the material and is equal to several tens of electron volts. This energy is sufficient for overcoming the binding force between the atoms and displacing an atom some distance from its lattice point. Both the vacancy and the displaced atom are highly mobile even at room temperature. If the two encounter each other in the course of their motion through the crystal, they may re-combine, move to the surface of the crystal, or remain fixed in defects that do not derive from radiation, such as impurity atoms, dislocations, grain boundaries, and cleavages. If the energy acquired by an atom exceeds ℰ₀ by a factor of tens or hundreds, then the initially displaced atom can interact with other atoms as it moves through the crystal and produce a cascade of displaced atoms.

Clusters of simple radiation defects may form as a result of precipitation. The formation of clusters is most likely in those cases where the radiation takes the form of the high-energy particles that initiate cascade processes. In such cases, even small initial clusters may serve as centers for the aggregation of simple lattice defects. The growth of vacancy clusters converts the clusters into pores. However, this process cannot continue indefinitely; on the one hand, it is limited by the relative decrease in the precipitation surface of the vacancies, while on the other, it is limited by the conditions of thermal equilibrium. Spherical pores are unstable in metals and are compressed into a platelet on one of the most compact atomic layers of the crystal. The platelet eventually forms dislocation loops.

The most complete information on radiation defects in crystals is obtained by irradiating materials at very low temperatures (as low as a few °K). The radiation defects formed are “frozen,” and their motion through the crystal is maximally retarded. Upon gradual warming, a restoration of the original properties of the material is often observed in stages. The study of the nature and rate of the restoration of the original properties over time at the temperature providing the sharpest change in properties between two stages (isothermal annealing) permits a determination of the energy of activation for the motion of radiation defects and a determination of the features of the property transformations. Radiation defects may also be observed directly, for example, by using an electron or field-ion microscope.

The study of radiation defects in crystals has great practical importance. Various construction materials and fissionable materials used in nuclear reactors and materials on board spacecraft in the earth’s radiation belts are subjected to irradiation by beams of neutrons, protons, electrons, and gamma rays. In order to predict the operational characteristics of materials, it is necessary to possess a knowledge of the various types of radiation defects and a knowledge of defect transformations, thermal stability, and the effect of radiation on the properties of materials. Such knowledge also aids in the development of materials that are resistant to radiation.