Due to relatively large concentrations of conduction electrons, materials with metallic bonding typically do not exhibit sensitivity to ionizing radiation. On the other hand, semiconductor and insulating materials can be strongly affected by ionizing radiation by vari­ous mechanisms that lead to either enhanced or sup­pressed defect accumulation.159 Some materials such as alkali halides, quartz, and organic materials, are susceptible to displacement damage from radiolysis reactions.65,160-163 In materials that are not suscepti­ble to radiolysis, significant effects from ionizing radi­ation can still occur via modifications in point defect migration behavior. Substantial reductions in point defect migration energies due to ionization effects have been predicted, and significant microstructural changes attributed to ionization effects have been observed in several semiconductors and inorganic insulator materials.18,159,164-169 The effect of ionizing radiation can be particularly strong for electron or light ion beam irradiations of certain ceramic materi­als since the amount of ionization per unit displace­ment damage is high for these irradiation species; the ionization effect per dpa is typically less pronounced for heavy ion, neutron, or dual ion beam irradiation. Figure 20 summarizes the effect of variations in the ratio of ionizing to displacive radiation (achieved by varying the ion beam mass) on the dislocation loops

density and size in several oxide ceramics.94,169,170

The loop density decreases rapidly when the ratio of ionizing to displacive radiation (depicted in Figure 20 as electron-hole pairs per dpa) exceeds a material — dependent critical value, and the corresponding loop size simultaneously increases rapidly.

Numerous microstructural changes emerge in mate­rials irradiated with so-called swift heavy ions that produce localized intense energy deposition in their ion tracks. Defect production along the ion tracks is observed above a material-dependent threshold value for the electronic stopping power with typical values of 1-50keVnm-1.159,171-175 The microstructural changes are manifested in several ways, including dislocation loop punching,176 creation of amorphous tracks with typical diameters of a few nm,159,173,174,177-180

atomic disordering,176,181,182 crystalline phase transfor-

mations,171 destruction of preexisting small dislocation

50

40

30

20

10

Figure 20 Effect of variations in ionizing to displacive radiation on the dislocation loop density and size in ion-irradiated MgO, Al2O3, and MgAl2O4. Adapted from Zinkle, S. J. Radiat Eff. Defects Solids 1999, 148, 447-477; Zinkle, S. J. J. Nucl. Mater. 1995, 219, 113-127; Zinkle, S. J. In Microstructure Evolution During Irradiation; Robertson, I. M., Was, G. S., Hobbs, L. W., Diaz de la Rubia, T., Eds. Materials Research Society: Pittsburgh, PA, 1997; Vol. 439, pp 667-678.

_p

(a) 20 nm

Figure 21 Plan view microstructure of disordered ion tracks in MgAl2O4 irradiated 430 MeV Kr ions at room temperature to a fluence of 6 x 1015 ions per square meter (isolated ion track regime) under (a) weak dynamical bright field and (b) g = (222) centered dark field imaging conditions (tilted 10° to facilitate viewing of the longitudinal aspects of the ion tracks). High-resolution TEM and diffraction analyses indicate disordering of octahedral cations (but no amorphization) within the individual ion tracks. Adapted from Zinkle, S. J.; Skuratov, V. A. Nucl. Instrum. Methods B 1998, 141(1-4), 737-746;

Zinkle, S. J.; Matzke, H.; Skuratov, V. A. In Microstructural Processes During Irradiation; Zinkle, S. J., Lucas, G. E.,

Ewing, R. C., Williams, J. S., Eds. Materials Research Society: Warrendale, PA, 1999; Vol. 540, pp 299-304.

loops,176 and formation of nanoscale hillocks and sur­rounding valleys183’184 at free surfaces. Annealing of point defects occurs for irradiation conditions below the material-dependent threshold electronic stopping power for track creation,159,180,185,186 whereas defect production occurs above the stopping power threshold.159,171,173,175,178,180,183,185,186 The swift heavy ion annealing and defect production phenomena are

observed in both metals and alloys171,175,183,185,186 as well as nonmetals.159,172,173,178-180,187-190 Defect pro­duction by swift heavy ions is of importance for
understanding the radiation resistance of current and potential fission reactor fuel systems, including the mechanisms responsible for the finely polygo — nized rim effect18 ,191 in UO2 and radiation stability of inert matrix fuel forms.182,189,191 The swift heavy ion defect production mechanism is generally attributed to thermal spike178,192 and self-trapped exciton187 effects. Figure 21 shows examples of the plan view (i. e. along the direction of the ion beam) microstructure of dis­ordered ion tracks in MgAl2O4 irradiated with swift

176,182

heavy ions.