Displacement damage can occur in materials when the energy transferred to lattice atoms exceeds a critical value known as the threshold displacement energy (Ed), which has a typical value of 30-50 eV.8,18,40 Figure 2 shows an example of the effect of bombard­ing energy on the microstructure of CeO2 during electron irradiation near room temperature.41 The loop density increases rapidly with increasing energy
above 200 keV, suggesting that 200 keVelectrons trans­fer elastic energy that is slightly above the threshold displacement energy. High-resolution microstructural analysis determined that the dislocation loops were associated with aggregation of oxygen ions only (i. e., no Ce displacement damage) for electron energies up to 1250 keV, whereas perfect interstitial-type disloca­tion loops were formed for electron energies of 1500 keV and higher. Therefore, the corresponding displacement energies in CeO2 are ^30 and ^50 eV for the O and Ce sublattices, respectively.41

A wide range of PKA energies can be achieved during irradiation, depending on the type and energy of irradiating particle. For example, the average PKA energies transferred to a Cu lattice for 1 MeV electrons, protons, Ne ions, Xe ions, and neutrons are 25 eV, 0.5 keV, 9 keV, 50 keV, and 45 keV, respec — tively.42 Irradiation of materials with electrons and light ions introduces predominantly isolated SIAs and vacancies (together known as Frenkel pairs) and small clusters of these point defects, because of the low average recoil atom energies of ~0.1-1 keV. Con­versely, energetic neutron or heavy ion irradiations produce energetic displacement cascades that can lead to direct formation of defect clusters within isolated displacement cascades due to more ener­getic average recoil atom energies that exceed 10 keV. Figure 3 compares the weighted PKA energy values for several irradiation species.40,42

These differences in PKA energy produce signifi­cant changes in primary damage state that can have a pronounced effect on the microstructural evolution observed during irradiation. As briefly mentioned in Section 1.03.3.1, the defect production efficiency per dpa determined from electrical resistivity mea­surements during irradiation near absolute zero and MD simulation studies is significantly lower (by about

a factor of 3-4) for energetic displacement cascade conditions compared to isolated Frenkel pair condi­tions, due to pronounced in-cascade recombination and clustering processes.38,39 MD computer simula — tions43—46 and in situ or postirradiation thin foil exper­imental studies13,14,47,48 (where interaction between different displacement damage events is minimal due to the strong influence of the surface as a point defect sink) have found that defect clusters visible by transmission electron microscopy (TEM) can be produced directly in displacement cascades if the average PKA energy exceeds 5-10 keV. Irradiations with particles having significantly lower PKA ener­gies typically produce isolated Frenkel pairs and sub­microscopic defect clusters that can nucleate and coarsen via diffusional processes. The microstructural evolution of an irradiated material is controlled by different kinetic equations if initial defect clustering occurs directly within the displacement cascade (~0.1—1 ps timescale) versus three-dimensional ran­dom walk diffusion to produce defect cluster nucle — ation and growth, particularly ifsome ofthe in-cascade created defect clusters exhibit one-dimensional glide 49-52 As discussed in Chapter 1.13, Radiation Damage Theory, this can produce significant differ­ences in the microstructural evolution for features such as voids and dislocation loops. Figure 4 compares the microstructure produced in copper following irra­diation near 200 °C with fission neutrons53 and 1 MeV electrons.54,55 Vacancies and SIAs are fully mobile in copper at this temperature. The 1 MeV electron pro­duces a steady flux of point defects that leads to the

creation of a moderate density of large faulted interstitial loops. On the other hand, the creation of SFTs and small dislocation loops directly in fission neutron displacement cascades creates a high density (~2 x 1023 m~ ) of small defect clus­ters, and the high point defect sink strength asso­ciated with these defect clusters inhibits the growth of dislocation loops. As shown in Figure 4, the net result is a dramatic qualitative and quantitative

difference in the irradiated microstructure due to differences in the PKA spectrum.

Electron microscopy48’56 and binary collision48’57 and MD simulation45 studies have found that irradi­ation with PKA energies above a critical material — dependent value of ^ 10-50 keV results in formation of multiple subcascades (rather than an ever — increasing single cascade size), with the size of the largest subcascades being qualitatively similar to an isolated cascade at a PKA energy near the critical value. Figure 5 compares MD simulations of the peak displacement configurations of PKAs in iron with energies ranging from 1 to 50 keV.58 At low PKA energies’ the size of the displacement cascade increases monotonically with PKA energy. When the PKA energy in Fe exceeds a critical value of ~10 keV, multiple subcascades begin to appear, with the largest subcascade having a size comparable to the 10 keV cascades. The number of subcascades increases with increasing PKA energy, reaching ^5 subcascades for a PKA energy of 50 keV in Fe. A fortunate conse­quence of subcascade formation is that fission reactor irradiations (~1 MeV neutrons) can be used for ini­tial radiation damage screening studies of potential future fusion reactor (~14 MeV neutrons) materials, since both would have comparable primary damage subcascade structures.59,60 Further details on the ef­fect of PKA spectrum on primary damage formation
are given in Chapter 1.11, Primary Radiation Damage Formation.