The damage function refers to the number of FPs created within the first several picoseconds of the primary recoil event. At longer times, defects migrate from their nascent sites and interact with other defects and microstructural features. As noted earlier, many radiation effects, such as radiation-enhanced diffusion, segregation, and void swelling, depend more strongly on the number of defects that escape their nascent cascades and migrate freely in the lattice before annihilating, trapping, or forming defect clusters. The same general approach used to determine the damage function has been employed to determine the relative fraction of freely migrating defects, that is, e/nNRT, as illustrated by Figure 14. Here, the relative number of Si atoms segregating to the surface during irradiation, per dpa, is plotted versus a characteristic energy of the recoil spectrum, T1/2. It is seen that the fraction decreases rapidly with increasing recoil energy. Simi­lar experiments were performed using radiation — enhanced diffusion, as described in Section 1.07.2.

While ion irradiation has proved extremely useful in illustrating the spectral effects on freely migrating

defects, extracting quantitative information about freely migrating defects from such experiments is difficult. These measurements, unlike the damage function, require very high doses, and several dpa; the buildup of the sink structure must be adequately taken into account. It is also difficult to estimate, for example, how many interstitials are required to transport one Si atom to the surface. We mention in passing that experiments performed using ordering kinetics in order-disorder alloys have provided a more direct measure of the number of freely migrat­ing defects (vacancies in this case), as these experi­ments require doses less than «10~7dpa so that no damage build-up can occur.25 These experiments show similar effects of primary recoil spectrum on the fraction of freely migrating defects, although the fractions of such defects were found to be somewhat higher in these experiments, «5-10%. These frac­tions are in good agreement with radiation-enhanced diffusion experiments using self-ions on Ni, when the effect of sink strength is taken into account.26