Fig. 1-3 Electron excitation and atomic collisions by energetic ion irradiation of a solid. Energetic ions cause the electron excitation along the ion beam path and atomic displacement, resulting in the production of interstitial atoms and vacancies in the solid.

Fig. 1-4 Change in the defect concentration versus ion fluence in Ni during 84 MeV C ion irradiation and during subsequent 100 MeV I ion irradiation at liquid helium temperature. About a half the defects produced by C ion irradiation are quickly annihilated by I ion irradiation.

When solid material is irradiated by some kinds of radiation, the quality of the material is sometimes degraded considerably (radiation damage). An understanding of the mechanism of radiation damage is important for the development of useful materials. When a high energy ion is injected into the lattice of a metal crystal, it passes through the lattice exciting electrons near the ion orbit and transfering the ion energy to them, or the ion directly collides with an atom of the lattice. In the latter case, the atom is ejected from the lattice. The ejected atom collides with another atom and the process is repeated. The defects of interstitial atoms and vacancies are produced as a cascade (Fig. 1-3). To know which of these processes is dominant is very important and up until now it was believed that radiation damage in metals is explained by the direct collision of atoms. However, in experiments for Ni, Pt, and so on at an extremely low temperature (-265 degrees cent.) where the movement of defects is frozen, we found that the effect of electron excitation dominates. This is clearly shown in the experiment of the irradiation of Ni crystal by carbon and iodine ions sequentially as follows; (1) by irradiating the Ni crystal with C ions, Ni atoms are ejected from the lattice but their movement is frozen because of the extremely low temperature, which results in the accumulation of defects with time (Fig. 1-4 left), but (2) if after the irradiation by C ions, the crystal is irradiated by iodine ions which have a higher electron excitation effect than carbon, the energy of the iodine ions effectively diminishes the defects and the number of the defects decreases quickly. Thus, to decrease the defects, only a small amount of an irradiation with iodine ions suffices because of their high electron excitation effects (Fig. 1-4 right).

Reference A. Iwase et al., Effect of Electron Excitation on Radiation Damage in fcc Metals, Nucl. Instrum. Methods B90, p.322-329(1994).

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