High energy cascades in gold as studied by high energy self-ion irradiation

Abstract

Primary knock-on atom (PKA) energy spectrum extends up to several hundreds of keV in fusion reactor materials irradiated with 14 MeV neutrons. When we are going to evaluate materials behavior in an expected d-Li type intense neutron source, fundamental knowledge on effects of high energy PKA on formation of cascade damage, microstructural evolution and properties of irradiated materials is required. In this study, 170 MeV self-ion irradiation of thin foils of gold were performed to estimate effects of very high energy PKA on formation of defect clusters by cascade damage and its interactions. Defect clusters of vacancy type were observed in the thin foils of gold irradiated to 5 × 10 13–1 × 10 15 ions/m 2. In the case of irradiation with 170 MeV self-ions of which the projected range exists at 6.3 μm from the ion incident surface, films of gold, 1.1, 3.0, 4.3, 6.3 and 7.2 μm in thickness, were placed in front of the 50 nm thick specimens to change PKA energy spectrum within the specimens. The number of vacancy clusters within a cluster group formed by a PKA varied with the thickness of gold film. High energy PKA was found to increase the number of defect clusters. However, size distributions of defect clusters were not strongly dependent on PKA energies. Interactions of high energy cascades result in the appearance of new defect clusters near the existing defect cluster groups in the higher dose range. Dose dependence of defect cluster density was similar to that observed in 14 MeV and fission neutron irradiated specimens. The contribution of PKA higher than 400 keV to the interactions of cascades is estimated from the calculated PKA energy spectrum and low energy self-ion irradiation data.