Low-dose neutron irradiation damage in copper

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Abstract
Transmission electron microscopy has been used to follow how the damage structure as a whole evolves from the earliest stages (1021-2 × 1022 n m−2 (E > 1 MeV)) in neutron-irradiated copper over a range of temperatures from 523 to 623 K. A major feature was the observation of an inhomogeneous structure from the lowest doses consisting of high- and low-damage regions. At 523 K the high-damage regions (HDR) were particularly well developed and consisted of tangles of dislocations decorated by interstitial and vacancy dislocation loops, which grew in size as the dose increased. At 573 and 623 K the HDR were more widely spaced, with the constituent dislocations being relatively unjogged, loosely tangled and not decorated by dislocation loops. The relatively undamaged regions between the coarse dislocation structure (LDR) contained small dislocation loops, stacking-fault tetrahedra (SFT) and/or voids. The defect types depend on neutron dose and irradiation temperature. At 623 and 573 K SFT and voids coexist, and there is a trend for voids to become the dominant sink as the dose increases. At 623 K only SFT were observed at doses up to 1022 n m−2 (E > 1 MeV), while at a dose of 2 × 1022 n m−2 (E > 1 MeV) the defect population consisted of large voids with very few SFT. The dose dependences of the concentration and size distribution of the different components of the LDR at the different temperatures were complex. Notable features were that the SFT size distributions exhibited little dependence on dose and temperature, while bimodal void-size distributions were observed at 573 and 623 K. While it has not been possible to interpret the results in terms of a detailed quantitative treatment, insight was gained into the interdependence of the different components of the damage structure and the factors controlling their formation. In particular, the competition between SFT and voids as sinks for vacancies has a major influence on damage structure evolution, and this aspect is analysed in some detail, as is the role of residual gases in stabilizing the observed void populations.

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