Low-dose neutron irradiation damage in copper. I. Vacancy loop production

Abstract

In this paper vacancy loop production in neutron-irradiated pure copper is described. Single crystals with an [011] orientation have been irradiated at 355K and 523–623 K with doses of up to 2 × 10²²nm⁻² in the materials test reactor PLUTO. Subsequent observations in the transmission electron microscope of material irradiated at 355 K revealed that the damage structure consists of a uniform distribution of dislocation loops which are predominantly vacancy in nature. An analysis has been carried out to quantify the link between the cascades produced by energetic neutron lattice-atom collisions and the observed vacancy loop population. This has exposed the need to know the threshold energy, E _T, required to generate a cascade that collapses to a loop of minimim visible size. Taking 6 keV and 10keV as limiting values for E _T, the corresponding values of the recoil spectrum averaged defect yield, Y _N, and cascade efficiency, ε_N, are 0·08 to 0·11 and 0·49 to 0·37, respectively. At irradiation temperatures of 523–623 K the damage structure has two components: a coarse dislocation structure decorated by dislocation loops, stacking fault tetrahedra and voids. It is shown that the fall-off in vacancy loop numbers at elevated temperatures is broadly consistent with loop lifetimes during elevated temperature irradiations being governed by the thermal emission of vacancies.