Figure 1. Schematic view of the system for the present study.

Using this system, we extracted the dynamic behaviour of fast-migrating atomic-size single SIAs, which cannot be directly traced. Within a high-voltage electron microscope, under high-energy electron irradiation, pairs of an SIA and vacancy are produced spatially homogeneously at a constant rate in a tungsten specimen. Individual SIAs migrate and react with other objects, such as immobile vacancies (leading to mutual annihilation), impurity atoms (SIA–impurity complex formation), SIA–impurity complexes (heterogeneous SIA–cluster nucleation), other SIAs (homogeneous SIA–cluster nucleation), SIA clusters (SIA–cluster growth), and surfaces (SIA escape to surfaces). The formation process of nanoscale SIA clusters, which can be directly imaged by TEM, reflects numerous SIA reactions whose rates strongly depend on SIA dynamics, such as migration dimension (3D or 1D), migration frequency, and reaction radius with other objects. In the present study, the dynamics of TEM-invisible SIAs were extracted from direct observation of the formation process of TEM-visible SIA clusters. The displayed microstructure within the specimen is a snapshot (irradiation time: 331.14 s) produced by an object kinetic Monte Carlo (OKMC) simulation of SIA reactions for SIA dynamics parameters that reproduced the experimental data.