Interstitial clusters are known to cause frequent one-dimensional (1D) jumps (stepwise positional changes) under electron irradiation around room temperature. The distance of 1D jumps in iron was examined in detail through in-situ observation using high-voltage electron microscopy. The 1D jump distance was found to be longer for smaller clusters in specimens of higher purity, although the distance did not depend on the irradiation beam intensity and electron energy. The distribution of the 1D jump distance was well described by the distribution of the free path of interstitial clusters migrating through randomly distributed impurity atoms. The 1D jump process is considered as fast 1D diffusion of interstitial clusters at low activation energy from the point where the cluster detrapped from an impurity atom to the point where the cluster was trapped again by another impurity. Electron irradiation provides a trigger for causing 1D migration by detrapping of clusters from impurity atoms.
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