Electron displacement damage in cobalt in a high voltage electron microscope

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Abstract
If a lattice atom receives, as the result of a collision with an incident projectile, an energy in excess of the displacement threshold energy Ed , then it is displaced from its lattice site, thereby creating at least one vacancy and one interstitial atom. High voltage electron microscopes with beam energies sufficient to transfer to a lattice atom an energy E> Ed are now available. Reported here are the results of an investigation on the formation and growth of visible defects (believed to be interstitial dislocation loops) which were produced in cobalt at 300°K during observation in the 750 kv high voltage electron microscope at the Cavendish Laboratory, University of Cambridge. Many of the features regarding the nucleation of the defects, the growth of the dislocation loops and the extent of a surface denuded layer are consistent with that expected from a simple theory of loop formation. Using the onset of visible damage as a criterion, the displacement threshold energy has been determined for various directions of incidence of the electron beam in both hexagonal close-packed and face-centred cubic cobalt. In the hexagonal close-packed case E d = 23, 30 and 33 ev for incidence along ⟨ll20⟩, ⟨10l0⟩ and ⟨0001⟩ respectively, and in the face-centred cubic case E d = 23 and 30 ev for incidence along ⟨110⟩ and ⟨100⟩ respectively.