The voids forming a three dimensional lattice in a molybdenum specimen irradiated at 550°C to a fast neutron fluence of approximately 1x1022ncm-2 were investigated for the first time by the field ion microscope (FIM) technique. The numerical values obtained for the mean void diameter, the number density, and mean spacing are in approximate agreement with transmission electron microscope observations. Controlled field evaporation sequences through 16 voids revealed complex atomic displacements around each void much of which is a result of the void intersecting the surface. The advantages of the FIM technique for the study of high number densities (>5x1017cm-3) of voids are discussed and the potentialities of the FIM atom probe for the solution of problems concerning the nucleation and stabilization of voids are also considered.
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