In-situ characterization of embedded metallic nano-clusters with a positron beam

Positron measurements on model systems of aluminum with thin embedded layers (0.5nm to 100nm) of different materials (Au, Cr and Cu) are reported. It is shown how the positron affinity of different metals affects the measurements sensitivity. It is demonstrated that temperature dependent Doppler broadening on gold clusters embedded in aluminum reveals clustering and re-organization processes. When a positron is implanted into a sample, it thermalizes rapidly (within ps) and starts to diffuse over up to several hundred lattice constants. During the diffusion it can be trapped at irregularities in the crystal matrix. Hence it acts as a ''nanoprobe'' with a highly increased sensitivity e.g. for vacancy-like defects and metallic agglomerations. The non-destructiveness of this method enables in-situ observation of agglomeration growth at different temperatures. The usage of a mononergetic positron beam assures the positron implantation into the bulk where surface effects do not influence the measurement.