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Dipartimento di Fisica - Politecnico di Milano

Variable energy positron annihilation spectroscopy (VEPAS)

In this apparatus positrons generated from the decay of a radioactive source are slowed down to energies of about 1 eV in a first stage; then, they are accelerated to energies up to 20 keV and focused into a spot of a few millimeter on the surface of the sample. The energy can be regulated by modifying the electrostatic potentials of a series of electrodes; this allows the positrons to be implanted in the sample in layers with variable thickness (up to about 1 µm in silicon). The setup, which is shown in the figure during its installation,  is used to investigate defects in open volumes (vacancies and nanocavities) formed in the sample after deposition or ionic implantation treatments. Spatial resolution for analysis of density profiles of defects is of the order of ten nanometers.