Development and application of a positron microbeam at JAEA

We have developed a positron microbeam using magnetic lenses based on the commercial scanning electron microscope. The minimum beam diameter was 1.9 μm on target. Using this apparatus, the Doppler broadening of annihilation radiation (DBAR) measurements were performed around the crack tip introduced by a stress corrosion cracking of a stainless steel. The peak intensities of the DBAR spectra (S parameter) increases at the further region of the tip of the crack. This shows that vacancy type defects may be generated as crack precursor. S parameter measurements were also performed on Si crystals near the melting point. In a very narrow temperature range near the melting point, the S parameter increased until melting. This indicates the formation of thermal vacancies. (author)