Study of Gaseous Impurity Elements in Cavities with Use of Positrons and Positronium

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In metals and semiconductors, there are cavities which are induced by bombardment of high energy particle or produced during sample preparation processes. In silicate compounds (zeolites and sepiolite) there are structural cavities. These cavities have been object of basic research because of their important roles in physical, mechanical and chemical properties such as dimensional stability of heavily irradiated metals, and catalytic behavior of synthetic zeolites.In this study positrons are used to investigate the interiors of cavities, especially state and behavior of gaseous element molecules within them.1) Positronium in cavities (voids) in vanadium Positron ACAR (angular correlation of annihilation radiation) and life measurements have been done on neutron-irradiated vanadium samples with controlled impurity contents of oxygen or carbon. It has been shown that contamination of voids with oxygen impurities, presumably on the void internal surface, leads to Ps formation, but not wit … More h carbon impurities. From the annealing behavior of narrow component due to para-Ps, the Ps formation increases with annealing temperature up to 600 ﾟC but subsequently decreases with increasing the temperature. This suggests that segregation of oxygen impurities onto the void surface up to 600 ﾟC followed by resolution into the matrix. Lifetime experiments shows a very rapid Ps ortho-para conversion rate and its large temperature dependence.2)Positron annihilation in zeolites and sepiolite. Positron annihilation ACAR measurements have been carried out for series of synthetic zeolite powders and single fibers of natural sepiolite. All of the samples showed marked narrow para-Ps peaks af the top of the ACAR curves. From a simple model calculation, the Ps atoms are found to exist as free particle in the structural cavities, such as the alpha-cages in the MS-3A zeolite and the 1-dimensional cavity channel in sepiolite. Oxygen molecules influence motion and reactions of Ps in these cavities.Cavities in neutron-irradiated nickel and stainless steel and in graphite have been studied by positron annihilation method. Less