Positron annihilation lifetime spectroscopy in nickel ferrite and iron oxide nanopowders

Abstract

Abstract In this study, a positron annihilation lifetime spectrometer was set up and its resolution was optimized. The spectrometer was a fast-slow arrangement with the time resolution of 250 ps. To analyze the obtained positron annihilation lifetime spectrum and extract the lifetime components and their intensities, the Pascual software was used. Positrons production was done from a source of radioactive NaCl with 20 μCi activity enclosed in a 7μm thick Mylar foil. For the determination of source corrections, positron lifetime spectrum measurement was carried out on defect-free Aluminum samples and Mylar foils. The positron annihilation lifetime spectrum in nickel ferrite and iron oxide nanopowders was measured. The shortest component was attributed to the annihilation of the nonlocalized positrons in the samples. The intermediate lifetime was related to the annihilation of positrons in octahedral and tetrahedral cationic vacancies in the spinel structure and, partly, due to the annihilation of positrons in the surface of nanoparticles and vacancy clusters. The longest component was due to the ‘‘pick-off” annihilation of orthopositronium atoms in the large free volumes of the intergranular regions of the nanoparticles.