Positron Annihilation Spectroscopy (PAS)

Abstract

A positron is an anti-particle of an electron and is annihilated with the electron in a sample. When lattice defects with open-volume exist in the samples, positrons are trapped there. Therefore, positron annihilation spectroscopy (PAS) is used to investigate lattice defects in samples. In this chapter, we will introduce the principles of the PAS technique and show some examples of how PAS was applied to studies on hydrogen storage materials. The topics are the formation mechanism of lattice defects by the initial hydrogenation in LaNi 5-based alloys and the effect of hydrogen charging on the formation of lattice defects by tensile strain (hydrogen embrittlement). PAS showed that both vacancies and dislocations are introduced by the initial hydrogenation in LaNi 4.5 M 0.5 and by plastic deformation in pure Fe. The presence of hydrogen enhanced the increase in the concentration of vacancies rather than the dislocation density in pure Fe.