Spin conversion of positronium in NiO/AlO catalysts observed by coincidence Doppler broadening technique

Zhang, H. J.*; Chen, Z. Q.*; Wang, S. J.*; Kawasuso, Atsuo; Morishita, Norio

Positronannihilation spectroscopy was used to study the microstructure and surface properties of the pores inside the NiO/AlO catalysts. The positron lifetime spectrum comprises two short and two long lifetime components. The two long lifetime components correspond to ortho-positronium (o-Ps) annihilated in microvoids and large pores, respectively. With increasing NiO content, both the latter lifetime and intensity show continuous decrease. Meanwhile, the para-positronium (p-Ps) intensity, obtained from coincidence Doppler broadening spectra, increases gradually with NiO content. The different variation in o-Ps and p-Ps intensity suggests the ortho-para conversion of Ps in NiO/AlO catalysts. The electron-spin-resonance measurements reveal that the ortho-para conversion of Ps is induced by the unpaired electrons of the paramagnetic centers of NiO.