Chemical Quenching of Positronium in CuO/Al₂O₃ Catalysts

CuO/Al₂O₃ catalysts were prepared by mixing CuO and γ−Al₂O₃ nanopowders. Microstructure and chemical environment of the catalysts are characterized by positron annihilation spectroscopy. The positron annihilation lifetime measurements reveal two long lifetime components τ₃ and τ₄, which correspond to ortho−positronium (o-Ps) annihilating in microvoids and large pores, respectively. With increasing CuO content from 0 to 40 wt%, both τ₄ and its intensity I₄ show significant decrease, which indicates quenching effect of o−Ps. The para-positronium (p-Ps) intensities derived from multi-Gaussian fitting of the coincidence Doppler broadening spectra also decrease gradually with increasing CuO content. This excludes the possibility of spin-conversion of positronium. Therefore, the chemical quenching by CuO is probably responsible for the decrease of o-Ps lifetime. Variation in the o-Ps annihilation rate λ₄ (1/τ₄) as a function of CuO content can be well fitted by a straight line, and the slope of the fitting line is (1.83±0.05)×10⁷ s⁻¹.