Chemical Quenching of Positronium in CuO/Al2O3 Catalysts

CuO/Al2O3 catalysts were prepared by mixing CuO and gamma-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 tau(3) and tau(4), which correspond to ortho-positronium (o-Ps) annihilating in microvoids and large pores, respectively. With increasing CuO content from 0 to 40 wt%, both tau(4) and its intensity I-4 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 decreases 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 lambda(4) (1/tau(4)) 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) x 10(7) s(-1).