Three-gamma annihilation of ortho-positronium in NiO/γ-Al2O3 catalysts detected by positron lifetime and coincidence Doppler broadening measurements

摘要

The pore structure of NiO/γ-Al2O3 catalysts is characterized by positron lifetime and Doppler broadening measurements. A very long lifetime τ4 of 92 ns is resolved from the positron lifetime spectrum measured for pure Al2O3, which could be attributed to the ortho-positronium (o-Ps) lifetime in large pores. It was also found that the fitted lifetime τ4 and its corresponding intensity I4 obtained from the lifetime spectra both decrease with narrowing energy window of the stop channel in the fast-fast coincidence lifetime measurement system. This suggests that the ultra long lifetime is primarily due to the self annihilation of o-Ps which emits three gamma-rays. Such 3y annihilation is further evidenced by measuring the Doppler broadening of annihilation gamma rays in coincidence with the prompt gamma rays (1.28 MeV) emitted from the ~(22)Na positron source. In NiO/γ-Al2O3 catalysts both the lifetime τ4 and its intensity I4 decreases with increasing NiO content (from 3 wt% to 40 wt%), which indicates decreasing of the number of 3y events. The 3y annihilation parameter analyzed from the coincidence Doppler broadening spectrum shows consistent decrease with increasing NiO content.