Formation and annihilation of positronium in silica aerogels under atmosphere of oxygen and nitrogen mixture

Formation and annihilation of positronium in silica aerogels under flowing nitrogen and oxygen mixtures with various ratios were studied by positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening spectroscopy (CDBS). The annihilation rates, effective cross section for o-Ps quenching on oxygen molecules and effective number of electrons per oxygen molecule available for 2 γ -annihilation of positron in o-Ps have been measured by PALS. It is found that the longest o-Ps lifetime decreases remarkably and the center of positron 2 γ -annihilation peak becomes narrower and narrower as the oxygen partial pressure increases. The results show o-Ps quenching on oxygen molecules occurs through spin conversion, which simultaneously results in a remarkable decrement in its intensity. However, the total Ps formation probability is enhanced with increasing oxygen partial pressure, which is interpreted for higher probability of Ps formation on an oxygen molecule.