Conversion of ortho-positronium in low-density oxygen gas

Conservation of ortho-positronium in oxygen has been investigated by the high-resolution 2 gamma angular correlation method. The oxygen pressure was low enough to suppress possible conversion of para-positronium to ortho-positronium. Silica aerogel was used to form a sufficient number of positronium atoms in a restricted region necessary for the high-resolution angular correlation measurements. The existence of an inelastic conversion process has been reconfirmed. The cross sections for the elastic and inelastic conversion processes have been estimated to be (1.0+or-0.3)*10 -19 cm 2 and some 10 -17 cm 2 , respectively.