Angular correlation of annihilation radiation (ACAR) from silica-powder pellets and silica aerogel has been measured in order to investigate the slowing down of free positronium (Ps) atoms by collisions with silica grains and gas molecules. The data for the pellets and the aerogel in vacuum show that the slowing down of parapositronium (p-Ps) in the free space between the silica grains depends on the number of collisions and hence on the mean distance between the grains. The momentum distribution of orthopositronium (o-Ps) shows further slowing down because of its long lifetime. From the ACAR data obtained from specimens of aerogel 611ed with gases (He, Ne, Ar, Kr, Xe, H2, CH4, CO2, and iso-C4Hqo), the momentum-transfer cross sections between Ps and the gas molecules are estimated. It is concluded that the Ps kinetic energy is transferred only to the translational motion of the gas molecules, i.e. , the excitations of vibration and rotation of the molecules are negligible. PACS number(s): 36.10.Dr, 34.50. -s, 78.70.Bj I. INTR. ODU CTIONIt was observed in the mid 1960s that the angular correlation of annihilation radiation (ACAR) and positron lifetime spectra in metal oxide and metal Huoride powders [1,2] showed formation of positronium (Ps). In 1968, Paulin and Ambrosino [3] reported that the Ps component in the positron lifetime spectra for silica powders depends on the grain diameter. It was postulated that the Ps atoms form inside the grains and then disuse out of them [4]. Paulin and Ambrosino also observed the eA'ect of air on the o-Ps annihilation. Following this, silica powders were used for investigating the interactions between Ps and paramagnetic gases [5 -8].

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