The momentum-transfer cross section for positronium-He scattering has been determined by analysing the thermalization process of orthopositronium in the gas. The momentum distribution of the positronium is observed by using the one-dimensional angular correlation of the annihilation radiation method. Silica aerogel is used to form a sufficient amount of positronium in a restricted region necessary for the high-resolution measurements. The average energies of orthopositronium for mean lifetimes ranging from 3.2 to 86 ns are determined by applying static magnetic fields. A thermalization model which includes the momentum-transfer cross section as an adjustable parameter is fitted to the average energy. The cross section thus obtained is σ m = (11 ± 3) × 10 −16 cm 2 for positronium in the energy range below 0.3 eV.

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