MOMENTUM-TRANSFER CROSS-SECTION FOR SLOW POSITRONIUM-HE SCATTERING

The momentum-transfer cross section for positronium-He scattering has been determined by analysing the thermalization process of orthopositr onium in the gas. The momentum distribution of the positronium is obse rved by using the one-dimensional angular correlation of the annihilat ion radiation method. Silica aerogel is used to form a sufficient amou nt of positronium in a restricted region necessary for the high-resolu tion measurements. The average energies of orthopositronium for mean l ifetimes ranging from 3.2 to 86 ns are determined by applying static m agnetic fields. A thermalization model which includes the momentum-tra nsfer cross section as an adjustable parameter is fitted to the averag e energy. The cross section thus obtained is sigma(m) = (11 +/- 3) x 1 0(-16) cm(2) for positronium in the energy range below 0.3 eV.