Effect of permanent dipole moment of molecules onpositronium thermalization in the gases

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The scattering of positronium (Ps) atoms by atoms or molecules is a fundamental process that provide an attractive testing ground for scattering theories. Recently, it has become possible to extract useful qualitative information on Ps interactions with atoms or molecules in the energy range below 1eV using the angular correlation of annihilation radiation (ACAR) method combined with the use of silica aerogel.In this project, we have observed Ps thermalization process in Ne, H_2, N_2, CH_4, and C_2H_4 gases. By analyzing the data, we determined the momentum-transfer cross sections between Ps and these molecules to be (10*7)×10^<-16>cm^2 for Ne, (17*5)×10^<-16>cm^2 for H_2, (26*8)×10^<-16>cm^2 for N_2, (17*8)×10^<-16>cm^2 for Ch_4, and (23*7)×10^<-16>cm^2 for C_2H_4. We have also compared the Ps thermalization process in neo-C_5H_<12> is slower than that in neo-C_5H_<12>. This indicates that the effect of permanent dipole of iso-C_5H_<12> is small for Ps thermalization.