Simplified model for positronium-helium scattering

Using a local effective potential to account for electron exchange, we have analyzed the zero-energy scattering of ortho-positronium by helium atoms.The effective potential was chosen so that the results obtained in the static approximation agreed with the static-exchange results of Fraser and Kraidy.Correlation was introduced in two different approximations which agree well.We find a decrease in the scattering length a from 1.72 a to 1.3 , -0 9 a 0 and an increase of more than a factor of 2 in the quenching rate of the positronium (from Zeff -0.042 to 0.10), as compared with the static exchange approximation.This gives improved agreement with the experimental value Zeff a. 0.18.