On approximations involved in the theory of positron transport in gases in electric and magnetic fields

A multi term theory for solving Boltzmann's equation is briefly reviewed and used to test various concepts and approximate expressions for the determination of the positron transport properties in neutral molecular gases in crossed electric and magnetic fields. Among many important approximations which have found their way into contemporary positron studies, the following are particularly discussed: (1) is the approximation of using the cross sections for the electron scattering to describe the positron behavior satisfactory, (2) how accurate is two term approximation for solving Boltzmann's equation in the context of positron studies, and (3) what is the domain of applicability of Langevin elementary transport theory and Tonks' theorem for positrons in electric and magnetic fields. We highlight the limitations, range of applicability and inadequacies of such assumptions for positrons in H2 and N2. It is pointed out that there is no real alternative to the accurate multi term theory and/or Monte Carlo simulations if high precision is required. It is demonstrated that if the demands for accuracy associated with some of these approximations are relaxed, results may not be even qualitatively correct.