CONSTRAINTS OF THE MULTIPLE-SCATTERING THEORY OF MOLIERE IN MONTE-CARLO SIMULATIONS OF THE TRANSPORT OF CHARGED-PARTICLES

The breakdown of Moliere's multiple-scattering theory for short pathle ngths occurring during Monte Carlo simulations with charged particles is demonstrated. It has been found that in certain conditions where th e theory is assumed to be valid, significant distortions of the angula r distribution occur that make the sampling of the polar angle questio nable in numerous steps of Monte Carlo simulations. The limits of the theory have been investigated, both using a large number of terms in t he Moliere's series and using steps of Moliere's theory where 1/B expa nsions are not involved. At B=4.5 the commonly accepted 3-term series expansion yields differences up to +/-6% compared with the evaluation of the complete Moliere angular distribution, and up to 7 terms in the series are needed in order to achieve agreement within +/-2%. One per cent agreement requires B=10. Numerical values of the full distributio n are given in terms of Moliere's parameters B and reduced angle theta . By using the general dependence. of the distribution results are val id for both electron and proton Monte Carlo simulations in any materia l.