Monte Carlo Simulation Of Electron Bremsstrahlung And Multiple Elastic Scattering

Monte Carlo simulation has been widely used in particle transport theory.The accuracy of Monte Carlo simulation of particle transport depends on the accuracy of physical processes used in the simulation.Previously some efforts have been done to use accurate and efficient methods to describe the Monte Carlo simulation of electron transport in general purpose electron transport codes but still there is need to improve the efficiency and accuracy of physical models used in Monte Carlo electron transport codes.In Monte Carlo simulation of electron transport,electron bremsstrahlung and multiple elastic scattering are the most time consuming and complicated phenomena.Therefore,in this work,accurate and efficient methods for electron bremsstrahlung and multiple scattering have been developed and implemented in a general purpose Monte Carlo Program for Nuclear and Radiation Simulation called SuperMC.Focusing on these aspects the main research and innovative work of this dissertation are followings.Firstly,a fast and accurate algorithm for the Monte Carlo simulation of electron bremsstrahlung based on SuperMC has been developed.Efficient and accurate methods have been used by which the angular distribution and energy of emitted bremsstrahlung photons are sampled.A discussion of the sampling efficiency and accuracy of this new-ly developed algorithm is given.Photon energy is sampled according to scaled energy-loss differential cross sections(DCSs)tabulated by Seltzer and Berger(1983).A novel hybrid model for photon angular distribution by low as well as high energy incident electrons has been developed.The model uses Tsai full form of angular distribution function with atomic form factors for high energy incident electrons,while for elec-trons with kinetic energy less than 500 keV,a simple,efficient and accurate analytical distribution function has been proposed.The later uses adjustable parameters deter-mined from the fitting of numerical values of the shape functions tabulated by Kissel et al.(1983).The efficiency of sampling photon energy is at least 80%.Our angular sampling algorithm for high energy electron bremsstrahlung based on Tsai distribution function is more efficient(sampling efficiency at least 70%)in the useful photon energy range than the sampling technique proposed by Alex F.Bielajew et al.(1989)Secondly,an efficient and accurate model has been developed based on Goudsmit-Saunderson theory of multiple elastic scattering for angular distribution of electrons and positrons.Differential cross sections of electrons and positrons by neutral atoms have been calculated by using Dirac partial wave program ELSEPA.The Legendre coeffi-cients are accurately computed by using Gauss-Legendre integration method.Final-ly,a novel hybrid method which combines rejection sampling and fitting function for sampling angular distribution has been developed.The model uses efficient rejection sampling method for low energy electrons(<500 keV)and larger path lengths(103 mean free paths).For small path lengths,a simple,efficient and accurate analytical dis-tribution function has been proposed.The later uses adjustable parameters determined from the fitting of Goudsmith-Saunderson angular distribution.The efficiency of rejec-tion sampling algorithm is at least 50%for electron kinetic energies less than 500 keV and longer path lengths(>500 mean free paths).Monte Carlo Simulation results are then compared with measured angular distributions of Ross et.al(2008).Comparison shows that our results are in good agreement with experimental measurements.The results showed that the proposed algorithm provides accurate and efficien-t methods for sampling energy and direction of bremsstrahlung photons and angular distribution of multiple scattering.Hybrid model provides very accurate treatment of bremsstrahlung photon angular distribution by low as well as high energy incident elec-trons.It is worth mentioning that the approaches used for sampling energy and direction of bremsstrahlung photons and angular distribution of multiple scattering can be used for a wide range of incident electron energies and materials and hence suitable for gen-eral purpose Monte Carlo simulations.