Generalized Particle Concept for Adjoint Monte Carlo Calculations of Coupled Gamma-Ray-Electron-Positron Transport

Adjoint Monte Carlo methods for coupled transport are developed. The phase-space is extended by the introduction of an additional discrete coordinate (particle type of so-called generalized particle). The generalized particle concept allows the treatment of the transport of mixed radiation as a process with only one particle outgoing from a collision regardless of the physical picture of the interaction. In addition to the forward equation for the generalized particle, the adjoint equation is also derived. The proposed concept is applied to the adjoint equation of the coupled gamma-ray-electron-positron transport. Charged particle transport is considered in continuous slowing down approximation and Molière's theory of multiple scattering, for which special adjoint sampling methods are suggested. A new approach to simulation of fixed-energy secondary radiation is implemented into the generalized particle concept. This approach performs fixed-energy secondary radiation simulation as the local energy estimator through the intermediate state with fixed energy. A comparison of forward and adjoint calculations for energy absorption shows the same results for radionuclide energies with and without electron equilibrium. Adjoint methods show greater efficiency in thin slabs.