Monte Carlo Variance Reduction for Energy Profile Calculation at the Electron Emission Due to Photon Radiation

A variance reduction method has been developed for the Monte Carlo calculation of electron emission energy profile induced by photon radiation. The spatial control of particle weight was exclusively investigated. It was derived that the photon weight in the electron range at an electron detection surface should be equal to the electron weight that is determined to be inversely proportional to the electron adjoint function. Therefore, the preliminary Monte Carlo calculation of the forward electron-only problem with the uniform electron source over the maximum electron range, maximum allowed energy, and all solid angles was conducted to create the photon and electron weight window. The photon weight window more than the maximum electron range away from the electron detection surface was made constant. Monte Carlo simulations of photon and electron coupled-transport were conducted for slab materials with photons normally incident on one side and the electron energy profile to be evaluated on the other side. Numerical results show that efficiency gain with respect to the simulation with no weight control is significant for slabs of typical low and high atomic number materials even if taking into account time spent on the preliminary Monte Carlo calculation of the electron adjoint function.