Optimization of Monte Carlo calculations for photon sources from actinides

The majority of the photon source intensities for most actinides are below 100 keV, but the lower intensity lines above 100 keV become the dominant contributors to dose rates beyond even very thin (∼1-mm) shields of lead or stainless steel. A thin shield will attenuate almost all of the very low-energy photons so the higher energy portion becomes the dominant contributor to the dose, even though it may be orders of magnitude lower in intensity. A monte Carlo calculation without optimization will waste most of the computer time on the lower energy (uninteresting) photons. Optimization of the selection of the source energy can be quite complicated because the near-optimum probability of selecting the higher energies increases with the thickness of the shields. Additional complications include the potential necessity of optimization int he selection of the spatial location of source points, and the often encountered situation where calculated dose rates are needed beyond various thicknesses of shields. Optimization of the Monte Carlo calculation for generic sources and shields is addressed in this paper