Monte Carlo radiation transport modeling studies were performed for a compact; high-resolution gamma-ray computed tomography system designed for imaging irradiated nuclear fuel. The system comprises a 60Co source – chosen for its highly penetrating 1173 keV and 1332 keV gamma rays – a pair of high-aspect-ratio pencil beam collimators, and an inorganic scintillator detector. While analog Monte Carlo methods can predict the detailed physics of gamma-ray radiation transport, these methods are computationally expensive for simulating entire radiographs, particularly when high resolution requirements demand that small detector solid angles and thick shielding be used. Two acceleration methods are proposed to rapidly model a transmission type gamma-ray tomography system. The first, a variance reduction technique, is based on performing Monte Carlo simulations with a monodirectionally-biased source, sampled from a characteristic sub-volume of the full source volume. This approach applies analytical correction factors to estimate the true count rate. Comparison of simulations using this acceleration approach with analog simulations of the fully isotropic, full-volume equivalent as well as deterministic calculations using the Beer-Lambert law and detector response characteristics show that both the Monte Carlo variance reduction technique and the deterministic calculations give reasonably accurate predictions of photopeak count rates with the level of agreement improving as the collimator aspect ratio increases. As such, these techniques can be used to reduce the computational cost by several orders of magnitude in radiation transport problems involving highly collimated gamma-ray radiography and tomography systems.
