Monte Carlo simulations in the optimisation of linac bunker shielding

Monte Carlo modelling is ideal for the determination of linear accelerator bunker shielding capabilities, especially when non-standard geometry and beam configurations are involved, making traditional manual calculations, as outlined in NCRP 151, less effective. This study developed a realistic model of a Varian TrueBeam linac head and its surrounding bunker using Geant4. Various modifications were implemented to enhance the bunker performance, such as adjustments to the wall thickness, maze geometry, shielding concrete type, and the addition of new shielding layers. This can identify potential weak spots and offer recommendations to enhance the cost and space efficiency of bunker design. The results show a strong agreeance between simulated and measured values. Overshielding of the primary and secondary barriers by 1 tenth value layer was observed. Changes to the maze geometry effectively reduced the dose to the maze entrance, while denser bunker materials improved photon attenuation within the walls. Inclusion of additional shielding layers allowed for significant reductions in required wall thickness, particularly with steel and concrete/lead layering. A combination of maze improvements reduced the dose to the maze entrance by up to 85 %. While this research highlights areas for improvement, further investigation is necessary to achieve a fully optimised bunker design.