Analysis of computational methods used to optimize transmission x-ray targets for 100- to 500-kV x-ray tubes.

While the process of designing a low-to-medium energy transmission X-ray target follows a similar general approach throughout the literature, the specific methodology can vary substantially. The goal of this study was to determine the sensitivity of X-ray target optimization to different methods of X-ray quantification and temperature modeling. Approach: Ultrathin (1.5- to 90-μm) tungsten transmission X-ray targets were modeled in TOPAS (v.3.8) and bombarded with electrons at energies of 100-500 keV. Simulations were run to compare the optimal target thickness according to various tallies of the emitted X-ray field, including primary beam fluence, energy fluence, air KERMA, dose to water in air, or dose to water at depth in phantom. The optimized targets for each energy were simulated again to calculate the spatial distribution of heat deposited within the target and the attached diamond or copper window. Finally, COMSOL Multiphysics® was used to compare temperature calculations with a volumetric heat source model versus a simplified surface heat source model. Main results: The target thickness optima varied significantly across the different X-ray quantities; whereas some metrics favored very thin targets (<10 μm) for higher, low-quality X-ray output, others favored thicker targets (>50 μm) weighted towards higher-energy X-ray fields. Heat calculations in the optimized targets showed that single-scattering Monte Carlo produced a greater energy deposition tally compared to multiple-scattering models. Lastly, finite element temperature calculations did not change significantly between the full spatial heat deposition model and the simplified surface heat source model. Significance: This study demonstrates the importance of selecting the proper quantity for target optimization to ensure the most translatable results for the device's intended use. This work also shows that temperature modeling can be greatly simplified to accelerate the design process while maintaining computational accuracy. The data presented here represent an important resource in transmission X-ray target modeling.