Purpose: The microfocus x-ray imaging suffers from the long scanning time due to the insufficient x-ray intensity limited by focal-spot power density. We propose a new design of microfocus x-ray target with ultra-thin tungsten film on a diamond substrate. Simulations and experiments are performed to show the enhancement of output x-ray flux.
Methods: Geant4 Monte Carlo simulation models were created for transmission x-ray target with different thicknesses of W film on 0.5 mm diamond substrate. 100 keV electron beam hit the target with a 100 Âµm focal spot. The angular x-ray flux and energy deposition at different depths were obtained. The energy deposition results were then used as input in finite-element heat-transfer simulation using COMSOL software. The optimal thickness of tungsten film was determined based on Monte Carlo and thermal simulation results. A W-diamond target was fabricated by depositing tungsten on a diamond substrate using PVD, installed in a microfocus x-ray source prototype, and tested.
Results: Monte Carlo simulation shows that although x-ray flux by 300-nm W target is about 25% of 5 Âµm target the energy deposited in tungsten is much lower. The majority of energy is deposited in the diamond layer. The energies of electrons interacting with the W layer mostly maintain their high energy and direction. The x-ray flux is highly directional and the spectrum is much harder than reflection target. Finite-element thermal simulation demonstrates the heat can be rapidly removed thanks to the high thermal conductivity of diamond. A W-diamond target with 300 nm W layer was fabricated by PVD. The target was examined with optical and SEM after tested in the microfocus x-ray source and found undamaged.
Conclusion: Due to the high x-ray production efficiency and better heat conduction in diamond, the W-diamond target with ultrathin W layer can potentially achieve higher focal-spot power density.
Funding Support, Disclosures, and Conflict of Interest: NIH/NIBIB 1R03EB024952-01