Description
Dark-field X-ray imaging has emerged as a powerful tool for visualizing microstructural features by visualizing X-ray scattering. This offers enhanced contrast for structures invisible to conventional attenuation or phase-contrast imaging [1, 2, 3]. Existing dark-field methods, such as grating- and speckle-based techniques, have been mainly confined to laboratory and micro-CT systems with micrometer resolution [4, 5]. Recently, dark-field imaging at the nanoscale using full-field Transmission X-ray Microscopy (TXM) has been demonstrated, unlocking new pathways for high-resolution imaging of scattering features [1]. Enhancing the capability of this technique even further, we introduce a directional dark-field TXM approach that retrieves anisotropic scattering information, enabling visualization of structural orientation at the nanometer scale.
The experimental setup has been realized at the nanotomography station of the P05 imaging beamline at PETRA III, operated by the Helmholtz-Zentrum Hereon. The TXM comprises a beam-shaping condenser, a Fresnel zone plate (FZP), and a dark-field aperture (DF-AP) [1]. Directional sensitivity is achieved by incorporating condenser apertures (C-APs), allowing selective illumination from specific directions. By sequentially covering parts of the condenser, we extract directionally dependent dark-field images, enabling the reconstruction of angular and magnitude maps of the scattering vector. This setup allows seamless switching between transmission, dark-field, and directional dark-field imaging modes with minimal system modification.
We validated the method using a Siemens star test pattern and a human primary tooth enamel sample. The Siemens star demonstrated the capacity to retrieve scattering orientations well below the optical resolution limit, while the enamel sample revealed the orientation of hydroxyapatite crystals within rod-like prisms, highlighting variations in crystal orientation across the enamel structure.
Our results highlight the potential of directional dark-field TXM for nanoscale imaging of anisotropic microstructures in materials science and biomedical research. The technique provides complementary insights into hierarchical structures at the nanoscale, such as fiber orientation or crystalline anisotropy, that are inaccessible with conventional attenuation- or phase-contrast imaging. Moreover, the approach paves the way for future developments in nano tensor tomography and high-resolution in situ studies.
[1] S. Wirtensohn et al., “Nanoscale dark-field imaging in full-field transmission X-ray microscopy,” Optica, vol. 11, no. 6, pp. 852–859, Jun. 2024, doi: 10.1364/OPTICA.524812.
[2] C. Jud et al., “X-ray dark-field tomography reveals tooth cracks,” Sci. Rep., vol. 11, no. 1, p. 14017, Jul. 2021, doi: 10.1038/s41598-021-93393-4.
[3] V. Revol et al., “Sub-pixel porosity revealed by x-ray scatter dark field imaging,” J. Appl. Phys., vol. 110, no. 4, 2011, doi: 10.1063/1.3624592.
[4] A. Gustschin et al., “High-resolution and sensitivity bi-directional x-ray phase contrast imaging using 2D Talbot array illuminators,” Optica, vol. 8, no. 12, p. 1588, Dec. 2021, doi: 10.1364/OPTICA.441004.
[5] Ö. Öztürk et al., “Defect detection in glass fabric reinforced thermoplastics by laboratory-based X-ray scattering,” Compos. Part B Eng., vol. 252, p. 110502, Mar. 2023, doi: 10.1016/j.compositesb.2023.110502.
