Description of the measurement method
While using common X-ray computed tomography the spatial absorption of a sample can be imaged, differential phase- and dark field contrast allow for analysing the phase shift and the small angle scattering induced by the specimen. At IAM-WK this is accomplished through a 2- and 3-grating Talbot-Lau interferometer with a micro focus X-ray tube respectively. Goal is to achieve a high resolution through cone beam geometry and small focal spot sizes. While a high resolution is needed for many materials science related problems, some may already be answered using a more integral approach given by the dark field signal.
Schematics of the phase / dark field contrast setup at IAM-WK ©06
|Field of view||10 cm x 10 cm|
|Exposure time||1 s - 10 s|
|Max. sample mass||50 kg|
|Max. sample size||10 cm x 10 cm|
The setup is located on a granite block in a radiation-proof, conditioned lead-cabin. The granite serves as a damping material against external vibrations. The samples are mounted on a rotation stage with 50 cm diameter.
Sample environment in the lead-cabin. Visible are phase- and analysator grating, in between is the rotation stage and the micro focus tube in the back. ©06
Example of application
While analysing friction contact materials for clutch discs made from several combinations of fibre reinforced paper, common absorption based computed tomography encounters its limits concerning the separability of both: carbon based fibres and paper. Using dark field imaging it was possible to distinguish between parts with high and low fibre contents. The latter exhibit less scattering and hence a lower contrast in the dark field signal.
Projection of friction contact material in dark field contrast ©06
Name: Dipl.-Phys. Jens Maisenbacher