Ultra fast micro-tomography

During the last decade micro-tomography has been extensively used at synchrotron sources in order to study the morphology of different kind of samples. Efforts have been made to improve the spatial resolution. The time resolution has been neglected restricting micro-tomography to static conditions or on systems evolving over very long timescales. In order to cover this gap ID15 has developed a fast micro-tomography system that is capable of collecting a full 3-D dataset in less than one second allowing in-situ studies of system evolving on the timescale of a few seconds.

High resolution micro-tomography setup.

The first micro-tomographic experiments at ID15 were performed approximately eight years ago. The measurements were performed using a classical tomography system: collecting step by step absorption ../images of the sample using monochromatic x-ray radiation. It immediately turned out that this type of design was not well suited for high energy imaging: data collection took from 4 to 8 hours due to very low efficiency of the scintillator screen. Also, the light optics were damaged by the radiation in a day. The origin of diffused colour-centres in the light optics is the Compton scattering in the lenses themselves, an effect which does not occur at low energies, where the colour-centres are produced by photoelectric absorption only within the region impinged by the direct x-ray beam. Three solutions were adopted in order to overcome these problems: i) the experiments were conducted in filtered white-beam radiation to increasing the x-ray flux, ii) the refractive objective was replaced by reflecting objective and a beam-stop was mounted on the secondary mirror to protect the camera and secondary optics, and iii) the YAG:Ce scintillator was replaced by LAG:Eu scintillator, which has better efficiency at high energies. As a result tomographic scans were performed in less than a minute with a spatial resolution of ~2µm.

By optimizing the CCD-detector (back illuminated chip with many readout ports using frame transfer mode), the architecture of the data acquisition system (continuous scans) and the rotation stage (high-precision high-speed air-bearing rotary stage) it is possible to collect a full tomographic dataset at 2µm spatial resolution in a few seconds (flux limited) and at ~2µm resolution in less than one second (detector limited) even when the synchrotron operates in 4 bunch mode. Recently phase contrast experiments with polychromatic high-energy radiation have been exploited with very encouraging results. The white high-energy x-ray beam does not have any temporal coherence, but it still has spatial coherence due to the small source size. The projections can be collected at one sample to detector distance and the phase information recovered in the near field region. This is practically always the case at high energies. Therefore, phase-contrast tomography can be performed at same speed as absorption contrast tomography. Furthermore, the absorbed dose is small due to the small x-ray absorption at high energies which allows the measurement of systems sensitive to radiation damage like biological samples.

In order to obtain complementary information about the sample the imaging setup has been integrated with the angular dispersive diffraction setup using an area detector. The use of these combined techniques provides the user with information about the evolution of morphology, texture, phase changes etc. The setup is equipped with a furnace working under controlled atmosphere up to 1250ºC. A loading device with a maximum load of 7.5kN, developed in collaboration with the Institute National Polytechnique de Grenoble, is available for in-situ tensile-compression micro-tomographic experiments.