Synchrotron CT for high-resolution examination of large objects

A synchrotron is a special infrastructure that includes a number of particle accelerators. Conventional X-ray tubes also exploit the acceleration of ions. However, unlike conventional X-ray sources, synchrotron X-ray imaging accelerates electrons to near the speed of light in three stages: First, they are accelerated to nearly 99% of the speed of light by a linear accelerator. These subatomic particles are then injected into a booster synchrotron, where the energy is further increased to 6 GeV. Packets of these accelerated electrons are then injected into the large storage ring every 100 ms and held at speed in an ultra-high vacuum.

Inside the large storage ring, a variety of permanent magnetic systems - called insertion devices (wigglers, undulators) and deflection magnets - are installed to keep the accelerated electron beam as coherent, i.e., focused, as possible and thus increase the brilliance. Brilliance measures the number of photons per unit time and relative to the area of the beam profile. Synchrotrons deliver extremely high brilliance in this process due to their special architecture, in about 12 orders of magnitude above medical X-ray sources, enabling sharp images at high resolutions.

While the insertion devices manipulate the beam along the straight portions of the ring accelerator, the deflection magnets guide the beam around the curved sections. As the electron beam passes through the deflection magnets, the electrons in the beam lose velocity and release this energy in the form of X-rays. These X-rays are then guided out of the storage ring tangentially at specified points and into a beamline. The X-ray beam arriving there possesses the beam properties of extremely high brilliance and strong coherence held in the ring.

In addition, the high frequency with which X-rays can be extracted from the storage ring allows an in situ and, if necessary, in operando view of the object, which therefore enables time-resolved experiments.

Phase contrast CT is performed at beamline BM18 in particular. This is a special form of X-ray imaging in which the interaction of the wave of the X-ray radiation with the sample is recorded directly. This results in an exaggeration of the edges, which are thus typically recorded much sharper than with regular absorption contrast CT.

Artifact correction, numerical recovery of the absorption information, and tomographic reconstruction from the individual projections are then performed. Especially in industrial applications, this also accumulates a considerable amount of data, which can hardly be processed conventionally, which is why a three-dimensional compression developed at Fraunhofer EZRT will also be used in the future.

Based on the electron beam from the storage ring, a variety of experiments can be performed on the respective beam lines. Depending on the design, the individual beam lines further filter and modify the beam to tailor its spectrum to the specific application. This enables multiple imaging techniques to be performed with one beam, including absorption contrast tomography, phase contrast tomography, diffraction imaging, spectroscopy, etc.

The large number of possible imaging techniques results in a correspondingly large number of application fields:

• From an industrial point of view, the added value of synchrotron lies mainly in the imaging of complex components made of materials that are difficult to transmit radiation through, the detection of defects and other flaws within fuel cells, or the analysis of novel materials.
• Art and cultural objects and their investigation with reference to forgery detection, the uncovering of overpainted images, and the analysis of ancient dyes.
• The technology is also becoming increasingly important in the field of archaeology and paleontology, where fine details and images of fossils can be produced more sharply than with classical methods.
• Similarly, biological objects from agriculture are often examined, e.g., to study disease infestations of plants with high precision.

A current example of synchrotron CT can be found in the investigation of novel battery materials, where the distribution and structure of tiny particles in the anode material is of particular interest. The figure shows a hierarchical tomography of a flat Li-ion battery pouch cell: Starting from a scan of the entire object with a voxel size of 42 μm, regions of particular interest are determined incrementally. After entering new settings concerning the detector used, object positioning, and others, another CT scan is performed. Thus, region-of-lnterest scans with voxel sizes of 24.63 μm, 9.9 μm, and 2.24 μm are obtained. These scans show in small selected regions of the same size that the increasing resolution helps to identify the different structures of the battery, i.e., the separator plates and the cathode and anode particles.