2.2.

The Diamond-Manchester Imaging branchline

2.2.3

Full-field microscope (TXM)

Details of the pilot instrument TXM are described elsewhere [7](see figure 2), the essential characteristics are summarized in this section. For the optical layout of the instrument the working distance between sample and objective lens is larger than 50 mm to allow for space of dedicated sample environments. A field of view close to 100 μm provides ease for sample preparation and corresponds to the microscope’s depth of focus. Tomography is applicable over the entire field of view for the sample. The large field of view is enabled by a specific microscope condenser optic, called ‘beam shaper’. This device consists of an array of diffractive optics, each segment focuses on the sample plane.

Figure 2:

Scheme of optical layout of TXM (top), and photo of pilot instrument (bottom) [7].

The instrument provides imaging in Zernike phase contrast mode. The early version of the instrument operated with monochromatic beam of a Si (111) monochromator and the PCO4000 detector system of the micro-tomography setup. Two sets of X-ray optics for 8.3 and 11.5 keV (collaboration C. David, PSI) are currently used. Most recently more broadband radiation from a multilayer monochromator (MLM) has been successfully tested and a more efficient, medium-resolution detector is used. Three different multilayer systems (Mo-B₄C; Ru-B₄C; V-B₄C) of the monochromator cover the energy range of the beamline with energy bandwidth ΔE/E of 10^-3 to 10^-2. The flux is increased by about a factor of 20 compared to monochromatic beam. The Hamamatsu C12849-101U X-ray sCMOS detector has a 2048x2048 array with 6.5 micron pixel size and a 10 μm thick Gadox scintillator. With the current version of the instrument typical exposure times are in the range of some hundred millisecond and full tomograms can be recorded within minutes.

2.3.

The coherence branchline

The spatial resolution for imaging in direct space is either limited by the detector resolution (for in-line phase contrast) or the X-ray optics used (the outermost zone width of the zone plates of the transmission microscope). Techniques such as ptychography and Bragg-CDI operating in the far-field/reciprocal space overcome these limitations. Currently ptychography and Bragg-CDI are developed at the coherence branchline for user operation. The basic instrumentation is similar to the imaging branch in terms of sample and detector stages currently used [8, 9]. For both experiments –Ptychography and Bragg-CDI-different optics such as Fresnel-Zone plates (FZP) and Kirkpatrik-Baez (KB) mirrors are available for focusing the beam to spot sizes between 50 nm to 10 μm. The distance between sample and detector can be large: in the case of ptychography up to 18m and for Bragg-CDI up to 8 m (with some limitations in the angular range for the latter). The variability in terms of spot size and detector-sample distance provides a zooming capability for the branchline, which is a unique feature for this type of techniques. The detectors for the branchline are based on the single photon counting MediPixIII chip and a pixel size of 55 μm. The EXCALIBUR detector has been developed in junction with STFC [10, 11], measures a 2k x1.5k pixel array, which can be read out with 100 Hz in continuous mode and 1 kHz in burst mode. The detector is either held on a translation stage or for Bragg-CDI on a robot arm (see figure 3). The latter provides the necessary stability and drifts of few microns over the time scale of hours [8]. Today, ptychography can be performed with a rate faster than 10 Hz and a resolution better than 30 nm, using specific scanning software originally developed for serial crystallography [12]. In the near future stages for the sample will be upgraded with piezo stages (ptychography), for Bragg-CDI the angular range will be increased. The ptychography and Bragg-CDI measurements can also be complemented with fluorescence information. Currently a single element Vortex X-ray Detector with XMap readout is used. The maximum detectable energy is 20 keV.

Figure 3:

Layout scheme for experiments on I13 coherence branch (left) and photo of experiment and robot arms.

3.1

Nuclear graphite

Graphite is a moderator material for neutrons in nuclear reactors. Its long term stability is highly relevant for the industrial use in fission power stations. The micro-structure is studied under different conditions, such as heat and mechanical load. The rig and the sample environment used for the measurements are shown in figure 4. Pressure and torsion can be applied with an Open-Frame Rig (Deben; see figure 4 left). Two rotation stages, placed above and below the sample, (see figure 4, left blue part) can be operated indepently and the distance between both can be changed. Two infrared lamps focus from opposite directions onto the sample (figure 4 centre and right). Two encapsuled glass cylinder enclose and secure the radioactive material.

Figure 4:

Sample environment for studying nuclear graphite. Left: torsion rig with two independent rotation stages above and below the sample Centre and right: double glass enclosure for sample and surrounding heating units

First results of this study are shown in figure 5 [13]. The experiment was carried out with polychromatic ‘pink-beam’, taking 2001 projections with 75ms exposure time. Using an effective pixel size of 2.6μm on the detector, the resolution is estimated to be around 6μm. The measurements compare the micro-structure of unirradiated and irradiated graphite, both samples heated up to 300° C and exposed to a load of 450 N. The significant structural difference between the two types of samples is obvious as shown in the reconstructed sections. Radiation and radiolytic oxidation cause a significant increase of the material’s porosity and simultaneous compression of filler particles (see circle in figure 5). The results serve for theoretical models for predicting the change of the thermal expansion coefficient – and therefore the expansion behavior - for graphite rods during their lifetime.

Figure 5:

Section through reconstructed volume of unirradiated (left) and irradiated (right) nuclear graphite [13].

3.2

Ice cream, CO₂ storage and Batteries

Some more examples for experiments with dedicated sample environment are listed in the following and details are provided in the corresponding publications. One original experiment is about ice cream. The taste of ice cream is significantly influenced by the microstructure of its components. A specific box has been designed for the experiment with the capabilities to go through several melting-freezing cycles to induce the typical changes which may occur [see figure 6 left) [14]. The results show the coarsening of both the air cells and ice crystals in ice cream, which finally result in a change in taste.

Figure 6:

Sample environment for study of ice cream (left); storage of supercritical CO2 in brine and sandstone studied with high-pressure cell and ‘pink’ beam (right).

The first fast scans (some seconds per tomogram) at the beamline were carried out on the study of CO₂ storage in brine and sandstone. For this experiment a dedicated system has been designed, consisting of a pumping system putting up a pressure of up to 100 bars and the container with the sandstone-brine-CO₂ mix (setup and ‘pink’ beam see figure 6, right) [15-17]. With the setup it has been possible to learn about the connectivity between the different components, which is essential to understand the mechanism of storing CO₂.

Battery materials require special enclosures for protecting the often very reactive composites such lithium from the outer atmosphere. The material has to be at the same time sufficiently transparent for X-rays. Initially kapton cells were used and are now replaced by more advanced designs using other cell materials[18, 19].

3.3

Fukushima fallout particles

The Fukushima reactor accident in 2011 produced nuclear fallout. The particles found in the following provide valuable information for possible accident scenarios. The collected samples were sealed in double walled kapton containers (see figure 7, left) for the experiments on the I13 coherence branchline. The ptychography and fluorescence measurements permit to learn about the morphology and the chemical composition of the sample. While the ptychography data provides a 2D resolution of about 30 nm, the spectroscopic data are in the micron range. Ideally, the ptychographic data may help improving the resolution of the spectroscopic information.

Figure 7:

Sample environment and measurements of particles from Fukushima fallout Left: Double wall container for radioactive samples for ptychographic measurements. Centre: object phase reconstructed from a 2D ptychography scan. Right: 3D Distribution of the manganese obtained from the fluorescence data.

3.4

Sample environments and Manchester collaboration

For the future it is planned to provide a choice of different sample environments for user operation, to be booked online or requested jointlyin with the beamtime application. Many of these sample environments are purchased by the renewed Manchester collaboration. This collaboration ensured the completion and the proper staffing of the imaging branchline in the past and continues supporting the scientific program at I13L now.