High-speed hard X-ray microscopic movies at a free-electron laser
A group of researchers has for the first time performed high-speed microscopy using an X-ray laser at the European XFEL in Schenefeld near Hamburg, Germany. The method allows for observations of processes that take place at speeds up to a few kilometres per second, paving the way for 3D microscopic movies of fast phenomena, with important potential industrial applications.
Such movies could show what happens during complex processes with a resolution at the sub-micrometre level, which is less than the diameter of a human hair, while also teasing out hidden internal details. While most other applications of X-ray lasers are based on the short wavelength of their X-ray flashes, making images that reach atomic resolution possible, this use takes advantage of the penetrating properties of X-rays.
The resulting images, which are on the microscopic rather than atomic scale, reveal the internal structures of complex processes such as fluid cavitation at high speed.
The research, which has been published in the journal Optica, was led by scientists from the Center for Free-Electron Laser Science (CFEL) in Hamburg (a collaboration between DESY, Universität Hamburg, and the Max Planck Society) and the European XFEL and involves scientists from P.J. Safárik University in Slovakia, Lund University in Sweden, Diamond Light Source and University College London in the UK, the Karlsruhe Institute of Technology in Germany, and the European Synchrotron Radiation Facility (ESRF) in France.
For the experiment, the team used the SPB/SFX experiment station at the European XFEL. Using a bright optical laser, they caused a water-filled glass capillary tube of 300 millionths of a metre (300 micrometres) diameter to explode, which they then filmed with several X-ray pulses. The results revealed the shape, orientation, and velocities of the water droplets and glass shards in each image captured, calculated with a variety of different methods. The images' resolution, clearly showing moving objects in the single micrometre scale, was also compared favourably to a similar study performed by the team at ESRF which could record motion on the scale of kilometres per second.
The X-ray pulses generated by the European XFEL can be used for making observations of a variety of samples, particularly at the atomic scale-"molecular movies" that film chemical reactions taking place. X-ray microscopy is a different use of the facility's properties: Since X-rays can penetrate deeply into materials, as seen with medical X-rays, they can also be used to see inside structures of varying sizes, providing advantages over similar methods using visible light. Recently, some synchrotron X-ray sources began to use this property in order to observe fast processes at the microscopic scale. In this experiment, the scientists were able to make movies with better contrast and resolution than what is possible at synchrotrons.
The application of X-ray laser megahertz microscopy is expected to be of high interest for industry, helping develop more resistant and longer-lasting materials. The microscopic inner workings of fluid injection through nozzles, novel syringes, and microfluidic systems could be seen in slow motion. Industrial processes such as water jet cavitation or liquid jet atomization for spraying may be studied. More general cavitation phenomena and shockwaves could be observed and analysed with this method as well.
Lead author Patrik Vagovic, a scientist at CFEL, European XFEL, and the Institute of Physics at the Czech Academy of Sciences, said: "Because the European XFEL generates several orders of magnitude more photons per pulse than any synchrotron radiation facility, and in comparison with all other existing hard X-rays FEL's provides megahertz repetition rate of light pulses, we can push this method to further boundaries. For many samples, where there was previously just a simulation or only surface information of what's happening at the microscopic level at short timescales, we would for the first time have a tool for making direct observations of volumetric motion."
Additionally, a procedure using the European XFEL's extremely high brightness could allow for multiple simultaneous movies of the same sample viewed from different angles-meaning a sample could at once be shown in slow motion, at the microscopic level, and in 3D.
Prof. Adrian Mancuso, SPB/SFX leading scientist, said: "This is an important technique for observing faster-than-microsecond processes in materials at sub-micrometre length scales. This opens up possibilities for experiments that presently can't be performed anywhere else."
Source: European XFEL GmbH