From the discovery to modern applications of X-rays
A central issue in soft matter physics is the characterization of particles in solution and their dynamic properties. The foundation to address this question was laid in 1895 when Wilhelm Röntgen discovered X-ray radiation. He observed the glow of fluorescent materials in the vicinity of a shielded cathode ray tube. Due to the lack of knowledge about the nature of this radiation, he named the unknown rays “X-rays”. A short time later, he discovered that radiation emitted from the cathode ray tube is capable of penetrating human tissue, but not bone. This discovery resulted in what is today probably the most famous X-ray image (Fig.2a) which shows the left hand of his wife Anna Röntgen. The newly discovered radiation aroused great interest in science, medicine and many other fields. In the following years, great efforts has been made to improve X-ray tubes due to their versatile applications. Half a century after their discovery, the generation of X-rays was also observed in synchrotrons. The simplified working principle of a synchrotron light source is that pre-accelerated electrons (very close to the speed of light) are transferred to a storage ring in which they are kept on a closed-loop path with a constant energy. At dedicated points of the electron storage ring, strong magnets force the electrons to undergo oscillations and emit X-rays. The X-ray radiation from synchrotron light sources differs in many aspects from the radiation from X-ray tubes. The most important ones are the much higher brilliance provided by synchrotrons, the coherence of the radiation and the free choice of photon energy. These properties allow scientists to study time-dependent processes in soft matter systems, like e.g. the diffusion of proteins in solution. However, in comparison to X-ray tubes, the costs for constructing and operating a synchrotron are immensely high and they require a lot more space. Today’s storage rings range in circumference from hundreds of meters to kilometers (see Fig. 3) whereas X-ray tubes fit easily in your pocket.
Using X-rays to characterize particles and their interaction in solution
Even though the advancement of X-ray tubes is tremendous (Fig. 1b), their radiation is not sufficient to resolve dynamic processes on nanometer length scales. The information about the shape and size of particles in solution as well as their interactions are “hidden” in the scattering pattern (Fig.1). This scattering pattern arises from the interaction of the X-rays with the molecules in solution. The shape and size of the irradiated particles can be extracted from the scattering pattern of a strongly diluted solution by radial integration of the 2D detector image. Knowing the shape and size of the molecules allows to study particle-particle interactions for higher concentrations. As already indicated, the scattering pattern is recorded by a 2D X-ray detector. The detector can be compared to a conventional camera. It also creates a 2D image, but is sensitive to X-rays rather than visible light. Using an incoherent beam results in a smeared out scattering pattern, i.e. loss of the grainy structure, of the detector image. However, the temporal evolution of the grainy structure contains the information about the systems dynamics and comes with further requirements to the detector. To visualize fast processes on nanometer length scales, the detector must have the appropriate time resolution, like a slow-motion camera.
Combining static and dynamic information - X-ray photon correlation spectroscopy
To draw a complete picture of the complex phenomena in soft matter systems, it is crucial to also characterize the dynamic processes. This can be achieved by using the coherent scattering method X-ray photon correlation spectroscopy (XPCS) which is schematically described in Fig.1. The coherent scattering pattern (speckle pattern) of a sample is directly related to the spatial arrangement of the scattering molecules in the irradiated volume. Consequently, the temporal changes in the speckle pattern relate to their movement or displacement in solution. In an XPCS experiment, the temporal correlation of the speckle patterns with themselves is investigated which is called auto correlation. The timescale on which the correlation vanishes is a measure for the velocity of the moving molecules in solution and provides information about diffusion coefficients of the system. In our case, we applied XPCS on phase separating protein systems. A picture of the setup is shown in Fig. 4. Under changing pressure conditions, the proteins’ attraction increases and leads to the formation of two liquid phases which is called liquid-liquid phase separation, where one phase has a higher protein concentration compared to the other. Those liquid-liquid phase separations (LLPS) are fundamental in many physiological processes. For example, LLPS is regarded as precursor for the fibrillation of brain cells which leads to neurodegenerative diseases like Alzheimer. Observing the temporal changes in the speckle pattern during LLPS allowed us to track the formation, growth and diffusion of the concentrated protein phase. These observations are only possible due to the high coherent flux provided by today’s synchrotron radiation sources. However, to investigate the diffusion of single protein molecules it requires faster detectors and an even higher coherent flux. Improved detectors are currently under design (see  and references therein). With 4th generation synchrotron radiation sources  a higher coherent flux will become available.
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