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Online Annual Report 2014

Scientific Highlights

Dresden Researchers Offer Insights into Static Mixers' Flow Patterns

An illustration of a static mixer for industrial purposesOne of the chemical industry's most common processes is the dispersion and dissolution of gases in liquids. Therefore, so-called static mixers are increasingly used. The method involves elaborate arrangements of mixing elements like helical blades or cross-bars for mixing of various fluids like gases and liquids directly inside the tubing. The details of these mixing processes are still not unveiled. Computer simulations that are frequently used here are not powerful enough since flows are far too dynamic.

For this reason, researchers at the HZDR Institute of Fluid Dynamics have started using a new method called ultrafast X-ray tomography, a method that involves a rapidly moving X-ray source to irradiate the flow from many different angles. From individual projections, the scientists are afterwards able to reconstruct cross-sectional images. This means that 1,000 images per second are not even an issue. Even individual gas bubbles dispersed in the liquid and their pathways through the mixing segments can be visualized. Researchers are particularly interested in the bubble size distribution patterns since the mass transfer occurs across the bubbles' surfaces. The desired gas bubbles are small and highly dispersed as they act to intensify it.

With this research, the Rossendorf scientists were able to show that flow turbulence and centrifugal forces compete in the presence of helical elements. This in turn affects the mixture and the bubbles’ dispersion. On the one hand, the turbulence acts to break the bubbles. On the other, the bubbles coalesce as the centrifugal forces separate the lighter gaseous phase from the liquid, the heavier material. From these findings, the scientists are able to draw conclusion about the components' ideal arrangements and about the required mixing segment length. This way, the facilities' energy efficiency can be optimized.

  • Publication: S. Rabha, M. Schubert, F. Grugel, M. Banowski, U. Hampel, „Visualization and quantitative analysis of dispersive mixing by a helical static mixer in upward co-current gas-liquid flow”, Chemical Engineering Journal, 262, 527-540 (2015, DOI: 10.1016/j.cej.2014.09.019)
  • Contact: Dr. Markus Schubert, Institute of Fluid Dynamics

Combination of Different Methods Reveals New Information about Uranyl Compounds

Uran(VI)-Komplexe lassen sich zur Lumineszenz anregen

Through a combination of mathematical and experimental methods, researchers at the Helmholtz-Zentrum Dresden-Rossendorf were able to glean surprising insights into the uranyl(VI) hydrolysis, the basic model for examining uranium chemistry in aqueous systems. For the secure storage of highly radioactive waste in permanent repositories, knowledge of how the toxic materials interact with their environment is critical and scientists are drawing on a range of different spectroscopy-based methods to figure out the specifics of these interactions. As such, the chemical structure, the binding, and thus the dispersal behavior of actinides can be decoded.

Because of their high sensitivity, methods involving luminescence spectroscopy are particularly well-suited to studying uranyl(VI) systems. However, an analysis of the collected data has proved challenging, which is why researchers at the Institute of Resource Ecology are using parallel factor analysis (PARAFAC), which they adapted for uranyl hydrolysis. The Dresden scientists collected data like pH measurements using time-resolved laser-induced fluorescence spectroscopy and assembled it into a three-dimensional data cube. This allowed them to be the first to actually show a consistent image of the uranyl(VI) system they had analyzed, identifying five important uranyl(VI) hydrolysis compounds and characterizing them using spectroscopy.

The results refute the long-held belief that distinguishing between uranyl(VI) compounds by way of excitation spectroscopy at wavelengths of less than 370 nanometers is impossible. Not only were the researchers able to confirm their findings with the help of quantum chemical calculations, but for the first time ever, they were actually able to show an existing relationship between the luminescence signal and the chemical structure. This work is helping lay the foundation for the study of more complex uranyl(VI) systems. In addition, the combination of cutting-edge luminescence spectroscopy and theoretical methods could potentially be applied to a number of systems involving additional actinides and rare earths.

  • Publication: B. Drobot, R. Steudtner, J. Raff, G. Geipel, V. Brendler, S. Tsushima, „Combining luminescence spectroscopy, parallel factor analysis and quantum chemistry to reveal metal speciation – a case study of uranyl(VI) hydrolysis”, Chemical Science, 2015, 6, 964-972 (2014, DOI: 10.1039/C4SC02022G)
  • Contact: Björn Drobot, Institute of Resource Ecology

New Form of Cancer Treatment Starts in Dresden

Neue Krebstherapie in Dresden - Behandlung

Dresden's Carl Gustav Carus University Hospital is the very first site in East Germany to be banking on proton-based radiotherapy in the fight against cancer. By mid-December 2014, the first set of cancer patients were treated with radiotherapy. This form of cancer treatment involves shooting the body with protons at two-thirds the speed of light. Unlike is the case with conventional X-ray radiotherapy, these tiny particles don't actually unfold their maximum potential until they have reached the tumor, so that the surrounding tissue remains less damaged. The facility is a joint project by the University Hospital Carl Gustav Carus, the TU Dresden's Faculty of Medicine, and the HZDR.

The scientists have taken the approach of uniting research and treatment under one roof. Scientific findings are extrapolated and applied directly to the treatment - and, vice versa, the practical insights gleaned during treatment also make their way back into the research lab. This cycle is supposed to help speed up the transfer of research knowledge to clinical application. The scientists are hoping to continue to refine the use of protons in cancer therapy over the next several years with a clear focus on the patient and far from commercial dictates.

The National Center for Radiation Research in Oncology - OncoRay provides them with a cutting-edge infrastructure. In addition to its proton therapy facility, the Center also houses an experimental hall on an area of 250 square meters. Here, HZDR researchers are working closely with their partners on a brand new method for accelerating particles. Highly intense laser light is supposed to replace the need for electromagnetic fields that are traditionally used. The hope is that this will help reduce the overall size of the facilities while reducing their price tag, which in turn would help make them more ubiquitously available in the everyday hospital setting.

A Simple Method for Measuring Particle Ranges Improves Precision of Proton Therapy

Distribution of prompt emitted gamma radiation

Proton beams might improve the fight against cancer as their highest destructive potential can be focused on a specific target within the body. Just like bullets a proton beam has a specific range. As long as the initial speed of particles entering the patient's body is set correctly, they unfold their greatest destructive potential precisely within the tumor following their deceleration inside the body – just at the point, at which they stop. However, this is at the same time a challenge. Even a congested nose at the time of the preliminary assessment can distort the data on whose basis the radiation treatment is planned, which could potentially mean the beam may end up missing its target. This is why scientists search for methods for obtaining precise measurements of the protons' actual range.

A team of Dresden researchers has developed a surprisingly simple technology. The high speed of the protons triggers nuclear reactions, which causes gamma radiation. The previous concepts try measuring this radiation using complex and costly detector systems in order to trace the beam's path. By contrast, the new method relies on time measurements that merely need a single detector. Researchers at the HZDR and at the OncoRay Center use an effect which so far has been viewed as a potential source of error: protons require a very short but finite period of time to reach their target inside the body.

Their deceleration time depends on the braking distance - that is, on the range. The gamma radiation generated in the process is emitted within a timespan that grows longer with increasing range. This is easy to record. If the measured time spectra deviate from the modeled ones, the beam does not reach its target with high enough precision. After only a few seconds, the beam can be shut off. The researchers were able to experimentally confirm their hypotheses. The technology could potentially help decrease the safety margin around the tumor down to a few millimeters, which increases the efficacy of the treatment.

  • Publication: C. Golnik, F. Hueso-González, A. Müller, P. Dendooven, W. Enghardt, F. Fiedler, T. Kormoll, K. Roemer, J. Petzoldt, A. Wagner, G. Pausch, „Range assessment in particle therapy based on prompt γ-ray timing measurements”, Physics in Medicine and Biology, 59, 5399-5422 (2014, DOI: 10.1088/0031-9155/59/18/5399)
  • Contacts: Dr. Guntram Pausch, OncoRay; Dr. Fine Fiedler, Institute of Radiation Physics

Magnetic Fields and Lasers Help Unveil Graphene's Secret

Auger scattering in graphene leading to redistribution of electrons

With its tensile strength greater than steel and its conductance of electricity and heat better than copper, graphene is being hailed as a "miracle material." A two-dimensional material consisting of only a single layer of carbon atoms, it is both flexible and nearly transparent, and several million times thinner than a sheet of paper. Following its discovery ten years ago, scientists realized early on that graphene's energy states within a magnetic field - called its Landau levels - behave differently from semiconductors. But the electron dynamics of graphene in the presence of magnetic fields had not before been studied.

In order to uncover the dynamics of graphene-based charge carriers, HZDR researchers have exposed the "miracle material" to a magnetic field, examining it using light pulses from their free-electron laser. In the process, they discovered a seemingly paradoxical phenomenon: Of all things, it was the energy level, into which a laser continued to pump new electrons, that, little by little, began to empty out. The culprits behind this unusual redistribution behavior were collisions between electrons. Although scientists are familiar with this effect, which is known as Auger scattering, nobody was expecting it to be quite this powerful, capable of emptying out an entire energy level.

Looking ahead, this discovery may prove useful for the development of a new kind of laser capable of producing light at a desired set wavelength in the infrared and terahertz ranges. For the longest time, this kind of Landau level laser was considered unfeasible, but now, thanks to graphene, this semiconductor physicists' dream could become a reality.

  • Publication: M. Mittendorff, F. Wendler, E. Malic, A. Knorr, M. Orlita, M. Potemski, C. Berger, W.A. de Heer, H. Schneider, M. Helm, S. Winnerl: „Carrier dynamics in Landau quantized graphene featuring strong Auger scattering“, Nature Physics 11, 75-81 (2015, DOI: 10.1038/nphys3164)
  • Contact: Dr. Stephan Winnerl, Institute of Ion Beam Physics and Materials Research

Cosmic Jets of Young Stars Formed by Magnetic Fields

Cosmic jets forming from a young star

Astrophysical jets are counted among the most spectacular of our universe's phenomena: from the center of black holes, quasars, or protostars, these rays of matter project far out into space, at times at a distance of several light years. What we're talking about here is a thin, straight emission of matter emanating from the center of a disc-shaped cluster made up of cosmic gas and dust. Together with colleagues from Europe, America, and Asia, HZDR researchers have realized an experiment that allowed to generate these types of jets from magnetic fields and as a result they can be modeled.

For their experiments, the researchers simulated the origination process of a jet in the lab: To this end, they targeted intense laser light on a plastic sample. This transformed the previously solid plastic object into a conductive plasma - a sort of rapidly spreading "hot cloud" made up of electrons and ions, which represented a young star's cluster of matter - on a much smaller scale, of course. This way, the scientists were able to extrapolate the results they had obtained in the lab to real-life conditions as they exist in the universe.

At the same time, the plasma was exposed to a very strong pulsed magnetic field, which was produced using a specially designed pulse generator by the Dresden High Magnetic Field Laboratory. This, then, turned out to be the experiment's deciding trick as - according to the physicists' hypothesis - the normally widely scattered plasma was supposed to focus in the magnetic field, and form a hollow interior. As predicted, this in turn generated a shockwave, from which an ultrathin beam - a jet - began to project. The experimental data corresponded nicely to previously made astronomical observations of actual jets in the universe.

  • Publication: B. Albertazzi, A. Ciardi, M. Nakatsutsumi, T. Vinci, J. Béard, R. Bonito, J. Billette, M. Borghesi, Z. Burkley, S. N. Chen, T. E. Cowan, T. Herrmannsdörfer, D. P. Higginson, F. Kroll, S. A. Pikuz, K. Naughton, L. Romagnani, C. Riconda, G. Revet, R. Riquier, H.-P. Schlenvoigt, I. Skobelev, A. Faenov, A. Soloviev, M. Huarte-Espinosa, A. Frank, O. Portugall, H. Pépin, J. Fuchs, “Laboratory formation of a scaled protostellar jet by coaligned poloidal magnetic field”, Science, 346, 325-328 (2014, DOI: 10.1126/science.1259694)
  • Contact: Prof. Dr. Thomas E. Cowan, Director Institute of Radiation Physics