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34121 Publications

Protracted fluvial recovery from medieval earthquakes, Pokhara, Nepal

Stolle, A.; Bernhardt, A.; Schwanghart, W.; Andermann, C.; Schönfeldt, E.; Seidemann, J.; Adhikari, B. R.; Merchel, S.; Rugel, G.; Fort, M.; Korup, O.

River response to strong earthquake shaking in mountainous terrain often entails the flushing of sediments delivered by widespread co-seismic landsliding. Detailed mass-balance studies following major earthquakes in China, Taiwan, and New Zealand suggest fluvial recovery times ranging from several years to decades. We report a detailed chronology of earthquake-induced valley fills in the Pokhara region of western-central Nepal, and demonstrate that rivers continue to adjust to several large medieval earthquakes to the present day, thus, challenging the notion of transient fluvial response to seismic disturbance. The Pokhara valley features one of the largest and most extensively dated sedimentary records of earthquake-triggered sedimentation in the Himalayas, and independently augments paleo-seismological archives obtained mainly from fault trenches and historic documents.
New radiocarbon dates from the catastrophically deposited Pokhara Formation document multiple phases of extremely high geomorphic activity between ~700 and ~1700 AD, preserved in thick sequences of alternating fluvial conglomerates, massive mud and silt beds, and cohesive debris-flow deposits. These dated fan-marginal slackwater sediments indicate pronounced sediment pulses in the wake of at least three large medieval earthquakes in ~1100, 1255, and 1344 AD. We combine these dates with digital elevation models, geological maps, differential GPS data, and sediment logs to estimate the extent of these three pulses, which are characterized by sedimentation rates of ~200 mm yr-1 and peak rates as high as 1,000 mm yr-1. Some 5.5 to 9 km3 of material infilled the pre-existing topography, and is now prone to ongoing fluvial dissection along major canyons. Contemporary river incision into the Pokhara Formation is rapid (120-170 mm yr-1), triggering widespread bank erosion, channel changes, and very high sediment yields of the order of 103 to 105 t km-2 yr-1, which by far outweigh bedrock denudation rates inferred from cosmogenic 10Be inventories in river sands. The rapid infill of about a dozen tributary valleys displaced river channels, and caused them to re-incise into bedrock along steep epigenetic gorges. We conclude that the Pokhara Formation offers a unique archive of medieval earthquakes as well as the associated protracted fluvial response that may have been ongoing for up to 900 years.

Keywords: earthquake; AMS; dating; radiocarbon; Be-10

  • Lecture (Conference)
    European Geosciences Union (EGU) General Assembly, Vienna, Austria, 17.-22.04.2016., 17.-22.04.2016, Wien, Österreich

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Permalink: https://www.hzdr.de/publications/Publ-23052
Publ.-Id: 23052


Geomorphic legacy of medieval Himalayan earthquakes in the Pokhara Valley

Schwanghart, W.; Bernhardt, A.; Stolle, A.; Hoelzmann, P.; Adhikari, B. R.; Andermann, C.; Tofelde, S.; Merchel, S.; Rugel, G.; Fort, M.; Korup, O.

The Himalayas and their foreland belong to the world’s most earthquake-prone regions. With millions of people at risk from severe ground shaking and associated damages, reliable data on the spatial and temporal occurrence of past major earthquakes is urgently needed to inform seismic risk analysis. Beyond the instrumental record such information has been largely based on historical accounts and trench studies. Written records provide evidence for damages and fatalities, yet are difficult to interpret when derived from the far-field. Trench studies, in turn, offer information on rupture histories, lengths and displacements along faults but involve high chronological uncertainties and fail to record earthquakes that do not rupture the surface. Thus, additional and independent information is required for developing reliable earthquake histories.
Here, we present exceptionally well-dated evidence of catastrophic valley infill in the Pokhara Valley, Nepal. Bayesian calibration of radiocarbon dates from peat beds, plant macrofossils, and humic silts in fine-grained tributary sediments yields a robust age distribution that matches the timing of nearby M>8 earthquakes in ~1100, 1255, and 1344 AD. The upstream dip of tributary valley fills and X-ray fluorescence spectrometry of their provenance rule out local sediment sources. Instead, geomorphic and sedimentary evidence is consistent with catastrophic fluvial aggradation and debris flows that had plugged several tributaries with tens of meters of calcareous sediment from the Annapurna Massif >60 km away.
The landscape-changing consequences of past large Himalayan earthquakes have so far been elusive. Catastrophic aggradation in the wake of two historically documented medieval earthquakes and one inferred from trench studies underscores that Himalayan valley fills should be considered as potential archives of past earthquakes. Such valley fills are pervasive in the Lesser Himalaya though high erosion rates reduce preservation potential. Further studies may wish to seek such remnants of prehistoric earthquakes using extensive sedimentological work as well as numerical age control.

Keywords: earthquake; Nepal; radiocarbon; AMS; Be-10

  • Lecture (Conference)
    European Geosciences Union (EGU) General Assembly, 17.-22.04.2016, Wien, Österreich

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Permalink: https://www.hzdr.de/publications/Publ-23051
Publ.-Id: 23051


Making SiMn and TiO2 ferromagnetic at room temperature

Semisalova, A. S.; Rylkov, V. V.; Nikolaev, S. N.; Tugushev, V. V.; Zhou, S.; Potzger, K.; Smekhova, A.; Perov, N.; Granovsky, A.

During the last two decades the enormous efforts were put into the creation, understanding and manipulation of room temperature ferromagnetism (RT FM) in semiconductors. The utilization of spin functionality hand in hand with electrical charge-based electronics opens the wide field of phenomena combining brand-new physics and extensive potential for applications in the next generation logic device and storage. In this talk, recent experimental results on RT FM in Si1-xMnx and Ti1-xCo(V)xO2, the promising materials for hybrid semiconductor spintronics will be reported. Si1-xMnx mosaic (polycrystalline) thin films prepared by pulsed laser deposition exhibit unusual magnetic characteristics - The Curie temperature TC in nonstoichiometric Si1-xMnx alloys (x ~ 0.52-0.55) is one order of magnitude higher (TC ~ 300 K) as compared to stoichiometric MnSi (TC ~ 30 K). The mechanism of the high-temperature FM is still not clear. The FM exchange is associated with the formation of defects with localized magnetic moments coupled via spin fluctuations of itinerant electrons in the host. Also we suppose that structural defects caused, in particular, by small sizes of crystallites have a strong influence on the formation of Si1-xMnx phase with high temperature FM. In case of TiO2, Co and V dopants have been used to create a RT FM dilute magnetic oxide. The study of magnetic, magnetotransport and magneto-optical properties of magnetron sputtered Ti1-xCo(V)xO2–δ (x ∼ 1 at. %) revealed a different mechanism responsible for ferromagnetic response at RT. For Ti1-xVxO2−δ the magnetic properties are determined mainly by structural defects, whereas in Ti1-xCoxO2−δ the magnetic moments of Co play a main role.

  • Invited lecture (Conferences)
    9th International Conference on Magnetic and Superconducting Materials, 01.-3.05.2015, Antalya, Turkey

Permalink: https://www.hzdr.de/publications/Publ-23050
Publ.-Id: 23050


Approval procedures for clinical trials in the field of radiation oncology

Simon, M.; Habeck, M.; Büttner, D.; Habeck, U.; Nölling, T.; Krause, M.; Brix, G.; Willich, N.; Wenz, F.; Schmidberger, H.; Debus, J.; Baumann, M.

BACKGROUND AND PURPOSE:

Application of ionizing radiation for the purpose of medical research in Germany needs to be approved by the national authority for radiation protection (Bundesamt für Strahlenschutz, BfS). For studies in the field of radiation oncology, differentiation between use of radiation for "medical care (Heilkunde)" versus "medical research" frequently leads to contradictions. The aim of this article is to provide principle investigators, individuals, and institutions involved in the process, as well as institutional review or ethics committees, with the necessary information for this assessment. Information on the legal frame and the approval procedures are also provided.
METHODS:

A workshop was co-organized by the German Society for Radiation Oncology (DEGRO), the Working Party for Radiation Oncology (ARO) of the German Cancer Society (DKG), the German Society for Medical Physics (DGMP), and the German Cancer Consortium (DKTK) in October 2013. This paper summarizes the results of the workshop and the follow-up discussions between the organizers and the BfS.
RESULTS:

Differentiating between "Heilkunde" which does not need to be approved by the BfS and "medical research" is whether the specific application of radiation (beam quality, dose, schedule, target volume, etc.) is a clinically established and recognized procedure. This must be answered by the qualified physician(s) ("fachkundiger Arzt" according to German radiation protection law) in charge of the study and the treatments of the patients within the study, taking into consideration of the best available evidence from clinical studies, guidelines and consensus papers. Among the important parameters for assessment are indication, total dose, and fractionation. Radiation treatments applied outside clinical trials do not require approval by the BfS, even if they are applied within a randomized or nonrandomized clinical trial. The decision-making by the "fachkundigem Arzt" may be supported on request by an opinion given by the DEGRO Expert Committee for clinical trials.
CONCLUSION:

An important aim for promoting clinical research and patient care in radiation oncology is to further professionalize planning and implementation of clinical trials in this field. Correct assessment, at an early stage, whether a trial needs to be approved by the BfS may reduce unneccesary costs and reduce the time needed for the approval procedure for those trials which need to be assessed by the BfS.

Permalink: https://www.hzdr.de/publications/Publ-23049
Publ.-Id: 23049


Evaluating the use of optimally respiratory gated 18F-FDG-PET in target volume delineation and its influence on radiation doses to the organs at risk in non-small-cell lung cancer patients

Wijsman, R.; Grootjans, W.; Troost, E. G.; van der Heijden, E. H.; Visser, E. P.; de Geus-Oei, L.-F.; Bussink, J.

OBJECTIVE:

This radiotherapy planning study evaluated tumour delineation using both optimally respiratory gated and nongated fluorine-18 fluorodeoxyglucose-PET (F-FDG-PET).
METHODS:

For 22 non-small-cell lung tumours, both scans were used to create the nongated and gated (g) gross tumour volumes (GTVg) together with the accompanying clinical target volumes (CTV) and planning target volumes (PTV). The size of the target volumes (TV) was evaluated and the accompanying radiotherapy plans were created to study the radiation doses to the organs at risk (OAR).
RESULTS:

The median volumes of GTVg, CTVg and PTVg were statistically significantly smaller compared with the corresponding nongated volumes, resulting in a median TV reduction of 0.5 cm (interquartile range 0.1-1.2), 1.5 cm (-0.2 to 7.0) and 2.3 cm (-0.5 to 11.3) for the GTVg, CTVg and PTVg, respectively. For the OAR, only the percentage of lung (GTV included) receiving at least 35 Gy was significantly smaller after gating, with a median difference in lung volume receiving at least 35 Gy of 5.7 cm (interquartile range -0.8 to 30.50).
CONCLUSION:

Compared with nongated F-FDG-PET, the TVs obtained with optimally respiratory gated F-FDG-PET were significantly smaller, however, without a clinically relevant difference in radiation dose to the OAR.

Permalink: https://www.hzdr.de/publications/Publ-23048
Publ.-Id: 23048


Determination of the exchange stiffness in ultrathin magnetic films by magnonic patterning and ferromagnetic resonance

Lenz, K.; Langer, M.; Wagner, K.; Sebastian, T.; Schultheiss, H.; Lindner, J.; Fassbender, J.

In ultrathin films of below 20 nm thickness, it is hardly possible to determine the exchange constant A, since perpendicular standing spin waves (PSSWs) are shifted up to inaccessibly high energies. In this work, a method is presented to analytically determine the exchange stiffness constant D = 2A/MS using ferromagnetic resonance (FMR) and magnonic patterning. Usual FMR measurements, however, are not influenced by the value of D, since no exchange energy is involved in uniform precession. To overcome this problem a coupling mechanism, such as two-magnon scattering (TMS), can be employed to couple exchange dominated in-plane spin waves with the uniform mode.
In our approach lateral magnetic surface patterning was carried out to artificially induce TMS. Subsequent FMR measurements give access to the spin wave spectra of backward volume modes, and thus, to the exchange stiffness constant D.

Keywords: Magnetism; FMR; magnonics

  • Poster
    603. WE-Heraeus Seminar: Magnonics: Spin Waves Connecting Charges, Spins and Photons, 05.-8.1.2016, Bad Honnef, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-23047
Publ.-Id: 23047


Spontaneous pattern formation on ion irradiated semiconductor surfaces

Facsko, S.; Ou, X.; Wang, X.; Hübner, R.; Grenzer, J.

Low energy ion irradiation of semiconductor surfaces induces the formation of periodic surface patterns under particular conditions. These nanostructured surfaces exhibit periodici- ties in the range of a few tens to hundreds of nanometers and are promising templates for producing nanostructured thin films [1]. During ion irradiation the surfaces are driven out of equilibrium by continuous creation of displacements in the sub-surface region. At room tem- perature (RT) the accumulation of created displacements leads to amorphization of the irradi- ated semiconductor surfaces. Under these conditions periodic ripple patterns with wave vec- tor parallel to the ion beam direction are observed frequently for ion irradiation at incidence angles between 50° and 70° to the surface normal [2]. At normal incidence dot or hole pat- terns with hexagonal symmetry are observed for specific semiconductors, i.e. GaSb [3], InSb, GaP, or for special irradiation conditions, e.g. Ga+ or Bi3+ irradiation of Ge [4, 5].
In Fig. 1 different patterns on ion irradiated Ge (001) surfaces are shown. Although the Ge (001) surface is thermodynamically stable at all temperature used in the experiments, ion irradiation induces a surface instability which is counterbalanced by surface smoothing via different relaxation mechanisms, e.g. surface diffusion, ion enhanced surface diffusion, sur- face viscous flow, etc. As a result a wavelength selection in the surface roughness manifests itself as a periodic surface pattern. For off-normal angle of incidence ripple patterns are
At higher temperatures than RT, however, point defects created by the displacements in the ion collision cascade can diffuse longer distances, thus, vacancies and interstitial recom- bine or diffuse to the surface more effectively. Eventually, at temperatures higher than the recrystallization temperature, defects in the sub-surface region are annealed or diffuse to the surface before a second ion creates new defects in the same area and the surface remains crystalline. However, the average density of surface vacancies and ad-atoms is much higher than the corresponding densities in thermal equilibrium resulting in a much higher entropy. In this regime, ion irradiation creates an excess of vacancies on the crystalline surface due to sputtering. Thus, the surfaces morphology is determined primarily by vacancy kinetics alt- hough the kinetics of ad-atoms also play an important role.
In this contribution we present investigations of the evolution of Ge surfaces with dif- ferent surface orientation irradiated at temperatures above the recrystallization temperature. The irradiations were done with 1 keV Ar+ ions at normal incidence at temperatures above 250°C which has been established to be the temperature at which the Ge surface remains crystalline even after prolonged irradiation. The samples were cut from epi-ready Ge wafers with (001) and (111) surface orientation. Irradiations were performed in a UHV chamber with a base pressure in the range of 10-8 mbar with a beam from a Kaufman ion source. During irradiation the chamber is flooded with Ar up to a pressure of 3x10-4 mbar. The flux was 1.7x1015 cm-2s-1 and the applied fluence was in the range of 1017 – 1019 cm-2.
The formation of these patterns on crystalline surfaces can be understood in analogy to the formation of 3D structures in homoepitaxy. In molecular beam epitaxy (MBE) the contin- uous deposition of atoms can lead to growth of self-organized 3D nanostructures [5]. One of the possible surface instability, which is responsible for the formation of islands or mounds is caused by the Ehrlich-Schwoebel (ES) barrier, i.e. an additional diffusion barrier for ad- atoms to cross terrace steps. Due to this effect the arriving atoms are trapped on a terraces and can again nucleate to form new terraces.
The same mechanism is also active on ion irradiated surfaces when the temperatures is above the recrystallization temperature. In this case bulk defects are dynamically annealed and amorphization is prevented. Now, ion sputtering is creating vacancies on the crystalline surface and the surfaces morphology is determined by vacancy kinetics. The diffusion of va- cancies is also biased by the ES barrier like the diffusion of ad-atoms. Consequently, the 3D growth turns into 3D erosion. The resulting structures are inverse pyramids which are grow- ing into the surface. The symmetry of these patterns is given by the crystal symmetry. In Fig. 3 zooms of AFM images and the 2D slope distributions of the surface patterns on Ge (001) and Ge (111) are shown, respectively. The detailed facet analysis of the patterns by the 2D slope distribution reveals that on Ge (001) {105} facets with a polar angle of 11° exhibiting a four-fold symmetry are formed, whereas on Ge (111) {356} facets with a polar angle of 15° are formed with a three-fold symmetry. These facets are not know to be thermodynamically stable facets in growth conditions. The {105} facets have only been observed in heteroepi- taxy, where they are stabilize by strain due to the lattice mismatch. In the case of ion erosion no strain is expected [8]. Hence, it can be concluded that these are non-equilibrium facets which are determined by the kinetics of vacancies induce by ion irradiation.
For the description of the pattern formation and evolution in reverse epitaxy a continuum equation can be used which combines the effects of ion irradiation and effective diffusion
currents due to the ES barrier on the crystalline surface. For normal incidence irradiation it is know that smoothing mechanisms dominate thus we can omit an instability term induced by the curvature dependent sputtering or ion induced mass redistribution [9]. By choosing the adequate ES barrier induced surface currents and including also a conserved Kardar-Parisi- Zhang term a remarkable qualitative agreement to the experiments is achieved for both sur- face orientations. Ge (001) and Ge (111), respectively (see Fig. 4) [7].

1. J. Fassbender, T. Strache, M.O. Liedke, D. Marko, S. Wintz, K. Lenz, A. Keller, S. Facsko, I. Monch, J. McCord, New Journal of Physics 11, 125002 (2009).
2. W.L. Chan and E. Chason, J. Appl. Phys. 101, 121301 (2007).
3. S. Facsko, T. Dekorsy, C. Koerdt, C. Trappe, H. Kurz, A. Vogt, and H.L. Hartnagel, Science 285, 1551 (1999).
4. M. Fritzsche, A. Muecklich, S. Facsko, Appl. Phys. Lett. 100, 223108 (2012).
5. Böttger, K.-H. Heinig, L. Bischoff, B. Liedke, R. Hübner, and W. Pilz, Phys. Status Solidi (RRL), 501 (2013).
6. C. Teichert, Phys. Rep. 365, 335 (2002).
7. X. Ou, A. Keller, M. Helm, J. Fassbender, and S. Facsko, Phys. Rev. Lett. 111, 016101 (2013).
8. X. Ou and S. Facsko, Nucl. Instr. Meth. B 341, 13 (2014).
9. C.S. Madi, E. Anzenberg, K.F. Ludwig, and M.J. Aziz, Phys. Rev. Lett. 106, 066101 (2011).

Keywords: ion induced nanopatterns

  • Invited lecture (Conferences)
    22nd International Conference on Ion-Surface Interactions, 19.-22.08.2015, Moskow, Russia
  • Lecture (Conference)
    8th International Meeting on Recent Developments in the Study of Radiation Effects in Matter, 21.-23.09.2015, Kerteminde, Denmark

Permalink: https://www.hzdr.de/publications/Publ-23046
Publ.-Id: 23046


Reverse Epitaxy on Semiconductor Surfaces

Facsko, S.; Ou, X.; Hübner, R.; Grenzer, J.; Heinig, K.-H.

Arrays of semiconductor nanostructures are emerging as building blocks for next generation of electronic and optoelectronic nano-devices. In molecular beam epitaxy (MBE) the continuous deposition of atoms can lead to growth of self-organized 3D nanostructures. One of the possible surface instabilities, which is responsible for this kind of growth, is caused by the Ehrlich-Schwoebel (ES) barrier, i.e. an additional diffusion barrier for ad-atoms to cross terrace steps [1]. The arriving atoms are trapped on terraces and can again nucleate to form new terraces. This mechanism leads to the growth of pyramidal mounds on the surface with facets corresponding to energetically favored crystal planes. An analogous mechanism is also observed on ion irradiated surfaces. However, ion sputtering leads to the erosion of the surfaces and at room temperature semiconductor surfaces become amorphous. At these conditions various periodic patterns are observed. [2,3] For device fabrication, a crystalline surface of high quality is indispensable.
In this talk, we demonstrate single crystal elemental (Si and Ge) and compound semiconductor (III-V) nanostructure pattern formation based on a “reverse epitaxy” process. Vacancies created during ion beam irradiation at elevated temperature distribute according to the crystallographic anisotropy, which results in an orientation-dependent pattern formation on single crystal semiconductor surfaces. This process shows nicely the equivalence of epitaxy with deposited adatoms and “reverse epitaxy” with ion induced surface vacancies on semiconductors. The formation of these patterns is interpreted as the result of a surface instability due to an Ehrlich-Schwoebel barrier for ion induced surface vacancies. The potential application of reverse epitaxy on fabrication of UUV optical grating and of metallic nanowires will be discussed.

[1] P. Politi, G. Grenet, A. Marty, A. Ponchet, J. Villain, Phys. Rep. 324, 271 (2000).
[2] S. Facsko, T. Dekorsy, C. Koerdt, C. Trappe, H. Kurz, A. Vogt, and H. L. Hartnagel, Science 285, 1551 (1999).
[3] W. L. Chan and E. Chason, J. Appl. Phys. 101, 121301 (2007).

Keywords: ion induced nanopatterns

  • Invited lecture (Conferences)
    8th International Workshop on Nanoscale Pattern Formation at Surfaces, 12.-16.07.2015, Krakow, Poland

Permalink: https://www.hzdr.de/publications/Publ-23045
Publ.-Id: 23045


First clinical application of a prompt gamma based in vivo proton range verification

Richter, C.; Pausch, G.; Barczyk, S.; Priegnitz, M.; Keitz, I.; Thiele, J.; Smeets, J.; Vander Stappen, F.; Bombelli, L.; Fiorini, C.; Hotoiu, L.; Perali, I.; Prieels, D.; Enghardt, W.; Baumann, M.

Background and Purpose: To improve precision of particle therapy, in vivo range verification is highly desirable. Methods based on prompt gamma rays emitted during treatment seem promising but have not yet been applied clinically. Here we report on the worldwide first clinical application of prompt gamma imaging (PGI) based range verification.
Material and Methods: A prototype of a knife-edge shaped slit camera was used to measure the prompt gamma ray depth distribution during a proton treatment of a head and neck tumor for seven consecutive fractions. Inter-fractional variations of the prompt gamma profile were evaluated. For three fractions in-room control CTs were acquired and evaluated for dose relevant changes.
Results: The measurement of PGI profiles during proton treatment was successful. Based on the PGI information, inter-fractional global range variations were in the range of ±2 mm for all evaluated fractions. This is in agreement with the control CT evaluation showing negligible range variations of about 1.5 mm.
Conclusions: For the first time, range verification based on prompt gamma imaging was applied for a clinical proton treatment. With the translation from basic physics experiments into clinical operation, the potential to improve the precision of particle therapy with this technique has increased considerably.

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Permalink: https://www.hzdr.de/publications/Publ-23044
Publ.-Id: 23044


Interaction of Highly Charged Ions with Surfaces and Nanomembranes

Facsko, S.; Wilhelm, R. A.; Gruber, E.; Ritter, R.; Heller, R.; Aumayr, F.

Highly charged ions (HCI) release a large amount of potential energy (the stored ionization energy) when interacting with solids. This energy is deposited into a very small volume directly at the surface via multiple charge exchanges on a fs time scale leading to a highly excited electronic system. Especially ionic crystals have shown a predisposition to potential energy effects due to their low conductivity and their strong electron phonon coupling. On CaF_2 surfaces the formation of hillocks induced by the potential energy of a single highly charged Xe^{q+} ion has been observed for charge states higher than q > 27. The formation of these hillocks can be attributed to local melting [1]. In contrast, on surfaces of KBr one monolayer deep pits are formed by defect mediated desorption also showing a threshold behavior in the pit formation [2].
The interaction of HCI with thin membranes is particularly interesting because the pre-equilibrium interaction regime can be accessed for thicknesses below a few nm. In 1 nm carbon nano membranes (CNM) for instance, holes are produced by the passage of highly charged Xe^{q+} ions [3]. For the formation of these holes a threshold in the potential energy of the HCI exists that depends on the kinetic energy. In order to elucidate the formation mechanism we examined the charge state and the energy loss of the Xe^{q+} ions after their passage through the CNM. Surprisingly, two distinct exit charge distributions were observed [4]. Part of the ions are passing the membrane with almost now charge loss, whereas the other part looses most of their charge. Apparently, the measured charge distribution reflects two different impact parameter regimes. Ions with trajectories far away of any C atom of the membrane can stabilize only few electrons and exit therefore in a high charge state, whereas ions with trajectories close to a C atom can capture a large amount of electrons and exit the membrane in a low charge state. The different impact parameter regimes are also connected to different energy losses: ions with large impact parameters are practically not stopped, whereas ions in close collisions exhibit high stopping force which is strongly dependent on the incident charge state.
[1] A. El-Said, R. Wilhelm, R. Heller, S. Facsko, C. Lemell, G. Wachter, J. Burgdorfer, R. Ritter, and F. Aumayr, Phys. Rev. Lett. 109, 117602 (2012).
[2] R. Heller, S. Facsko, R.A. Wilhelm, and W. Moller, Phys. Rev. Lett. 101, 096102 (2008).
[3] R. Ritter, R.A. Wilhelm, M. Stöger-Pollach, R. Heller, A. Mücklich, U. Werner, H. Vieker, A. Beyer, S. Facsko, A. Gölzhäuser, F. Aumayr, Appl. Phys. Lett. 102, 063112 (2013).
[4] R.A. Wilhelm, E. Gruber, R. Ritter, R. Heller, S. Facsko, F. Aumayr, Phys. Rev. Lett. 112, 153201 (2014).

Keywords: highly charged ions

  • Invited lecture (Conferences)
    MRS Spring Meeting, 06.-10.04.2015, San Francisco, USA

Permalink: https://www.hzdr.de/publications/Publ-23043
Publ.-Id: 23043


Transition from Pits to Mounds in Ion Induced Patterning of Germanium

Facsko, S.; Ou, X.

Low energy ion irradiation drives surfaces out of equilibrium by continuous creation of displacements in the sub-surface region. At room temperature the accumulation of displacements leads to the amorphization of the irradiated surfaces and self-organized ripple pattern perpendicular to the ion beam direction are formed for incidence angles higher than 50° [1]. At normal incidence irradiation smoothing dominates and no pattern are observed for low energy ion irradiation. At higher temperatures, point defects created by the displacements in the ion collision cascade can diffuse longer distances, thus vacancies and interstitial recombine more effectively or diffuse to the surface. Finally, at temperatures higher than the recrystallization temperature, all defects in the sub-surface region are annealed before an ion creates new defects and the surface remains crystalline. The average density of surface vacancies and ad-atoms on the surface is, however, much higher than the corresponding densities in thermal equilibrium resulting in a much higher entropy.
In this regime, ion irradiation creates an excess of vacancies on the crystalline surface due to sputtering and the surfaces morphology is determined primarily by their kinetics. The diffusion of vacancies is biased by the Ehrlich-Schwoebel barrier, i.e. an additional barrier for crossing terrace steps, similar to the diffusion barrier of ad-atoms known from growth by molecular beam epitaxy. Consequently, ion sputtering leads to the erosion of 3D structures in a “reverse epitaxy” process. The resulting patterns are arrays of inverse pyramids growing into the Ge surface [1,2]. The morphology of these patterns is given by the crystal symmetry of the surface. Hence, checkerboard patterns appear on the Ge (001) surface Here, we show that the inverse pyramid pattern on Ge(001) surface, which is observed for normal incidence ion irradiation at higher temperatures, turns into a pyramidal mound pattern at incidence angles between 50° and 70° with respect to the surface normal, and finally, into ripple patterns above 80° incidence. All irradiations were performed at 350° C with 1 keV Ar+ at a fluence of 1x1018 cm-2 from a Kaufman ion source.
The observed transition from pit to mound patterns in reverse epitaxy can be understood by assuming a transition from vacancy dominated pattern formation to ad-atom dominated pattern formation. Therefore, at incidence angles above 50° the pattern resemble mound patterns observed in growth. Furthermore, the transition to ripples patterns at higher incidence angles is ascribed to a shadowing instability at these grazing incidence angles.

[1] A. Keller and S. Facsko, Materials 2010, Vol. 3, Pages 4811-4841 3, 4811 (2010).
[2] X. Ou, A. Keller, M. Helm, J. Fassbender, and S. Facsko, Phys. Rev. Lett. 111, 016101 (2013).
[3] X. Ou and S. Facsko, Nucl. Instr. Meth. B 341, 13 (2014).

Keywords: ion induced nanostructures

  • Lecture (Conference)
    21st International Workshop on Inelastic Ion-Surface Collisions, 18.-23.10.2015, San Sebastian, Spanien

Permalink: https://www.hzdr.de/publications/Publ-23042
Publ.-Id: 23042


Pre-equilibrium Dynamics of Highly Charged ions at Surfaces and Carbon Nanomembranes

Facsko, S.; Wilhelm, R.; Gruber, E.; Heller, R.; Aumayr, F.

The interaction of HCI with thin membranes is particularly interesting because the pre-equilibrium interaction regime can be accessed for thicknesses below a few nm. In 1 nm carbon nano membranes (CNM) for instance, holes are produced by the passage of highly charged Xe$^{q+}$ ions. For the formation of these holes a threshold in the potential energy of the HCI exists that depends on the kinetic energy. In order to elucidate the formation mechanism we examined the charge state and the energy loss of the Xe$^{q+}$ ions after their passage through the CNM. Surprisingly, two distinct exit charge distributions were observed. Part of the ions are passing the membrane with almost now charge loss, whereas the other part looses most of their charge. Apparently, the measured charge distribution reflects two different impact parameter regimes. Ions with trajectories far away of any C atom of the membrane can stabilize only few electrons and exit therefore in a high charge state, whereas ions with trajectories close to a C atom can capture a large amount of electrons and exit the membrane in a low charge state. The different impact parameter regimes are also connected to different energy losses: ions with large impact parameters are practically not stopped, whereas ions in close collisions exhibit high stopping force which is strongly dependent on the incident charge state.

Keywords: highly charged ions

  • Lecture (others)
    Kolloquiumsvortrag, 12.11.2015, Leipzig, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-23041
Publ.-Id: 23041


Defects in zinc oxide grown by pulsed laser deposition

Ling, F. C. C.; Wang, Z.; Ho, L. P.; Younas, M.; Anwand, W.; Wagner, A.; Su, S. C.; Shan, C. X.

ZnO films are grown on c-plane sapphire using the pulsed laser deposition method. Systematic studies on the effects of annealing are performed to understand the thermal evolutions of the defects in the films. Particular attention is paid to the discussions of the ZnO/sapphire interface thermal stability, the Zn-vacancy related defects having different microstructures, the origins of the green luminescence (~2.4-2.5 eV) and the near band edge (NBE) emission at 3.23 eV.

Keywords: ZnO; pulsed laser deposition; Zn-vacancy; green luminescence; near band edge emission; positron annihilation spectroscopy

Permalink: https://www.hzdr.de/publications/Publ-23037
Publ.-Id: 23037


Ferromagnetism of MnxSi1-x (x~0.5) films grown in the shadow geometry by pulsed laser deposition method

Nikolaev, S.; Semisalova, A.; Rylkov, V.; Tugushev, V.; Zenkevich, A.; Vasiliev, A.; Pashaev, E.; Chernoglazov, K.; Chesnokov, Y.; Likhachev, I.; Perov, N.; Matveyev, Y.; Novodvorskii, O.; Kulatov, E.; Bugaev, A.; Wang, Y.; Zhou, S.

The results of a comprehensive study of magnetic, magneto-transport and structural properties of nonstoichiometric MnxSi1-x (x=0.51-0.52) films grown by the Pulsed Laser Deposition (PLD) technique onto Al2O3(0001) single crystal substrates at T = 340 C are present. A highlight of used PLD method is the non-conventional ("shadow") geometry with Kr as a scattering gas during the sample growth. It is found that the films exhibit high-temperature (HT) ferromagnetism (FM) with the Curie temperature TC ~ 370 K accompanied by positive sign anomalous Hall effect (AHE); they also reveal the polycrystalline structure with unusual distribution of grains in size and shape. It is established that HT FM order is originated from the bottom interfacial self-organizing nanocrystalline layer. The upper layer adopted columnar structure with the lateral grain size >50 nm, possesses low temperature (LT) type of FM order with ТС ~ 46 K and contributes essentially to the magnetization at T < 50 K. Under these conditions, AHE changes its sign from positive to negative at T < 30K. We attribute observed properties to the synergy of distribution of MnxSi1-x crystallites in size and shape as well as peculiarities of defect-induced FM order in shadow geometry grown polycrystalline MnxSi1-x (x~0.5) films.

Keywords: Si-Mn alloys; High-temperature ferromagnetism; Anomalous Hall effect

Permalink: https://www.hzdr.de/publications/Publ-23036
Publ.-Id: 23036


Modifications of gallium phosphide single crystals using slow highly charged ions and swift heavy ions

El-Said, A. S.; Wilhelm, R. A.; Heller, R.; Akhmadaliev, S.; Schumann, E.; Sorokin, M.; Facsko, S.; Trautmann, C.

GaP single crystals were irradiated with slow highly charged ions (HCI) using 114 keV 129Xe(33–40)+ and with various swift heavy ions (SHI) of 30 MeV I9+ and 374 MeV–2.2 GeV 197Au25+. The irradiated surfaces were investigated by scanning force microscopy (SFM). The irradiations with SHI lead to nanohillocks protruding from the GaP surfaces, whereas no changes of the surface topography were observed after the irradiation with HCI. This result indicates that a potential energy above 38.5 keV is required for surface nanostructuring of GaP. In addition, strong coloration of the GaP crystals was observed after irradiation with SHI. The effect was stronger for higher energies. This was confirmed by measuring an increased extinction coefficient in the visible light region.

Keywords: GaP; Swift heavy ions; Slow highly charged ions; Nanostructures

Downloads:

Permalink: https://www.hzdr.de/publications/Publ-23035
Publ.-Id: 23035


A setup for transmission measurements of low energy multiply charged ions through free-standing few atomic layer films

Smejkal, V.; Gruber, E.; Wilhelm, R. A.; Brandl, L.; Heller, R.; Facsko, S.; Aumayr, F.

We report the design and testing of a setup for transmission measurements of multiply charged ions through free-standing films with a thickness of a few atomic layers. The in- vestigation thereof can yield deeper insight into charge equilibration and pre-equilibrium stopping phenomena which can ultimately be used to specifically tailor and modify these materials.

Keywords: transmission measurements; highly charged ions; ion surface interaction; graphene; carbon nano membranes; ion charge loss; ion energy loss; equilibrium charge state

Downloads:

Permalink: https://www.hzdr.de/publications/Publ-23033
Publ.-Id: 23033


Polarity dependence of Mn incorporation in (Ga,Mn)N superlattices

Tropf, L.; Kunert, G.; Jakieła, R.; Wilhelm, R. A.; Figge, S.; Grenzer, J.; Hommel, D.

In the context of recent efforts to combine high Mn concentrations in (Ga,Mn)N with a pronounced p- type carrier density, (Ga,Mn)N/GaN:Mg-superlattices have been fabricated using plasma-assisted mole- cular beam epitaxy. Profiles of the dopant atomic densities in the heterostructures are obtained by secondary ion mass spectroscopy. They show an abrupt drop of two to three orders of magnitude in both Mn and Mg concentrations after the first GaN:Mg layer above a critical Mg-flux. Scanning electron microscopy before and after selective etching reveals a polarity inversion from originally Ga-face to N- face GaN in samples in which high Mg fluxes were applied. From our observations, we are able to draw an analogy between the impurity incorporation laws of Mg and Mn.

Keywords: Doping; Molecular beam epitaxy; Nitrides; Magnetic materials

Permalink: https://www.hzdr.de/publications/Publ-23032
Publ.-Id: 23032


Interaction of multiply charged ions with single layer graphene Part I: Charge exchange and energy loss (Conference Paper)

Smejkal, V.; Gruber, E.; Kralik, M.; Wilhelm, R. A.; Heller, R.; Facsko, S.; Aumayr, F.

The exit charge state distribution and the energy loss of slow multiply charged ions transmitted through single layers of graphene and 1 nm thick carbon nanomembranes is analyzed.

Permalink: https://www.hzdr.de/publications/Publ-23031
Publ.-Id: 23031


Imaging-Based Treatment Adaptation in Radiation Oncology

Troost, E. G.; Thorwarth, D.; Oyen, W. J.

In many tumor types, significant effort is being put into patient-tailored adaptation of treatment to improve outcome and preferably reduce toxicity. These opportunities first arose with the introduction of modern irradiation techniques (e.g., intensity-modulated radiotherapy) combined with functional imaging for more precise delineation of target volume. On the basis of functional CT, MRI, and PET results, radiation target volumes are altered during the course of treatment, or subvolumes inside the primary tumor are defined to enhance the dosing strategy. Moreover, the probability of complications to normal tissues is predicted using anatomic or functional imaging, such as in the use of CT or PET to predict radiation pneumonitis. Besides focusing, monitoring, and adapting photon therapy for solid tumors, PET also has a role in verifying proton-beam therapy. This article discusses the current state and remaining challenges of imaging-based treatment adaptation in radiation oncology.

Permalink: https://www.hzdr.de/publications/Publ-23029
Publ.-Id: 23029


Is integrated transit planar portal dosimetry able to detect geometric changes in lung cancer patients treated with volumetric modulated arc therapy?

Persoon, L. C.; Podesta, M.; Hoffmann, L.; Sanizadeh, A.; Schyns, L. E.; de Ruiter, B. M.; Nijsten, S. M.; Muren, L. P.; Troost, E. G.; Verhaegen, F.

BACKGROUND:

Geometric changes are frequent during the course of treatment of lung cancer patients. This may potentially result in deviations between the planned and actual delivered dose. Electronic portal imaging device (EPID)-based integrated transit planar portal dosimetry (ITPD) is a fast method for absolute in-treatment dose verification. The aim of this study was to investigate if ITPD could detect geometric changes in lung cancer patients.
MATERIALS AND METHODS:

A total of 460 patients treated with volumetric modulated arc therapy (VMAT) following daily cone beam computed tomography (CT)-based setup were visually inspected for geometrical changes on a daily basis. Forty-six patients were subject to changes and had a re-CT and an adaptive treatment plan. The reasons for adaptation were: change in atelectasis (n = 18), tumor regression (n = 9), change in pleural effusion (n = 8) or other causes (n = 11). The ITPDs were calculated on both the initial planning CT and the re-CT and compared with a global gamma (γ) evaluation (criteria: 3%\3mm). A treatment fraction failed when the percentage of pixels failing in the radiation fields exceeded 10%. Dose-volume histograms (DVHs) were compared between the initial plan versus the plan re-calculated on the re-CT.
RESULTS:

The ITPD threshold method detected 76% of the changes in atelectasis, while only 50% of the tumor regression cases and 42% of the pleural effusion cases were detected. Only 10% of the cases adapted for other reasons were detected with ITPD. The method has a 17% false-positive rate. No significant correlations were found between changes in DVH metrics and γ fail-rates.
CONCLUSIONS:

This study showed that most cases with geometric changes caused by atelectasis could be captured by ITPD, however for other causes ITPD is not sensitive enough to detect the clinically relevant changes and no predictive power of ITPD was found.

Permalink: https://www.hzdr.de/publications/Publ-23025
Publ.-Id: 23025


Single organ metastatic disease and local disease status, prognostic factors for overall survival in stage IV non-small cell lung cancer: Results from a population-based study.

Hendriks, L. E.; Derks, J. L.; Postmus, P. E.; Damhuis, R. A.; Houben, R. M.; Troost, E. G.; Hochstenbag, M. M.; Smit, E. F.; Dingemans, A. M.

PURPOSE:

To analyse the prognostic impact on overall survival (OS) of single versus multiple organ metastases, organ affected, and local disease status in a population based stage IV non-small cell lung cancer (NSCLC) cohort.
METHODS:

In this observational study, data were analysed of all histologically confirmed stage IV NSCLC patients diagnosed between 1 January 2006 and 31 December 2012 registered in the Netherlands Cancer Registry. Location of metastases before treatment was registered. Multivariable survival analyses [age, gender, histology, M-status, local disease status, number of involved organs, actual organ affected] were performed for all patients and for an (18)fluorodeoxyglucose-positron emission tomography ((18)FDG-PET)-staged subgroup.
RESULTS:

11,094 patients were selected: 60% male, mean age 65years, 73% adenocarcinoma. Median OS for 1 (N=5676), 2 (N=3280), and ⩾3 (N=2138) metastatically affected organs was 6.7, 4.3, 2.8months, respectively (p<0.001). Hazard ratio (HR) for 2 versus 1 organ(s) was 1.33 (p<0.001), for ⩾3 versus 1 organ(s) 1.91 (p<0.001). Results were confirmed in the (18)FDG-PET-staged cohort (N=1517): patients with single organ versus 2 and ⩾3 organ metastases had higher OS (8.6, 5.7, 3.8months, HR 1.40 and 2.17, respectively, p<0.001). In single organ metastases, OS for low versus high TN-status was 8.5 versus 6.5months [HR 1.40 (p<0.001)]. (18)FDG-PET-staged single organ metastases patients with low TN-status had a superior OS than those with high TN-status (11.6 versus 8.2months, HR 1.62, p<0.001).
CONCLUSION:

Patients with single organ metastases stage IV NSCLC have a favourable prognosis, especially in combination with low TN status. They have to be regarded as a separate subgroup of stage IV disease.

Keywords: Local disease status; Metastases; Non-small cell lung cancer; Prognosis; Stage IV

Permalink: https://www.hzdr.de/publications/Publ-23023
Publ.-Id: 23023


Increasing the Therapeutic Ratio of Stereotactic Ablative Radiotherapy by Individualized Isotoxic Dose Prescription

Zindler, J. D.; Thomas, C. R.; Hahn, S. M.; Hoffmann, A. L.; Troost, E. G. C.; Lambin, P.

To obtain a favorable tradeoff between treatment benefits and morbidity ("therapeutic ratio"), radiotherapy (RT) dose is prescribed according to the tumor volume, with the goal of controlling the disease while respecting normal tissue tolerance levels. We propose a new paradigm for tumor dose prescription in stereotactic ablative radiotherapy (SABR) based on organ-at-risk (OAR) tolerance levels called isotoxic dose prescription (IDP), which is derived from experiences and limitations of conventionally fractionated radiotherapy. With IDP, the radiation dose is prescribed based on the predefined level of normal tissue complication probability of a nearby dose-limiting OAR at a prespecified dose-volume constraint. Simultaneously, the prescribed total tumor dose (TTD) is maximized to the technically highest achievable level in order to increase the local tumor control probability (TCP). IDP is especially relevant for tumors located at eloquent locations or for large tumors in which severe toxicity has been described. IDP will result in a lower RT dose or a treatment scheduled with more fractions if the OAR tolerance level is exceeded, and potential dose escalation occurs when the OAR tolerance level allows it and when it is expected to be beneficial (if TCP < 90%). For patients with small tumors at noneloquent sites, the current SABR dose prescription already results in high rates of local control at low toxicity rates. In this review, the concept of IDP is described in the context of SABR.

Permalink: https://www.hzdr.de/publications/Publ-23022
Publ.-Id: 23022


Interactive Plasma Simulations on Next Generation Supercomputers for Everybody

Huebl, A.; Widera, R.; Zenker, E.; Worpitz, B.; Burau, H.; Pausch, R.; Grund, A.; Matthes, A.; Garten, M.; Eckert, C.; Debus, A.; Bussmann, M.

Subject to change: we will cover our fundamental, performance portable building blocks (alpaka) that power kernels in PIConGPU and PMacc. PMacc is our particle-mesh library with reusable, light-weight containers and event scheduling for many/multi-core hardware. Combining our open source libraries with C++ meta-programming and our open data standard suitable for extreme I/O load in HPC (openPMD) we will visualize the whole plasma simulation environment with a live simulation.

Keywords: Simulation; Plasma; LPA; GPU; PIConGPU; HPC; Open

  • Invited lecture (Conferences)
    Swiss Platform for Advanced Scientific Computing (PASC) Conference 2016, 08.-10.06.2016, Lausanne, Schweiz

Permalink: https://www.hzdr.de/publications/Publ-23021
Publ.-Id: 23021


Functional DNA origami nanostructures for nanoelectronics and Photonics

Teschome, B.; Facsko, S.; Keller, A.; Kerbusch, J.

Nanodevices based on DNA origami-based nanowires

  • Lecture (Conference)
    Group meeting Aarhus University, 22.05.2015, Aarhus, Denmark

Permalink: https://www.hzdr.de/publications/Publ-23020
Publ.-Id: 23020


Arrangement and characterization of functional DNA origami nanostructures for nanoelectronics

Teschome, B.; Facsko, S.; Kerbusch, J.; Hübner, R.; Gothelf, K. V.; Keller, A.

In this work, we will highlight some of results from our work on the arrangement and characterization of functional DNA origami nanostructures for nanoelectronics. First, a new compelling approach to generate ordered arrays of DNA origami nanotubes on topographically patterned Si surfaces will be introduced. Then, the high-yield synthesis of high-density gold nanoparticle (AuNP) arrangements on DNA origami nanotubes with few unbound background nanoparticles will be presented. The high yield of AuNP assembly was achieved by careful control of the buffer concentration and the hybridization time on Si surface. Finally, also the assembly of heterogeneous nanostructures, i.e. 5 nm gold nanoparticles (AuNPs) and 10 nm semiconductor quantum dots (QDs), on a single DNA origami will be demonstrated.

  • Invited lecture (Conferences)
    IHRS NanoNet Annual Workshop 2015, 30.09.-02.10.2015, Lohmen, Germany

Permalink: https://www.hzdr.de/publications/Publ-23019
Publ.-Id: 23019


Above Room Temperature Ferromagnetism in Dilute Magnetic Oxide Semiconductors

Semisalova, A. S.; Orlov, A.; Smekhova, A.; Gan’Shina, E.; Perov, N.; Anwand, W.; Potzger, K.; Lähderanta, E.; Granovsky, A.

In this chapter, we will survey early and recent experimental results on magnetic properties of dilute magnetic oxide semiconductors, focusing on TiO2-δ:Co and TiO2-δ:V. Room temperature ferromagnetism was observed in both types of thin film samples fabricated by RF sputtering, but their magnetic properties appeared to be quite different. Magnetic moments in case of TiO2-δ:Co are mostly associated with local polarization of Co ions and induced defects. There is an evidence of intrinsic ferromagnetism in the case of low Co content (<1 at.%). Room temperature ferromagnetism was observed in TiO2-δ:V at V content from 3 up to 18 at.% in the whole resistivity range from 10e-3 up to 10e6 Ω cm. Positron annihilation spectroscopy revealed a correlation between magnetization and concentration of the negatively charged defects in TiO2-δ:V thin films. The origin of room temperature ferromagnetism in these systems is discussed. Besides, the recent research findings in ZnO-based magnetic semiconductors are briefly discussed with focus on defect-induced ferromagnetism.

Keywords: Dilute magnetic oxides; Dilute magnetic semiconductors; Above room temperature ferromagnetism; Defect-induced ferromagnetism; Positron annihilation spectroscopy

  • Book chapter
    Zhukov, A.: Novel Functional Magnetic Materials. Fundamentals and Applications. Springer Series in Materials Science 231, Switzerland: Springer International Publishing, 2016, 978-3-319-26104-1, 187-219
    DOI: 10.1007/978-3-319-26106-5_5

Permalink: https://www.hzdr.de/publications/Publ-23018
Publ.-Id: 23018


Structure-Correlated Exchange Anisotropy in Oxidized Co80Ni20 Nanorods

Liebana-Vinas, S.; Wiedwald, U.; Elsukova, A.; Perl, J.; Zingsem, B.; Semisalova, A.; Salgueirino, V.; Spasova, M.; Farle, M.

Rare earth-free permanent magnets for applications in electro-magnetic devices promise better sustainability and availability and lower prices. Exploiting the combination of shape, magnetocrystalline and exchange anisotropy in 3D-metals can pave the way to practical application of nanomagnets. In this context, we study the structural and magnetic properties of Co80Ni20 nanorods with a mean diameter of 6.5 nm and a mean length of 52.5 nm, prepared by polyol reduction of mixed cobalt and nickel acetates. Structural analysis shows crystalline rods with the crystallographic c-axis of the hexagonal close-packed (hcp) phase parallel to the long axis of the Co80Ni20 alloy rods, which appear covered by a thin oxidized face-centered cubic (fcc) shell. The temperature dependence of the surprisingly high coercive field and the exchange bias effect caused by the antiferromagnetic surface oxide indicate a strong magnetic hardening due to alignment of anisotropy axes. We identify a temperature dependent local maximum of the coercive field at T = 250 K, which originates from noncollinear spin orientations in the ferromagnetic core and the antiferromagnetic shell. This might be useful for building four way magnetic switches as a function of temperature.

Permalink: https://www.hzdr.de/publications/Publ-23017
Publ.-Id: 23017


Discrimination, correlation and provenance of Bed I tephrostratigraphic markers, Olduvai Gorge, Tanzania, based on multivariate analyses of phenocryst compositions

Habermann, J. M.; Mchenry, L. J.; Stollhofen, H.; Tolosana-Delgado, R.; Stanistreet, I. G.; Deino, A. L.

The chronology of Pleistocene flora and fauna, including hominin remains and associated Oldowan industries in Bed I, Olduvai Gorge, Tanzania, is primarily based on 40Ar/39Ar dating of intercalated tuffs and lavas, combined with detailed tephrostratigraphic correlations within the basin. Although a high-resolution chronostratigraphy has been established for the eastern part of the Olduvai Basin, the western subbasin is less well known due in part to major lateral facies changes within Bed I combined with discontinuous exposure. We address the correlation difficulties using the discriminative power of the chemical composition of the major juvenile mineral phases (augite, anorthoclase, plagioclase) from tuffs, volcaniclastic sandstones, siliciclastic units, and lavas. We statistically evaluate these compositions, obtained from electron-microprobe analyses, applying principal component analysis and discriminant analysis to develop discriminant models that successfully classify most Bed I volcanic units. The correlations resulting from integrated analyses of all target minerals provide a basin-wide Bed I chemostratigraphic framework at high lateral and vertical resolution, consistent with the known geological context, which expands and refines the geochemical databases currently available. Correlation of proximal ignimbrites at the First Fault with medial and distal Lower Bed I successions of the western basin enables assessment of lateral facies and thickness trends that corroborate Ngorongoro Volcano as the primary source for Lower Bed I, whereas Upper Bed I sediment supply is mainly from Olmoti Volcano. Compositional similarity between Tuff IA, Bed I lava, and Mafic Tuffs II and III single-grain fingerprints, together with north- and northwestward thinning of Bed I lava, suggests a common Ngorongoro source of these units. The techniques applied herein improve upon previous work by evaluating compositional affinities with statistical rigor rather than primarily relying on visual comparison of bivariate plots.

Keywords: Olduvai; Ngorongoro fan; Principal component analysis; Discriminant analysis; Chemostratigraphy

Permalink: https://www.hzdr.de/publications/Publ-23015
Publ.-Id: 23015


Lectures concerning uranium pollution, time-resolved laser-induced fluorescence spectroscopy (TRLFS), and radiometric dating

Baumann, N.

Within a comprehensive course in Radioecology, 5 special lectures were held in the Department of Applied Radiation and Isotopes, Faculty of Science, Kasetsart University, Chatuchak, Bangkok. Topics of these 5 lectures were “Kinds of radiation and radioactive decay”, “Natural occurring radionuclides and natural radiation”, “Age determination by radioactive decay and mass spectrometry”, “Speciation of radionuclides and time-resolved laser-induced fluorescence spectroscopy (TRLFS)”, and “Uranium as basement of nuclear energy production”. As a precondition for obtaining Credit Points for the students in the audience, a questionnaire was created concerning these 5 topics.

Keywords: Uranium; TRLFS; Radiometric Dating

  • Invited lecture (Conferences)
    Course in Radioecology, Department of Applied Radiation and Isotopes, Faculty of Science, Kasetsart University, 22.-26.02.2016, Chatuchak, Bangkok, Thailand

Permalink: https://www.hzdr.de/publications/Publ-23014
Publ.-Id: 23014


Single crystal growth, structural characteristics and magnetic properties of chromium substituted M-type ferrites

Shlyk, L.; Vinnik, D. A.; Zherebtsov, D. A.; Hu, Z.; Kuo, C.-Y.; Chang, C.-F.; Lin, H.-J.; Yang, L.-Y.; Semisalova, A. S.; Perov, N. S.; Langer, T.; Pöttgeng, R.; Nemrava, S.; Niewa, R.

Two different types of fluxes, namely sodium based and chloride based fluxes were used to grow Cr substituted barium and strontium hexaferrite ferrite crystals, (Sr,Ba)Fe12 − xCrxO19 at comparatively low temperatures of about 1300 °C. The sodium based flux led to growth of larger crystals up to 5 mm, but with only minor Cr contents x ≤ 0.07. From the chloride based flux the obtained Cr contents are significantly higher with x = 5.7 (Sr) and x = 3.4 (Ba), however, crystals reach only sizes in the sub-mm range. X-ray absorption spectroscopy data support exclusively isovalent substitution of Fe3+ by Cr3+ even for very low Cr contents. 57Fe Mößbauer spectroscopy reveals Cr to preferentially occupy the six-fold by oxygen coordinated site at 12k and, to a lower degree, 2a and 4f2 in space group P63/mmc. All characteristic magnetic properties drop upon Cr substitution, e. g., the Curie temperature from 728 K for pure BaFe12O19 to 465 K for BaFe8.6Cr3.4O19, the saturation magnetization from 71 emu/g to 29.7 emu/g and the coercive field from 363 Oe to 45 Oe.

Keywords: Hexaferrites; Chromium; Mößbauer; Magnetism

Permalink: https://www.hzdr.de/publications/Publ-23013
Publ.-Id: 23013


Layer-to-layer compression and enhanced optical properties of few-layer graphene nanosheet induced by ion irradiation

Shang, Z.; Tan, Y.; Zhou, S.; Chen, F.

We report on the first experimental study of the layer-to-layer compression and enhanced optical properties of few-layer graphene nanosheet by applying ion irradiation. The deformation of graphene layers is investigated both theoretically and experimentally. It is observed that after the irradiation of energetic ion beams, the space between separate graphene layers is reduced due to layer-to-layer compression, resulting in tighter contact of the graphene sheet with the surface of the substrate. This processing enables enhanced interaction of the graphene with the evanescent-field wave near the surface, which induces reinforced polarization-dependent light absorption of the graphene. Utilizing the ion-bombarded graphene nanosheets as saturable absorbers, we have realized efficient Q-switched waveguide lasing with enhanced performance through the interaction of the graphene and evanescent field.

Keywords: Graphene; Absorption; Q switching; Optical properties; Waveguides

Permalink: https://www.hzdr.de/publications/Publ-23012
Publ.-Id: 23012


Detectability of local range shifts in double scattered proton irradiation with a prompt gamma slit camera

Priegnitz, M.; Nenoff, L.; Barczyk, S.; Vander Stappen, F.; Hotoiu, L.; Smeets, J.; Fiedler, F.; Pausch, G.; Richter, C.

no abstract available

  • Poster
    55th Annual Conference PTCOG 55, 22.-28.05.2016, Prague, Czech Republic

Permalink: https://www.hzdr.de/publications/Publ-23011
Publ.-Id: 23011


Evaluation of four Sigma-1 PET radiotracers in nonhuman primates

Cai, Z.; Baum, E.; Bois, F.; Holden, D.; Lin, S.-F.; Chen, Y.; Fischer, S.; Jia, H.; Brust, P.; Huang, Y.

Objectives: Sigma-1 receptor (S1R) is resident to mitochondrial-associated endoplasmic reticulum and plasma membranes with implications in a variety of diseases including Alzheimer's disease, ALS, and cancer. Previous PET S1R radiotracers are characterized by slow kinetics that impedes their use for human brain imaging. Recently a series of spirocyclic piperidine-based ligands showed great promise as S1R PET imaging probes, based on their high selectivity towards S1R and good binding characteristics in rodents or porcine. [1-3] Here, we report the first monkey PET imaging studies of four ligands (1-4) in this series to assess their pharmacokinetic and in vivo binding properties, and to select the most suitable tracer for advancing to humans.
Methods:
Each tracer was injected as a bolus (~5 mCi) to the same rhesus monkey.
Baseline scans were obtained on a Siemens FOCUS 220 scanner over 4 h. Two hour blocking scans were performed with administration of SA4503 (0.5 mg/kg) [4] before tracer injection. Arterial blood was drawn during each scan for metaboite analysis by HPLC and construction of the plasma input functions. Regional brain time-activity curves (TACs) were analyzed by one-tissue (1T), two-tissue (2T), and multilinear analysis-1 (MA1) models to obtain regional volumes of distribution (VT). The free fraction (fp) in plasma was meassured via ultrafiltration method. Log D of each tracer was also determined.
Results:
Fast metabolism of the tracers was observed in rhesus monkeys, with ~ 35%, 18%, and 19% parent fraction, respectively, for 1 (2), 3 and 4 at 60 min post-injection. Plasma fP values were 2%, 8%, and 17%, for 1 (2), 3 and 4, consistent with their respective measured Log D values of 2.80, 2.55, and 2.50. In the brain, all four tracers showed high and fast uptake. Tissue activity washout was rapid for 2 and 4, and much slower for 1 and 3, in line with their respective in vitro S1R binding affinities. Both the 1T and MA1 kinetic models provided good fits of regional TACs, and reliable VT estimates with low errors. Across all regions, 1T VT values were greatest for 3, follwed by 1, 4, and 2. The highest VT values were in the cingulate gyrus for all tracers. Ligand 4 showed the greatest differential uptake across different brain regions. SA4503 at the dose of 0.5 mg/kg blocked ~85% (2) and ~95% (4) of radiotracer binding, respectively.

  • Lecture (Conference)
    SNMMI 2016 Annual Meeting, 11.-15.06.2016, San Diego, California, USA
  • Abstract in refereed journal
    Journal of Nuclear Medicine 57(2016)2, 81P

Permalink: https://www.hzdr.de/publications/Publ-23010
Publ.-Id: 23010


A Johann-type X-ray emission spectrometer at the Rossendorf beamline

Kvashnina, K. O.; Scheinost, A. C.

This paper gives a detailed description of the Johann-type X-ray emission spectrometer recently installed and tested at the Rossendorf Beamline (ROBL) of the European Synchrotron Radiation Facility (ESRF). The spectrometer consists of a single spherically bent crystal analyzer and avalanche photodiode detector positioned on the vertical Rowland cycle of 1m diameter. The hard X-ray emission spectrometer (~ 5 – 25 keV) operates at atmospheric pressure and covers the Bragg angles of 65°–89°. The instrument has been tested at high and intermediate incident energies, i.e. at the Zr K-edge and at the Au L3-edge, in the second experimental hutch of ROBL. The spectrometer is constructed for studying nuclear materials and environmental applications by high energy resolution X-ray absorption and X-ray emission spectroscopies.

Permalink: https://www.hzdr.de/publications/Publ-23009
Publ.-Id: 23009


Evaluation of Actinide(IV)-Silica colloids mobility under repository conditions - the NuWaMa Project

Hildebrand, H.; Weiss, S.; Zaenker, H.; Kulenkampff, J.; Lippmann-Pipke, J.; Videnska, K.; Červinka, R.

Colloidal transport in the near-field and far-field of repositories is considered as one potential pathway for migration of (radio-)toxic components in case of groundwater intrusion. Recently, the in-situ formation of actinide(IV)-silica colloids (dp< 20 nm) was discovered under typical conditions for nuclear waste repositories in granitic formations (Dreissig et al. (2011), Hennig et al. (2013), Husar et al. (2015). These colloids show long-term stability over years and could therefore play a significant role in radionuclide migration since silica is an ubiquitous compound. So far, there is only little knowledge about the reactive transport of actinide(IV)-silica colloids under repository conditions. Within the NuWaMa project (intended start in January 2016), a new close collaboration between the Helmholtz-Zentrum Dresden - Rossendorf (HZDR) and the ÚJV Řež will be established and joint research focused on this problem will be intensified. First transport experiments using packed columns with crushed granite and distilled water amended with Th(IV)-silica colloids gave hints of mobility of the colloids under certain conditions. These experiments are conducted using conventional analytics such as ICP/MS and light scattering techniques for detection of the colloids in the column effluent. The aim of the project is also to evaluate this transport in more detail, including also by positron emission tomography (PET) with its unrivaled sensitivity and robustness for non-destructive quantitative spatio-temporal measurements. HZDR empowered “GeoPET” for its applicability in opaque/geological media (e.g. Kulenkampff et al., (2008), Kulenkampff et al.,(2015) , see Fig. 1). Zirconium radionuclides (positron-emitter and analogues for tetravalent actinides)shall be used for visualization of colloidal transport on column scale and, if applicable, also in a fractured rock sample.
The aim of the study, applied techniques and first results are intended to be shown as a poster presentation.

  • Poster
    BELBaR-Conference 2016 "Clay Colloids in Aqueous Systems", 03.-04.02.2016, Berlin, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-23007
Publ.-Id: 23007


Structure and chemistry of surface-doped Pt:SnO2 gas sensing materials

Degler, D.; Pereira De Carvalho, H. W.; Kvashnina, K.; Grunwaldt, J.-D.; Barsan, U. W. N.

Surface-doped Pt:SnO2 was synthesized by impregnation of calcined SnO2 made by an aqueous sol-gel route. The structure of the introduced Pt-dopant and its behaviour during gas exposure were examined by in-situ and operando X-ray absorption spectroscopy. The results reveal that Pt forms a nano-sized PtO2 phase, which was not found for bulk and surface doped materials, studied previously. In a comparative investigation of undoped and Pt-doped SnO2 gas sensors the performance and the surface chemistry were investigated, the latter one using operando FT-IR spectroscopy. The results prove a strong influence of the different Pt structures on the surface chemistry of SnO2, providing the basis for an understanding on the varying sensor performance of differently synthesized Pt:SnO2 gas sensing materials

Permalink: https://www.hzdr.de/publications/Publ-23006
Publ.-Id: 23006


Crystal-field and covalency effects in uranates: x-ray spectroscopic study

Butorin, S.; Kvashnina, K.; Smith, A.; Popa, K.; Martin, P.

The electronic structure of U(V) and U(VI) containing uranates NaUO3 and Pb3UO6 was studied using an advanced technique, namely x-ray absorption spectroscopy (XAS) in the high-energyresolution uorescence-detection (HERFD) mode. Thanks to a significant reduction of the core-hole lifetime broadening, the crystal-field splitting of the 5f shell were probed directly in the HERFDXAS spectra collected at the U 3d edge, which is not possible with conventional XAS. In addition, the charge-transfer satellites resulting from the U 5f-O 2p hybridization were clearly resolved. The crystal-field parameters, 5f occupancy, and degree of covalency of the chemical bonding in these uranates were estimated using the Anderson impurity model by calculating the U 3d HERFD-XAS, conventional XAS, core-to-core (U 4f-to-3d transitions) resonant inelastic x-ray scattering (RIXS) and U 4f x-ray photoelectron spectra, respectively. The crystal field was found to be strong in these systems, while the 5f occupancy was determined to be 1.32 and 0.84 electrons in the ground state for NaUO3 and Pb3UO6, respectively, thus indicating a significant covalent character for these compounds.

Downloads:

  • Secondary publication expected

Permalink: https://www.hzdr.de/publications/Publ-23005
Publ.-Id: 23005


High-resolution x-ray absorption spectroscopy as a probe of crystal-field and covalency effects in actinide compounds

Butorin, S. M.; Kvashnina, K.; Vegelius, J. R.; Meyer, D.; Shuh, D. K.

Applying the high-energy-resolution uorescence-detection (HERFD) mode of x-ray absorption spectroscopy (XAS) we were able for the first time to probe the crystalline electric field (CEF) splittings of the 5f shell directly in the HERFD-XAS spectra of actinides. Using ThO2 as an example, the data measured at the Th 3d edge were interpreted within framework of the Anderson impurity model. Since the charge-transfer satellites were also resolved in the HERFD-XAS spectra, their analysis revealed that ThO2 is not an ionic compound as previously believed. The Th 6d occupancy in the ground state was estimated to be as twice as much compared to that for the Th 5f states. We demonstrate that HERFD-XAS now allows for characterization of the CEF interaction and degree of covalency in the ground state of actinide compounds as it is extensively done for 3d transition metal systems.

  • Open Access Logo Proceedings of the National Academy of Sciences of the United States of America 113(2016)29, 8093-8097
    DOI: 10.1073/pnas.1601741113

Permalink: https://www.hzdr.de/publications/Publ-23004
Publ.-Id: 23004


Valence fluctuations of europium in the boride Eu4Pd29+xB8

Gumeniuk, R.; Schnelle, W.; Ahmida, M.; Abd-Elmeguid, M.; Kvashnina, K.; Tsirlin, A.; Leithe-Jasper, A.; Geibel, C.

We synthesized a high quality sample of the boride Eu4Pd29+xB8 (x = 0.76) and studied its structural and physical properties. Its tetragonal structure was solved by direct methods and confirmed to belong to the Eu4Pd29B8 type. All studied physical properties indicate a valence fluctuating Eu state, with a valence decreasing continuously from about 2.9 at 5 K to 2.7 at 300 K. Maxima in the T dependence of the susceptibility and thermopower at around 135 K and 120 K, respectively, indicate a valence fluctuation energy scale on the order of 300 K. Analysis of the susceptibility evidences some inconsistencies when using the ionic interconfigurational fluctuation (ICF) model, thus suggesting a stronger relevance of hybridization between 4f and valence electrons compared to standard valence-fluctuating Eu systems.

Permalink: https://www.hzdr.de/publications/Publ-23003
Publ.-Id: 23003


Kondo effect and thermoelectric transport in CePd3Bex

Gumeniuk, R.; Schnelle, W.; Kvashnina, K.; Leithe-Jasper, A.

The physical properties of the series CePd3Bex (0 ≤ x ≤ 0.47) have been studied. Introducing Be into CePd3 results in a drastic reduction of the Seebeck coefficient from 100 μV K−1 at 300 K to -2 μV K−1, respectively. Paramagnetism of Ce3+ free ions and metallic conduction dominate the physical properties. A structural transition at x = 0.25 is accompanied by a significant lowering of the Kondo temperature and leads to a successive suppression of the thermoelectric performance of CePd3Bex with increasing x.

Keywords: crystal structure; Kondo effect; thermoelectric properties; XAS

Permalink: https://www.hzdr.de/publications/Publ-23002
Publ.-Id: 23002


Creating and imaging nanosized magnets using HIM and TEM holography

Hlawacek, G.; Röder, F.; Bali, R.; Wintz, S.; Hübner, R.; Bischoff, L.; Lichte, H.; Potzger, K.; Lindner, J.; Fassbender, J.

Besides imaging, gas field ion source (GFIS) based microscopes [1] are used for materials modification. This usually is based on the use of high fluence to either mill the sample material or implant Nobel gas ions into the target material [2]. Here, we present a novel route utilizing a Helium Ion Microscope (HIM) to form nano–sized magnets of arbitrary shape using very low fluences (6 × 1014 cm−2) of 20 keV–25 keV Neon ions. The fine Neon beam available in the HIM is used to locally switch 40nm thin Fe60Al40 films from the well ordered paramagnetic B2 structure into the ferromagnetic A2 structure [3, 4]. Planar structures potentially useful for applications such as spin valves or other spin–transport devices have been formed this way. Kerr Microscopy and off–axis TEM holography has been used to analyze the resulting magnetic nano–structures. Results on the energy depended depth of magnetization as well as on the lateral definition of the magnetic structures due to scattering are presented.

  • Lecture (Conference)
    62nd AVS meeting, 18.-23.10.2015, San Jose, USA

Permalink: https://www.hzdr.de/publications/Publ-23000
Publ.-Id: 23000


High resolution surface patterning with the Helium Ion Microscope

Hlawacek, G.

In the past years Helium Ion Microscopy (HIM) [1] has become a mature imaging and nano-modification technique. The method is best known for its high resolution imaging capabilities. In addition it provides excellent charge compensation capabilities and a high surface sensitivity [2]. With the introduction of Ne as an working gas for the used Gas Field Ion Source (GFIS) also fast and high resolution nanomachining has become possible. In the following I would like to give a brief introduction of the technique. Subsequently, I will present examples of materials modification with a highly focused Helium or Neon beam.

  • Invited lecture (Conferences)
    XXII International Conference on Ion-Surface Interactions, 20.-24.08.2015, Moskau, Russia

Permalink: https://www.hzdr.de/publications/Publ-22999
Publ.-Id: 22999


Application of Helium Ion Microscopy to surface science problems

Hlawacek, G.; Veligura, V.; Jankowski, M.; van Gastel, R.; Wormeester, R.; Zandvliet, H. J. W.; Poelsema, B.

Helium Ion Microscopy (HIM) is well known for its high lateral resolution and unique nanomachining capabilities. In addition it is a very surface sensitive technique and therefore ideally suited to answer scientific questions in surface and interface science. I will give a brief introduction of the technique followed by a selection of problems related to surface and interface science.
The high surface sensitivity of HIM allowed us to measure the thickness of thin carbon layers present on gold nanorods. On the other hand one can use back scattered helium (BSHe) particles to reveal buried interfaces such as the diffusion front of a Pd2Si layer covered by more than 100 nm of SiO2.
The orientation of a sample can be determined using channeling. I will show that with a simple geometrical model channeling directions can be predicted with sufficient accuracy to align the He beam parallel to low index directions. Exploiting channeling into a crystalline sample the background from the substrate can be suppressed, thus enhancing the surface sensitivity even further. This has been used in a recent study of the surface reconstruction observed in the case of a few ML of Ag deposited on Pt(111). Based on a change of the work function of 25meV across the atomically flat terraces we can distinguish Pt rich from Pt poor areas and visualize the single atomic layer high steps between the terraces. Utilizing channeling/dechanneling and the exceptional surface sensitivity of the HIM we can measure the periodicity of the hcp/fcc pattern formed in the 2 ML thick Ag alloy layer. A periodicity of 6.65 nm along the <-1-12> surface direction has been measured. In terms of crystallography a hcp domain is obtained through a lateral displacement of a part of the outermost layer by 1/√3 of a nearest neighbor spacing along <-1-12>.

  • Invited lecture (Conferences)
    1st International Conference on Applied Surface Science, 27.-30.07.2015, Shanghai, China

Permalink: https://www.hzdr.de/publications/Publ-22998
Publ.-Id: 22998


On the way to a quantitative analytical ion microscopy

Klingner, N.; Hlawacek, G.; Heller, R.; Facsko, S.

Analytical Ion Micrsocope

  • Lecture (Conference)
    Gas Ion Microscopy User Group Meeting 2015, 13.07.2015, Dublin, Ireland

Permalink: https://www.hzdr.de/publications/Publ-22997
Publ.-Id: 22997


Helium Ion Microscopy

Hlawacek, G.

In the past years Helium Ion Microscopy (HIM) [1] has become a mature imaging and nano-modification technique. The method is best known for its high resolution imaging capabilities. In addition it provides excellent charge compensation capabilities and high surface
sensitivity [2]. With the introduction of Ne as an working gas for the used gas field ion source (GFIS) also fast high resolution nanomachining has become possible. In the following I would like to give a brief introduction to the technique. Subsequently I want to highlight the
importance of channeling to achieve the best possible imaging conditions and maximize the surface sensitivity. Finally I want to present selected results on high and low fluence He and Ne milling and implantation.

  • Invited lecture (Conferences)
    mmc2015, 29.06.-02.07.2015, Manchester, United Kingdom

Permalink: https://www.hzdr.de/publications/Publ-22996
Publ.-Id: 22996


Materials analysis using channeling and ionoluminescence in a helium ion microscope

Hlawacek, G.

Helium ion Microscopy1 is a versatile microscopy technique that provides high resolution imaging and nano-machining in combination with a high surface sensitivity and large depth of focus. It utilizes a narrow beam of He+ ions to achieve a lateral resolution of less than 0.5 nm. Backscattered Helium ions (BSHe) and secondary electrons (SE) can be used to obtain an image of the specimen.
When using crystalline samples channeling of the particles can occur. This effect can be exploited in several ways in the HIM. First of all it is possible to map out the different channeling directions and intensities and thus obtain information on the crystal structure of the sample. A simple geometrical model is introduced that can predict the channeling directions and relative intensities observed in the HIM2. By exploiting channeling and making use of the dechanneling contrast thin surface layers can be made visible in SE as well BSHe images3. We used this to observe composition and structural changes in a 2 ML thin silver layer on Pt(111). Work function differences as small as 40 meV between Ag and Pt rich areas on the surface reveal the position of mono—atomic surface steps. A regular arrangement of areas with reduces the channeling probability reveals the surface reconstruction of the top 2—3 ML which has a periodicity of only 5.8 nm.
Ionoluminescence on the other hand allows to obtain information on defects in the bulk of the material. I will show results obtained for a variety of materials including semiconductors4, rare earth containing perovskites and ionic crystals. The types of defects were identified and the influence of the scanning conditions on the IL signal has been investigated5. We used IL to map out the interaction volume of the beam in NaCl, and demonstrate the possibility of subsurface patterning. In our setup using a 35 keV He+ beam and NaCl only 3 vac/nm-2 are needed to obtain a detectable IL signal6.

  • Invited lecture (Conferences)
    IBA2015, 14.-19.06.2015, Opatija, Croatia

Permalink: https://www.hzdr.de/publications/Publ-22995
Publ.-Id: 22995


Helium Ion Microscopy of atomic steps and surface reconstruction

Hlawacek, G.; Jankowski, M.; van Gastel, R.; Wormeester, H.; Zandvliet, H.; Poelsema, B.

Helium Ion Microscopy (HIM) [1] is well known for its exceptional imaging and nanofabrication capabilities. HIM has an unprecedented surface sensitivity, and channeling can be utilized to maximise the signal to noise ratio. We demonstrate the resolving power of the technique using a thin (2 ML) silver layer on Pt(111). The obtained HIM results are compared to results obtained by low energy electron microscopy, spot profile analysis low energy electron diffraction (SPA-LEED), and atomic force microscopy phase contrast. In HIM single atom layer high steps can be visualized as a result of a work function change—across the otherwise atomically flat terraces—of only ~20 meV. By utilizing the dechanneling contrast mechanism [2] also the surface reconstruction of this thin surface layer can be revealed. We find a threefold periodic structure of channeling (fcc stacking) and dechanneling (hcp stacking) areas.
The periodicity—measured along the h112i surface direction—of this structure is 5.8 nm. This is in excellent agreement with values obtained by SPA-LEED [3].

  • Lecture (Conference)
    Physics Boat, 09.-11.06.2015, Helsinki, Stockholm, Finland, Sweden

Permalink: https://www.hzdr.de/publications/Publ-22994
Publ.-Id: 22994


He-ion Microscopy

Hlawacek, G.

Tutorial on Helium Ion Microscopy

  • Invited lecture (Conferences)
    Physics boat 2015, 08.06.2015, Helsinki, Finland

Permalink: https://www.hzdr.de/publications/Publ-22993
Publ.-Id: 22993


Relaxation dynamics in graphene: Surprising Coulomb scattering effects

Winnerl, S.

We report on Coulomb scattering effects in graphene and Landau-quantized graphene.

  • Lecture (others)
    3rd Workshop of the SPP "Graphene", 17.-20.05.2015, Kremmen, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-22992
Publ.-Id: 22992


Relaxation dynamics in graphene studied by THz radiation from the free-electron laser FELBE

Winnerl, S.

The free-electron laser FELBE, which is operated as a user facility, provides tunable radiation in the mid infrared and terahertz spectral range (wavelength: 4 – 230 µm) in form of ps pulses. It is driven by a superconducting accelerator that enables continuous pulsing operation at a repetition rate of 13 MHz, making it highly attractive for many experiments. We briefly review a few types of experiments including non-perturbative nonlinear spectroscopy and near-field microscopy on systems like excitons in semiconductor quantum wells or electrons confined in quantum dots. Mainly we discuss time-resolved spectroscopy on graphene and in particular Landau quantized graphene. Here evidence for extremely efficient Auger scattering is found that can actually deplete a level that at the same time is optically pumped.

Keywords: Graphene; ultrafast dynamics; FEL

  • Lecture (others)
    Semninar an der Uni Manchester, 25.11.2015, Manchester, United Kingdom

Permalink: https://www.hzdr.de/publications/Publ-22991
Publ.-Id: 22991


Coulomb scattering in the vicinity of the Dirac point in graphene

Winnerl, S.

After a brief introduction into time-resolved spectroscopy we focus on polarization-resolved pump-probe experiments at different infrared photon energies. Here an anisotropy induced by the polarization of the pump beam is observed. For photon energies below the optical photon energy (~200 meV) the relaxation dynamics is dominated by Coulomb scattering. The experiments show directly that non-collinear scattering, which leads to an isotropic distribution, is remarkably slow at low fluences, namely on a timescale of 5 ps. The findings are in good agreement with microscopic theory. The timescale of this thermalization is very attractive for applications where hot carriers in graphene are exploited in detectors or modulators of infrared radiation.

Keywords: Graphene; ultrafast dynamics; Coulomb scattering

  • Invited lecture (Conferences)
    International workshop on many-body phenomena in graphene, 26.-27.10.2015, Gothenburg, Schweden

Permalink: https://www.hzdr.de/publications/Publ-22990
Publ.-Id: 22990


Coulomb Scattering in Graphene

Winnerl, S.; Mittendorff, M.; Otto, J.; Wendler, F.; Winzer, T.; Malic, E.; Knorr, A.; Schneider, H.; Helm, M.

Coulomb interaction is the main mechanism that transforms a nonequilibrium carrier distribution in graphene into a hot Fermi-Dirac distribution. In many experiments carriers are assumed to be thermalized on a timescale well below 100 fs. However, we have recently observed that the carrier distribution is strongly anisotropic in k-space when graphene is excited with near infrared pulses (photon energy 1.5 eV, duration 30 fs). The anisotropy induced by the polarization of light vanishes on a timescale of 150 fs due to scattering with optical phonons [1]. Exciting with photon energies of 75 meV, i.e. well below the optical phonon energy (~200 meV), strongly quenches the scattering via phonons and allows one to study pure Coulomb scattering in the vicinity of the Dirac point. At low fluences the transition from the anisotropic distribution to an isotropic one is very slow (~5 ps at 10 K) since Coulomb scattering is predominantly collinear and thus preserves the anisotropic distribution. At higher fluences the strength of Coulomb scattering increases, resulting in a faster decay of the anisotropy. In a second experiment we study the relaxation dynamics in Landau quantized graphene [2].
By applying circularly polarized radiation in the pump-probe experiments individual low-index Landau level transitions can be addressed. Here a surprising effect is revealed, namely a change in sign of a pump-probe signal with respect to the expectation considering single-particle interactions only (cf.Fig.1). This is caused by strong Auger scattering that depletes the zeroth Landau level even though it is optically pumped at the same time.

[1] M. Mittendorff. et al. Nano Lett., 14 2014, 1504
[2] M. Mittendorff. et al. Nature Phys., 11 2015, 75

Keywords: Graphene; ultrafast spectroscopy; carrier dynamics; Coulomb scattering; Landau quantization

  • Lecture (Conference)
    Nanospectroscopy for two-dimensional materials, 08.-10.09.2015, Chemnitz, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-22989
Publ.-Id: 22989


Strong Auger scattering in Landau-quantized graphene

Winnerl, S.; Mittendorff, M.; Wendler, F.; Malic, E.; Knorr, A.; Schneider, H.; Helm, M.

While the carrier dynamics in graphene in absence of magnetic fields is well researched in a large spectral range ranging from UV to THz, the dynamics in Landau quantized graphene is almost unexplored. We investigate the carrier dynamics within the system of Landau levels (LLs) of index n = -1, n = 0 and n = 1 by pump-probe experiments complemented by microscopic modelling. Using circularly polarized mid-infrared radiation (photon energy 75 meV) allows one to selectively excite the two energetically degenerate transitions LL-1 → LL0 and LL-1 → LL0, respectively. Surprisingly, induced transmission is observed in one configuration of pumping and probing with opposite configuration (cf. Fig 1 (c) and (d)). Considering single particle Pauli blocking, one would expect induced absorption in this case. The sign change indicates that LL0 is depleted by strong Auger scattering, even though it is optically pumped at the same time.
We discuss the role of carrier-carrier and carrier-phonon scattering in Landau quantized graphene and provide an outlook on the application potential of this system for tunable THz lasers.

References
[1] M. Mittendorff, F. Wendler, E. Malic, A. Knorr, M. Orlita, M. Potemski, C. Berger, W. A. de Heer, H. Schneider, M. Helm, and S. Winnerl, " Nature Phys. 11, 75 (2015).

Keywords: Graphene; Landau quantization; Auger scattering; Terahertz

  • Lecture (Conference)
    Graphene Week 2015, 22.-26.06.2015, Manchester, United Kingdom

Permalink: https://www.hzdr.de/publications/Publ-22988
Publ.-Id: 22988


Einblick in die Relaxationsdynamik von Graphen mittels Infrarot-Kurzzeitspektroskopie: überraschende Coulomb-Streueffekte

Winnerl, S.

Graphen, ein zweidimensionaler Kohlenstoffkristall weist eine Bandstruktur ohne Bandlücke auf. Im Bereich kleiner Energien ist die Dispersion linear und Elektronen und Löcher verhalten sich perfekt symmetrisch. Ihr Verhalten entspricht masselosen Dirac-Fermionen. Nach einem kurzen Überblick über grundlegende Eigenschaften von Graphen und dem Anwendungspotential dieses Materials stellen wir einige unserer Experimente zur Ladungsträgerdynamik vor.
Wird Graphen mit kurzen, linear polarisierten nahinfraroten Lichtpulsen angeregt, so beobachten wir eine im k-Raum anisotrope Ladungsträgerverteilung. Erstaunlicher Weise bleibt diese Anisotropie für 150 fs erhalten, obwohl die Elektron-Elektron Streuzeit in dem System viel kürzer ist. Wir zeigen, dass Energie- und Impulserhaltung für diese Anisotropie-erhaltende Wirkung der Coulomb-Streuung verantwortlich sind und eine isotrope Ladungsträgerverteilung durch Elektron-Phonon-Streuung erreicht wird. Verwenden wir zur Anregung Strahlung mit Photonenergie kleiner als die optische Phonon-Energie, so bleibt die Anisotropie auf der Skala von 1 – 10 ps erhalten. Für diese Experimente dient der Freie-Elektronenlaser FELBE als Strahlungsquelle.
Schließlich diskutieren wir die Ladungsträgerdynamik zwischen Landau-Niveaus von Graphen im Magnetfeld. Hierfür wurde zirkular polarisierte Infrarotstrahlung von FELBE eingesetzt. Hier gibt es aufgrund der Coulomb-Wechselwirkung einen sehr verblüffenden Effekt, nämlich dass ein Landauniveau entvölkert wird, obwohl es durch optisches Pumpen mit Ladungsträgern befüllt wird.
Zusammenfassend zeigen unsere Experimente, dass man mit Licht unterschiedlicher Wellenlänge und unterschiedlicher Polarisationszustände tiefe Einblicke in die Dynamik stark Coulomb-wechselwirkenden Elektronen in Graphen erhalten kann.

Keywords: Graphen; Relaxationsdynamik; Kurzzeit-Spektroskopie

  • Invited lecture (Conferences)
    26. Edgar Lüscher Seminar 2015, 07.02.2015-13.01.2016, Klosters, Schweiz

Permalink: https://www.hzdr.de/publications/Publ-22987
Publ.-Id: 22987


Terahertz spectroscopy at HZDR

Schneider, H.

There is no abstract.

  • Invited lecture (Conferences)
    International consortium on terahertz photonics and optoelectronics conference, 16.-17.12.2015, Moskau, Russland

Permalink: https://www.hzdr.de/publications/Publ-22986
Publ.-Id: 22986


Applications of Helium Ion Microscopy

Hlawacek, G.

HIM is well known for its exceptional imaging and nanofabrication capabilities. After a brief introduction of the gas field ion source and the ion microscope, I will present a wide range of results obtained with either the Twente UHV Orion+ or the NanoFab at the HZDR in Dresden. Special emphasis will be given to the use of channeling and the role of defects created by the energetic ion beam. Ionoluminescence is used to obtain information on the latter. Helium Ion Microscopy has an unprecedented surface sensitivity. Recent results obtained on thin silver layers on Pt(111) demonstrate that work function differences as small as ~20 meV as well as surface reconstructions can be visualized. Finally, some preliminary results of Neon based materials modification and cross section preparation will be presented.

  • Invited lecture (Conferences)
    49th Annual Meeting of the Israel Society for Microscopy, 17.-18.05.2015, Bar Ilan, Israel

Permalink: https://www.hzdr.de/publications/Publ-22985
Publ.-Id: 22985


Microscopy of (electronic) materials

Hlawacek, G.

Microscopy of (electronic) materials

  • Invited lecture (Conferences)
    Festveranstaltung 30 Jahre Werkstoffe der Elektronik, 25.03.2015, Leoben, Österreich

Permalink: https://www.hzdr.de/publications/Publ-22984
Publ.-Id: 22984


Exploiting channelling in Helium Ion Microscopy

Hlawacek, G.

Helium ion Microscopy1⁠ is a versatile microscopy technique that provides high resolution imaging and nano-machining in combination with a high surface sensitivity and large depth of focus. It utilizes a narrow beam of He+ ions to achieve a lateral resolution of less than 0.5 nm. Backscattered Helium ions (BSHe) and secondary electrons (SE) can be used to obtain an image of the specimen. While the first one will provide information of the bulk the latter is extremely surface sensitive2⁠.
When using crystalline samples channeling of the particles can occur. This effect can be exploited in several ways in the HIM. First of all it is possible to map out the different channeling directions and intensities and thus obtain information on the crystal structure of the sample. A simple geometrical model is introduced that can predict the channeling directions and relative intensities observed in the HIM. Such a map of the channeling directions for a fcc metal is presented in figure 13⁠. Channeling is also important for many imaging applications. The contrast on thin surface layers in SE mode can be enhanced when channeling is considered. For BSHe images the situation is more complicated as the signal is dominated by the bulk. Only heavy element adlayers on light element substrates can easily be imaged in BSHe mode. However, the dechanneling contrast also allows the visualization of light elements on heavy element substrates4⁠. In figure 2 a thin organic layer on a silicon wafer is made visible in SE and BSHe mode. By exploiting channeling and making use of the dechanneling contrast thin surface adlayers can be made visible in SE as well BSHe images5⁠.

  • Invited lecture (Conferences)
    EBSD 2015, 30.-31.03.2015, Glasgow, United Kingdom

Permalink: https://www.hzdr.de/publications/Publ-22983
Publ.-Id: 22983


Mechanisms of metal induced crystallization analyzed by in situ Rutherford Backscattering Spectroscopy

Wenisch, R.; Hanf, D.; Lungwitz, F.; Heller, R.; Hübner, R.; Gemming, S.; Krause, M.

Metal induced crystallization (MIC) is a promising technique for low temperature thin film transistor fabrication and graphene synthesis. In MIC, a transition metal acts as seed for the crystallization of an amorphous group IV element. Bond screening near the interface and facilitation of nucleation are recently discussed as mechanisms for MIC. So far, in situ studies have been performed using X-ray diffraction, which is sensitive to the degree of crystallinity but lacks depth resolution. A better insight into the MIC mechanisms requires depth resolved in situ studies in order to determine the concentration profiles of the diffusing components.

Here, the Si/Ag and C/Ni bilayer systems are studied. They are annealed at temperatures of up to 750 °C. Simultaneously, the layer composition and the compositional profiles are investigated with in situ Rutherford backscattering spectroscopy revealing the diffusion kinetics of the components. Both, the quick initial nucleation and the ensuing growth processes are investigated. Further characterization is performed employing in vacuo Raman spectroscopy revealing the phase structure of the resulting films and scanning electron microscopy to investigate the surface structure.

  • Poster
    DPG Frühjahrstagung, 15.-20.03.2015, Berlin, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-22982
Publ.-Id: 22982


In situ Study of Metal Induced Crystallization Processes for Low-Dimensional Materials Synthesis

Wenisch, R.; Heller, R.; Hanf, D.; Hübner, R.; Lungwitz, F.; Schumann, E.; Gemming, S.; Krause, M.

Metal induced crystallization (MIC) is a promising technique for thin film transistor fabrication and graphene synthesis. In MIC, a transition metal catalyzes the crystallization of the amorphous phase of a group IV element by bond screening near the interface and facilitation of nucleation. So far, in situ studies have been performed using X-ray diffraction which is sensitive to the degree of crystallinity. However this technique lacks depth resolution and is therefore unable to track diffusion and layer exchange.
Here, the Si/Ag and C/Ni bilayer systems are studied. The samples are annealed at temperatures of up to 750 °C. Simultaneously, depth profiles of the elements are investigated by in situ Rutherford backscattering spectroscopy revealing the diffusion kinetics. The changes in the phase structure are explored by in situ Raman spectroscopy. Both the quick initial nucleation and ensuing growth processes are investigated. Scanning electron microscopy provides insight to the surface morphology.

  • Poster
    International Winterschool on Electronic Properties of Novel Materials, 08.-13.03.2015, Kirchberg in Tirol, Österreich

Permalink: https://www.hzdr.de/publications/Publ-22981
Publ.-Id: 22981


Ion Beam Analysis in the Helium Ion Microscope

Heller, R.; Klingner, N.; Facsko, S.; von Borany, J.; Gnauck, P.; Hlawacek, G.

Helium ion microscopes (HIM) have turned into a frequently used imaging device in several laboratories around the world. Beside a sub nano-meter resolution and its high field of depth the latest generation of HIM devices (Zeiss Orion NanoFab) offers the ability to make use of Neon ions enabling additional opportunities in terms of surface modifications on the nm scale [1].

While the image generation in a HIM is based on evaluating the amount of secondary electrons the information carried by the back-scattered He/Ne projectiles (BSP) is not taken into consideration at the moment. Thus the HIM offers excellent topographic imaging capabilities but chemical information (in terms of elemental composition) of the surface is not accessible. Nevertheless back-scattered particles carry that information and may be used to provide additional contrast mechanism(s). First attempts to measure BSP energy spectra were done by Sijbrandij et al. [2] and gave evidence for the general feasibility but also revealed that a quantitative chemical analysis of thin layers would require development of more sophisticated detection concepts than those used in their experiments (silicon surface barrier detector).

In the present contribution we show the development and the implementation of a Time-of-Flight back-scattering spectrometry (ToF-BS) technique within our HIM. Pulsing the primary ion beam by using the existing beam blanker with a customized pulsing electronics enables us to generate pulses as short as below 10ns. BSP detection is done by means of a micro channel plate detector. Our measurements demonstrate that this technique is capable to achieve an energy resolution as good as 2keV (for 30keV He incident ions) by simultaneously keeping the spatial resolution in the order of a few 100nm. We further show that with some slight modification the presented setup can be utilized to acquire ToF spectra of sputtered particles as well, thus enabling lateral resolved ToF-SIMs within the HIM.

[1] G. Hlawacek, V. Veligura, R. van Gastel, and B. Poelsema, J. Vac. Sci. Technol. B 32(2), 2014, 020801.
[2] S. Sijbrandij, B. Thompson, J. Notte, B. W. Ward and N. P. Economou, J. Vac. Sci. Technol. B, 26(6), 2008, 2103-2106

  • Lecture (Conference)
    22nd International Conference on Ion Beam Analysis, 16.06.2015, Opatija, Croatia

Permalink: https://www.hzdr.de/publications/Publ-22980
Publ.-Id: 22980


KMC simulation of irradiation-induced nanostructure evolution in Oxide Dispersion Strengthened Fe-Cr alloys

Liedke, B.; Posselt, M.; Murali, D.; Abdou, J. M.; Claisse, A.; Olsson, P.

Oxide Dispersion Strengthened (ODS) steels are considered as one of the most promising candidates for structural materials in next generation nuclear fusion reactors and future nuclear fission reactors [1]. The ODS materials consist of a ferritic or ferritic/martensitic Fe-Cr matrix filled with yttria-based oxide particles and is fabricated during mechanical alloying and hot consolidation processes. It is well known that their extraordinary properties such as high-temperature creep strength as well as high dose ion/neutron irradiation resistance are due to formation of small Y-Ti-O clusters with a size of few nanometers. Besides their significant effect on reduction of dislocations and grain-boundaries mobility, the nanoclusters also act as traps for point defects like vacancies, interstitials and helium, which may be typically generated in a nuclear reactor. It is still under debate what the formation mechanisms of the nanoclusters are and why they prove such high temperature and radiation damage stability.
Experimental methods typically applied to investigate the issues stated above cannot fully reflect the atomic-scale of the nanoclusters, as well as the mechanisms related to their formation, evolution and destruction upon radiation damage. Therefore, atomistic computer experiments can significantly contribute to a general understanding.
In this work, kinetic Monte Carlo (KMC) technique is applied to study evolution of Y-Ti-O nanoclusters in a bcc-Fe and FeCr matrix. Starting from a uniform distribution of O, Y, Ti atoms in the matrix at first a stationary state is produced by high temperature annealing. Such a state is characterized by a certain population of Y-Ti-O nanoclusters. Then vacancies and interstitials are introduced in order to simulate ion and neutron irradiation taking into account realistic conditions, and the evolution of the nanostructure is studied. The parameters for the atomic interactions used in KMC were obtained recently by first-principle Density-Functional-Theory calculations and applied in Metropolis Monte Carlo simulations on energetics, structure and composition of the Y-Ti-O nanoclusters [2].
1. G. R. Odette, M. J. Alinger, B. D. Wirth, Annu. Rev. Mater. Res. 38, 471 (2008)
2. M. Posselt, D. Murali, B. K. Panigrahi, Modelling Simul. Mater. Sci. Eng. 22, 085003 (2014)

Keywords: Oxide Dispersion Strengthened steels; ODS; kinetic Monte-Carlo; density functional theory

  • Lecture (Conference)
    Advances in Materials and Processing Technologies (AMPT), 14.-17.12.2015, Madird, Spain

Permalink: https://www.hzdr.de/publications/Publ-22979
Publ.-Id: 22979


Development of a new radiofluorinated quinoline analog for PET imaging of phosphodiesterase 5 (PDE5) in brain.

Liu, J.; Wenzel, B.; Dukic-Stefanovic, S.; Teodoro, R.; Ludwig, F.-A.; Deuther-Conrad, W.; Schröder, S.; Chezal, J.-M.; Moreau, E.; Brust, P.; Maisonial-Besset, A.

Phosphodiesterases (PDEs) are enzymes that play a major role in cell signalling by hydrolysing the second messengers cyclic adenosine monophosphate (cAMP) and/or cyclic guanosine monophosphate (cGMP) throughout the body and brain. Altered cyclic nucleotide-mediated signalling has been associated with a wide array of disorders, including neurodegenerative disorders. Recently, PDE5 has been shown to be involved in neurodegenerative disorders such as Alzheimer's disease but its precise role has not been elucidated yet. To visualize and quantify the expression of this enzyme in brain, we developed a radiotracer for specific PET imaging of PDE5. A quinoline based lead compound has been structurally modified resulting in the fluoroethoxymethyl derivative ICF24027 with high inhibitory activity towards PDE5 (IC50 = 1.89 nM). Radiolabelling with fluorine-18 was performed by a one-step nucleophilic substitution reaction using a tosylate precursor (RCY(EOB) = 12.9 ± 1.8%; RCP >99%; SA(EOS) = 70-126 GBq/µmol). In vitro autoradiographic studies of [18F]ICF24027 on different mouse tissue as well as on porcine brain slices demonstrated a moderate specific binding to PDE5. In vivo studies in mice revealed that [18F]ICF24027 was metabolized under formation of brain penetrable radiometabolites making the radiotracer not suitable for PET imaging of PDE5 in brain.

Keywords: PDE5; PET imaging; Fluorine-18; quinoline; Micellar chromatography

Permalink: https://www.hzdr.de/publications/Publ-22978
Publ.-Id: 22978


NMR shift and relaxation measurements in pulsed high-field magnets up to 58 T

Kohlrautz, J.; Reichardt, S.; Green, E. L.; Kühne, H.; Wosnitza, J.; Haase, J.

Nuclear magnetic resonance (NMR) experiments at fields up to 58 T in pulsed magnets at the Dresden High Magnetic Field Laboratory are reported. The challenge to resolve NMR shifts in these timedependent fields is addressed for the first time, and it is shown that this can indeed be accomplished with high precision with an internal reference. As a result, signal averaging is possible during a single magnetic field pulse, but also for multiple pulses. Thus, even very weak signals can in principle be recorded and their shifts can be determined. In a second set of experiments, the measurement of nuclear relaxation is investigated. Using adiabatic inversion with the inherent time dependence of the magnetic field and small-angle inspection, it is shown that relaxation measurements are possible, as well. The shift experiments were performed with 27Al NMR on a mixture of aluminum metal and a Linde type A zeolite. For the relaxation studies, 27Al NMR and 69Ga NMR on the metals aluminum and gallium were preformed, respectively.

Permalink: https://www.hzdr.de/publications/Publ-22977
Publ.-Id: 22977


Electron spin resonance modes in a strong-leg ladder in the Tomonaga-Luttinger liquid phase

Ozerov, M.; Maksymenko, M.; Wosnitza, J.; Honecker, A.; Landee, C. P.; Turnbull, M. M.; Furuya, S. C.; Giamarchi, T.; Zvyagin, S.

Magnetic excitations in the strong-leg quantum spin ladder compound (C7H10N)2CuBr4 (known as DIMPY) in the field-induced Tomonaga-Luttinger spin-liquid phase are studied by means of high-field electron spin resonance (ESR) spectroscopy. The presence of a gapped ESR mode with unusual nonlinear frequency-field dependence is revealed experimentally. Using a combination of analytic and exact-diagonalization methods, we compute the dynamical structure factor and identify this mode with longitudinal excitations in the antisymmetric channel. We argue that these excitations constitute a fingerprint of the spin dynamics in a strong-leg spin-1/2 Heisenberg antiferromagnetic ladder and owe their ESR observability to the uniform Dzyaloshinskii-Moriya
interaction.

Permalink: https://www.hzdr.de/publications/Publ-22976
Publ.-Id: 22976


Subcritical transition to turbulence of a precessing flow in a cylindrical vessel

Herault, J.; Gundrum, T.; Giesecke, A.; Stefani, F.

The transition to turbulence in a precessing cylindrical vessel is experimentally investigated. Our measurements are performed for a nearly resonant configuration with an initially laminar flow dominated by an inertial mode with azimuthal wave number m = 1 superimposed on a solid body rotation. By increasing the precession ratio, we observe a transition from the laminar to a non-linear regime, which then breakdowns to turbulence for larger precession ratio. Our measurements show that the transition to turbulence is subcritical, with a discontinuity of the wall-pressure and the power consumption at the threshold ϵLT. The turbulence is self-sustained below this threshold, describing a bifurcation diagram with a hysteresis. In this range of the control parameters, the turbulent flows can suddenly collapse after a finite duration, leading to a definitive relaminarization of the flow. The average lifetime 〈τ〉 of the turbulence increases rapidly when ϵ tends to ϵLT.

Keywords: DRESDYN; precession; turbulence; transition; bifurcation

Permalink: https://www.hzdr.de/publications/Publ-22975
Publ.-Id: 22975


Magneto-elastic coupling across the first-order transition in the distorted kagome lattice antiferromagnet Dy3Ru4Al12

Henriques, M. S.; Gorbunov, D. I.; Kriegner, D.; Valiska, M.; Andreev, A. V.; Matej, Z.

Structural changes through the first-order paramagnetic–antiferromagnetic phase transition of Dy3Ru4Al12 at 7 K have been studied by means of X-ray diffraction and thermal expansion measurements. The compound crystallizes in a hexagonal crystal structure of Gd3Ru4Al12 type (P63/mmc space group), and no structural phase transition has been found in the temperature interval between 2.5 and 300 K. Nevertheless, due to the spin-lattice coupling the crystal volume undergoes a small orthorhombic distortion of the order of 2 x 10-5 as the compound enters the antiferromagnetic state. We propose that the first-order phase transition is not driven by the structural changes but rather by the exchange interactions present in the system.

Permalink: https://www.hzdr.de/publications/Publ-22974
Publ.-Id: 22974


High-resolution gamma-ray tomography for two-phase flow investigations in centrifugal pumps

Bieberle, A.; Schäfer, T.; Neumann, M.; Hampel, U.

This article presents high-resolution gamma-ray computed tomography (HireCT) measurement technique for quantitative investigations of two-phase flow in authentically operated centrifugal pumps. For instance in nuclear power plants single liquid phase designed centrifugal pumps are used for the emergency cooling in a hypothetic accident scenario. Thus, their operation is essential to keep the reactor under control. This contribution includes the determination of measurement accuracy and selected results of two-phase flow studies in a commercially available industrial centrifugal pump. The HireCT scanner uses 137Cs as isotopic source emitting high energy gamma photons of 662 keV. This offers imaging as well as quantitative investigations in technical devices with dense walls, such as centrifugal pumps. CT scans have been applied especially in the impeller region of the operating industrial centrifugal pump. To observe gas-liquid phase distributions within a sharply mapped impeller wheel, which rotates with nominal speed of 1480 rpm, time-averaging rotation-synchronized computed tomography has been applied. Therefore, data projections of the entire radiation detector arc are continuously acquired with a sampling rate of 22 kHz. The detector contains 320 single scintillation detectors with an active area of 2 mm in width and 4 mm in height as well as a detection efficiency of about 75% for 662 keV gamma photons. The measuring accuracy was evaluated by a sophisticated mockup. For measurement intervals of 5 min a measuring accuracy of ±1% absolute could be provided. Identified phase fraction distributions in measurements at an authentically operated centrifugal pump at various inlet gas fractions offered gas phase buffering areas within the impeller chambers that apparently interfere the hydrodynamic processes and, thus, the transport of the liquid. Eventually, the HireCT have been proven as a suitable tool for two-phase flow investigations in rapidly rotating objects.

Keywords: Centrifugal pumps; gamma-ray computed tomography; two-phase flow

  • Contribution to proceedings
    Specialist Workshop on Advanced Instrumentation and Measurement Techniques for Nuclear Reactor Thermal Hydraulics (SWINTH), 15.-17.06.2016, Livorno, Italien
  • Lecture (Conference)
    Specialist Workshop on Advanced Instrumentation and Measurement Techniques for Nuclear Reactor Thermal Hydraulics (SWINTH), 15.-17.06.2016, Livorno, Italien

Permalink: https://www.hzdr.de/publications/Publ-22973
Publ.-Id: 22973


Active optical metasurfaces based on defect-engineered phase-transition materials

Rensberg, J.; Zhang, S.; Zhou, Y.; Mcleod, A. S.; Schwarz, C.; Goldflam, M.; Liu, M.; Kerbusch, J.; Nawrodt, R.; Ramanathan, S.; Basov, D. N.; Capasso, F.; Ronning, C.; Kats, M.

Active, widely tunable optical materials, such as phase-transition materials, have enabled rapid advances in photonics and optoelectronics, especially in the emerging field of meta-surfaces and meta-devices. Here, we demonstrate that spatially selective defect engineering on the nanometer scale can transform phase-transition materials into optical metasurfaces. Using ion irradiation through nanometer-scale masks, we selectively defect-engineered the insulator-metal transition of vanadium dioxide, a prototypical correlated phase-transition material whose optical properties change dramatically depending on its state. Using this robust technique, we demonstrated several optical metasurfaces, including tunable absorbers with artificially induced phase co-existence and tunable polarizers based on thermally triggered dichroism. Spatially selective nanoscale defect engineering represents a new paradigm for active photonic structures and devices.

Keywords: metasurfaces; metamaterials; meta-devices; defect engineering; phase-transition materials

Permalink: https://www.hzdr.de/publications/Publ-22972
Publ.-Id: 22972


Tailored magnetic fields for controlling the electrochemical deposition of metal

Mutschke, G.; Mühlenhoff, S.; Yang, X.; Eckert, K.; Tschulik, K.; Uhlemann, M.; Fröhlich, J.; Bund, A.

The contribution will give an overview of recent results regarding the influence of magnetic fields on the electrochemical deposition of metal. Magnetic fields give rise to forces on the electrolyte which, if properly applied, can be useful. Lorentz forces have been known long-since for causing convection of the electrolyte, and thus, to affect mass transfer. Magnetic gradient forces are well established for the purpose of magnetic particle separation and also influence electrolytes that consist of para- or diamagnetic ions and molecules. The presentation will mainly discuss two different ways of controlling the metal deposition by tailored magnetic fields. First it will be shown that Lorentz forces originating from specific inhomogeneous magnetic field configurations can be utilized for improving the uniformity of the metal deposit at vertical electrodes, despite of the influence of buoyancy [1].
On the contrary, magnetic fields can also be beneficial for obtaining a desired non-uniform deposition at length scales down to the micrometer range. Here, the Kelvin force resulting from small-scale gradient fields can be utilized. Depending on the electrolyte composition, structured deposits of paramagnetic and also of diamagnetic metal ions can be obtained, despite of the small modulus of the magnetic susceptibility of the latter. In both examples, analytical reasoning, simulations and experimental results of lab-scale electrochemical systems will be presented which elucidate the interplay of forces, the electrolyte flow and the effect on mass transfer [2-3].

References:
[1] S. Mühlenhoff et al.; On the homogenization of the thickness of Cu deposits by means of MHD convection within small dimension cells. Electrochem. Comm. 36 (2013) 80–83.
[2] G. Mutschke et al.; Comment on "Magnetic Structuring of Electrodeposits". Phys. Rev. Lett. 109 (2012) 229401.
[3] M. Uhlemann et al.; Structured Electrodeposition in Magnetic Gradient Fields. Eur. Phys. J. Spec. Top. 220 (2013) 287-302.

Keywords: electrochemistry; electrolysis; metal deposition; magnetic field; Lorentz force; Kelvin force; buoyancy; convection

  • Invited lecture (Conferences)
    2015 International Chemical Congress of Pacific Basin Societies (PACIFICHEM 2015), 15.-20.12.2015, Honolulu, USA

Permalink: https://www.hzdr.de/publications/Publ-22971
Publ.-Id: 22971


Multi-isotope comparison of 3He, 21Ne, and 36Cl moraine ages from the high-altitude central Puna Plateau, NW Argentina (24°S)

Luna, L.; Bookhagen, B.; Niedermann, S.; Merchel, S.; Rugel, G.; Scharf, A.; Strecker, M.

Glacial deposits on the high-altitude, arid Puna Plateau of northwestern Argentina document past changes in climate, but the associated geomorphic features have never been directly dated. The plateau is situated in the “Arid Diagonal,” the hyper-arid transition zone between the Westerlies precipitation dominated southern Andes, and the South American Summer Monsoon controlled central Andes. Despite the climatically critical position of the Puna Plateau, paleoclimate data for the region is extremely sparse. This study provides direct age control of glacial moraine deposits from the central Puna Plateau (24°S) at elevations of 4500-5000 m through cosmogenic surface exposure dating. The volcanic lithologies of the deposits additionally allow for comparison of production rates from multiple cosmogenic isotope systems at low latitude and high elevation. Moraine boulders were dated using cosmogenic 3He from pyroxene, 21Ne from quartz, and 36Cl from feldspars. Preliminary data suggests that the most extensive glaciation occurred more than 80 ka ago, and that an additional prominent advance occurred at ~39 ka. In addition, comparison of isotope production ratios from low latitude and high elevation will contribute to better constrained production rates, particularly for 36Cl, for which global production rate estimates are highly variable. This study documents Quaternary climate changes on the Puna Plateau, while at the same time improving production rate agreement between multiple cosmogenic isotope systems.

Keywords: AMS; TCN; cosmogenic dating; paleoclimate; feldspars; quartz; pyroxene

  • Poster
    European Geosciences Union (EGU) General Assembly, 17.-22.04.2016, Vienna, Österreich

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Permalink: https://www.hzdr.de/publications/Publ-22970
Publ.-Id: 22970


Radiative particle-in-cell simulations - plasma based light sources and synthetic spectroscopy

Pausch, R.; Debus, A.; Huebl, A.; Steiniger, K.; Widera, R.; Bussmann, M.

We present simulated far field radiation spectra from plasma based light sources such as high harmonics generation (HHG) during laser foil irradiation and betatron oscillation during laser-wakefield-acceleration (LWFA). The synthetic spectra allow quantifying both total radiation flux of these light sources and the occurring radiation background. We also present applications of far field radiation as spectroscopic diagnostic of the plasma dynamics occurring during LWFA and a variety of plasma instabilities, such as Kelvin-Helmholtz and the Rayleight-Taylor instability. This so-called synthetic diagnostic allows probing the electron dynamics by predicting the emitted radiation spectra thus emulating real experiments.

In order to obtain these results, we developed an in-situ method to compute spectrally and angularly resolved far field radiation based on Liénard-Wiechert potentials. By computing radiation concurrently to simulating the plasma dynamics, using the particle-in-cell code PIConGPU, we are able to take into account all $sim10^{10}$ electrons simulated, thus allowing to quantify both coherent and incoherent radiation. Furthermore, spectra can be computed for thousand of observation directions and frequencies simultaneously. We show that the code’s capability of resolving radiation processes temporally is an indispensable tool for linking the evolution of the plasma dynamics to the emitted radiation.

Keywords: PIConGPU; LWFA; HHG; KHI; radiation; spectra; synthetic diagnostics

  • Poster
    Novel Light Sources from Laser-Plasma Interactions, 20.-24.04.2015, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-22969
Publ.-Id: 22969


Silicon photomultiplier readout of a monolithic 270×5×5 cm3 plastic scintillator bar for time of flight applications

Reinhardt, T. P.; Gohl, S.; Reinicke, S.; Bemmerer, D.; Cowan, T. E.; Heidel, K.; Röder, M.; Stach, D.; Wagner, A.; Weinberger, D.; Zuber, K.

The detection of 200-1000MeV neutrons requires large amounts, ∼100 cm, of detector material because of the long nuclear interaction length of these particles. In the example of the NeuLAND neutron time-of-flight detector at FAIR, this is accomplished by using 3000 monolithic scintillator bars of 270×5×5 cm3 size made of a fast plastic. Each bar is read out on the two long ends, and the needed time resolution of sigma_t < 150 ps is reached using fast timing photomultipliers. In the present work, it is investigated whether silicon photomultiplier (SiPM) photosensors can be used instead. Experiments with a picosecond laser system were conducted to determine the timing response of the SiPM-preamplifier assembly. The response of the full system including the scintillator was studied using 30MeV single electrons provided by the ELBE superconducting electron linac. The ELBE data were matched by a simple Monte Carlo simulation, and they were found to obey an inverse-square-root scaling law. In the electron beam tests, a time resolution of sigma_t = 136 ps was reached with a pure SiPM readout, well within the design parameters for NeuLAND.

Keywords: Time of flight detector; Neutron detector; FAIR; Nuclear Astrophysics; Silicon Photomultiplier; Avalanche Photodiode; Picosecond laser system; Photomultiplier

Permalink: https://www.hzdr.de/publications/Publ-22968
Publ.-Id: 22968


Liquid metal batteries

Weber, N.

Liquid metal batteries consist of a stable density stratification of two liquid metals, separated by a likewise liquid salt, which is working as electrolyte. A potentially very long lifetime, extreme current densities and a low price are just a few advantages of such cells, making them a promising candidate for stationary energy storage. Simulations of the fluid flow in liquid metal batteries are presented.

  • Poster
    International Summer School Storage 4 Energy S4E 2015, 05.-10.10.2015, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-22967
Publ.-Id: 22967


Synthetic in situ radiation diagnostics in particle-in-cell codes – from the lab to the stars with a GPU-accelerated code

Pausch, R.; Debus, A.; Steiniger, K.; Huebl, A.; Widera, R.; Schramm, U.; Bussmann, M.

We present the synthetic in situ radiation diagnostic included in the 3D3V particle-in- cell code PIConGPU. It provides spectrally and angularly resolved far field radiation and thus translates the simulated plasma dynamics to radiation observables accessible in experiments.
Our radiation diagnostics algorithm based on Liénard-Wiechert potentials is capable of predicting non-linear Thomson scattering of both sub-relativistic and relativistic electrons in plasmas. The multi-GPU-based implementation and its direct integration into the particle-in-cell code PIConGPU not only results in high processing speeds - up to 7.2 Peta FLOPS on TITAN - but also enables to compute the radiation for all ~10^10 simulated macro particles, thousands of frequencies and hundreds of observation directions. Such performance permits to cover spectral ranges from infrared to X-ray with virtual radiation detectors covering an entire “skymap”, whereas taking into account all particles allows us to make quantitative predictions of the emitted radiation power for both coherent and incoherent plasma radiation occurring simultaneously. We discuss in detail how our multi-GPU-based implementation overcomes bandwidth, processing speed and disk space constraints seen in CPU- based radiation codes by replacing the offline fast Fourier transform over several tens of Petabytes of particle trajectories with an in situ Fourier transform.
We demonstrate PIConGPU’s capabilities using simulated spectra of plasma based light sources and accelerators, realized in laboratories today, and instabilities, occurring in astrophysical jets. As examples, we show a quantitative analysis for high harmonics generation (HHG) during laser foil irradiation, an explorative search for spectral signatures of laser wakefield acceleration (LWFA) and a time-dependent study of Kelvin-Helmholtz instability (KHI) spectra.

Keywords: PIConGPU; radiation; LWFA; KHI; HHG; spectra; sky map

  • Poster
    ICNSP 2015 - International Conference on Numerical Simulation of Plasmas, 12.-14.08.2015, Golden, USA
  • Poster
    HZDR PhD Seminar 2015, 02.-04.11.2015, Altenberg, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-22966
Publ.-Id: 22966


Energy filter for tailoring depth profiles in semiconductor doping application

Csato, C.; Krippendorf, F.; Akhmadaliev, S.; von Borany, J.; Han, W.; Siefke, T.; Zowalla, A.; Rüb, M.

This work presents the physics and technology of a micromechanically fabricated ‘‘energy filter’’ for doping applications. This energy filter is capable of producing pre-defined tailored doping profiles by a single monoenergetic ion implantation. The functional principle of the energy filter is explained using a simple model. Pattern transfer is being investigated for two different filter-substrate distances. Different aspects of the filter’s temperature behavior during irradiation are discussed. Finally, the results of an entire wafer area implantation are presented and discussed.

Keywords: Implantation; Ion; Energy filter; Silicon carbide; Doping homogeneity

Permalink: https://www.hzdr.de/publications/Publ-22965
Publ.-Id: 22965


Synthesis and radiofluorination of 4-(6-fluoro-fluoren-9-on-2-yl)-1,4-diazabicyclo[3.2.2]nonane for imaging of alpha-7 nicotinic acetylcholine receptor (α7 nAChR) by PET

Scheunemann, M.; Teodoro, R.; Wenzel, B.; Deuther-Conrad, W.; Brust, P.

Aim: Until now, the two fluorinated dibenzothiophene sulfones [18F]ASEM(1) and its para fluorinated isomer [18F]DBT10(2) have been developed from a novel class of α7 nicotinic acetylcholine receptor (nAChR) ligands based on the antiviral drug tilorone. The aim of this study is to investigate the effect of substituting the SO2 by a CO linkage on the radiofluorination efficiency of the corresponding fluoren-9-one analogues and on radioligand parameters.
Methods: FLN-19 was prepared from 3-nitro-fluoren-9-one via bromination, fluorodenitration and Pd catalyzed cross coupling in 25% overall yield. A precursor with a nitro leaving group for radiolabelling was similarly obtained. Binding affinity for human nAChRs was evaluated in vitro by radioligand displacement experiments. [18F]FLN-19 was obtained via nucleophilic aromatic substitution. In vitro autoradiography of [18F]FLN-19 on pig brain slices was performed.
Results: FLN-19 binds with high affinity and selectivity to α7 nAChR (Ki = 1.18 nM, 288 nM, and 64.0 nM for α7, α4β2, and α3β4 nAChR, respectively). The highest nitro-to-fluoro conversion was obtained in DMF under microwave assisted heating (labeling yields ≈ 90%), and [18F]FLN-19 was prepared in ≥ 97% radiochemical purity. Binding of [18F]FLN-19 on pig brain slices was significantly reduced by α7 nAChR-specific ligands.
Conclusions: With [18F]FLN-19, a first fluorenone derived α7 nAChR radioligand was readily prepared, efficiently radiolabelled and successfully tested in first in vitro studies.

References

1. Horti A. G. et al. J Nucl. Med., 2014, 55, 672-677.
2. Teodoro R. et al. Molecules, 2015, 20, 18387-18421.

  • Lecture (Conference)
    Nuklearmedizin2016, 54. Jahrestagung der DGN, 20.-23.04.2016, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-22964
Publ.-Id: 22964


Theoretical investigation of in situ k-restore processes for damaged ultra-low-k dielectrics

Förster, A.; Wagner, C.; Schuster, J.; Gemming, S.

Ultra-low-k (ULK) materials are essential for today's production of integrated circuits (ICs). However, during the manufacturing process, the ULK's low dielectric constant (k-value) increases due to the replacement of hydrophobic species with hydrophilic groups. We investigate the use of plasma enhanced fragmented silylation precursors to repair this damage. The fragmentation of the silylation precursors octamethylcyclotetrasiloxane (OMCTS) and bis(dimethylamino)-dimethylsilane (DMADMS) and their possible repair reactions are studied using density functional theory (DFT) and molecular dynamics (MD) simulations.

Keywords: k-Restore; Low-k materials; DFT; MD; Plasma repair

Permalink: https://www.hzdr.de/publications/Publ-22963
Publ.-Id: 22963


Radiosynthesis and biological evaluation of the new PDE10A radioligand [18F]AQ28A

Wagner, S.; Teodoro, R.; Deuther-Conrad, W.; Kranz, M.; Scheunemann, M.; Fischer, S.; Wenzel, B.; Egerland, U.; Hoefgen, N.; Steinbach, J.; Brust, P.

Cyclic nucleotide phosphodiesterase 10A (PDE10A) regulates the level of the second messengers cAMP and cGMP in particular in brain regions assumed to be associated with neurodegenerative and psychiatric diseases. A better understanding of the pathophysiological role of the expression of PDE10A could be obtained by quantitative imaging of the enzyme by positron emission tomography (PET). Thus, in this study we developed, radiolabeled and evaluated a new PDE10A radioligand, 8-bromo-1-(6-[18F]fluoropyridin-3-yl)-3,4-dimethylimidazo[1,5-a]quinoxaline ([18F]AQ28A). [18F]AQ28A was radiolabeled by both nucleophilic bromo-to-fluoro or nitro-to-fluoro exchange using K[18F]F-K2.2.2-carbonate complex with different yields. Using the superior nitro precursor, we developed an automated synthesis on a Tracerlab FX F-N module and obtained [18F]AQ28A with high radiochemical yields (33±6%) and specific activities (96-145 GBq/µmol) for further evaluation. Initially, we investigated the binding of [18F]AQ28A to the brain of different species by autoradiography and observed the highest density of binding sites in striatum, the brain region with the highest PDE10A expression. Subsequent dynamic PET studies in mice revealed a region-specific accumulation of [18F]AQ28A in this region, which could be blocked by pre-injection of the selective PDE10A ligand MP-10. In conclusion, the data suggest [18F]AQ28A is a suitable candidate for imaging of PDE10A in rodent brain by PET.

Keywords: PDE10 radioligand; 18F-labeled radioligand; automated synthesis; nitro precursor; bromo precursor; Animal PET/MR

  • Open Access Logo Journal of Labelled Compounds and Radiopharmaceuticals 60(2017)1, 36-48
    DOI: 10.1002/jlcr.3471

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Permalink: https://www.hzdr.de/publications/Publ-22962
Publ.-Id: 22962


Synthesis of fluorine-containing PDE10A-Inhibitors as potential Ligands for Positron Emission Tomography (PET).

Franz, L.; Scheunemann, M.; Wagner, S.; Lang, M.; Brust, P.; Briel, V.

Phosphodiesterases (PDE ́s) are second messenger hydrolysing enzymes and important regulators of signal transduction mediated by these molecules. PDE10A, a cAMP and cGMP sensitive hydrolase, is primarily expressed in the striatum and was identified as drug target for the therapy of diverse disorders in the central nervous system (CNS) [1] like schizophrenia or chorea huntington [2]. Recently, 1-arylimidazo[1,5-a]quinoxalines have been reported to be potent and selective inhibitors of PDE10A [3]. In terms of a potential use as 18F-labelled PET imaging agent new substituted derivatives were synthesized. It has been shown that the methoxy substituted inhibitors are prone to metabolic oxidation, which leads to a loss of inhibitory potency or ability to cross the blood brain barrier [3,4].
To improve the metabolic stability of inhibitors the methoxy function in position 6 was exchanged by chlorine. In the first synthesis step chlorine was introduced at position 6 by electrophilic aromatic substitution. An electron deficient system was generated in step 2 by oxidation of the amine to a nitro function to allow the nucleophilic aromatic substitution of fluorine by 4-methylimidazole in step 3. Afterwards, the amine was recovered by acidic reduction with elementary iron in step 4 and acetylated in step 5. Cyclisation in step 6 was realized by a Bischler-Napieralski reaction. The derivatization of the 1-arylimidazo[1,5-a]quinoxaline was focused on position 1 and 8. Finally, the fluoro-pyridinyl-group was introduced by Suzuki-coupling with the corresponding boronic acid at the brominated positions to afford the mono- or disubstituted pyridinyl derivatives. All compounds were characterized by high performance liquid chromatography, nuclear magnetic resonance spectroscopy and mass spectrometry. It is expected that the new chlorinated derivatives have the same pharmaceutical effects as their methoxy analogues.

  • Poster
    Annual Meeting of the German Pharmaceutical Society - DPhG, 23.-25.09.2015, Düsseldorf, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-22961
Publ.-Id: 22961


Quantifying the interaction of seismicity and gas transport in fractured hard rock at earthquake focal depth (DAFGAS-II)

Lippmann-Pipke, J.; Erzinger, J.

Es ist kein Abstract vorhanden.

  • Other report
    Potsdam: GFZ, 2015
    13 Seiten

Permalink: https://www.hzdr.de/publications/Publ-22960
Publ.-Id: 22960


Alles im Fluss

Helm, M.; Michel, P.; Gensch, M.; Wagner, A.

Der supraleitende Elektronenbeschleuniger ELBE erzeugt als Sekundärstrahlung auch Infrarot- und THz-Photonen, Positronen, Neutronen und MeV-Röntgenquanten.

Keywords: ELBE; Beschleuniger; Freie-Elektronenlaser; Positronen

  • Physik Journal 15(2016)1, 29-34

Permalink: https://www.hzdr.de/publications/Publ-22959
Publ.-Id: 22959


Regular flow reversals in Rayleigh-Benard convection in a horizontal magnetic field

Tasaka, Y.; Igaki, K.; Yanagisawa, T.; Vogt, T.; Zuerner, T.; Eckert, S.

MHD Rayleigh-Benard convection was studied experimentally using a liquid metal inside a box with square horizontal cross section and aspect ratio five. Systematic flow measurements were performed by means of ultrasonic velocity profiling that can capture time variations of instantaneous velocity profiles. Applying a horizontal magnetic field organizes the convective motion into a flow pattern of quasi-two dimensional rolls arranged parallel to the magnetic field. The number of rolls has the tendency to decrease with increasing Rayleigh number Ra and to increase with increasing Chandrasekhar number Q. We explored convection regimes in a parameter range, at 2x10^3 < Q < 10^4 and 5x10^3 < Ra < 3x10^5, thus extending the regime diagram provided by Yanagisawa et al. (Physical Review E, 2013). Three new regimes were identified, whereas the regime of regular flow reversals in which five rolls periodically change the direction of their circulation with gradual skew of the roll axes can be considered as the most remarkable one. The regime appears around a range of Ra=Q = 10, where irregular flow reversals were observed in Yanagisawa et al. (2013). We performed the POD analysis on the spatio-temporal velocity distribution and detected that the regular flow reversals can be interpreted as a periodic emergence of a 4-rolls state in a dominant 5-rolls state. The POD analysis

Keywords: Rayleigh-Benard convection

Permalink: https://www.hzdr.de/publications/Publ-22958
Publ.-Id: 22958


Ge nanoparticle formation in ZrO2/Ge and SiN/Ge superlattices by flash lamp annealing

Rebohle, L.; Seidel, S.; Wutzler, R.; Prucnal, S.; Hübner, R.; Helm, M.; Skorupa, W.; Lehninger, D.; Heitmann, J.; Klemm, V.; Rafaja, D.

Semiconductor nanocrystals in dielectric matrices are of great interest for a broad range of applications, especially in the field of photon management in solar cells and for non-volatile memories. In this work we investigate the formation of crystalline Ge nanoparticles in superlattice stacks by flash lamp annealing. In detail, amorphous ZrO2/Ge and SiN/Ge superlattices confined by two SiO2 layers on Si were produced by magnetron-sputtering and plasma-enhanced chemical vapor deposition, respectively. Raman and TEM investigations reveal that, depending on the original Ge layer thickness, crystalline Ge nanoparticles with different aspect ratios will be formed under annealing. As shown by electrical measurements, these layers feature large charge trapping capabilities. We compare these two types of layer systems with regard to the formation process of the Ge nanoparticles, the trapped charge density, the memory window and the retention. Finally, the perspectives for non-volatile memories are discussed, if these layer stacks are downscaled to current device dimensions.

Keywords: Ge nanoparticles; amorphous ZrO2; flash lamp annealing; non-volatile memory

  • Lecture (Conference)
    EMRS Spring Meeting 2015, 11.-15.05.2015, Lille, France

Permalink: https://www.hzdr.de/publications/Publ-22957
Publ.-Id: 22957


Probleme der SUV-Quantifizierung in der Positronenemissionstomografie und mögliche Lösungswege/Problems of SUV Quantification in Positron Emission Tomography and Possible Solutions

van den Hoff, J.; Hofheinz, F.

Die Genauigkeit und Reproduzierbarkeit des Standardized Uptake Value (SUV) zur quantitativen Auswertung von Ganzkörper-PET/CT-Untersuchungen wird von messtechnischen und biologischen Faktoren beeinflusst, welche die Aussagekraft des SUV einschränken, insbesondere seine Eignung als Surrogat des Glukosemetabolismus in der FDG-PET. In messtechnischer Hinsicht wird der SUV unmittelbar von der Güte der Kreuzkalibrierung zwischen PET-Gerät und Dosiskalibrator wie auch von der Genauigkeit des Patientengewichts beeinflusst. Adäquate Qualitätskontrolle vorausgesetzt können diese Einflussgrößen aber beherrscht werden. Eine grundsätzliche Beschränkung bei kleinen Läsionen stellen Partialvolumeneffekte dar, welche sich gar nicht oder nur mit ungenügender Verlässlichkeit korrigieren lassen. Bei Läsionsgrößen unterhalb etwa des 3-Fachen der räumlichen Auflösung ist eine sinnvolle SUV-Bestimmung nur noch sehr bedingt möglich. Neben messtechnischen Faktoren beeinflussen 2 biologische Effekte maßgeblich die Verlässlichkeit der SUV-Bestimmung, zum einen die nicht unbeträchtliche Zeitabhängigkeit des SUV, zum anderen die Tatsache, dass der SUV sich proportional zum gegebenen Tracerangebot im arteriellen Blut ändert, wobei Letzteres selbst nach Normierung auf SUV-Einheiten eine beträchtliche Variabilität zwischen verschiedenen Untersuchungen aufweist. Diese biologischen Faktoren führen zu einer schlechten Test/Retest-Stabilität des SUV, was insbesondere nachteilig für Verlaufsuntersuchungen oder die schwellwertbasierte Unterscheidung zwischen benignen und malignen Läsionen ist. Beide Effekte können durch den Übergang vom SUV zum Tumor-zu-Blut Standard Uptake Ratio (SUR) eliminiert werden. Der SUR ist bildgestützt leicht zu bestimmen, sofern ein großes Gefäß (Aorta, Herzventrikel) in der Untersuchung miterfasst wird. Der Scanzeit-korrigierte SUR weist eine im Vergleich zum SUV deutlich verbesserte lineare Korrelation zur metabolischen Umsatzrate auf, wodurch sich die diagnostische Aussagekraft der PET erhöht, wie erste klinische Studien belegen.

Abstract
Accuracy and reproducibility of the Standardized Uptake Value (SUV) for quantitative evaluation of whole body PET/CT investigations is influenced by technical and biological factors which limit its value, notably suitability of the SUV as a surrogate of glucose metabolism in FDG PET. On the technical side the SUV is directly influenced by the quality of the cross calibration between the PET system and the dose calibrator as well as by accuracy of the assumed patient weight. Given adequate quality control these factors can be handled. A principal limitation in small lesions are partial volume effects, which cannot be corrected at all or only with insufficient reliability. For lesion sizes below approximately three times the given spatial resolution a meaningful SUV determination is only possible with substantial reservations. Beside the technical factors, 2 biological factors strongly influence the reliability of SUV determination, the notable time dependency of the SUV on the one hand and on the other hand the fact that the SUV changes in proportion to the given tracer supply in the arterial blood, where the latter exhibits substantial variability even after normalization to SUV units. These biological factors lead to poor test/retest stability of the SUV which is especially disadvantageous for follow-up investigations or threshold based discrimination between benign and malign lesions. Both effects can be eliminated by a transition from SUV to the tumor to blood Standard Uptake Ratio (SUR). Image-based determination of the SUR is easy, provided a large vessel (aorta, heart ventricle) is covered by the investigation. In comparison to SUV, the scantime corrected SUR exhibits a distinctly improved linear correlation to the metabolic uptake rate which increases the diagnostic power of PET as could be demonstrated in first clinical studies.

Keywords: Positronenemissionstomografie; PET/CT; Standard Uptake Value; SUV; Standard Uptake Ratio; SUR/positron emission tomography; PET/CT; Standard Uptake Value; SUV; Standard Uptake Ratio; SUR

Permalink: https://www.hzdr.de/publications/Publ-22956
Publ.-Id: 22956


Shallow Boron Emitters in Crystalline Silicon through in-Diffusion by Flash Lamp Annealing

Riise, H. N.; Schumann, T.; Azarov, A.; Hübner, R.; Skorupa, W.; Svensson, B. G.; Monakhov, E.

Flash Lamp Annealing (FLA) is a technique in which Si can be heated to temperatures close to and above its melting point within a few milliseconds [1, 2] and it has been shown to be suitable for annealing of implantation-induced damage [2, 3] and for activation of implanted dopants [4]. Recently, FLA was also proved to be effective in forming shallow Phosphorous (P) emitters in Si through diffusion from a P surface source deposited by spin coating [5].
In this work, it is demonstrated that shallow Boron (B) emitters can be formed in crystalline Silicon (Si) by spin coating and subsequent in-diffusion using FLA. A 300 μm Float Zone mono-crystalline Si wafer was spin-coated at 6000 rpm for 30 seconds by a polyboron spin-on diffusant (Filmtronics B155 SOD) before being processed with FLA. After heat treatment by FLA, the film was oxidized in HNO3:H2SO4 (1:1) before being removed by HF. Secondary Ion Mass Spectrometry (SIMS), sheet resistance measurements and Transmission Electron Microscopy (TEM) analysis were performed to determine the B diffusion profile, the sheet resistance and crystal quality of the samples, respectively.
Annealing for 10 and 20 ms with an energy density of 93-105 J/cm2 leads to B emitter depths of 140- 200 nm and peak B concentrations of 1-3·1020cm−3. Sheet resistance values below 200 Ω/ indicate high dopant activation. These values are well suited for e.g. emitters in crystalline Si solar cells as the shallow emitters will only absorb photons with a wavelength below 420 nm [6] and most of the available sunlight will be absorbed in the base of the cell while the low sheet resistance gives a low series resistance. High-resolution TEM images of the surface and junction regions did not show any crystal defects demonstrating that the FLA treatment does not induce high defect concentrations in the samples. TEM did however reveal a rough surface resulting from the etching treatment to remove the SOD.
Annealing for 10 and 20 ms with energy densities below 90 J/cm2 produce even shallower profiles with a maximum B extension of <100 nm while the peak concentration still remains above 1·1020cm−3 whilst the sheet resistance increases to 300-3000 Ω/. In conclusion, spin-coating with subsequent in- diffusion by FLA is thus a versatile technique with possibility to tailor the emitter depth in Si while still keeping the peak concentration high.
References
[1] H. A. Bomke, H. L. Berkowitz, M. Harmatz, S. Kronenberg, R. Lux, Applied Physics Letters 33, 955 (1978).
[2] J. T. Lue, Applied Physics Letters 36, 73 (1980).
[3] R. Klabes, et al., Physica Status Solidi A: Applications and Materials Science 66, 261 (1981).
[4] T. Ito, et al., Japanese Journal of Applied Physics, Part 1: Regular Papers, Brief Communications & Review Papers 41, 2394 (2002).
[5] H. B. Normann, et al., Applied Physics Letters 102, 132108 (2013).
[6] M. A. Green, Solar Energy Materials & Solar Cells 92, 1305 (2008).

  • Poster
    2015 MRS Spring Meeting & Exhibit, 06.-10.04.2015, San Francisco, CA, USA

Permalink: https://www.hzdr.de/publications/Publ-22955
Publ.-Id: 22955


Nanoripple patterning under medium energy implantation using metal foreign atoms

Redondo-Cubero, A.; Palomares, F. J.; Lorenz, K.; Mücklich, A.; Hübner, R.; Vázquez, L.

Ion beam sputtering (IBS) is a universal phenomenon that can be used for the production of nanopatterns in a wide range of materials and scales. Many semiconductor systems are suitable for this kind of processing, but Si is certainly the most studied one due to its technological relevance and mono-elemental nature [1]. In the last years, the key role of metal impurities for the initial formation of the pattern has been clearly established [2], changing the field in a significant way. Still, several questions remain open, such as the segregation effect of metal silicides [3], the relevance of preferential sputtering for the different metal species [4], or the threshold metal concentration needed for nanopatterning at given experimental conditions. Most of these works are restricted to low energetic beams (0.5-5 keV) produced with conventional ion guns and different set-ups to induce indirect metal codeposition [5]. However, in order to have an appropriate control of the metal species more dedicated systems, where metal could be also directly incorporated, are becoming essential. In this communication, we will present our recent experimental works on IBS nanopatterning of Si at medium energies (40 keV) with simultaneous metal incorporation [6]. In order to understand the influence of the metal on the pattern formation we study three different experimental systems produced with (a) direct metal implantation, (b) indirect metal co-deposition, i.e., with simultaneous irradiation of a metallic plate adjacent to the target, and (c) with non-metal implantation (used as a reference). In all cases, irradiation was carried out in a high-flux ion implanter using an incidence angle of 60º with respect to the target surface normal and for different ion fluences. The dynamics of the pattern is studied using atomic force microscopy (AFM) to characterize the pattern morphology, and particularly to quantify the surface roughness and pattern wavelength. Metal content was determined with Rutherford backscattering spectrometry and the formation of silicides mapped with X-ray photoelectron spectroscopy. In addition, we performed current sensing AFM as well as transmission electron microscopy analysis of the metal containing samples in order to disclose the formation of any compositional pattern and its eventual correlation with the morphological one. We will discuss the main differences arising from the different metal incorporation paths, paying special attention to effects such as geometrical shadowing, the threshold contents required to trigger the pattern in every case and the formation of metal silicides. [1] J. Muñoz-García et al., Mater. Sci. Eng. R-Rep. 86, 1 (2014) [2] C. Madi et al., Phys. Rev. Lett. 101, 246102 (2008) [3] M. Engler et al., Nanotechnology 25, 115303 (2014) [4] R. Gago et al., Nanotechnology 25, 415301 (2014) [5] K. Zhang et al., Nanotechnology 25, 085301 (2014) [6] A. Redondo-Cubero et al. Phys. Rev. B 86, 085436 (2012)

  • Invited lecture (Conferences)
    2015 MRS Spring Meeting & Exhibit, 06.-10.04.2015, San Francisco, CA, USA

Permalink: https://www.hzdr.de/publications/Publ-22954
Publ.-Id: 22954


TEM investigation of barrier-like anodic oxide films on aluminium

Schneider, M.; Lämmel, C.; Hübner, R.; Michaelis, A.

In a previous work, the authors investigated the formation of thin barrier-like oxide films on aluminium using pulse anodizing [1]. The electrochemical experiments have shown differences in the properties and behaviour of the anodic films depending on the pulse frequency. Furthermore, the results suggest a relationship to the microstructure (e.g. thickness or composition) of the oxide films. Therefore, the authors have completed the former investigation by new research using transmission electron microscopy (TEM) and ellipsometry. All results were compared with electrochemical measurements. Again, the anodic films were formed at 3VAg/AgCl with various pulse frequencies in an acetate buffer solution (pH5.9). The aluminium was chemical vapour deposited on silicon wafers to get a smooth and well-defined surface which is favourable for subsequent analytical analysis (TEM, ellipsometry). The TEM investigation confirms the tendency that the oxide thickness decreases with increasing pulse frequency. The film thicknesses determined by coulometry and microscopy fit very well assuming a native oxide layer of approximately 1nm. Additionally, the Al:O ratio across the film thickness clearly depends on the pulse frequency. A model concept explaining this fact will be presented. Furthermore, there are no indications to the formation of crystalline domains within the oxide layers deposited at various pulse frequencies.
[1] M. Schneider, C. Lämmel, C. Heubner, A. Michaelis: Surface and Interface Analysis 45 (2013) 1497-1502

  • Lecture (Conference)
    VII Aluminium Surface Science & Technology Conference, 17.-21.05.2015, Madeira Island, Portugal

Permalink: https://www.hzdr.de/publications/Publ-22953
Publ.-Id: 22953


Three-Dimensional Hydrodynamic Numerical Simulations of Gas-Liquid Flows in Reactor with Ceramic Foams as Internals.

Subramanian, K.; Baldota, R. D.; Zalucky, J.; Schubert, M.; Lucas, D.; Hampel, U.

Ceramic foams are promising alternatives for packing internals used in chemical engineering processes due to their high porosity and high specific surface area, which results in low pressure drop and high catalytic utilization of the packing. The application of solid foam packings as catalyst carriers, particularly for gasliquid systems, are not yet well understood. Due to their highly porous nature, it is very tough to understand the interplay between hydrodynamic behaviour and process performance [1]. The aim of this work is to perform three-dimensional Computational Fluid Dynamics (CFD) simulations of the evolving gas-liquid flow patterns considering ceramic foams as column internals and to validate them with experimental X-ray tomographic studies The closures applied for trickle bed simulation studies are modified according to the hydraulic/geometric properties of the ceramic foam considering the flow domain as porous. The influence of the liquid and gas drag is taken into consideration via Relative Permeability approach [2] and added as external source term to the liquid and gas momentum equations separately. The influence of dispersion forces are further included inorder to steady the liquid spreading and liquid saturation. The geometrical parameters of the ceramic foam packing are estimated using µCT image analysis. Empirical correlations proposed in earlier experimental studies are included in 3D CFD models. Initial results of trickle bed reactor simulations were presented earlier [3]. In this work, the validation of CFD simulation with X-ray tomographic experimental studies will be presented and discussed in detail.

Reference 1 :: S. Calvo., d. Beugre.,m.Crine., a. Leonard., p. Marchot.,d.Toye.,phase distribution measurements in metallic foam packing using x-ray radiography and micro-tomography, chemical engineering and processing, 48, 1030–1039 (2009).
Reference 2 :: A.E. Saez,, R.G.Carbonell, Hydrodynamic parameters for gas-liquid cocurrent flow in packed beds,AIChE Journal, Vol. 31, No.1, 52-62 (1985).
Reference 3 :: K.Subramanian, M. Schubert, D. Lucas, U. Hampel, Closures for simulation of gas-liquid flows in solid foam structures, ISCRE23 & APCRE7, Sep 7 – 10 2014, Bangkok, Thailand.

Keywords: CFD; Foam; Multiphase; Porous media

  • Lecture (Conference)
    10th European Congress of Chemical Engineering, 27.09.-01.10.2015, Nice, France

Permalink: https://www.hzdr.de/publications/Publ-22952
Publ.-Id: 22952


36Cl and 129I at ASTER and DREAMS

Rugel, G.; ASTER-Team; Braucher, R.; Merchel, S.; Pavetich, S.; Scharf, A.; Ziegenrücker, R.

At ASTER (Accélérateur pour les Sciences de la Terre, Environnement, Risques) and DREAMS (DREsden Accelerator Mass Spectrometry) sophisticated ion sources are used for 36Cl and 129I. Both facilities have dedicated 36Cl chemistry labs. At DREAMS it is also used for storage and pressing of AgCl in sample holders (SHs). Most 36Cl-AMS labs, reduce the isobar 36S from the SH by a labor-intensive AgBr-backing. Though, at ASTER and DREAMS only ultrapure Ni and Cu is used, respectively. To find out the pros and cons of the two materials, we have (a) exposed AgCl pressed in Ni and Cu to air (in the dark). After 2 h only, cauliflower-type NiCl2 (analysed by EDX) has been formed from AgCl and Ni preventing any later AMS, whereas AgCl in Cu after 3 days looks unweathered and is still measurable. (b) compared S-decline in AgCl pressed in Ni and Cu (ASTER SHs). After ∼5 min S decreases by a factor of ∼5 for both reaching the same low S-rate after 20 min. However, S is higher at 5-20 min in Cu showing that Cu is contaminated at the surface. High S is not seen at all at DREAMS for DREAMS Cu SHs. Thus, chemical etching and controlled storage of Cu SHs might be a cheaper and better alternative for 36Cl-AMS.
For 129I AMS a sophisticated tuning strategy is minimising sputtering of any iodine containing material at DREAMS.

Keywords: AMS; cross-contamination; ASTER; sulphur; sulfur

  • Poster
    DPG Frühjahrstagung des Arbeitskreises Atome, Moleküle, Quantenoptik und Plasmen (AMOP), 29.02.-04.03.2016, Hannover, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-22950
Publ.-Id: 22950


Time-Resolved Two Million Year Old Supernova Activity Discovered in the Earth’s Microfossil Record

Bishop, S.; Ludwig, P.; Egli, R.; Chernenko, V.; Deveva, B.; Faestermann, T.; Famulok, N.; Fimiani, L.; Gomez, J.; Hain, K.; Korschinek, G.; Hanzlik, M.; Merchel, S.; Rugel, G.

Using accelerator mass spectrometry, we have conducted a search for live, supernova-produced, 60Fe atoms within biogenically produced magnetite (Fe3O4)crystals contained in two Pacific Ocean sediment cores. We have found a time-resolved 60Fe signal in both sediment cores, above background, centered at approximately 2.1 Myr ago and spanning approximately 800 kyr duration (full width half maximum). The onset of this signal coincides with a known marine extinction event at the Pleiocene/Pleistocene boundary, and its shape will require eventual astrophysical interpretation to understand.

Keywords: AMS; supernova; magneto-fossil

  • Lecture (Conference)
    DPG Frühjahrstagung des Fachverbandes Physik der Hadronen und Kerne und des Arbeitskreises Beschleunigerphysik, 14.-18.03.2016, Darmstadt, Deutschland
  • Invited lecture (Conferences)
    Max-Planck-Institute for Physics (MPP) Colloquium, 17.05.2016, München, Deutschland
  • Invited lecture (Conferences)
    Seminar Geophysics Faculty of the Kazan Federal University, 22.03.2016, Kazan, Tatarstan, Russia
  • Invited lecture (Conferences)
    Summer Institute Using Particle Physics to understand and image the Earth Geoneutrinos, Muography, Cosmogenic Nuclides, 11.-21.07.2016, L’Aquila, Italy

Permalink: https://www.hzdr.de/publications/Publ-22949
Publ.-Id: 22949


Alternative fabrication routes towards oxide dispersion strengthened (ODS) steels and model alloys

Bergner, F.

The talk is focussed on fabrication, in particular non-standard or alternative fabrication routes towards ODS steels. There is a natural reference for that, namely state-of-the-art ODS steels produced by the ‘classical’ PM route, which consists of the production of a pre-alloy, gas atomization, mechanical alloying, consolidation and thermal, mechanical or thermo-mechanical treatment. The motivation to consider alternative routes is derived from problems such as the multi-step/multi-parameter character of the PM process and the resulting high production costs, limited throughput and limited reproducibility. These problems can be addressed by way of improving, simplifying, skipping or combining individual steps, by way of reconsidering alloy fabrication from the melt or by looking for hybrid routes. It is demonstrated that there is a wide and fascinating potential for alternative fabrication routes towards ODS steels. For this to be achieved, each of the steps involved in the classical PM route has to be carefully reconsidered. Moreover, the liquid metal route and hybrid routes composed of PM and LM elements have to be taken into account. Significant progress has been made on particular process steps. These include substitution of mechanical alloying, scaling up of SPS and the contactless excitation of cavitation.

Keywords: ODS steels; Fabrication

  • Lecture (Conference)
    Advances in Materials and Processing Technologies (AMPT 2015), 14.-17.12.2015, Madrid, Spain

Permalink: https://www.hzdr.de/publications/Publ-22948
Publ.-Id: 22948


Late Pleistocene outburst floods from Issyk Kul, Kyrgyzstan?

Rosenwinkel, S.; Landgraf, A.; Korup, O.; Schwanghart, W.; Volkmer, F.; Dzhumabaeva, A.; Merchel, S.; Rugel, G.; Preusser, F.

Elevated shorelines and lake sediments surrounding Issyk Kul, the world’s second largest mountain lake, record fluctuating lake levels during Quaternary times. Together with bathymetric and geochemical data, these markers document alternating phases of lake closure and external drainage in the Late Pleistocene. The uppermost level of lake sediments requires a former blockage of the lake’s western outlet through the Boam gorge. Previous studies hypothesised that failures of Pleistocene ice or landslide dams in the gorge generated partial outburst floods of Issyk Kul. We test this hypothesis by exploring possible links between late Quaternary lake levels and outbursts. We dated stranded shorelines using 14C in shells, snails, and plant detritus, as well as sand lenses in delta and river sediments using Infrared Stimulated Luminescence. Our dates are consistent with lake levels expanding into Boam gorge between ~46 ka and 22 ka. Cosmogenic 10Be and 26Al exposure ages of fan terraces containing erratic boulders downstream of the gorge constrain the timing of possible outburst floods to 22-24 ka, postdating a highstand of Issyk Kul. A flow competence analysis gives a peak discharge of >104 m3 s–1 for entraining and transporting these boulders. Palaeoflood modelling, however, shows that naturally dammed lakes unconnected to Issyk Kul could have produced such high discharges upon sudden emptying. Hence, although our data are consistent with hypotheses of catastrophic outburst flooding, we caution against directly these to Pleistocene lake levels of Issyk Kul. Average lake-level changes of up to 90 mm yr–1 in the past 150 years were highly variable without any outburst event, so that attributing catastrophic lake-level drops to dam breaks is ambiguous using sedimentary archives alone. Nevertheless, the Pleistocene flood events that we reconstruct are among the largest reported for the Tien Shan mountains, and motivate further research into the palaeoflood hydrology in Central Asia.

Keywords: outburst flood; lake-level changes; palaeoflood; Issyk Kul; Kyrgyzstan; cosmogenic nuclide; exposure age; AMS

  • Earth Surface Processes and Landforms 42(2017), 1535-1548
    DOI: 10.1002/esp.4109

Permalink: https://www.hzdr.de/publications/Publ-22947
Publ.-Id: 22947


Effect of neutron flux on the characteristics of irradiation-induced nanofeatures and hardening in pressure vessel steels

Wagner, A.; Bergner, F.; Chaouadi, R.; Hein, H.; Hernández-Mayoral, M.; Serrano, M.; Ulbricht, A.; Altstadt, E.

Effects of neutron flux (or dose rate) on the characteristics of irradiation-induced nanofeatures in neutron-irradiated pressure vessel steels were occasionally reported. Such effects let one expect that the mechanical properties mediated by the operation of the nanofeatures as dislocation obstacles would also depend on flux. However, there are controversial views on the presence of flux effects on mechanical properties. The present approach is based on the investigation of pairs of samples from the same batch of material for a number of RPV steels including different levels of Cu as well as base and weld materials. The samples of each pair were irradiated at about the same temperature but different fluxes up to about the same fluence, thus automatically revealing potential flux effects. Small-angle neutron scattering and Vickers hardness testing were applied to characterize the nm-scale solute clusters and the resulting irradiation hardening. A number of analytic models of cluster evolution, namely deterministic growth and coarsening, and hardening as well as combinations thereof were applied to interpret the trends extracted from the experimental results. It is found that there are indeed trends of the cluster size and volume fraction as functions of flux but no resolvable trend for hardening. The absence of a flux effect on hardening can be rationalized in terms of size and number density of solute clusters that enter the hardening expressions. Size and number density depend on flux in opposite directions and, therefore, partly cancel out. Flux effects for the whole set of experimental data including low- and high-Cu base metals and welds can be consistently described by a combination of deterministic growth and dispersed barrier hardening using a unique value of the obstacle strength of solute clusters.

Keywords: Pressure vessel steel; Neutron flux; SANS; Microstructure; Hardening

Permalink: https://www.hzdr.de/publications/Publ-22946
Publ.-Id: 22946


Experimental characterisation of a novel, compact high-field beamline for application in laser-driven ion beam therapy

Kroll, F.; Karsch, L.; Masood, U.; Pawelke, J.; Schürer, M.; Schramm, U.

Introduction:
Compact laser-driven ion accelerators are a potential alternative to complex, large and expensive conventional accelerators. High-power short-pulse lasers, impinging on e.g. thin metal foils enable multi-MeV ion acceleration on µm length and ps time scale. The generated ion bunches (typically protons) show unique beam properties, like ultra-high pulse dose. Nevertheless, laser accelerators still require substantial development in reliable beam generation and transport. We present first experimental results on a beamline prototype based on high-field magnet technology specifically designed for capture and transport of laser-accelerated particles.

Material and methods:
Since the mid-1950s, pulsed high-field magnets serve as versatile research tools for solid state physics and material research. Recently developed pulsed magnet technology, specifically designed to meet the demands of laser acceleration [1], open up new research opportunities: We present a pulsed solenoid for effective collection and focusing of laser-accelerated ions which could function as a first component in a laser-driven gantry system [2]; furthermore, we present a dipole magnet for beam deflection and energy dispersion. The magnets were combined to form a first, pulsed high-field beamline and are powered by portable pulse. Characterization of both magnets has been carried out using a 10 MeV proton beam from a conventional Tandetron accelerator at Helmholtz-Zentrum Dresden – Rossendorf (HZDR). The transported beam was detected by means of radiochromic film and scintillator.

Result:
The transport experiments clearly show the functionality of both magnets: The solenoid focuses the large, divergent ion beam to a millimeter-sized spot showing no aberration. The dipole deflects the protons by a 45° angle and can be used as energy selection device via energy dispersion. For the several 10 µs long proton bunches no effect of the time structure of the magnetic pulse (~ several 100 µs) was observed thus making the magnets well suited for even shorter, laser-driven ion bunches. The beam position accuracy, after passing the beamline, was measured to be of the same order as the beam position fluctuations itself, showing the precision of the installed beamline. The maximum magnetic field strength achieved was 20 T for the solenoid and 5 T for the dipole. For operation at higher proton energies, i.e. above 200 MeV, an increase by a factor of only 1.5 – 2 is requested.

Summary:
Our experimental results show that compact high-field magnets can be used to precisely guide charged particle bunches. The pulsed nature of laser-accelerated particles is matched by the magnet technology. The maximum field strengths reached are sufficient for experiments with protons of several 10 MeV kinetic energy. For clinically relevant energies above 200 MeV, however, a slight increase is required. Experimental studies at a laser-accelerator are scheduled.
References
[1] T. Burris-Mog, et al., Laser accelerated protons captured and transported by a pulse power solenoid, PRSTAB 14, 121301 (2011)
[2] U. Masood, et al., A compact solution for ion beam therapy with laser accelerated protons, Appl. Phys. B 117, 41 (2014)

Keywords: ion beam therapy; beamline; pulsed magnet; laser acceleration

  • Lecture (Conference)
    46. Jahrestagung der Deutschen Gesellschaft für Medizinische Physik (DGMP) e. V., 09.-12.09.2015, Marburg, Deutschland
  • Open Access Logo Contribution to proceedings
    46. Jahrestagung der Deutschen Gesellschaft für Medizinische Physik (DGMP) e. V., 09.-12.09.2015, Marburg, Deutschland
    46. Jahrestagung der Deutschen Gesellschaft für Medizinische Physik (DGMP) e. V. - Abstractband, 978-3-9816508-8-4

Permalink: https://www.hzdr.de/publications/Publ-22945
Publ.-Id: 22945


Nanostructured BN–Mg composites: features of interface bonding and mechanical properties

Kvashnin, D. G.; Krasheninnikov, A. V.; Shtansky, D.; Golberg, P. B. S. D.

Magnesium (Mg) is one of the lightest industrially used metals. However, wide applications of Mg-based components require a substantial enhancement of their mechanical characteristics. This can be achieved by introducing small particles or fibers into the metal matrix. Using first-principles calculations, we investigate the stability and mechanical properties of a nanocomposite made of magnesium reinforced with boron nitride (BN) nanostructures (BN nanotubes and BN monolayers). We show that boron vacancies at the BN/Mg interface lead to a substantial increase in BN/Mg bonding establishing an efficient route towards the development of BN/Mg composite materials with enhanced mechanical properties.

Keywords: BN nanotubes; composite materials

Permalink: https://www.hzdr.de/publications/Publ-22944
Publ.-Id: 22944


Silicon and silicon-nitrogen impurities in graphene: Structure, energetics, and effects on electronic transport

Ervasti, M. M.; Fan, Z.; Uppstu, A.; Krasheninnikov, A. V.; Harju, A.

We theoretically study the atomic structure and energetics of silicon and silicon-nitrogen impurities in graphene. Using density-functional theory, we get insight into the atomic structures of the impurities, evaluate their formation energies and assess their abundance in realistic samples. We find that nitrogen, as well as oxygen and hydrogen, are trapped at silicon impurities, considerably altering the electronic properties of the system. Furthermore, we show that nitrogen doping can induce local magnetic moments resulting in spin-dependent transport properties, even though neither silicon nor nitrogen impurities are magnetic by themselves. To simulate large systems with many randomly distributed impurities, we derive tight-binding models that describe the effects of the impurities on graphene π electron structure. Then by using the linear-scaling real-space Kubo-Greenwood method, we evaluate the transport properties of large-scale systems with random distribution of impurities, and find the fingerprintlike scattering cross sections for each impurity type. The transport properties vary widely, and our results indicate that some of the impurities can even induce strong localization in realistic graphene samples.

Keywords: graphene; defects; electronic transport

Permalink: https://www.hzdr.de/publications/Publ-22943
Publ.-Id: 22943


Evaluation and Improvement of MR-based Attenuation Correction for PET/MR

Schramm, G.

This chapter is an introduction to the fields of Positron Emission Tomography (PET), hybrid Positron Emission Tomography and Magnetic Resonance Imaging (PET/MR), attenuation correction (AC), and MRI-based attenuation correction (MRAC). Since this work represents a cumulative dissertation, the aim of this chapter is to provide the required background to understand the intentions and the context of the three publications [Schramm et al., 2013a,b, 2014] dealing with the evaluation and improvement of MRAC. These publications are included in chapter 3 of this work.

  • Doctoral thesis
    TU Dresden, 2015
    Mentor: Prof. Jörg van den Hoff
    107 Seiten

Permalink: https://www.hzdr.de/publications/Publ-22942
Publ.-Id: 22942


Super-Resonant Infrared Near-Field Microscopy

Lang, D.; Uhlig, T.; Kehr, S. C.; Eng, L. M.; Helm, M.

Scattering-type scanning near-field optical microscopy (s-SNOM) is an AFM-based technique for achieving nanoscale resolution even at infrared wavelengths [1]. s-SNOM thus is of valuable impact when investigating low-dimensional conductors or semiconductors. Nevertheless, the scattered near-field signal strongly depends on the dielectric function of both tip and sample. In order to enhance this signal strength we face two options, by either tuning the tip or sample into resonance using appropriate or tunable laser light sources, and resonant tip materials.


In this work we use a CO2-laser with a tunable center wavelength from 9.7 µm to 11.3 µm as an infrared excitation source in combination with self-prepared AFM particle-tips as probes [2]. The tip particles consist of spherical silicon carbide (SiC), silicon nitride (Si3N4) or silicon oxide (SiO2) nanoparticles with a diameter of ~ 60 nm. Those materials show phonon resonances in or around the CO2-laser wavelength range and thus enhance the signal significantly. We explored here the scenario when using both resonant tips AND samples, hence resulting in a tip-sample coupled super-resonance where both the tip and the sample contribute to the signal-enhancement. Accordingly, a significantly increased near-field image contrast and resolution is expected in this case.

References:

[1] S.C. Kehr et al., Nat. Commun. 2, 249 (2011).

[2] M.T. Wenzel et al., Optics Express 16 (16), 12302-12312 (2008).

Keywords: CO2-laser; near-field infrared microscopy; phonon resonance

  • Poster
    79. DPG-Jahrestagung und DPG-Frühjahrstagung, 15.-20.03.2015, Berlin, Deutschland
  • Poster
    German THz Conference 2015, 08.-10.06.2015, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-22941
Publ.-Id: 22941


AMS measurements of 10Be, 26Al and 41Ca at DREAMS

Scharf, A.; Akhmadaliev, S.; Arnold, M.; Bohleber, P.; Leya, I.; Merchel, S.; Rugel, G.; Smith, T.; Ziegenrücker, R.; Zipf, L.

DREAMS, the DREsden AMS-facility, is performing routine accelerator mass spectrometry for the isotopes 10Be, 26Al, 36Cl, 41Ca and 129I. Sample ratios of 10Be/9Be as low as 8 x 10-15 (background-corrected) have been measured and an exposure age of about 330 years of a boulder of 3000 t in Nepal could be determined [1]. We could demonstrate that a by-product of ice core drilling, so-called drilling chips, are also suitable for 10Be analysis of ice cores, instead of using valuable ice core samples. A set of several in-house 26Al and 41Ca standards has been made traceable to primary standards by cross-calibration [2]. Numerous 26Al and 41Ca concentrations of meteorites could be determined, but for marine sediments there is still a need for a low-level (10-13) 26Al standard. ICP-MS measurements have shown that the steel pins used to fix the CaF2 sample material in the cathodes have high K-concentrations of (44.6 ± 2.2) μg/g. By replacing the steel pins with copper pins the 41K background during 41Ca measurements could be lowered by a factor of three.
Ref. : [1] W. Schwanghart et al., Science (2015), DOI:10.1126/ science.
aac9865. [2] G. Rugel et al., Nucl. Instr. Meth.B (2015), in review.

Keywords: AMS; DREAMS; exposure age; ice cores; meteorites

  • Poster
    DPG-Frühjahrstagung 2016, Sektion AMOP, 29.02.-04.03.2016, Hannover, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-22940
Publ.-Id: 22940


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