Krause B., Miljevic B., Aschenbrenner T., Piskorska-Hommel E., Tessarek C., Barchuk M., Buth G., Donfeu Tchana R., Figge S., Gutowski J., Hanschke D., Kalden J., Laurus T., Lazarev S., Magalhaes-Paniago R., Sebald K., Wolska A., Hommel D., Falta J., Holy V., Baumbach T.

in Journal of Alloys and Compounds, 585 (2014) 572-579. DOI:10.1016/j.jallcom.2013.09.005

Abstract

The structure and morphology of uncapped and capped InGaN quantum dots formed by spinodal decomposition was studied by AFM, SEM, XRD, and EXAFS. As result of the spinodal decomposition, the uncapped samples show a meander structure with low Indium content which is strained to the GaN template, and large, relaxed Indium-rich islands. The thin meander structure is responsible for the quantum dot emission. A subsequently deposited low-temperature GaN cap layer forms small and nearly unstrained islands on top of the meander structure which is a sharp interface between the GaN template and the cap layer. For an InGaN cap layer deposited with similar growth parameters, a similar morphology but lower crystalline quality was observed. After deposition of a second GaN cap at a slightly higher temperature, the surface of the quantum dot structure is smooth. The large In-rich islands observed for the uncapped samples are relaxed, have a relatively low crystalline quality and a broad size distribution. They are still visible after capping with a low-temperature InGaN or GaN cap at 700 C but dissolve after deposition of the second cap layer. The low crystalline quality of the large islands does not influence the quantum dot emission but is expected to increase the number of defects in the cap layer. This might reduce the performance of complex devices based on the stacking of several functional units. © 2013 Elsevier B.V. All rights reserved.

Butcher B.A., Zaldivar-Riveron A., Van De Kamp T., Rolo T.D.S., Baumbach T., Quicke D.L.J.

in Zootaxa, 3860 (2014) 449-463. DOI:10.11646/zootaxa.3860.5.4

Abstract

Copyright © 2014 Magnolia Press. Two new species of the parasitic wasp genus Mesocentrus Szépligeti (Betylobraconinae) are described. One based on a new species from Papua New Guinea, the other the first Palaearctic member of the subfamily based on a 30+ mya, species from Baltic amber. The second species is illustrated using synchrotron X-ray microtomography. Whereas the extant Betylobraconinae are restricted to Australia, New Guinea and New Caledonia, their ancestral distribution is now known to have extended considerably further. A key to the four species of Mesocentrus known from Papua New Guinea is provided. Both species possess some putatively plesiomorphic characters absent in other extant Mesocentrus spp. The new extant species differs in having a considerably larger number of antennal segments and a less laterally depressed frons, while the extinct one has the clypeus separated from the face dorsally and strongly developed hypoclypeal depression. Availability of sequence data for this species enabled further analysis of the relationships of the subfamily, which we present in a phylogenetic analysis additionally including the release of a number of new sequences of related taxa.

Cheng Y., Altapova V., Helfen L., Xu F., Dos Santos Rolo T., Vagovi P., Fiederle M., Baumbach T.

in Journal of Physics: Conference Series, 463 (2013), 012038. DOI:10.1088/1742-6596/463/1/012038

Abstract

X-ray computed laminography has been developed as a non-destructive imaging technique for inspecting laterally extended objects. Benefiting from a parallel-beam geometry, high photon flux of synchrotron sources and modern high-resolution detector systems, synchrotron radiation computed laminography (SRCL) results in a powerful three-dimensional microscopy technique. SRCL can be combined with different contrast modes, such as absorption, phase and dark-field contrasts, in order to provide complementary information for the same specimen. Here we show the development of SRCL at the TopoTomo beamline of the ANKA light source. A novel instrumentation design is reported and compared to the existing one. For this design, experimental results from different contrast modalities are shown. © Published under licence by IOP Publishing Ltd.

Yang X., Jejkal T., Pasic H., Stotzka R., Streit A., Van Wezel J., Rolo T.D.S.

in Proceedings of the 2013 21st Euromicro International Conference on Parallel, Distributed, and Network-Based Processing, PDP 2013 (2013) 86-93, 6498537. DOI:10.1109/PDP.2013.21

Abstract

In this paper, the method of data intensive computing is studied for large amounts of data in computed tomography (CT). An automatic workflow is built up to connect the tomography beamline of ANKA with the large scale data facility (LSDF), able to enhance the data storage and analysis efficiency. In this workflow, this paper focuses on the parallel computing of 3D computed tomography reconstruction. Different from the existing reconstruction system with filtered back-projection method, an algebraic reconstruction technique based on compressive sampling theory is presented to reconstruct the data from ultrafast computed tomography with fewer projections. Then the connected computing resources at the LSDF are used to implement the 3D CT reconstruction by distributing the whole job into multiple tasks executed in parallel. Promising reconstruction images and high computing performance are reported. For the 3D X-ray CT reconstruction, less than six minutes are actually required. LSDF is not only able to organize data efficiently, but also can provide reconstructed results to users in nearly instantaneous time. After integration into the workflow, this data intensive computing method will largely improve the data processing for ultrafast computed tomography at ANKA. © 2013 IEEE.

Mangold S., Steininger R., Rolo T.D.S., Gottlicher J.

in Journal of Physics: Conference Series, 430 (2013), 012130. DOI:10.1088/1742-6596/430/1/012130

Abstract

Full field transmission spectroscopic imaging is a powerful method for screening complex and large systems to map the distribution of valence state and electronic configuration of elements. Here, we describe an optimized hardware set-up and data evaluation tool chain for spectroscopic X-ray imaging. We tested this system successful on a ferruginous quartz (SiO2) sample.

Zabler S., Ershov A., Rack A., Garcia-Moreno F., Baumbach T., Banhart J.

in Acta Materialia, 61 (2013) 1244-1253. DOI:10.1016/j.actamat.2012.10.047

Abstract

Semi-solid melts exhibit a very unpredictable rheology and filling dynamics, when injected into thin-walled components. Optimization of the process requires an insight into the casting process during injection. For this purpose we injected semi-solid an Al-Ge alloy into two different thin channel geometries while recording high resolution radiographs at fast frame rates (up to 1000 images per s). Comparison of a bottleneck channel, which has previously been used for slower experiments, with a right-angle turn geometry reveals a significant influence of the channel shape on the flow behaviour of the particle-liquid mixture. While the bottleneck is quickly sealed with densified solid, turbulences in the right-angle turn apparently permit solid particles and clusters to move conjointly with the liquid and thus achieve a more complete filling. Single particle trajectories and rapid break-up of solid skeletons in such a system have been observed for the first time in situ. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Myagotin A., Voropaev A., Helfen L., Hanschke D., Baumbach T.

in IEEE Transactions on Image Processing, 22 (2013) 5348-5361, 6631501. DOI:10.1109/TIP.2013.2285600

Abstract

Computed laminography (CL) was developed to use X-rays from synchrotron sources for high-resolution imaging of the internal structure of a flat specimen from a series of 2-D projection images. The projections are acquired by irradiation of the sample under different rotation angles where the object rotation axis is inclined with respect to the beam direction. This yields for laterally extended objects a more uniform average transmitted intensity during sample rotation compared with computed tomography (CT). The reconstruction problem of CL cannot be reduced to a data-efficient 2-D case (as for parallelbeam CT) since each single slice perpendicular to the rotation axis requires a 2-D region on the detector as input data for all projection directions. This paper describes a computationally efficient reconstruction procedure based on filtered backprojection (FBP) adapted to the CL acquisition geometry. From the Fourier slice theorem, we derive a framework for analytic image reconstruction and outline implementation details of the generic FBP algorithm. Different approaches reducing the reconstruction time by means of parallel and distributed computations are considered and evaluated. © 2013 IEEE.

Moosmann J., Altapova V., Helfen L., Hanschke D., Hofmann R., Baumbach T.

in Journal of Physics: Conference Series, 425 (2013), 192003. DOI:10.1088/1742-6596/425/19/192003

Abstract

Considering a pure and not necessarily weak phase object, we review a noniterative and nonlinear single-distance phase-retrieval algorithm. The latter exploits the fact that a well-known linear contrast-transfer function, which incorporates all orders in object-detector distance, can be modified to yield a quasiparticle dispersion. Accepting a small loss of information, this algorithm also retrieves the high-frequency parts of the phase in an artefact free way. We point out an extension of this highly resolving quasiparticle approach for mixed objects by assuming a global attenuation-phase duality. Tomographically reconstructing two developmental stages in Xenopus laevis, we compare our approach with a linear algorithm, based on the transport-of-intensity equation, which suppresses high-frequency information.

Alaribe L., Fauler A., Cecilia A., Santos Rolo T.D., Fiederle M., Burger A.

in Materials Research Society Symposium Proceedings, 1576 (2013). DOI:10.1557/opl.2013.1143

Abstract

Much has been reported on the excellent performance of the Eu2+ activated SrI2-scintillator in spectroscopic applications, like the high light yield (97 660 ph/MeV) and good energy resolution (2.7% FWHM at 662 keV). The exploitation of these properties for other application fields is limited by the hygroscopic nature of the SrI2. Single crystal scintillating screens exhibit high spatial resolution, this combined with the high density, high effective atomic number, and the high light yield of the SrI2 could be used for high resolution X-ray imaging. Some of the questions we tried to answer in this work are the following: owing to the excellent performance of the SrI2-scintillator in spectroscopic applications, how would it perform in X-ray imaging applications. X-ray images are described based on their (spatial) resolution and contrast, how would they look like when recorded using the SrI2-scintillator detector. First a packaging technique was developed that protected the hygroscopic screens during the measurements. Our results show a high resolution of the images obtained with thin SrI2-scintillator screens both in 2D radiography and 3D tomography measurements. With these results, we think that the SrI2-scinitillator is not only a candidate for spectroscopic applications, but also for high resolution X-ray imaging purposes. Copyright © Materials Research Society 2013A.

Alaribe L., Fauler A., Cecilia A., Santos Rolo T.D., Fiederle M., Burger A.

in International Journal of Microwave and Wireless Technologies, 1576 (2013). DOI:10.1557/opl.2013.1143

Abstract

© Materials Research Society 2013. Much has been reported on the excellent performance of the Eu2+ activated SrI2-scintillator in spectroscopic applications, like the high light yield (97 660 ph/MeV) and good energy resolution (2.7% FWHM at 662 keV). The exploitation of these properties for other application fields is limited by the hygroscopic nature of the SrI2. Single crystal scintillating screens exhibit high spatial resolution, this combined with the high density, high effective atomic number, and the high light yield of the SrI2 could be used for high resolution X-ray imaging. Some of the questions we tried to answer in this work are the following: owing to the excellent performance of the SrI2-scintillator in spectroscopic applications, how would it perform in X-ray imaging applications. X-ray images are described based on their (spatial) resolution and contrast, how would they look like when recorded using the SrI2-scintillator detector. First a packaging technique was developed that protected the hygroscopic screens during the measurements. Our results show a high resolution of the images obtained with thin SrI2-scintillator screens both in 2D radiography and 3D tomography measurements. With these results, we think that the SrI2-scinitillator is not only a candidate for spectroscopic applications, but also for high resolution X-ray imaging purposes.