M. Kobas

ETH Zurich, Zürich, ZH, Switzerland

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Publications (23)39.43 Total impact

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    ABSTRACT: PILATUS X-ray detectors are in operation at many synchrotron beamlines around the world. This article reports on the characterization of the new PILATUS3 detector generation at high count rates. As for all counting detectors, the measured intensities have to be corrected for the dead-time of the counting mechanism at high photon fluxes. The large number of different bunch modes at these synchrotrons as well as the wide range of detector settings presents a challenge for providing accurate corrections. To avoid the intricate measurement of the count rate behaviour for every bunch mode, a Monte Carlo simulation of the counting mechanism has been implemented, which is able to predict the corrections for arbitrary bunch modes and a wide range of detector settings. This article compares the simulated results with experimental data acquired at different synchrotrons. It is found that the usage of bunch mode specific corrections based on this simulation improves the accuracy of the measured intensities by up to 40% for high photon rates and highly structured bunch modes. For less structured bunch modes, the instant retrigger technology of PILATUS3 detectors substantially reduces the dependency of the rate correction on the bunch mode. The acquired data also demonstrate that the instant retrigger technology allows for data acquisition up to 15 million photons per second per pixel.
    Journal of Synchrotron Radiation 05/2015; 22(3). DOI:10.1107/S1600577515003288 · 3.02 Impact Factor
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    ABSTRACT: PILATUS systems are well established as X-ray detectors at most synchrotrons. Their single photon counting capability ensures precise measurements, but introduces a short dead time after each hit, which becomes significant for photon rates above a million per second and pixel. The resulting loss in the number of counted photons can be corrected for by applying corresponding rate correction factors. This article presents a Monte-Carlo simulation, which computes the correction factors taking into account the detector settings as well as the time structure of the X-ray beam at the synchrotron. For the PILATUS2 detector series the simulation shows good agreement with experimentally determined correction factors for various detector settings at different synchrotrons. The application of more accurate rate correction factors will improve the X-ray data quality at high photon fluxes. Furthermore we report on the simulation of the rate correction factors for the new PILATUS3 systems. The successor of the PILATUS2 detector avoids the paralysation of the counter, and allows for measurements up to a rate of ten million photons per second and pixel. For fast detector settings the simulation is capable of reproducing the data within one to two percent at an incoming photon rate of one million per second and pixel.
    Journal of Physics Conference Series 03/2013; 425(6):062002. DOI:10.1088/1742-6596/425/6/062002
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    ABSTRACT: The PILATUS detector system is widely used for X-ray experiments at third-generation synchrotrons. It is based on a hybrid technology combining a pixelated silicon sensor with a CMOS readout chip. Its single-photon-counting capability ensures precise and noise-free measurements. The counting mechanism introduces a short dead-time after each hit, which becomes significant for rates above 10(6) photons s(-1) pixel(-1). The resulting loss in the number of counted photons is corrected for by applying corresponding rate correction factors. This article presents the results of a Monte Carlo simulation which computes the correction factors taking into account the detector settings as well as the time structure of the X-ray beam at the synchrotron. The results of the simulation show good agreement with experimentally determined correction factors for various detector settings at different synchrotrons. The application of accurate rate correction factors improves the X-ray data quality acquired at high photon fluxes. Furthermore, it is shown that the use of fast detector settings in combination with an optimized time structure of the X-ray beam allows for measurements up to rates of 10(7) photons s(-1) pixel(-1).
    Journal of Synchrotron Radiation 05/2012; 19(Pt 3):347-51. DOI:10.1107/S0909049512003950 · 3.02 Impact Factor
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    ABSTRACT: At the Paul Scherrer Institute PSI an X-ray single photon counting pixel detector (PILATUS) based on the hybrid-pixel detector technology was developed in collaboration with SPring-8. The detection element is a 320 or 450 μm thick silicon sensor forming pixelated pn-diodes with a pitch of 172 μm×172 μm. An array of 2×8 custom CMOS readout chips are indium bump-bonded to the sensor, which leads to 33.5 mm×83.8 mm detective area. Each pixel contains a charge-sensitive amplifier, a single level discriminator and a 20 bit counter. This design realizes a high dynamic range, short readout time of less than 3 ms, a high framing rate of over 200 images per second and an excellent point-spread function. The maximum counting rate achieves more than 2×106 X-rays/s/pixel.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 11/2010; 623(1):204-206. DOI:10.1016/j.nima.2010.02.196 · 1.32 Impact Factor
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    Th. WEBER, M. KOBAS, W. STEURER
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    ABSTRACT: Diffuse inter-layers from a decagonal quasicrystal with nominal composition Al 70 Co 12 Ni 18 were investigated at 1120 K, 1070 K and 300 K. Patterson maps calculated from the inter-layers are interpreted such that the main units for correlated displacements along a 5 are structure motifs (‘clusters’) having a diameter of about 15 Å. At 1120 K, displacements of the clusters are uncorrelated along quasiperiodic directions, while they form about 42 Å-sized super-clusters at lower temperatures. The arrangement but not the inner structure of the super-clusters differs significantly at 1070 K and 300 K. Further, a first approach to the atomic structure of the 15 Å cluster is presented.
    Ferroelectrics 09/2010; 2004(Vol. 305):213-216. DOI:10.1080/00150190490462810 · 0.38 Impact Factor
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    ABSTRACT: The hybrid pixel technology combines silicon sensors with CMOS-processing chips by a 2D micro bump-bonding interconnection technology developed at Paul Scherrer Institute [C. Broennimann, E.F. Eikenberry, B. Henrich, R. Horisberger, G. Huelsen, E. Pohl, B. Schmitt, C. Schulze-Briese, M. Suzuki, T. Tomizaki, H. Toyokawa, A. Wagner. J. Synchrotron Rad. 13 (2005) 120 [1]; T. Rohe, C. Broennimann, F. Glaus, J. Gobrecht, S. Heising, M. Horisberger, R. Horisberger, H.C. Kaestl, J. Lehmann, S. Streuli, Nucl. Instr. and Meth. Phys. Res. A 565 (2006) 303 [2]]. PILATUS hybrid pixel detectors like other instruments [X. Llopart, M. Campell, R. Dinapoli, D. San Segundo, E. Pernigotti. IEEE Trans. Nucl. Sci. 49 (2002) 2279 [3]; N. Boudet, J.-F. Berar, L. Blanquart, P. Breugon, B. Caillot, J.-C. Clemens, I. Koudobine, P. Delpierre, C. Mouget, R. Potheau, I. Valin, Nucl. Instr. and Meth. Phys. Res. A 510 (2003) 41 [4]] are operating in the so-called “single photon counting mode”: Every X-ray quantum is directly converted into an electrical signal and counted by the detector system. Several prototype detectors in various geometries were produced, tested and established at different synchrotron beamlines worldwide. We explain the technology and present some recent highlights from various fields of applications.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 08/2009; DOI:10.1016/j.nima.2009.03.200 · 1.32 Impact Factor
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    ABSTRACT: This is the first of two parts, where we report the structure determination of a novel family of cluster-based intermetallic phases of unprecedented complexity: cF444-Al(63.6)Ta(36.4) (AT-19), a = 19.1663 (1) A, V = 7040 A3, cF(5928-x)-Al(56.6)Cu(3.9)Ta(39.5), x = 20 (ACT-45), a = 45.376 (1) A, V = 93,428 A(3) and cF(23,256-x)-Al(55.4)Cu(5.4)Ta(39.1), x = 122 (ACT-71), a = 71.490 (4) A, V = 365,372 A3. The space group is F43m in all three cases. These cluster-based structures are closely related to the class of Frank-Kasper phases. It is remarkable that all three structures show the same average structure that resembles the cubic Laves phase.
    Acta crystallographica. Section B, Structural science 07/2009; 65(Pt 3):308-17. DOI:10.1107/S0108768109014001 · 2.10 Impact Factor
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    ABSTRACT: Pilatus is a silicon hybrid pixel detector system for detecting X-rays in single photon counting mode. The PILATUS II chip, fabricated in a radiation tolerant design with a standard 0.25 mum CMOS process, was used to construct multichip modules with a size of 84 times 34 mm<sup>2</sup> comprising 94'965 pixels. All calibrations and characterizations were carried out with monochromatic X-rays from a synchrotron source. In order to set any required threshold above the noise level between 2.14 keV and 22 keV the detector was calibrated with X-rays. An algorithm to adjust thresholds pixel-by-pixel and create trim files based on X-ray flat-field images was developed. The threshold dispersion was reduced from 343 eV to 36 eV by the means of trim files. An electronic noise of 447 eV has been measured. The PILATUS modules are suitable for various X-ray applications such as diffraction and imaging techniques.
    IEEE Transactions on Nuclear Science 07/2009; DOI:10.1109/TNS.2008.2009448 · 1.46 Impact Factor
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    ABSTRACT: How many of the theoretically densely distributed Bragg reflections of a quasicrystal can be observed employing an area detector and synchrotron radiation? How does the reflection density of a real quasicrystal change as a function of exposure time, and what is the minimum distance between reflections? What does the distribution of diffuse scattering look like? To answer these questions, the Bragg reflection density of a perfect icosahedral quasicrystal with composition Al(64)Cu(23)Fe(13) was measured employing a novel type of single-photon-counting X-ray pixel detector, PILATUS 6M, which allows noise-free data collection with the extraordinarily large dynamic range of 20 bit. The reflection density was found to be two orders of magnitude lower than expected for a strictly quasiperiodic structure. Moreover, diffuse scattering reflects significant structural disorder, breaking six-dimensional F-lattice symmetry. These findings have some implications for the interpretation of physical properties.
    Journal of Applied Crystallography 08/2008; 41:669-674. DOI:10.1107/s0021889808014386 · 3.95 Impact Factor
  • C. Broennimann, M. Kobas
    Acta Crystallographica Section A Foundations of Crystallography 08/2008; 64(a1):C112-C113. DOI:10.1107/S0108767308096396 · 2.07 Impact Factor
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    ABSTRACT: The MYTHEN detector is a one-dimensional microstrip detector with single photon counting readout optimized for time resolved powder diffraction experiments at the Swiss Light Source (SLS). The system has been successfully tested for many different synchrotron radiation applications including phase contrast and tomographic imaging, small angle scattering, diffraction and time resolved pump and probe experiments for X-ray energies down to 5keV and counting rate up to 3MHz. The frontend electronics is designed in order to be coupled to 50μm pitch microstrip sensors but some interest in enhancing the spatial resolution is arising for imaging and powder diffraction experiments.A test structure with strip pitches in the range 10–50μm has been tested and the gain and noise on the readout electronics have been measured for the different strip pitches, observing no large difference down to 25μm. Moreover, the effect of the charge sharing between neighboring strips on the spatial resolution has been quantified by measuring the Point Spread Function (PSF) of the system for the different pitches.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 06/2008; 591(1):163-166. DOI:10.1016/j.nima.2008.03.048 · 1.32 Impact Factor
  • M. Kobas, Ch. Broennimann
    Acta Crystallographica Section A Foundations of Crystallography 08/2007; 63(a1):s105-s105. DOI:10.1107/S0108767307097711 · 2.07 Impact Factor
  • Acta Crystallographica Section A Foundations of Crystallography 08/2007; 63(a1):s87-s87. DOI:10.1107/S0108767307098108 · 2.07 Impact Factor
  • T. Weber, M. Kobas, W. Steurer
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    ABSTRACT: Higher-dimensional and three-dimensional techniques are complementary approaches for modelling disorder and diffuse scattering in quasicrystals. The concept of phasonic disorder allows a higher-dimensional description of disorder and calculation of diffuse intensities without knowing the atomic structure of the quasicrystal. In three-dimensional space, disorder can be analyzed by application of Patterson techniques. Local order may be modelled quantitatively without considering medium or long-range order properties of the quasicrystal. Furthermore, a priori knowledge about the structure of clusters present in the crystal can be easily utilized with Patterson methods. Potential and limitations of the techniques are discussed and demonstrated by means of experimental data.
    Philosophical Magazine A 06/2007; 87(Nos. 18-21):2799-2805. DOI:10.1080/14786430701358657
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    ABSTRACT: A stable decagonal quasicrystal with nominal composition Al73Ir14.5Os12.5 has been grown from the aluminum-rich melt. It has an incongruent melting temperature of Tm = 1283 °C and does not transform into a periodic phase within the maximum applied annealing time of 8 weeks at 1000 °C. The phase equlibria in the aluminum-rich part of the system Al–Ir–Os have been explored. The X-ray diffraction patterns can be indexed by five reciprocal basis vectors with lengths , i = 1, …, 4, and . This corresponds to a quasilattice parameter (i.e. the edge length of the related Penrose tiling) of 2.501(2) Å and a translation period along the 10-fold axis of 16.821(8) Å. The 5D space group is P105mc. The structure of the decagonal phase has been solved by two different approaches, the 3D pseudo-approximant technique and the 5D charge-flipping method. It is closely related to those of decagonal Al–Os–Pd and Al–Ni–Ru and can be described as quasiperiodic packing of columnar clusters with 20.150 Å diameter. The clusters consist of eight quasiperiodic layers with stacking sequence …ABCDA′B′C′D′…, where the prime (′) denotes the layers generated by a 105 operation.
    ChemInform 04/2007; 428(1-2-428):164-172. DOI:10.1016/j.jallcom.2006.03.064
  • T. Weber, M. Kobas, W. Steurer
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    ABSTRACT: Diffuse scattering from decagonal Al70Co12Ni18 (Edagawa phase) was investigated by means of Patterson methods. Basically two systems of diffuse intensities can be observed. The first system is found in quasiperiodic layers containing Bragg reflections. Model calculations show that underlying disorder can equally well be explained as phasonic disorder in a five dimensional approach, as well as fivefold orientational disorder of clusters in three dimensions. A second set of diffuse intensities is observed in layers halfway between the Bragg layers. Disorder associated thereto shows some correlation to phasonic/orientational disorder, but a different temperature dependency of the respective disorder phenomena suggests that these correlations are accidental.
    Philosophical Magazine A 01/2006; 86(3-5):537-542. DOI:10.1080/14786430500253943
  • S. Deloudi, M. Kobas, W. Steurer
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    ABSTRACT: A structure model for decagonal Al–Co–Ni with 8 Å periodicity along the decagonal axis is proposed. The model agrees well with available experimental information, such as electron microscopic images and three-dimensional (3D) Patterson maps calculated from X-ray-diffraction data. The model is based on a novel columnar cluster with 20 Å diameter and symmetry 5/mm building the W-approximant, Al72.5Co20Ni7.5. The proposed cluster also allows modelling of the various types of disorder and superstructures found in decagonal Al–Co–Ni.
    Philosophical Magazine 01/2006; 86(3-5):581-585. DOI:10.1080/14786430500228044 · 1.43 Impact Factor
  • Miroslav Kobas, Thomas Weber, Walter Steurer
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    ABSTRACT: The hydrodynamic theory of phasonic and phononic disorder is applied successfully to describe the short-range disordered structure of a decagonal Al71.5Co14.6Ni13.9 quasicrystal (Edagawa phase, superstructure type I). Moreover, model calculations demonstrate that the main features of diffuse scattering can be equally well described by phasonic disorder and fivefold orientational disorder of clusters. The calculations allow us to distinguish the different cluster types published so far and the best agreement with experimental data could be achieved with the mirror-symmetric Abe cluster. Modeling of phason diffuse scattering associated with the S1 and S2 superstructure reflections indicate disorder of superclusters. The former show basically intercluster correlations inside quasiperiodic layers, while the latter exhibit intra- and inter-cluster correlations, both between adjacent and inside quasiperiodic layers. The feasibility, potential, and limits of the Patterson method in combination with the punch-and-fill method employed is shown on the example of a phasonic disordered rhombic Penrose tiling. A variation of the elastic constants does not change qualitatively the way phasonic disorder is realized in the local quasicrystalline structure. For the same model system it is also shown that phasonic fluctuations of the atomic surfaces yield average clusters in the cut space, which correspond to fivefold orientationally disordered clusters.
    Physical Review B 06/2005; 71(22-22). DOI:10.1103/pHYSrEVb.71.224206 · 3.66 Impact Factor
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    Miroslav Kobas, Thomas Weber, Walter Steurer
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    ABSTRACT: The three-dimensional (3D) difference Patterson (autocorrelation) function of a disordered quasicrystal (Edagawa phase) has been analyzed. 3D diffuse x-ray diffraction data were collected in situ at 300, 1070, and 1120 K. A method, the punch-and-fill technique, has been developed for separating diffuse scattering and Bragg reflections. Its potential and limits are discussed in detail. The different Patterson maps are interpreted in terms of intercluster correlations as a function of temperature. Both at high and low temperatures, the clusters decorate the vertices of the same quasiperiodic covering. At low temperatures, for the disordered part of the structure, short-range intercluster correlations are present, whereas at higher temperatures, medium-range intercluster correlations are formed. This indicates disorder mainly inside clusters at low temperatures, whereas at higher temperatures disorder takes place inside larger superclusters. Qualitatively, the Patterson maps may be interpreted by intercluster correlations mainly inside pentagonal superclusters below 1120 K, and inside the larger decagonal superclusters at 1120 K. The results of our diffraction study are published in two parts. Part I focuses on the 3D Patterson analysis based on experimental data, Part II reports modeling of structural disorder in decagonal Al-Co-Ni.
    Physical Review B 06/2005; 71(22-22). DOI:10.1103/PhysRevB.71.224205 · 3.66 Impact Factor
  • M. Kobas, T. Weber, W. Steurer
    Acta Crystallographica Section A Foundations of Crystallography 08/2004; 60. DOI:10.1107/S0108767304096254 · 2.07 Impact Factor