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At least 19 records

Simulation of kinematic Kikuchi diffraction patterns from atomistic structures

One of the limitations of atomistic simulations is that many of the computational tools used to extract structural information from atomic trajectories provide metrics that are not directly compatible with experiments for validation. In this work, to bridge between simulation and experiment, a method is presented to produce simulated Kikuchi diffraction patterns using data from atomistic simulations, without requiring a priori specification of the crystal structure or defect periodicity. The Kikuchi pattern simulation is based on the kinematic theory of diffraction, with Kikuchi line intensities computed via a discrete structure factor calculation. Reciprocal lattice points are mapped to Kikuchi lines using a geometric projection of the reciprocal space data. This method is validated using single crystal atomistic models, and the novelty of this approach is emphasized by simulating kinematic Kikuchi diffraction patterns from an atomistic model containing a nanoscale dislocation loop. Deviations in kinematic Kikuchi line intensities are explained considering the displacement field of the dislocation loop, as is done in diffraction contrast theory.

42 ENGINEERING↗

Grain refinement of cast zinc through magnesium inoculation: Characterisation and mechanism

It was previously found that peritectic-forming solutes are more favourable for the grain refinement of cast Al alloys than eutectic-forming solutes. In this work, we report that the eutectic-forming solute, Mg, can also significantly grain refine cast Zn. Differential thermal analysis (DTA) of a Zn–Mg alloy, in which efficient grain refinement occurred, evidenced an unexpected peak that appeared before the nucleation of η-Zn grains on the DTA spectrum. Based on extensive examination using X-ray diffraction, high resolution SEM and EDS, it was found that: (a) some faceted Zn–Mg intermetallic particles were reproducibly observed; (b) the particles were located at or near grain centres; (c) the atomic ratio of Mg to Zn in the intermetallic compound was determined to be around 1/2. Using tilting selected area diffraction (SAD) and convergent beam Kikuchi line diffraction pattern (CBKLDP) techniques, these faceted particles were identified as MgZn{sub 2} and an orientation relationship between such grain-centred MgZn{sub 2} particles and the η-Zn matrix was determined. Hence, the unexpected peak on the DTA spectrum is believed to correspond to the formation of MgZn{sub 2} particles, which act as effective heterogeneous nucleation sites in the alloy. Together with the effect of Mg solute on restricting grain growth, such heterogeneous nucleation is cooperatively responsible for the grain size reduction in Zn–Mg alloys. - Highlights: • A new eutectic-based grain refiner for the cast Zn was found. • The formation process of an intermetallic compound (MgZn{sub 2}) was characterised. • MgZn{sub 2} can act as potent heterogeneous nucleation sites above the liquidus. • A new OR between MgZn{sub 2} and η-Zn was determined using the CBKLDP technique.

36 MATERIALS SCIENCE↗

Accurate measurement of relative tilt and azimuth angles in electron tomography: A comparison of fiducial marker method with electron diffraction

Electron tomography is a method whereby a three-dimensional reconstruction of a nanoscale object is obtained from a series of projected images measured in a transmission electron microscope. We developed an electron-diffraction method to measure the tilt and azimuth angles, with Kikuchi lines used to align a series of diffraction patterns obtained with each image of the tilt series. Since it is based on electron diffraction, the method is not affected by sample drift and is not sensitive to sample thickness, whereas tilt angle measurement and alignment using fiducial-marker methods are affected by both sample drift and thickness. The accuracy of the diffraction method benefits reconstructions with a large number of voxels, where both high spatial resolution and a large field of view are desired. The diffraction method allows both the tilt and azimuth angle to be measured, while fiducial marker methods typically treat the tilt and azimuth angle as an unknown parameter. The diffraction method can be also used to estimate the accuracy of the fiducial marker method, and the sample-stage accuracy. A nano-dot fiducial marker measurement differs from a diffraction measurement by no more than ±1°.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

Low Thermal Gradient Czochralski growth of large CdWO{sub 4} crystals and electronic properties of (010) cleaved surface

The crystal growth of large high-quality inclusion-free CdWO4 crystals, 110 mm in diameter and mass up to 20 kg, has been carried out by the Low Thermal Gradient Czochralski (LTG Cz) technique. The high-purity CdWO{sub 4}(010) surface has been prepared by cleavage and high structural quality of the surface has been verified by RHEED, revealing a system of Kikuchi lines. The chemical state and electronic structure of the surface have been studied using X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). The total and partial densities of states of the CdWO{sub 4} tungstate were calculated employing the first-principles full potential linearized augmented plane wave (FP-LAPW) method. The results indicate that the principal contributors to the valence band of CdWO{sub 4} are the Cd 4d, W 5d and O 2p states which contribute mainly at the bottom, in the central portion and at the top of the valence band, respectively, with also significant contributions of the mentioned states throughout the whole CdWO{sub 4} valence-band region. With respect to the occupation of the O 2p states, the results of the FP-LAPW calculations are confirmed by comparison on a common energy scale of the XPS valence-band spectrum and the XES band representing the energy distribution of the O 2p states in this compound. Additionally, the FP-LAPW data allow us to conclude that the CdWO{sub 4} tungstate is a non-direct semiconductor. - Highlights: • Large high-quality CdWO{sub 4} crystals, up to 115 mm in diameter and mass up to 20 kg, have been grown. • The high-purity cleaved CdWO{sub 4}(010) surface has been studied by the XPS and XES methods. • The principal contributors to the CdWO{sub 4} valence band are the Cd 4d, W 5d and O 2p states.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

KSpaceNavigator (KSN)

Intuitive GUI for manipulating microscope stages, allowing to align crystallographic data with stages, allowing to align crystallographic data with stage coordinates and microscope images. Simulates kinematic diffraction patterns and Kikuchi line patterns. Simulated patterns can be displayed as overlay to actually measured data, allowing manual fingerprinting and angular alignment. Crystallographic data is fed to the program in form of CIF (crystallographic information file) files, which are available from many databases and cover virtually all crystal structure ever reported in any journal. Actual goniometer scales can be linearized by lookup tables, program can be used with any microscope goniometer, double tilt and tilt-rotation type. Software requirements: Win32 platform (XP); Compiler/Version: Borland C++ Builder 5; Type of files: Executable modules; Hardware requirement: PC

Duden, Thomas↗

Diffuse multiple scattering

A new form of diffraction lines similar to Rutherford, Kikuchi and Kossel lines has been identified. They can be used to eliminate the need for sample/source matching in Lonsdale’s triple convergent line method in lattice-parameter determination. A new form of diffraction lines has been identified, similar to Rutherford, Kikuchi and Kossel lines. This paper highlights some of the properties of these lines and shows how they can be used to eliminate the need for sample/source matching in Lonsdale’s triple convergent line method in lattice-parameter determination.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Dirac Magnon in Honeycomb Lattice Magnet NiTiO 3

We performed inelastic neutron scattering experiments on single-crystal samples of the honeycomb lattice magnet, ilmenite NiTiO 3 . Below the Néel temperature of 22 K, spin wave excitations with a band energy of 3.7 meV were observed. Further, the neutron energy spectra were well-reproduced by modeling the system as a ferromagnetic honeycomb lattice with antiferromagnetic interlayer coupling, using linear spin wave theory. Similar to another ilmenite CoTiO 3 , a crossing structure was observed at the K point, suggesting the presence of Dirac magnons in NiTiO 3 . Further calculations suggested the formation of Dirac nodal line.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Identification of sub-grains and low angle boundaries beyond the angular resolution of EBSD maps

A new method called ALGrId (Anti-Leak GRain IDentification) is proposed for the detection of sub-grains beyond the relative angular resolution of Electron Backscatter Diffraction maps. It does not use any additional information such as Kikuchi Pattern Quality map nor need data filtering. It uses a modified Dijkstra algorithm which seeks the continuous set of boundaries having the highest average disorientation angle. - Highlights: • ALGrId is a new method to identify sub-grains and low angle boundaries in EBSD maps. • Unlike classical methods, ALGrId works even beyond the relative angular resolution. • If the orientation noise peaks at 0.7°, ALGrid detects 0.4°-boundaries correctly. • In the same example, the classical algorithm identifies 1.1°-boundaries only.

36 MATERIALS SCIENCE↗

Circular dichroism in hard X-ray photoelectron diffraction observed by time-of-flight momentum microscopy

X-ray photoelectron diffraction (XPD) is a powerful technique that yields detailed structural information of solids and thin films that complements electronic structure measurements. Among the strongholds of XPD we can identify dopant sites, track structural phase transitions, and perform holographic reconstruction. High-resolution imaging of k ll -distributions (momentum microscopy) presents a new approach to core-level photoemission. It yields full-field k x -k y XPD patterns with unprecedented acquisition speed and richness in details. Here, we show that beyond the pure diffraction information, XPD patterns exhibit pronounced circular dichroism in the angular distribution (CDAD) with asymmetries up to 80%, alongside with rapid variations on a small k ll -scale (0.1 Å -1 ). Measurements with circularly-polarized hard X-rays (hν = 6 keV) for a number of core levels, including Si, Ge, Mo and W, prove that core-level CDAD is a general phenomenon that is independent of atomic number. The fine structure in CDAD is more pronounced compared to the corresponding intensity patterns. Additionally, they obey the same symmetry rules as found for atomic and molecular species, and valence bands. Further, the CD is antisymmetric with respect to the mirror planes of the crystal, whose signatures are sharp zero lines. Calculations using both the Bloch-wave approach and one-step photoemission reveal the origin of the fine structure that represents the signature of Kikuchi diffraction. To disentangle the roles of photoexcitation and diffraction, XPD has been implemented into the Munich SPRKKR package to unify the one-step model of photoemission and multiple scattering theory.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Growth of very large InN microcrystals by molecular beam epitaxy using epitaxial lateral overgrowth

Very thick InN (∼40 μm) was grown by molecular beam epitaxy using the epitaxial lateral overgrowth (ELO) technique. In some regions, the ELO of InN was observed as expected, indicating an important step toward fabricating quasi-bulk InN substrates. Interestingly, most parts of the sample consist of large flat-topped microcrystals and well-faceted microstructures. This is likely due to local growth condition variations during ELO, which is supported by an experiment where ELO of InN was performed on a substrate with various stripe mask patterns. TEM characterization of a flat top InN microcrystal revealed few stacking faults and only related threading dislocations. Defect-free small faceted microcrystals were also observed. The thick InN crystals show a narrow photoluminescence spectrum with a peak at 0.679 eV and linewidth of 16.8 meV at 4 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Niobium hyperfine structure in crystal calcium tungstate

A study of the niobium hyperfine structure in single crystal calcium tungstate was made by the combination of the technique of electron paramagnetic resonance and electron nuclear double resonance (EPR/ENDOR). The microwave frequency was about 9.4 GHz and the radio frequency from 20MHz to 70 MHz. The rare earth ions Nd(3+), U(3+), or Tm(3+) were added as the charge compensator for Nb(5+). To create niobium paramagnetic centers, the sample was irradiated at 77 deg K with a 10 thousand curie Co-60 gamma source for 1 to 2 hours at a dose rate of 200 K rads per hour and then transferred quickly into the cavity. In a general direction of magnetic field, the spectra showed 4 sets of 10 main lines corresponding to 4 nonequivalent sites of niobium with I = 9/2. These 4 sets of lines coalesced into 2 sets of 10 in the ab-plane and into a single set of 10 along the c-axis. This symmetry suggested that the tungsten ions are substituted by the niobium ions in the crystal.

Tseng, D. L.↗

Atomic Scattering Factor of the ASTRO-H (Hitomi) SXT Reflector Around the Gold's L Edges

The atomic scattering factor in the energy range of 11.2 - 15.4 keV for the ASTRO-H Soft X-ray Telescope (SXT) is reported. The large effective area of the SXT makes use of photon spectra above 10 keV viable, unlike most other X-ray satellites with total-reflection mirror optics. Presence of gold's L-edges in the energy band is a major issue, as it complicates the function of the effective area. In order to model the area, the reflectivity measurements in the 11.2 - 15.4 keV band with the energy pitch of 0.4 - 0.7 eV were made in the synchrotron beam-line Spring-8 BL01B1. We obtained atomic scattering factors f1 and f2 by the curve fitting to the reflectivities of our witness sample. The edges associated with the L-I, II, and III transitions are identified, of which the depths are found to be roughly 60 shallower than those expected from the Henkes atomic scattering factor.

ASTRO-H Soft X-ray↗

Progress in the development and understanding of a high poloidal-beta tokamak operating scenario for an attractive fusion pilot plant

Abstract The high poloidal-beta ( $$\beta _{\textrm{P}}$$ β P ) regime was first proposed as a high bootstrap current scenario for a steady-state fusion pilot plant (FPP) in the 1990s (Kikuchi in Nucl Fusion 30:265, 1990). Since then, there have been many theoretical, modeling, and experimental research activities on this topic. A joint DIII-D/EAST research team began exploring the high- $$\beta _{\textrm{P}}$$ β P regime in 2013, focusing on addressing the needs of attractive FPP design by taking advantage of the extensive diagnostic set and sophisticated plasma control system on DIII-D and the well-developed integrated modeling capability at General Atomics. The ultimate goal is to demonstrate such a scenario on EAST with truly long pulse and metal wall compatibility. This paper summarizes the highlights of the research results on DIII-D by the joint team in the past decade. Experimental evidence and modeling analysis show the high- $$\beta _{\textrm{P}}$$ β P scenario has great advantages in addressing key needs for an attractive FPP design, such as high-energy confinement quality at low rotation, excellent core-edge integration, high line-averaged density above the Greenwald limit, low disruption risk, and high bootstrap current fraction for steady-state operation. This provides a relatively safe and economical option to base an FPP design on that will lead to commercial fusion energy.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

The ASTRO-H SXT Performance to the Large Off-Set Angles

The X-ray astronomy satellite ASTRO-H, which is the 6th Japanese X-ray astronomy satellite and is renamed Hitomi after launch, is designed to observe celestial X-ray objects in a wide energy band from a few hundred eV to 600 keV. The Soft X-ray Telescopes (SXTs) onboard ASTRO-H play a role of collecting and imaging X-rays up to approximately 12 keV. Although the field of view of the SXT is approximately 15' (FWHM), due to the thin-foil-nested Wolter-I type optics adopted in the SXTs, X-rays out of the field of view can reach the focal plane without experiencing a normal double reflection. This component is referred to as 'stray light'. Owing to investigation of the stray light so far, 'secondary reflection' is now identified as the main component of the stray light, which is composed of X-rays reflected only by secondary reflectors. In order to cut the secondary reflections, a 'pre-collimator' is equipped on top of the SXTs. However, we cannot cut all the stray lights with the pre-collimator in some off-axis angle domain. In this study, we measure the brightness of the stray light of the SXTs at some representative off-axis angles by using the ISAS X-ray beam line. ASTRO-H is equipped with two modules of the SXT; one is for the Soft X-ray Spectrometer (SXS), an X-ray calorimeter, and the other is for the Soft X-ray Imager (SXI), an X-ray CCD camera. These SXT modules are called SXT-S and SXT-I, respectively. Of the two detector systems, the SXI has a large field of view, a square with 38' on a side. To cope with this, we have made a mosaic mapping of the stray light at a representative off-axis angle of 30' in the X-ray beam line at the Institute of Space and Astronautical Science. The effective area of the brightest secondary reflection is found of order approximately 0.1% of the on-axis effective area at the energy of 1.49 keV. The other components are not so bright (less than 5 x 10(exp -4) times smaller than the on-axis effective area). On the other hand, we have found that the effective area of the stray light in the SXS field of view (approximately 3' x 3') at large off-axis angles (greater than 15') are approximately 1(exp -4) times smaller than the on-axis effective area (approximately 590 sq cm at 1.49 keV).

X-rays↗

DISCOVERY OF DRAMATIC OPTICAL VARIABILITY IN SDSS J1100+4421: A PECULIAR RADIO-LOUD NARROW-LINE SEYFERT 1 GALAXY?

We present our discovery of dramatic variability in SDSS J1100+4421 by the high-cadence transient survey Kiso Supernova Survey. The source brightened in the optical by at least a factor of three within about half a day. Spectroscopic observations suggest that this object is likely a narrow-line Seyfert 1 galaxy (NLS1) at z = 0.840, however, with unusually strong narrow emission lines. The estimated black hole mass of ∼10{sup 7} M {sub ☉} implies bolometric nuclear luminosity close to the Eddington limit. SDSS J1100+4421 is also extremely radio-loud, with a radio loudness parameter of R ≅ 4 × 10{sup 2}-3 × 10{sup 3}, which implies the presence of relativistic jets. Rapid and large-amplitude optical variability of the target, reminiscent of that found in a few radio- and γ-ray-loud NLS1s, is therefore produced most likely in a blazar-like core. The 1.4 GHz radio image of the source shows an extended structure with a linear size of about 100 kpc. If SDSS J1100+4421 is a genuine NLS1, as suggested here, this radio structure would then be the largest ever discovered in this type of active galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗

Comprehensive Genetic Search to Clarify the Molecular Mechanism of Drug Resistance Identifies ASCL2-LEF1/TSPAN8 Axis in Colorectal Cancer

Background: Treatment-resistance genes limiting anticancer therapy have not been well clarified in colorectal cancer (CRC). We explored gene expression profiles to identify biomarkers for predicting treatment resistance to an anticancer drug in CRC. Methods: Six CRC cell lines were treated with phenylbutyrate (PB). The gene expression profiles were then compared using microarrays (harboring 54,675 genes), and genes associated with PB resistance were identified. Candidate genes were functionally examined in cell lines and clinically validated for treatment resistance in clinical samples. Results: Both DLD1 and HCT15 cells were PB resistant, while HCT116 cells were identified as PB sensitive. On microarray analysis, among the PB resistance-related genes, the expression of the genes ASCL2, LEF1, and TSPAN8 was clearly associated with PB resistance. PB-sensitive cells transfected with one of these three genes exhibited significant (P < 0.001) augmentation of PB resistance; ASCL2 induced expression of both LEF1 and TSPAN8, while neither LEF1 nor TSPAN8 induced ASCL2. RNA interference via ASCL2 knockdown made PB-resistant cells sensitive to PB and inhibited both genes. ASCL2 knockdown also played a critical role in sensitivity to treatment by 5-fluorouracil and radiotherapy in addition to PB. Finally, ASCL2 expression was significantly correlated with histological grade of rectal cancer with preoperative chemoradiation therapy. Conclusions: ASCL2 was identified as a causative gene involved in therapeutic resistance against anticancer treatments in CRC.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

Quantitative Infrared Image Analysis Of Thermally-Thin Cellulose Surface Temperatures During Upstream and Downstream Microgravity Flame Spread from A Central Ignition Line

Surface view calibrated infrared images of ignition and flame spread over a thin cellulose fuel were obtained at 30 Hz during microgravity flame spread tests in the 10 second Japan Microgravity Center (JAMIC). The tests also used a color video of the surface view and color images of the edge view using 35 millimeter 1600 Kodak Ektapress film at 2 Hz. The cellulose fuel samples (50% long fibers from lumi pine and 50% short fibers from birch) were made with an area density of 60 grams per square meters. The samples were mounted in the center of a 12 centimeter wide by 16 centimeter tall flow duct that uses a downstream fan to draw the air through the flow duct. Samples were ignited after the experiment package was released using a straight hot wire across the center of the 7.5 centimeter wide by 14 centimeter long samples. One case, at 1 atmosphere 35%O2 in N2, at a forced flow of 10 centimeters per second, is presented here. In this case, as the test progresses, the single flame begins to separate into simultaneous upstream and downstream flames. Surface temperature profiles are evaluated as a function of time, and temperature gradients for upstream and downstream flame spread are measured. Flame spread rates from IR image data are compared to visible image spread rate data. IR blackbody temperatures are compared to surface thermocouple readings to evaluate the effective emissivity of the pyrolyzing surface. Preheat lengths are evaluated both upstream and downstream of the central ignition point. A surface energy balance estimates the net heat flux from the flame to the fuel surface along the length of the fuel.

Olson, Sandra L.↗