252 research outputs found

    Diffractive triangulation of radiative point sources

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    We describe a general method to determine the location of a point source of waves relative to a twodimensional single-crystalline active pixel detector. Based on the inherent structural sensitivity of crystalline sensor materials, characteristic detector diffraction patterns can be used to triangulate the location of a wave emitter. The principle described here can be applied to various types of waves, provided that the detector elements are suitably structured. As a prototypical practical application of the general detection principle, a digital hybrid pixel detector is used to localize a source of electrons for Kikuchi diffraction pattern measurements in the scanning electron microscope. This approach provides a promising alternative method to calibrate Kikuchi patterns for accurate measurements of microstructural crystal orientations, strains, and phase distributions

    4D Imaging and Diffraction Dynamics of Single-Particle Phase Transition in Heterogeneous Ensembles

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    In this Letter, we introduce conical-scanning dark-field imaging in four-dimensional (4D) ultrafast electron microscopy to visualize single-particle dynamics of a polycrystalline ensemble undergoing phase transitions. Specifically, the ultrafast metal–insulator phase transition of vanadium dioxide is induced using laser excitation and followed by taking electron-pulsed, time-resolved images and diffraction patterns. The single-particle selectivity is achieved by identifying the origin of all constituent Bragg spots on Debye–Scherrer rings from the ensemble. Orientation mapping and dynamic scattering simulation of the electron diffraction patterns in the monoclinic and tetragonal phase during the transition confirm the observed behavior of Bragg spots change with time. We found that the threshold temperature for phase recovery increases with increasing particle sizes and we quantified the observation through a theoretical model developed for single-particle phase transitions. The reported methodology of conical scanning, orientation mapping in 4D imaging promises to be powerful for heterogeneous ensemble, as it enables imaging and diffraction at a given time with a full archive of structural information for each particle, for example, size, morphology, and orientation while minimizing radiation damage to the specimen

    Beam-induced backgrounds measured in the ATLAS detector during local gas injection into the LHC beam vacuum

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    Inelastic beam-gas collisions at the Large Hadron Collider (LHC), within a few hundred metres of the ATLAS experiment, are known to give the dominant contribution to beam backgrounds. These are monitored by ATLAS with a dedicated Beam Conditions Monitor (BCM) and with the rate of fake jets in the calorimeters. These two methods are complementary since the BCM probes backgrounds just around the beam pipe while fake jets are observed at radii of up to several metres. In order to quantify the correlation between the residual gas density in the LHC beam vacuum and the experimental backgrounds recorded by ATLAS, several dedicated tests were performed during LHC Run 2. Local pressure bumps, with a gas density several orders of magnitude higher than during normal operation, were introduced at different locations. The changes of beam-related backgrounds, seen in ATLAS, are correlated with the local pressure variation. In addition the rates of beam-gas events are estimated from the pressure measurements and pressure bump profiles obtained from calculations. Using these rates, the efficiency of the ATLAS beam background monitors to detect beam-gas events is derived as a function of distance from the interaction point. These efficiencies and characteristic distributions of fake jets from the beam backgrounds are found to be in good agreement with results of beam-gas simulations performed with the Fluka Monte Carlo programme

    An EBSD study of the deformation of service-aged 316 austenitic steel

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    Electron backscatter diffraction (EBSD) has been used to examine the plastic deformation of an ex-service 316 austenitic stainless steel at 297K and 823K (24 °C and 550 °C)at strain rates 3.5x10-3 to 4 x 10-7 s-1. The distribution of local misorientations was found to depend on the imposed plastic strain following a lognormal distribution at true strains 0.1. At 823 K (550 °C), the distribution of misorientations depended on the applied strain rate. The evolution of lattice misorientations with increasing plastic strain up to 0.23 was quantified using the metrics kernel average misorientation, average intragrain misorientation, and low angle misorientation fraction. For strain rate down to 10-5 s-1 all metrics were insensitive to deformation temperature, mode (tension vs. compression) and orientation of the measurement plane. The strain sensitivity of the different metrics was found to depend on the misorientation ranges considered in their calculation. A simple new metric, proportion of undeformed grains, is proposed for assessing strain in both aged and unaged material. Lattice misorientations build up with strain faster in aged steel than in un-aged material and most of the metrics were sensitive to the effects of thermal aging. Ignoring aging effects leads to significant overestimation of the strains around welds. The EBSD results were compared with nanohardness measurements and good agreement established between the two techniques of assessing plastic strain in aged 316 steel

    Mitochondrial Changes in Ageing Caenorhabditis elegans – What Do We Learn from Superoxide Dismutase Knockouts?

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    One of the most popular damage accumulation theories of ageing is the mitochondrial free radical theory of ageing (mFRTA). The mFRTA proposes that ageing is due to the accumulation of unrepaired oxidative damage, in particular damage to mitochondrial DNA (mtDNA). Within the mFRTA, the “vicious cycle” theory further proposes that reactive oxygen species (ROS) promote mtDNA mutations, which then lead to a further increase in ROS production. Recently, data have been published on Caenorhabditis elegans mutants deficient in one or both forms of mitochondrial superoxide dismutase (SOD). Surprisingly, even double mutants, lacking both mitochondrial forms of SOD, show no reduction in lifespan. This has been interpreted as evidence against the mFRTA because it is assumed that these mutants suffer from significantly elevated oxidative damage to their mitochondria. Here, using a novel mtDNA damage assay in conjunction with related, well established damage and metabolic markers, we first investigate the age-dependent mitochondrial decline in a cohort of ageing wild-type nematodes, in particular testing the plausibility of the “vicious cycle” theory. We then apply the methods and insights gained from this investigation to a mutant strain for C. elegans that lacks both forms of mitochondrial SOD. While we show a clear age-dependent, linear increase in oxidative damage in WT nematodes, we find no evidence for autocatalytic damage amplification as proposed by the “vicious cycle” theory. Comparing the SOD mutants with wild-type animals, we further show that oxidative damage levels in the mtDNA of SOD mutants are not significantly different from those in wild-type animals, i.e. even the total loss of mitochondrial SOD did not significantly increase oxidative damage to mtDNA. Possible reasons for this unexpected result and some implications for the mFRTA are discussed
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