Aluminium Nanowires: Influence of Work Hardening on Conductance Histograms

Conductance histograms of work-hardened Al show a series up to 11 equidistant peaks with a period of 1.15 +/- 0.02 of the quantum conductance unit G_0 = 2e^2/h. Assuming the peaks originate from atomic discreteness, this agrees with the value of 1.16 G_0 per atom obtained in numerical calculations by Hasmy et al.Comment: 4 pages, 4 figure

Effects of signal diffusion on x-ray phase contrast images

We discuss the problem of signal diffusion among neighbouring pixels in x-ray phase contrast imaging (XPCi) specifically for coded-aperture (CA) XPCi, but many of the discussed observations are directly transferable to other XPCi modalities. CA XPCi exploits the principle of pixel edge illumination by means of two CA masks. The first mask, placed in contact with the detector, creates insensitive regions between adjacent pixels; the second one, placed immediately before the sample, creates individual beams impinging on the boundaries between sensitive and insensitive regions on the detector, as created by the detector mask. In this way, edge illumination is achieved for all pixels of an area detector illuminated by a divergent and polychromatic beam generated by a conventional source. As the detector mask redefines the resolution properties of the detector, sample dithering can be used to effectively increase the system spatial resolution, without having to apply any post-processing procedure (e. g., deconvolution). This however creates artifacts in the form of secondary fringes (which have nothing to do with phase-related secondary fringes) if there is signal diffusion between adjacent pixels. In non-dithered images, signal diffusion between adjacent pixels causes a reduction in image contrast. This effect is investigated both theoretically and experimentally, and its direct implications on image quality are discussed. The interplay with the sample positioning with respect to the detector pixel matrix, which also has an effect on the obtained image contrast, is also discussed. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3606442

Phase Contrast Imaging with Coded Apertures Using Laboratory-Based X-ray Sources

X‐ray phase contrast imaging is a powerful technique that allows detection of changes in the phase of x‐ray wavefronts as they pass through a sample. As a result, details not visible in conventional x‐ray absorption imaging can be detected. Until recently the majority of applications of phase contrast imaging were at synchrotron facilities due to the availability of their high flux and coherence; however, a number of techniques have appeared recently that allow phase contrast imaging to be performed using laboratory sources. Here we describe a phase contrast imaging technique, developed at University College London, that uses two coded apertures. The x‐ray beam is shaped by the pre‐sample aperture, and small deviations in the x‐ray propagation direction are detected with the help of the detector aperture. In contrast with other methods, it has a much more relaxed requirement for the source size (it works with source sizes up to 100 μm). A working prototype coded‐aperture system has been built. An x‐ray detector with directly deposited columnar CsI has been used to minimize signal spill‐over into neighboring pixels. Phase contrast images obtained with the system have demonstrated its effectiveness for imaging low‐absorption materials

Atomic size oscillations in conductance histograms for gold nanowires and the influence of work hardening

Nanowires of different nature have been shown to self-assemble as a function of stress at the contact between two macroscopic metallic leads. Here we demonstrate for gold wires that the balance between various metastable nanowire configurations is influenced by the microstructure of the starting materials and we discover a new set of periodic structures, which we interpret as due to the atomic discreteness of the contact size for the three principal crystal orientations.Comment: This version corrects an error in attributing the three observed periods, and includes a comparison with recent model calculation

Correlative Microscopy of Morphology and Luminescence of Cu porphyrin aggregates

Transfer of energy and information through molecule aggregates requires as one important building block anisotropic, cable-like structures. Knowledge on the spatial correlation of luminescence and morphology represents a prerequisite in the understanding of internal processes and will be important for architecting suitable landscapes. In this context we study the morphology, fluorescence and phosphorescence of molecule aggregate structures on surfaces in a spatially correlative way. We consider as two morphologies, lengthy strands and isotropic islands. It turns out that phosphorescence is quite strong compared to fluorescence and the spatial variation of the observed intensities is largely in line with the amount of dye. However in proportion, the strands exhibit more fluorescence than the isotropic islands suggesting weaker non-radiative channels. The ratio fluorescence to phosphorescence appears to be correlated with the degree of aggregation or internal order. The heights at which luminescence saturates is explained in the context of attenuation and emission multireflection, inside the dye. This is supported by correlative photoemission electron microscopy which is more sensitive to the surface region. The lengthy structures exhibit a pronounced polarization dependence of the luminescence with a relative dichroism up to about 60%, revealing substantial perpendicular orientation preference of the molecules with respect to the substrate and parallel with respect to the strands

High resolution characterisation of microstructural evolution in Rbx_{x}Fe2−y_{2-y}Se2_{2} crystals on annealing

The superconducting and magnetic properties of phase-separated A$_x$Fe$_{2-y}$Se$_2$ compounds are known to depend on post-growth heat treatments and cooling profiles. This paper focusses on the evolution of microstructure on annealing, and how this influences the superconducting properties of Rb$_x$Fe$_2-y$Se$_2$ crystals. We find that the minority phase in the as-grown crystal has increased unit cell anisotropy (c/a ratio), reduced Rb content and increased Fe content compared to the matrix. The microstructure is rather complex, with two-phase mesoscopic plate-shaped features aligned along {113} habit planes. The minority phase are strongly facetted on the {113} planes, which we have shown to be driven by minimising the volume strain energy introduced as a result of the phase transformation. Annealing at 488K results in coarsening of the mesoscopic plate-shaped features and the formation of a third distinct phase. The subtle differences in structure and chemistry of the minority phase(s) in the crystals are thought to be responsible for changes in the superconducting transition temperature. In addition, scanning photoemission microscopy has clearly shown that the electronic structure of the minority phase has a higher occupied density of states of the low binding energy Fe3d orbitals, characteristic of crystals that exhibit superconductivity. This demonstrates a clear correlation between the Fe-vacancy-free phase with high c/a ratio and the electronic structure characteristics of the superconducting phase.Comment: 6 figures v2 is exactly the same as v1. The typesetting errors in the abstract have been correcte

RadICAL stack: A localisation method for dynamic gamma/neutron fields

A variation of the RadICAL (Radiation Imaging Cylinder Activity Locator) system capable of operating in a dynamic environment, such as that created by active interrogation techniques, has been developed. RadICAL is a novel method for locating a radiological source using a rotating detector element. The detector geometry is that of a thin sheet and is rotated to present a constantly changing surface area to the source; it therefore generates a characteristic temporal response which can be used to determine the source direction. The time required to determine the direction of a source make it unsuitable for dynamic environments and so an alternative method is presented that uses a stack of identical scintillator slabs positioned at fixed horizontal angles around a central axis. By comparing count rates from each slab to a standard response curve, using a specially developed algorithm, the direction of a source can be determined without the need to rotate the detector. EJ-299-33 plastic scintillator was used to allow detection of separate neutron and gamma events in a mixed field through pulse shape discrimination. A four element detector was built and shown to achieve a positional accuracy of approximately 4.4 degrees when exposed to a 1.44MBq 137 Cs source at distances of up to 2m. The same detector was used to discriminate separate neutron and gamma events in a mixed field, which allows for the possibility of locating a neutron source within a gamma rich environment

Neutron/gamma pulse shape discrimination in EJ-299-34 at high flux

The effect of scintillator geometry on the quality of neutron/γ pulse shape discrimination (PSD) in EJ-299 plastic scintillator, using a digital charge integration PSD algorithm has been studied. It is shown that the PSD Figure of Merit (FOM) reduces as the geometry of the scintillator moves from a cube-like shape towards a flat panel shape. The PSD performance in this material at high flux irradiation is investigated with performance deteriorating at rates of ∼107 photons/s. The use of EJ-299 for security applications, with a focus on active interrogation environments is explored in conjunction with a system capable of neutron/γ separation and localisation

Initial teacher training to promote health and well-being in schools – a systematic review of effectiveness and barriers and facilitators

Objectives: to conduct a systematic review of effectiveness, and barriers and facilitators, of initial teacher training to promote health and wellbeing in schools. Design: systematic review of the literature Method: a total of 20 bibliographic databases were searched, including Medline, Embase, and the Social Science Citation Index. Studies were included if they reported research into the processes and/or outcomes of initial (pre?service) teacher training to promote health. Results: a total of 20 studies met the inclusion criteria, mainly from the UK and Australia. Twelve studies assessed outcomes, commonly using uncontrolled before and after assessment designs. Sixteen studies evaluated the processes of training. Training was diverse in terms of content, format and health topics. The studies demonstrated short?term increases in trainee teachers’ factual knowledge of health issues, a general increase in teachers’ confidence to teach about health and to identify and help children with specific health issues. There was an increase in teachers’ positive beliefs about their role in promoting children’s health. None of the studies assessed changes in pupil outcomes. The training was generally considered acceptable and adequate by trainee teachers. However, some of trainees felt that they still lacked knowledge and confidence to address sensitive health issues on entering teaching practice. Conclusion: this systematic review identified some evidence for the effectiveness of teacher training for health, and highlighted factors which facilitate and inhibit effective training. Further evaluation, using controlled trial designs with long?term follow?up of teacher and pupil outcomes, may enable teachers to effectively address the health and education needs of school pupils <br/

Performance of a novel wafer scale CMOS active pixel sensor for bio-medical imaging

Recently CMOS Active Pixels Sensors (APSs) have become a valuable alternative to amorphous Silicon and Selenium Flat Panel Imagers (FPIs) in bio-medical imaging applications. CMOS APSs can now be scaled up to the standard 20 cm diameter wafer size by means of a reticle stitching block process. However despite wafer scale CMOS APS being monolithic, sources of non-uniformity of response and regional variations can persist representing a significant challenge for wafer scale sensor response. Non-uniformity of stitched sensors can arise from a number of factors related to the manufacturing process, including variation of amplification, variation between readout components, wafer defects and process variations across the wafer due to manufacturing processes. This paper reports on an investigation into the spatial non-uniformity and regional variations of a wafer scale stitched CMOS APS. For the first time a per-pixel analysis of the electro-optical performance of a wafer CMOS APS is presented, to address inhomogeneity issues arising from the stitching techniques used to manufacture wafer scale sensors. A complete model of the signal generation in the pixel array has been provided and proved capable of accounting for noise and gain variations across the pixel array. This novel analysis leads to readout noise and conversion gain being evaluated at pixel level, stitching block level and in regions of interest, resulting in a coefficient of variation ≤ 1.9%. The uniformity of the image quality performance has been further investigated in a typical X-ray application, i.e. mammography, showing a uniformity in terms of CNR among the highest when compared with mammography detectors commonly used in clinical practise. Finally, in order to compare the detection capability of this novel APS with the currently used technology (i.e. FPIs), theoretical evaluation of the Detection Quantum Efficiency (DQE) at zero-frequency has been performed, resulting in a higher DQE for this detector compared to FPIs. Optical characterization, X-ray contrast measurements and theoretical DQE evaluation suggest that a trade off can be found between the need of a large imaging area and the requirement of a uniform imaging performance, making the DynAMITe large area CMOS APS suitable for a range of bio-medical applications