Two-dimensional radiographic imaging analysis

Two-dimensional imaging properties of locally isotropic and isoplanatic point spread functions are described in a general radiographic context. In particular, the radiographic response to a two-dimensional wedge-shaped object is shown to possess several properties which enable the identification of the wedge corner on the image, thereby alleviating the need for image enhancement techniques. Extensions to the location of flat-edged boundaries are also discussed. The potential dimensioning application of this analysis lies in the precise and objective location of points of interest on radiographs of more complex two-dimensional objects.     

Arbitrary order edge elements for electromagnetic scattering simulations using hybrid meshes and a PML

The use of arbitrary order edge elements for the simulation of two‐dimensional electromagnetic scattering problems on hybrid meshes of triangles and quadrilaterals is described. Single‐frequency incident waves, generated by a source in the far field, are considered and the solution is determined in the frequency domain. For numerical simulation, the solution domain is truncated at a finite distance from the perfectly conducting scatterer and the non‐reflecting boundary condition at the truncated boundary is imposed by the use of a perfectly matched layer (PML). Several examples are included to demonstrate the performance of the proposed procedure. Copyright © 2002 John Wiley & Sons, Ltd.     

Elastodynamic scattering by fluid-filled nonplanar cracks

A recent modification of the null field approach is adapted to the study of scattering of elastic waves by fluid-filled nonplanar cracks. The fluid-filled crack is modeled as a surface over which friction-free boundary conditions apply. A closed surface is formed by adding a fictitious surface, on which latter surface boundary conditions of welded contact are applied. The surface fields on the closed surface are expanded in vector spherical harmonics in a manner which takes the edge conditions into account. Some numerical results on farfield quantities, such as scattering cross sections and backscattering amplitudes (both in the frequency and time domains), are presented for rotationally symmetric cracks.     

Effects of sinusoidal crack-face perturbations on scattering of elastic waves

The scattering of a plane longitudinal wave from a two-dimensional crack, with a sinusoidal surface perturbation whose amplitude and wavelength are much smaller than the length of the crack, is investigated. The amplitude of the cylindrical body waves in the far field are calculated from a Kirchhoff approximation that utilizes the solution to the reflection from the sinusoidal surface profile of a semiinfinite solid. The results are compared to those for a flat crack, and conditions for significant differences of the amplitude as a function of the angle of observation are discussed. Characteristic changes in the scattered field produced by profiles with different amplitudes and periods are explained.     

Analysis of ultrasonic wave scattering for characterization of defects in solids: I. Spherical inclusions and reciprocity

This paper, as part of a series on elastic wave scattering, presents results of measurements and calculations on scattering of ultrasonic waves by a solid spherical inclusion (tungsten carbide) embedded in titanium alloy by the diffusion bonding process. Both direct scattering and mode-converted scattering angular distributions are reported for shear and compressional incident waves. The consequences upon the signals when transmitter and receiver were interchanged are explored in a reciprocity rule.     

Neutron scattering from fractals

We describe briefly the concept of fractal dimension as applied to both mathematical and statistical fractals. We then discuss the scattering of radiation from fractals and describe the results of small angle neutron scattering studies of the aggregation of small particles in fractal clusters.     

水下复合边缘扁平结构散射声场特性研究

通过采用边界积分方法并结合水声无源材料自由场测试等工程实际,对几种典型复合边缘结构形式的水下扁平结构的散射声场分布规律开展了进一步地研究与分析。在此基础上,尝试在不改变主体扁平矩板结构声学性能的前提下,提出通过附加边缘结构设计即物理解决途径上对自由场测试中存在的边缘效应进行抑制或去除,为提高测试精度（尤其是低频段测试精度）创造条件,也为边缘效应抑制机理和技术的研究与开发奠定基础。数值仿真与理论分析结果表明：1.复合附加边缘矩板结构的散射声场频域分布特性与多数附加边缘类型扁平矩板结构散射声场分布的总体规律基本一致,在趋势上是相似的,而边缘重数的增加加剧了前向散射与镜向散射声场间的差异性和不同程度地增大了声场的波动程度;2.附加边缘声学结构设计（如共振吸声结构或软表面声学处理、增加有效边缘重数等）对抑制结构边缘效应负面影响是有效的和可行的,且具有较好的可控性。     

Ill-posed and well-posed problems in inverse elastodynamic scattering for nondestructive evaluation

The practical significance of ill-posedness in a data reduction problem is reviewed. Inverse elastodynamic scattering is shown to be ill-posed in general, although suitably restricted problems may be well-posed. These results underscore the need to analyze carefully the errors of data reduction problems in NDE, and to focus attention on final results of an NDE exercise, rather than on intermediate steps.     

PiBrED—A novel bragg edge detector for neutrons

We present a single pixel prototype of a pixelated Bragg edge detector for neutron transmission measurements. The optical signal coming from a scintillator is collected by an optical fiber and is detected by an avalanche photodiode. A fast, Field Programmable Gate Array based, readout allows to obtain transmission spectra within reasonable acquisition times. The performances of the instrument have been tested by measuring the transmission spectra of iron powder samples with two different scintillators. The instrument accuracy in detecting the Bragg edges positions is comparable with the state of the art for similar devices.     

Sensitivity of peak positions to flight-path parameters in a deep-inelastic scattering neutron TOF spectrometer

The effects of small changes in flight-path parameters (primary and secondary flight paths, detector angles), and of displacement of the sample along the beam axis away from its ideal position, are examined for an inelastic time-of-flight (TOF) neutron spectrometer, emphasising the deep-inelastic regime. The aim was to develop a rational basis for deciding what measured shifts in the positions of spectral peaks could be regarded as reliable in the light of the uncertainties in the calibrated flight-path parameters. Uncertainty in the length of the primary or secondary flight path has the least effect on the positions of the peaks of H, D and He, which are dominated by the accuracy of the calibration of the detector angles. This aspect of the calibration of a TOF spectrometer therefore demands close attention to achieve reliable outcomes where the position of the peaks is of significant scientific interest and is discussed in detail. The corresponding sensitivities of the position of peak of the Compton profile, J(y), to flight-path parameters and sample position are also examined, focusing on the comparability across experiments of results for H, D and He. We show that positioning the sample to within a few mm of the ideal position is required to ensure good comparability between experiments if data from detectors at high forward angles are to be reliably interpreted.     

Computational model of edge effects in graphene nanoribbon transistors

We present a semi-analytical model incorporating the effects of edge bond relaxation, the third nearest neighbor interactions, and edge scattering in graphene nanoribbon field-effect transistors (GNRFETs) with armchair-edge GNR (AGNR) channels. Unlike carbon nanotubes (CNTs) which do not have edges, the existence of edges in the AGNRs has a significant effect on the quantum capacitance and ballistic I-V characteristics of GNRFETs. For an AGNR with an index of m=3p, the band gap decreases and the ON current increases whereas for an AGNR with an index of m=3p+1, the quantum capacitance increases and the ON current decreases. The effect of edge scattering, which reduces the ON current, is also included in the model. This article is published with open access at Springerlink.com     

First study of macroscopic neutron dark field imaging using scattering grids

Instead of using the phase grating concept for dark field imaging, macroscopic scattering grids were employed at the ANTARES neutron imaging facility. Two Cadmium grids with a 1 mm gap and 1.2 mm bar were adjusted in a distance of only a few cm in order to block the direct beam. Thus, by placing the samples between these two grids only neutrons that were scattered at the samples were transmitted. A linear motion of the coupled grids allowed scanning across the samples and obtaining complete scattering projections, which delivered surprisingly sharp images. The geometric relation between grids permits determination of the transmitted scattering angles.     

Numerical modelling of the scattering of elastic waves in plates

This paper describes the application of finite difference methods to the calculation of the scattering of elastic waves. The emphasis is on cracklike defects in plates, and it is shown that a common numerical technique can span a range of wavelengths from Lamb waves to ultrasonic waves with many reflections from the surfaces of the plate. Quantitative results are given for the scattering of Lamb waves and ultrasonic shear waves from surface-breaking cracks.     

Small angle neutron scattering and photoluminescence property of wet chemistry process synthesised ZnO nanoparticles

We report the synthesis of zinc oxide (ZnO) nanoparticles from aqueous solution at 25°C and subsequent heating of the solution at 115°C by the suitable selection of the solution chemistry and the control of the alkaline conditions. The structure of the synthesised ZnO particles was studied by X-ray diffraction (XRD), confirming the formation of Wurtzite structure. The optical property of synthesised ZnO nanoparticles is investigated through room temperature photoluminescence (PL) measurement. The PL of ZnO nanoparticles shows a strong UV emission band at approximately 385 nm, a blue–green band at approximately 473 nm and a very weak green band at approximately 554 nm, although polydispersity of the sample shows no presence on the PL spectrum. Small angle neutron scattering is used to determine the size and the size distribution of ZnO nanoparticles. The SANS data analysis and model fitting predict the size as about 18–20 nm, which is closely matched with XRD and transmission electron microscopy results with Gaussian distribution.     

Small-angle neutron scattering studies of hot-wire CVD a-Si:H

Several a-Si:H and a-Si:D films prepared by hot-wire chemical vapor deposition have been examined by small-angle neutron scattering (SANS) to search for H non-uniformity in this material. The SANS measurements were supplemented by small-angle X-ray scattering measurements. The differences in H/D detection sensitivity of these two techniques allow distinction of the scattering mechanisms. Two- or three-phase models are used to interpret the results quantitatively. Significant H non-uniformity, as well as a small fraction of microvoids, was found in the best-quality material. Samples grown with higher deposition rates or lower substrate temperatures have much larger void fractions. The size scale of the heterogeneity spans a range from 2 nm to more than 50 nm, with the largest features assigned to surface roughness.     

Local structural instability of high-Tc oxide superconductors studied by inelastic neutron scattering

The scattering function,S(Q, E), of superconducting La_1.85Sr_0.15CuO₄ and YBa₂Cu₃O₇ was measured over a wide range ofQ-E space by inelastic neutron scattering. Fine structures in their phonon density of states as well as theQ-dependences ofS(Q, E) were observed. TheQ-dependence ofS(Q, E) is very susceptive atT _c , with an anomalous behavior similar to a critical phenomenon. We have found that the anomalous behavior is related to the local structural instability, i.e., the 110 buckling motion of the Cu-O plane and the shortening of the distance between the apical oxygen and the copper atom in the plane.This work was done under the UK-Japan collaboration on neutron scattering at the Rutherford Appleton Laboratory in the United Kingdom.     

A cryostat for investigating inelastic scattering of neutrons on liquid He4 between 4.2 and 0.5 K

A cryostat is described which is designed for the research of inelastic scattering of neutrons on a large volume (3.5 l) of He⁴ between 4.2 and 0.5 K. Temperatures lower than 1.3 K are produced by means of pumping out vapour above liquid He³ (～ 330 cm³) with an adsorption pump (1 kg of coal CKT-2) located in the device itself. A minimum temperature is held during 140 h. The essential time for the cryostat start is about 10 h. The cryostat was used for the measurement of temperature dependence of the Bose-condensate density in He⁴.     

Determination of the scattering lengths of gallium isotopes by neutron interferometry with PNO-apparatus in JRR-3M

For carrying out experiments in the field of the so-called precise neutron optics (PNO), we have implemented special multi-purpose apparatus called the “PNO-apparatus” at JRR-3M. Making use of an Si triple-Laue (LLL) neutron interferometer with the PNO-apparatus, we successfully determined the coherent neutron scattering lengths of gallium isotopes, ⁶⁹Ga and ⁷¹Ga. The results are 8.053±0.013 fm for ⁶⁹Ga and 6.170±0.011 fm for ⁷¹Ga, respectively.     

Implementation of traction constraints in Bragg‐edge neutron transmission strain tomography

Several recent methods for tomographic reconstruction of stress and strain fields from Bragg‐edge neutron strain images have been proposed in the literature. This paper presents an extension of a previously demonstrated approach based on Gaussian process regression that enforces equilibrium in the method. This extension incorporates knowledge of boundary conditions, primarily boundary tractions, into the reconstruction process. This is shown to increase the rate of convergence and is more tolerant of systematic errors that may be present in experimental measurements. An exact expression for a central calculation in this method is also provided which avoids the need for the approximation scheme that was previously used. Convergence of this method for simulated data is compared to existing approaches and a reconstruction from experimental data is provided. Validation of the results to conventional constant wavelength strain measurements and comparison to prior methods show a significant improvement.     

Small angle neutron scattering from helium bubbles in metals

Small angle neutron scattering (SANS) has been widely used in investigating defects in metals, and in particular, to characterize the helium bubble population in implanted materials. The main advantage in using SANS is the non-destructive feature of the tests and the quantitative results obtained by averaging over a large sample volume. SANS is a powerful technique, very sensitive to microstructural changes and its use was of fundamental importance to show the bimodal distribution of the bubble population: in the vicinity of grain boundaries and free surface and inside grains, respectively. Here the most important applications of the SANS technique to the study of the helium bubbles in implanted materials are reviewed. Most of the work has been done on nickel samples, but also a ternary alloy Fe-Ni 15%wt-Cr 15%wt and a steel (MANET) with a more complicated structure have been successfully investigated. Different annealing treatments, isothermal and isochronal, were investigated in order to determine the active mechanisms of the bubble coarsening and their activation energies. From the SANS data the bubble size distributions have been determined, from which parameters such as mean radius and density of the bubble population have been calculated. The gas pressure inside the bubbles was also determined by the contrast variation technique in SANS and by a computational procedure, and an excellent agreement was found between the results. These results show a marked overpressure inside the bubbles as compared to thermodynamical equilibrium values of about 3 GPa. A comparison with results obtained by other techniques confirms the validity of SANS, which has to be considered as a complementary technique for its indirect image of the sample.