Polychromatic Beam Research Articles

Reconstruction algorithms for grain mapping by laboratory X-ray diffraction contrast tomography.

X-ray-based non-destructive 3D grain mapping techniques are well established at synchrotron facilities. To facilitate everyday access to grain mapping instruments, laboratory diffraction contrast tomography (LabDCT), using a laboratory-based conical polychromatic X-ray beam, has been developed and commercialized. Yet the currently available LabDCT grain reconstruction methods are either ill-suited for handling a large number of grains or require a commercial licence bound to a specific instrument. To promote the availability of LabDCT, grain reconstruction methods have been developed with multiple reconstruction algorithms based on both forward and back calculations. The different algorithms are presented in detail and their efficient implementation using parallel computing is described. The performance of different reconstruction methods is assessed on synthetic data. The code to implement all the described algorithms has been made publicly accessible with the intention of fostering the development of grain mapping techniques on widely available laboratory instruments.

Grating-free quantitative phase retrieval for x-ray phase-contrast imaging with conventional sources

X-ray phase-contrast imaging can display subtle differences in low-density materials (e.g. soft tissues) more readily than conventional x-ray imaging. However, producing x-ray phase images requires significant spatial coherence of the beam which in turn requires highly specialized sources such as synchrotrons, small and low power microfocus sources, or complex procedures, such as multiple exposures with several carefully stepped precision gratings. To find appropriate approaches for producing x-ray phase-contrast imaging in a clinically meaningful way, we employed a grating-free method that utilized a low-cost, coarse wire mesh and simple processing. This method relaxes the spatial coherence constraint and allows quantitative phase retrieval for not only monochromatic but also polychromatic beams. We also combined the mesh-based system with polycapillary optics to significantly improve the accuracy of quantitative phase retrieval.

Simple beam hardening correction method (2DCalBH) based on 2D linearization

Objective. The polychromatic nature of the x-ray spectrum in computed tomography leads to two types of artifacts in the reconstructed image: cupping in homogeneous areas and dark bands between dense parts, such as bones. This fact, together with the energy dependence of the mass attenuation coefficients of the tissues, results in erroneous values in the reconstructed image. Many post-processing correction schemes previously proposed require either knowledge of the x-ray spectrum or the heuristic selection of some parameters that have been shown to be suboptimal for correcting different slices in heterogeneous studies. In this study, we propose and validate a method to correct the beam hardening artifacts that avoids such restrictions and restores the quantitative character of the image. Approach. Our approach extends the idea of the water-linearization method. It uses a simple calibration phantom to characterize the attenuation for different soft tissue and bone combinations of the x-ray source polychromatic beam. The correction is based on the bone thickness traversed, obtained from a preliminary reconstruction. We evaluate the proposed method with simulations and real data using a phantom composed of PMMA and aluminum 6082 as materials equivalent to water and bone. Main results. Evaluation with simulated data showed a correction of the artifacts and a recovery of monochromatic values similar to that of the post-processing techniques used for comparison, while it outperformed them on real data. Significance. The proposed method corrects beam hardening artifacts and restores monochromatic attenuation values with no need of spectrum knowledge or heuristic parameter tuning, based on the previous acquisition of a very simple calibration phantom.

Optimization of pixel size and propagation distance in X-ray phase-contrast virtual histology

X-ray phase-contrast coupled to high-spatial resolution imaging systems provides a high sensitivity for distinguishing soft tissue structures in small samples, thus being suited for X-ray virtual histology. Propagation-based phase-contrast tomography can deliver a considerable gain in signal-to-noise ratio (SNR) at small pixel sizes when it is combined to a suitable phase retrieval filter. We optimized acquisition parameters, namely the propagation distance and the pixel size, with the aim of providing adequate spatial resolution and sensitivity for virtual histology of breast surgery specimens, scanned with a phase-contrast microtomography (μCT) system employing a commercial sCMOS detector at the SYRMEP beamline of the Italian synchrotron facility Elettra (Trieste, Italy). A pathological breast tissue sample was embedded in paraffin and imaged using a polychromatic synchrotron beam at an average energy of 24 keV. The high numerical optical aperture of the imaging system enabled to adjust the pixel size to 1, 2.5 and 4 μm. The scans were acquired at five sample-to-detector distances: 4.5, 150, 250, 500 and 1000 mm. SNR was measured in an homogeneous region portion of the μCT image for each combination of pixel size and propagation distance. Experimental results were compared to a theoretical model taking into account the actual point spread function of the employed imaging system. The measured gain of SNR associated with the application of the phase-retrieval matched the predictions for large Fresnel numbers (N F > 2). For each pixel size, an optimal range of propagation distances was found. Optimal μCT reconstructions were then compared with their respective histopatological images, showing an excellent visibility of relevant structures. The optimization performed in this study will allow to select the most appropriate geometrical configurations for future acquisitions of virtual histology images of different specimens via phase-contrast microtomography.

Prototyping experience with Ge micro-strip sensors for EDXAS experiments

Energy dispersive X-ray spectrometers provide a method for the fast data acquisition of X-ray absorption spectra. The intensity of the spectral components of a polychromatic beam is detected simultaneously after being dispersed in space, and being transmitted by an experimental sample. The cryogenic detector system called XH deploys a germanium micro-strip sensor to perform such measurements. Here, the geometrical position of each strip is correlated to a specific spectral component of the dispersed beam.The prototyping experience reported in this work is related to the upgrade of the cryostat unit of the XH detector system. This upgrade was carried out in collaboration with beam-line ID24 at the European Synchrotron Radiation Facility (ESRF).Two 90 mm wafers of p-type high purity germanium were manufactured. Wafers contained test structures and micro-strip sensors made of 1024 strips with 50μm pitch. Electrical and in-beam tests were performed to verify the performance of this technology. The measurements included capacitance–voltage characteristics; stress-tests like temperature cycling testing and burn-in tests; leakage current as a function of sensor temperature; charge-collection as a function of time; a test-run of a pump and probe experiment.Results are reported below showing that this sensor technology survived the extensive thermal and electrical stress-tests, and that it was able to measure the physical variations created by a laser shock on a sample of Fe–10%wtNi.

Polychromatic electric field knots

The polarization of a monochromatic optical beam lies in a plane, and in general, is described by an ellipse, known as the polarization ellipse. The polarization ellipse in the tight focusing (non-paraxial) regime forms non-trivial three-dimensional topologies, such as M\"obius and ribbon strips, as well as knots. The latter is formed when the dynamics of specific polarization states, e.g., circular polarization states, are studied upon propagation. However, there is an alternative method to generate optical knots: the electric field's tip can be made to evolve along a knot trajectory in time locally. We propose an intuitive technique to generate and engineer the path traced by the electric field vector of polychromatic beams to form different knots. In particular, we show examples of how tightly focused beams with at least three frequency components and different spatial modes can cause the tip of the electric field vector to follow, locally, a knotted trajectory. Furthermore, we characterize the generated knots and explore different knot densities upon free-space propagation in the focal volume. Our study may provide insight for designing current densities when structured polychromatic electromagnetic fields interact with materials.

Prompt-gamma activation analysis and neutron imaging of layered metal structures

We present here Monte Carlo computer simulations to broaden the scope of the prompt-gamma activation analysis (PGAA) from the elemental composition measurement of ideal, point-like, homogeneous samples towards non-homogeneous or heterogeneous but still regularly-shaped samples. The successful correction for the negative matrix effect related to the neutron self-shielding and gamma self-absorption is a key step in the quantitative analysis of layered metal structures. This geometry is relevant to practical cases where corrosion layer, patina, paint, coating, gilding, or segregation are present on the surface of a core metal. Linear neutron attenuation coefficients are measured in a polychromatic neutron beam by radiography, while the gamma attenuation calculations by MCNP6 simulations were validated by gamma transmission measurements. Finally, the method was successfully applied to correct for the PGAA matrix effect in double- and triple-layer metal stacks and a real structured numismatic sample.

A large beam high efficiency radio frequency neutron spin flipper.

A design for a radio frequency (RF) neutron spin flipper obtained from magneto-static and neutron spin transport simulations is presented. The RF flipper constructed from this design provides a flipping probability of 0.999 or better for a beam size 6cm wide and 15cm high and a wavelength band between 0.4 and 0.6nm. Three permanent magnet guide field sections with air gaps provide a linear field gradient along the beam propagation direction over a large cross-sectional area. An RF oscillator based on coupling the resonant coil of a Hartley oscillator to the excitation coil was developed, which provides a higher current and, thereby, a larger RF amplitude, as compared to a conventional RF power amplifier. Two opaque He3 neutron spin filters were employed to measure the flipping probability of the flipper with very high precision. A spatially uniform flipping probability of 0.9995(2) or higher was measured over the large cross-sectional area neutron guide. This RF neutron spin flipper will be employed in a polychromatic beam reflectometer at the National Institute of Standards and Technology Center for Neutron Research. This design can be applied to other polarized neutron instruments or applications requiring a very high continuous flipping probability of the neutron spin for a large cross-sectional area beam.

Investigation and comparison of neutron image quality using monochromatic and polychromatic beams of Tehran Research Reactor by Monte Carlo simulation

Neutron radiography (NR) is a non-destructive testing (NDT) method which uses a neutron beam for materials investigation in the research and industrial application. In this work, the quality of the neutron radiography system based on thermal monochromatic and polychromatic beam tubes of the Tehran Research Reactor (TRR) has been studied and compared using Monte Carlo simulation. MCNPX is a Monte Carlo radiation transport code with a built-in radiography feature that has been used The parameters of neutron images have been evaluated by several standard test samples qualitatively and quantitatively. The image analysis of standard samples shows that the thermal neutron content of the monochromatic beam is about 29.8% more than a polychromatic beam. Moreover, the simulation results have been validated by measured quality parameters of the polychromatic neutron beam at the E channel of TRR which are in good agreement. The results show that the monochromatic neutron beam with a wavelength of 2.9 Å can have a lower neutron intensity by 36% on average for iron, brass, copper, and polyethylene. However, there was no significant difference between polychromatic beam and neutron beam at a wavelength of 1.75 Å for these materials. Furthermore, a polychromatic beam provides a better performance to show different percentages of uranium enrichment of fuel rods in the images than the monochromatic beam. So that, the maximum difference is about 90.09% between transmitted neutron intensities of rods with enrichment of 12 and 20% by a polychromatic beam.

A theoretical study to focus a polychromatic synchrotron X-ray beam for microbeam radiation therapy

Microbeam radiotherapy (MRT) appears to target tumour cells while sparing normal healthy cells. However, it is in its preclinical stage at present. Clinical trials of MRT can be done in the Imaging and Medical Beamline (IMBL) of the Australian Synchrotron provided that the dose rate deliverable by the X-ray beam in Hutch 3B of the IMBL, where the required patient positioning equipment is located, is sufficiently high for MRT. At present the deliverable dose rate there is only 75Gys−1. However, a deliverable dose rate of between and including 375Gys−1 and 750Gys−1 is required for doing MRT clinical trials in Hutch 3B due to the irradiation profile inherent to MRT. The height of the focused beam in Hutch 3B must also be between and including 1.0 mm and 10 mm. A kinoform compound refractive lens (kinoform CRL) made of [3.5×1018Patomscm−3]-doped Si is proposed to be used in the Scenario where the X-ray beam, produced by a superconducting multipole wiggler source (SCMPW) operating at a maximum magnetic field of 4.000 T, is filtered by 0.6 mm of diamond before reaching the kinoform CRL. This is the Diamond-Optic Scenario. In this Scenario, the dose rate deliverable in Hutch 3B using the optic ranges from 380Gys−1 to 1300Gys−1 with beam heights ranging from 3.55 mm down to 1.0 mm, respectively. Also, the weighted average photon energies of the beam in Hutch 3B in this Scenario are almost entirely suitable for MRT in the IMBL. The X-ray beam in this Scenario can heat the P-doped Si optic to a few hundred degrees Kelvin above the hypothesised melting point of the P-doped Si due to its high power density. Therefore, the optic must be cooled lest it melt. The Diamond-Optic Scenario is not used routinely in the IMBL due to its excessive power. A variation of the Diamond-Optic Scenario, namely that the kinoform CRL be placed before sensitive components instead of after them is proposed for the routine use of this Scenario.

Restoring size of detonation nanodiamonds from small-angle x-ray scattering of polychromatic synchrotron radiation beam

Over the past two decades, LIH SB RAS and BINP SB RAS have jointly conducted experiments on time-resolved (TR) small-angle x-ray scattering (SAXS) with detonating high explosives. The purpose of these experiments is to restore the dynamics of carbon species condensation to diamond nanoparticles by analyzing series of SAXS patterns behind the detonation front measured in real time with fast detectors. This knowledge is crucial for the development of reliable detonation models. In this paper, we compare SAXS patterns of identical nanodiamond samples measured at the TR-SAXS extreme state of matter end-station (BINP SB RAS) in the static mode under realistic conditions simulating fast real-time measurements with polychromatic SR beam and traditional SAXS BioMUR beamline at the Kurchatov Synchrotron Radiation Source (NRC “Kurchatov Institute”) with monochromatic synchrotron radiation (SR) beam. These experiments confirm that the size of scattering inhomogeneities determined in dynamic experiments with single bunch exposure with polychromatic SR beam is correct.

Energy-dispersive X-ray micro Laue diffraction on a bent gold nanowire.

This article reports on energy-dispersive micro Laue (µLaue) diffraction of an individual gold nanowire that was mechanically deformed in three-point bending geometry using an atomic force microscope. The nanowire deformation was investigated by scanning the focused polychromatic X-ray beam along the nanowire and recording µLaue diffraction patterns using an energy-sensitive pnCCD detector that permits measurement of the angular positions of the Laue spots and the energies of the diffracted X-rays simultaneously. The plastic deformation of the nanowire was shown by a bending of up to 3.0 ± 0.1°, a torsion of up to 0.3 ± 0.1° and a maximum deformation depth of 80 ± 5 nm close to the position where the mechanical load was applied. In addition, extended Laue spots in the vicinity of one of the clamping points indicated the storage of geometrically necessary dislocations with a density of 7.5 × 1013 m-2. While µLaue diffraction with a non-energy-sensitive detector only gives access to the deviatoric strain, the energy sensitivity of the employed pnCCD offers absolute strain measurements with a resolution of 1%. Here, the residual strain after complete unloading of the nanowire amounted to maximum tensile and compressive strains of the order of +1.2 and -3%, which is comparable to the actual resolution limit. The combination of white-beam µLaue diffraction using an energy-sensitive pixel detector with nano-mechanical testing opens up new possibilities for the study of mechanical behavior at the nanoscale.

Optimizing laboratory X-ray diffraction contrast tomography for grain structure characterization of pure iron.

Laboratory diffraction contrast tomography (LabDCT) is a recently developed technique for 3D nondestructive grain mapping using a conical polychromatic beam from a laboratory-based X-ray source. The effects of experimental parameters, including accelerating voltage, exposure time and number of projections used for reconstruction, on the characterization of the 3D grain structure in an iron sample are quantified. The experiments were conducted using a commercial X-ray tomography system, ZEISS Xradia 520 Versa, equipped with a LabDCT module; and the data analysis was performed using the software package GrainMapper3D, which produces a 3D reconstruction from binarized 2D diffraction patterns. It is found that the exposure time directly affects the background noise level and thus the ability to distinguish weak spots of small grains from the background. With the assistance of forward simulations, it is found that spots from the first three strongest {hkl} families of a large grain can be seen with as few as 30-40 projections, which is sufficient for indexing the crystallographic orientation and resolving the grain shape with a reasonably high accuracy. It is also shown that the electron current is a more important factor than the accelerating voltage to be considered for optimizing the photon numbers with energies in the range of 20-60 keV. This energy range is the most important one for diffraction of common metals, e.g. iron and aluminium. Several suggestions for optimizing LabDCT experiments and 3D volume reconstruction are finally provided.

Effective modeling of high-energy laboratory-based x-ray phase contrast imaging utilizing absorption masks or gratings

Model refinements for the edge illumination x-ray phase contrast imaging method have been developed to improve simulation accuracy for high energy, polychromatic beams. High-energy x rays are desirable in imaging due to their penetrative power and, for biological samples, their lower dose deposition rate. Accurate models of such scenarios are required for designing appropriate imaging systems and to predict signal strength in complex settings such as clinical imaging or industrial quality assurance. When using optical components appropriate for high-energy x rays in a non-synchrotron setting, system performance was observed to deviate from that predicted by existing models. In this work, experimental data utilizing increasing thicknesses of a known filter material are used to illustrate the limitations of existing models and as validation for the new modeling features. Angular filtration of the cone beam was observed to be the most significant effect; however, specific features of the source and detector are also shown to affect system performance. We conclude by showing that a significantly improved agreement between experimental and simulated data is obtained with the refined model compared to previously existing ones.

Label-free X-ray estimation of brain amyloid burden

Amyloid plaque deposits in the brain are indicative of Alzheimer’s and other diseases. Measurements of brain amyloid burden in small animals require laborious post-mortem histological analysis or resource-intensive, contrast-enhanced imaging techniques. We describe a label-free method based on spectral small-angle X-ray scattering with a polychromatic beam for in vivo estimation of brain amyloid burden. Our findings comparing 5XFAD versus wild-type mice correlate well with histology, showing promise for a fast and practical in vivo technique.

ProMeSCT: A Proximal Metric Algorithm for Spectral CT

The acquisition of a set of spectral photon-counting computed tomography (spectral PC-CT) measurements aims at uncovering both the spatial and energetic characteristics of the imaged body, which widens the potential of tomography compared to classical computed tomography (CT). In the preclinical context, the use of polychromatic beams induces spectral mixing and, as a consequence, the reconstruction procedure requires specific algorithmic tools more complex than the standard ones used in CT. In this article, we propose a one-step inversion method to simultaneously separate and reconstruct the physical materials of an object observed in the context of spectral PC-CT. To do so, we carefully consider the underlying polychromatic model of the X-ray beam and combine it with a priori on the materials of the object to reconstruct. The simultaneous separation and reconstruction of materials is done by minimizing the resulting nonconvex ill-posed inverse problem. The dimensionality of the data and object materials worsens the computational complexity of the problem. We propose an efficient optimization algorithm based on a proximal forward-backward algorithm that is accelerated by a metric, which is specifically designed for spectral PC-CT. The efficiency of our method called ProMeSCT is demonstrated on results obtained on 3-D synthetic data with a simple regularization that encompasses the positivity of the quantities of interest.

Blind Separation Model of Multi-voltage Projections for the Hardening Artifact Correction in Computed Tomography

The hardening artifacts caused by polychromatic X-ray beams in computed tomography (CT) reconstructed images increase the difficulty of quantitative analysis. A method based on the blind separation of multi-voltage projections is proposed to correct the hardening artifacts. Compared to most correction methods, this method does not require prior knowledge of the X-ray spectrum or materials or segmentation, reprojection or multiple reconstructions. The core concept of the method is to decompose the polychromatic X-ray projections to obtain projections with a narrow energy interval. The polychromatic X-ray projections can be seen as the sum of projections with narrow energy intervals. The multi-voltage projection matrix is decomposed into three matrixes. Since the X-ray beam is exponentially attenuated, two of the three matrixes are in the exponential term. The decomposition is an expanded form of nonnegative matrix factorization. Because the X-ray scatter is a low-frequency signal, the objective function of the decomposition model is the sum of the variance of the residual projection at different angles. The iterative solution formula can be derived under Karush-Kuhn-Tucker (KKT) optimality conditions. In experiments with real samples, the hardening artifacts are reduced compared to those in directly reconstructed images, demonstrating the availability of the proposed method.

Geometric determination of direction of dislocations using synchrotron X-ray transmission topography.

When performing transmission polychromatic beam topography, the extensions to the line segments of the diffraction images of a straight dislocation are shown to intersect at a single point on the X-ray film. The location of this point, together with the diffraction pattern recorded on the film by synchrotron radiation, gives the crystallographic direction [hkl] of the dislocation unambiguously. The results of two synchrotron topography experiments are presented. Very long dislocations found in the center of a large 450 mm-diameter Czochralski silicon crystal align with the growth direction [001]. In the other silicon sample, the dislocations are of mixed type and along the [011] direction.

Design of Neutron Microscopes Equipped with Wolter Mirror Condenser and Objective Optics for High-Fidelity Imaging and Beam Transport.

We present and compare the designs of three types of neutron microscopes for high-resolution neutron imaging. Like optical microscopes, and unlike standard neutron imaging instruments, these microscopes have both condenser and image-forming objective optics. The optics are glancing-incidence axisymmetric mirrors and therefore suitable for polychromatic neutron beams. The mirrors are designed to provide a magnification of 10 to achieve a spatial resolution of better than 10 m. The resolution of the microscopes is determined by the mirrors rather than by the L/Dratio as in conventional pinhole imaging, leading to possible dramatic improvements in the signal rate. We predict the increase in the signal rate by at least two orders of magnitude for very high-resolution imaging, which is always flux limited. Furthermore, in contrast to pinhole imaging, in the microscope, the samples are placed far from the detector to allow for a bulky sample environment without sacrificing spatial resolution.

Electric Field Imaging Using Polarized Neutrons.

We experimentally demonstrate that electrically neutral particles, neutrons, can be used to directly visualize the electrostatic field inside a target volume that can be physically isolated or occupied. Electric field images are obtained using a spin-polarized neutron beam with a recently developed polarimetry method for polychromatic beams that permits detection of a small angular change in spin orientation. This Letter may enable a new diagnostic technique sensitive to the structure of electric potential, electric polarization, charge distribution, and dielectric constant by imaging spatially dependent electric fields in objects that cannot be accessed by other probes.