Neutron Transmission Imaging Research Articles

Direct inversion of the Longitudinal ray transform for 2D residual elastic strain fields

We examine the problem of Bragg-edge elastic strain tomography from energy resolved neutron transmission imaging. A new approach is developed for two-dimensional plane-stress and plane-strain systems whereby elastic strain can be reconstructed from its Longitudinal ray transform (LRT) as two parts of a Helmholtz decomposition based on the concept of an Airy stress potential. The solenoidal component of this decomposition is reconstructed using an inversion formula based on a tensor filtered back projection (FBP) algorithm whereas the potential part can be recovered using either Hooke’s law or a finite element model of the elastic system. The technique is demonstrated for two-dimensional plane-stress systems in both simulation, and on real experimental data. We also demonstrate that application of the standard scalar FBP algorithm to the LRT in these systems recovers the trace of the solenoidal component of strain and we provide physical meaning for this quantity in the case of 2D plane-stress and plane-strain systems.

Neutron transmission imaging system with a superconducting kinetic inductance detector

Abstract We optimized the design and operating conditions of our superconducting neutron detector to improve spatial resolution. We obtained the best spatial resolution of 10 μm when a Gd Siemens star pattern was mounted in close contact with the detector. Although there is a trade-off between a spatial resolution and an easiness of replacing samples, we built our superconducting neutron imaging system for measuring in both the room-temperature samples with a proper collimation ratio L/D for achieving a reasonable spatial resolution and a cryogenic temperature with the best spatial resolution for certain purposes. In this study, we obtained neutron transmission images of various samples when they were cooled down with the superconducting neutron detector. We compared the effect of a different sample-detector distance on a spatial resolution when the samples were placed either at cryogenic temperature or at room temperature. We also confirmed that our CB-KID sensor was able to observe the neutron transmission coefficient over wider energies of pulsed neutrons. We found the appearance of clear Bragg dips by the measurements of natural FeS2 single crystals and succeeded in mapping the distribution of differently-oriented crystals by choosing several Bragg dips of the FeS2 crystals to compose the transmission images.

Measurement of needle and armature dynamics in a gasoline direct injector by high-speed neutron imaging

In modern spark ignition engines, precise delivery of fuel with gasoline direct injection has become increasingly important in the effort to meet ever stricter efficiency and emissions regulations. Use of multiple small close-coupled injections has become more common in attempt to precisely control fuel distribution in the cylinder, but these strategies are hindered by nonlinear injection effects due to operation in the ballistic region of the solenoid-operated valve and due to armature bounce at the end of injection. Understanding the internal dynamics of the injector is crucial to minimizing and controlling non-linearity and shot-to-shot variation, but the experimental techniques available to date are capable only of tracking the position of either the top of the needle (via laser sensors or by monitoring current and voltage in the solenoid coil) or the bottom of the needle (via transparent nozzles or high-speed x-ray imaging). A complete picture of the axial and radial motion of valve needle has until now remained elusive. In this work, we present high-speed ensemble neutron transmission imaging of an entire 8-hole gasoline direct injector operating at 200 bar, allowing for both visualization and quantification of the actuation dynamics including lift and wobble of the valve ball, oscillation and bending of the valve needle, lift, rocking, and bounce of the armature, compression of the springs, and radial swelling of the solenoid during energization. Because neutrons offer high penetration through the metal injector while also being sensitive to the 1H nuclei in fuel molecules, it is also possible to simultaneously see the fluid dynamics of the injection process, including filling of the sac volume, emanation of the spray through the nozzle holes and into the downstream gas, and the formation and evolution of fuel films on the tip of the injector and the walls of the spray container.

Advanced Corrections of wavelength-resolved neutron transmission imaging

Advanced Corrections of wavelength-resolved neutron transmission imaging

A high-temperature and high-pressure cell for in situ visualization of reaction processes by neutron imaging

This study developed a high-temperature and high-pressure (HTHP) cell for in situ neutron imaging of hydrothermal reactions. The cell’s maximum temperature and pressure were 500 °C and 50 MPa, respectively, and its vessel for observing reactions comprised SUS316 stainless steel. Neutron transmission images were obtained to observe the behavior of sub- and supercritical water and the decomposition of two plastics (polypropylene and polyethylene) at HTHP. The images showed that water’s density and phase changed with temperature and pressure, affecting neutron transmission (and thus image brightness). The plastics began to melt and change shape at 150–200 °C, and they decomposed at 500 °C and 20 MPa. This study provides a basis for future research using the HTHP cell to examine various reactions such as the decomposition of biomass samples, the reforming of heavy oil, and the synthesis of nano-materials using sub- and supercritical water.

Orientation mapping of YbSn3 single crystals based on Bragg-dip analysis using a delay-line superconducting sensor

Recent progress in high-power pulsed neutron sources has stimulated the development of the Bragg-dip and Bragg-edge analysis methods using a two-dimensional neutron detector with high temporal resolution to resolve the neutron energy by the time-of-flight method. A delay-line current-biased kinetic inductance detector (CB-KID) is a two-dimensional superconducting sensor with a high temporal resolution and multi-hit capability. Here, it is demonstrated that a delay-line CB-KID with a 10B neutron conversion layer can be applied to high-spatial-resolution neutron transmission imaging and spectroscopy up to 100 eV. Dip structures are observed in the transmission spectra of YbSn3 single crystals, induced by Bragg diffraction and nuclear resonance absorption. The orientation mapping of YbSn3 crystals is successfully performed via the analysis of observed Bragg-dip positions in the transmission spectra.

Superconducting neutron transmission imaging for investigating a sequential change in phase separations of low-melting Wood’s metal

We built a new cryostat system for a current-biased kinetic inductance detector (CB-KID) to be able to place a sample at room temperature for conducting the neutron transmission imaging experiments with pulsed neutrons. A Wood’s metal alloy sample (Bi 50wt.%, Pb 25wt.%, Sn 12.5wt.%, Cd 12.5wt.%) of melting temperature 75.2°C was used to check a practical test by observing fine microstructures under beam power of 812 kW at beamline BL10 of J-PARC (MLF). The Wood’s metal is composed of four phases, of which one is a Cd-rich needle-like phase of an average width of 25 µm and length of 5 mm. Since Cd is a strong neutron absorber, it is suitable for observing the fine mosaic structure contrastingly in neutron transmission imaging. After neutron-transmission imaging with the Wood’s metal in the initial state, we subsequently melted the Wood’s metal sample and solidified it again by slow cooling during the same beam time. We were successful in observing an impressive change in morphology of the phases by neutron-transmission imaging. Room-temperature sample imaging is very convenient for possible users, and is expected to be very useful for applying our CB-KID system to versatile different materials of interests.

Crystalline Orientation of CaF2 window determined by Neutron Transmission Imaging using a Delay Line Current-Biased Kinetic-Inductance Detector

The combination of a high speed two-dimensional neutron detector and an intensive pulsed neutron source provides not only neutron transmission imaging but also information on crystal structures, orientations and constituent elements by analyzing neutron transmission spectra. The delay-line current-biased kinetic-inductance detector (CB-KID) is a two-dimensional superconducting neutron detector with high spatial and temporal resolutions, and multi-hit tolerance. We demonstrated that the delay-line CB-KID with a 10B neutron conversion layer can be applied for neutron transmission measurements up to 100 keV. We observed two-different dip structures in the transmission spectra in a CaF2 single crystal, i.e., one is from the Bragg diffraction and the other is from nuclear resonance absorption. We consider that some duplicated structures in resonance absorption dips are originating from a double-bunch structure of proton pulses for producing pulsed neutrons in the accelerator.

Higher order correction and spectral deconvolution of wavelength-resolved neutron transmission imaging at the CONRAD-2 instrument

This paper uses a Fourier self-deconvolution method for improving the wavelength resolution in transmission experiments at continuous neutron sources utilizing a double-crystal monochromator device to probe as well as correct the generation of higher-order neutron scattering in a monochromatic neutron beam. The cold neutron radiography CONRAD-2 equipment has been utilized to resolve the steel transmission spectra of changing BCC phase and FCC phase fractions. Therefore, both low and high-spectral resolution instruments with equivalent wavelength resolution have been proposed. The primary benefit of Fourier self-deconvolution is its ability to precisely narrow individual bands without modifying their relative position or the total band area. Thus, the resolution of the transmission spectrum has been improved by a factor of 3.16, and the info that the sample material comprises two crystallographic phases has been determined by the wavelength resolution improvement employing the deconvolution approach. Additionally, the slight variation in Bragg edge position for different phase fractions and the locations of the double phase Bragg edges have also been obtained using the ray-tracing simulation tool McStas. The high resolution neutron wavelength selection experiment with the ESS test_beamline (V20) instrument employing the neutron time-of-flight detection demonstrates the precision of the resolving steel Bragg edge.

Neutron resonance absorption imaging of simulated high-level radioactive waste in borosilicate glass

We performed a preliminary study of neutron resonance absorption imaging to investigate the spatial distribution of constituent elements in borosilicate glasses containing simulated high-level radioactive waste, in which elemental inhomogeneities affect the physical and chemical stabilities of the glass. Dips generated by the resonance absorptions of Rh, Pd, Na, Gd, Cs, and Sm were observed in the neutron transmission spectra of the glass samples. The spatial distributions of these elements were obtained from the neutron transmission images at the resonance energies. The distributions of Rh and Pd visualized the sedimentation of these platinum group elements. In contrast, the lanthanides (Gd and Sm) and Cs were uniformly dispersed. These results show that neutron resonance absorption imaging is a promising tool for characterizing borosilicate glasses and investigating the vitrification mechanism of high-level radioactive waste.

Visualization of Gas-Liquid Interfacial Behavior in a Narrow Channel Using High-Speed Neutron Imaging

Gas-liquid two-phase flow is a very complicated flow phenomenon that involves the interaction between gas and liquid phases. Recently, an accurate simulation technique of two-phase flow has been developed. However, the validation of the simulated results is insufficient due to less experimental data. It is especially difficult to measure interfacial behavior with significant spatio-temporal fluctuation. To understand such phenomena, measurement methods with high spatial and temporal resolutions are required. Neutron imaging is a powerful tool for two-phase flow visualization. It can be applied to flow measurement in an opaque channel and has applicability to the heated condition. In this study, two-phase flow in the narrow rectangular channel was visualized by high-speed neutron imaging. The dynamics of the interfacial structure of air-water flow were investigated from the neutron transmission images, and the liquid film behavior was observed qualitatively.

Development of a Bragg-edge Neutron Transmission Imaging Detector Combined with Micro-structured Boron Cathode and Glass Gas Electron Multiplier

Development of a Bragg-edge Neutron Transmission Imaging Detector Combined with Micro-structured Boron Cathode and Glass Gas Electron Multiplier

A new thermography using inelastic scattering analysis of wavelength-resolved neutron transmission imaging

Thermography using energy-dependent neutron transmission imaging can non-invasively and non-destructively visualize a real-space distribution of interior temperatures of a material in a container. Previously, resonance absorption broadening analysis and Bragg-edge shift analysis using energy-resolved neutron transmission have been developed, however some issues remain, e.g., imaging efficiency, substance limitation and temperature sensitivity. For this reason, we propose a new neutron thermography using the temperature dependence of inelastic scattering of cold neutrons. This method has some advantages, for example, the imaging efficiency is high because cold neutrons are measured with moderate wavelength resolution, and light elements can be analysed in principle. We investigated the feasibility of this new neutron thermography at pulsed neutron time-of-flight imaging instruments at ISIS in the United Kingdom and HUNS in Japan. A Rietveld-type transmission spectrum analysis program (RITS) was employed to refine temperature and atomic displacement parameters from the inelastic scattering cross-section analysis. Finally, we demonstrated interior thermography of an α-Fe sample of 10 mm thickness inside a vacuum chamber by using a neutron time-of-flight imaging detector at the compact accelerator-driven pulsed neutron source HUNS.

Higher Order Correction and Spectral Deconvolution of Wavelength Resolved Neutron Transmission Imaging at Conrad-2 Instrument

Higher Order Correction and Spectral Deconvolution of Wavelength Resolved Neutron Transmission Imaging at Conrad-2 Instrument

Reconstructing mass and enrichment of uranium shell based on associated particle imaging

In the field of arms control verification, the attributes of uranium shells often need to be measured and authenticated. In this paper, the feasibility and results of reconstructing the masses and enrichment of uranium shells based on the associated particle imaging are studied, aiming at constructing and testifying the theoretical framework of this method. First, the basic principles of this method are introduced. Second, the physical model of the neutron transmission imaging of uranium shells was constructed, and the feasibility of reconstructing the masses of uranium shells based on the neutron transmission imaging was proved with the uncertainties of the reconstruction results analyzed. Third, the physical model of the neutron induced fission imaging of uranium shells was constructed, and reconstructing the enrichment of uranium shells based on the neutron transmission imaging and neutron induced fission imaging was proved to be feasible, and the influencing factors of the uncertainties of the reconstruction results were analyzed. Meanwhile, a method of reconstructing the enrichment of uranium shells based on a single calibration line and a method of revising neutron attenuation and multiplication effect are proposed; based on the above two methods, the reconstruction uncertainties of the enrichment of uranium shells can reach the level of 3.78%.

Visualising Martensite Phase Fraction in Bulk Ferrite Steel by Superimposed Bragg-edge Profile Analysis of Wavelength-resolved Neutron Transmission Imaging

Visualising Martensite Phase Fraction in Bulk Ferrite Steel by Superimposed Bragg-edge Profile Analysis of Wavelength-resolved Neutron Transmission Imaging

Strain imaging of corroded steel fasteners using neutron transmission imaging

Failures in bolted structural connections have caused in the past the collapse of various steel structures exposed to weather conditions, including wind turbine towers, oil and gas platforms, and bridges. Corrosion and the resulting loss of preloaded tension in the bolted connection were the root cause of such catastrophic failures in many cases. However, it is still not fully understood how strain is re-distributed in corroded bolted connections, nor how different factors influence the redistribution. Measuring non-destructively the strain inside the assembly has been the main obstacle. To this end, in this paper, we employed energy-resolved neutron transmission imaging and we present strain maps in corroded bolted connections for the first time. We examined and compared the strain distribution measurements in corroded and non-corroded assemblies for different bolt-tightening techniques. The specimens under investigation were artificially corroded without introducing any additional mechanical loads. The resulting strain maps illustrate how strain is re-distributed and provide insight into the influence of the bolt-tightening method.

Neutron Imaging toward Epithermal Regime using a Delay Line Current-Biased Kinetic-Inductance Detector

Wide energy range neutron imaging and spectroscopy in a single experiment without changing the setup is available by a combination of a high-power pulsed neutron source and fast operating neutron detector. The delay-line current-biased kinetic-inductance detector (CB-KID) is a two-dimensional superconducting detector with high spatial and temporal resolutions, and multi-hit tolerance. We demonstrate that the delay-line CB-KID with 10B neutron conversion layer is applicable for neutron imaging and spectroscopy up to the neutron energy of 100 eV. High spatial neutron transmission image for a Sm-Sn mixed metal is successfully constructed below 0.3 eV. Concomitantly, we examine the neutron transmission up to 100 eV and identify observed resonance dips by Sm nuclei.

Recovering the second moment of the strain distribution from neutron Bragg edge data

Point by point strain scanning is often used to map the residual stress (strain) in engineering materials and components. However, the gauge volume and, hence, spatial resolution are limited by the beam defining apertures and can be anisotropic for very low and high diffraction (scattering) angles. Alternatively, wavelength resolved neutron transmission imaging has a potential to retrieve information tomographically about residual strain induced within materials through measurement in transmission of Bragg edges—crystallographic fingerprints whose locations and shapes depend on microstructure and strain distribution. In such a case, the spatial resolution is determined by the geometrical blurring of the measurement setup and the detector point spread function. Mathematically, reconstruction of the strain tensor field is described by the longitudinal ray transform; this transform has a non-trivial null-space, making direct inversion impossible. A combination of the longitudinal ray transform with physical constraints was used to reconstruct strain tensor fields in convex objects. To relax physical constraints and generalize reconstruction, a recently introduced concept of histogram tomography can be employed. Histogram tomography relies on our ability to resolve the distribution of strain in the beam direction, as we discuss in the paper. More specifically, Bragg edge strain tomography requires extraction of the second moment (variance about zero) of the strain distribution, which has not yet been demonstrated in practice. In this paper, we verify experimentally that the second moment can be reliably measured for a previously well characterized aluminum ring and plug sample. We compare experimental measurements against numerical calculation and further support our conclusions by rigorous uncertainty quantification of the estimated mean and variance of the strain distribution.

Neutron transmission imaging with a portable D-T neutron generator

A portable fast-neutron imaging system is being developed to provide complementary information to field X-ray imaging. Applications include inspection of vehicles and infrastructure for corrosion, measurement of material levels in containers, and inspection of munitions and suspicious packages. While fast-neuron imaging generally provides lower imaging resolution compared to X-rays, fast-neutron interaction cross-sections have a weak dependence on material Z. This enables imaging of low-Z materials inside high-Z materials. Here, we discuss the limitations and current improvements in fast-neuron imaging. Limitations in portable fast-neutron imaging systems include low D-T neutron generator output, low light production in ZnS(Cu) imaging scintillators, low resolution due to scintillator thickness and D-T spot size, and digital-panel darknoise that varies in time and position and that can be 100× larger than the neutron signal. We have made improvements in these areas through development of a segmented high light yield scintillator, panel noise mitigation techniques, and testing of new high-output, small spot size D-T neutron generators. The segmented high light yield fast-neutron scintillator demonstrated 5× increase in light compared to ZnS(Cu). An additional 2× improvement in signal-to-noise was demonstrated with panel-noise mitigation techniques. Our MCNP calculations also show good agreement with neutron imaging results We have demonstrated improvements in fast-neutron imaging through development of a segmented high light yield neutron scintillator, mitigation of digital panel noise, and preliminary testing with new high-output, small spot size D-T neutron generators. We have also demonstrated good results modeling fast-neutron images and scatter effects using MCNP.