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The MCP-based neutron counting detector is a novel device that allows high spatial resolution and time-resolved neutron radiography and tomography with epithermal, thermal and cold neutrons. Time resolution is possible by the high readout speeds of ~ 1200 frames/sec, allowing high resolution event counting with relatively high rates without spatial resolution degradation due to event overlaps. The electronic readout is based on a Timepix sensor, a CMOS pixel readout chip developed at CERN. Currently, a geometry of a quad Timepix detector is used with an active format of 28 × 28 mm2 limited by the size of the Timepix quad (2 × 2 chips) readout. Measurements of a set of high-precision micrometers test samples have been performed at the Imaging and Materials Science & Engineering (IMAT) beamline operating at the ISIS spallation neutron source (U.K.). The aim of these experiments was the full characterization of the chip misalignment and of the gaps between each pad in the quad Timepix sensor. Such misalignment causes distortions of the recorded shape of the sample analyzed. We present in this work a post-processing image procedure that considers and corrects these effects. Results of the correction will be discussed and the efficacy of this method evaluated.

The uncertainties and the stability of a neutron sensitive MCP/Timepix detector when operating in the event timing mode for quantitative image analysis at a pulsed neutron source were investigated. The dominant component to the uncertainty arises from the counting statistics. The contribution of the overlap correction to the uncertainty was concluded to be negligible from considerations based on the error propagation even if a pixel occupation probability is more than 50%. We, additionally, have taken into account the multiple counting effect in consideration of the counting statistics. Furthermore, the detection efficiency of this detector system changes under relatively high neutron fluxes due to the ageing effects of current Microchannel Plates. Since this efficiency change is position-dependent, it induces a memory image. The memory effect can be significantly reduced with correction procedures using the rate equations describing the permanent gain degradation and the scrubbing effect on the inner surfaces of the MCP pores.

This paper presents a proof-of-concept demonstration of triaxial strain tomography from Bragg-edge neutron imaging within a three-dimensional sample. Bragg-edge neutron transmission can provide high-resolution images of the average through thickness strain within a polycrystalline material. This poses an associated rich tomography problem which seeks to reconstruct the full triaxial strain field from these images. The presented demonstration is an important step towards solving this problem, and towards a technique capable of studying the residual strain and stress within engineering components. A Gaussian process based approach is used that ensures the reconstruction satisfies equilibrium and known boundary conditions. This approach is demonstrated experimentally on a non-trivial steel sample with use of the RADEN instrument at the Japan Proton Accelerator Research Complex. Validation of the reconstruction is provided by comparison with conventional strain scans from the KOWARI constant-wavelength strain diffractometer at the Australian Nuclear Science and Technology Organisation and simulations via finite element analysis.

Energy-resolved neutron imaging experiments conducted on the Small Angle Neutron Scattering (SANS) instrument, Bilby, demonstrate how the capabilities of this instrument can be enhanced by a relatively simple addition of a compact neutron counting detector. Together with possible SANS sample surveying and location of the region of interest, this instrument is attractive for many imaging applications. In particular, the combination of the cold spectrum of the neutron beam and its pulsed nature enables unique non-destructive studies of the internal structure for samples that are opaque to other more traditional techniques. In addition to conventional white beam neutron radiography, we conducted energy-resolved imaging experiments capable of resolving features related to microstructure in crystalline materials with a spatial resolution down to ∼0.1 mm. The optimized settings for the beamline configuration were determined for the imaging modality, where the compromise between the beam intensity and the achievable spatial resolution is of key concern.

Bragg-edge strain imaging from energy-resolved neutron transmission measurements poses an interesting tomography problem. The solution to this problem will allow the reconstruction of detailed triaxial stress and strain distributions within polycrystalline solids from sets of Bragg-edge strain images. Work over the last decade has provided some solutions for a limited number of special cases. In this paper, we provide a general approach to reconstruction of an arbitrary system based on a least squares process constrained by equilibrium. This approach is developed in two- dimensions before being demonstrated experimentally on two samples using the RADEN instrument at the J-PARC spallation neutron source in Japan. Validation of the resulting reconstructions is provided through a comparison to conventional constant wavelength strain measurements carried out on the KOWARI engineering diffractometer within ANSTO in Australia. The paper concludes with a discussion on the range of problems to be addressed in a three-dimensional implementation.

An approach for tomographic reconstruction of three-dimensional strain distributions from Bragg-edge neutron transmission strain images is outlined and investigated. This algorithm is based on the link between Bragg-edge strain measurements and the Longitudinal Ray Transform, which has been shown to be sensitive only to boundary displacement. By exploiting this observation we provide a method for reconstructing boundary displacement from sets of Bragg-edge strain images. In the case where these displacements are strictly the result of externally applied tractions, corresponding internal strain fields can then be found through traditional linear-static finite element methods. This approach is tested on synthetic data in two-dimensions, where the rate of convergence in the presence of measurement noise and beam attenuation is examined.

Energy-resolved neutron transmission imaging is utilised for in situ comparisons of strain distributions in fastened assemblies with regular and self-locking Spiralock® female threads. The strain maps measured within torqued steel bolts indicate that for a Spiralock® thread, the load is distributed over a larger section of the fastener, making this type of thread more suitable for fastening of assemblies subject to transverse vibrations.

A nondestructive study of the internal structure and compositional gradient of dissimilar metal-alloy welds through energy-resolved neutron imaging is described in this paper. The presence of Bragg edges in the measured neutron transmission spectra can be used to characterize the internal residual strain within the samples and some microstructural features, while neutron resonance absorption provides the possibility to map the degree of uniformity in mixing of the participating alloys and intermetallic formation within the welds. This paper demonstrates the potential of neutron energy-resolved imaging to measure all these characteristics simultaneously in a single experiment with sub-mm spatial resolution.

Energy-resolved neutron imaging is used for a nondestructive study of bulk internal microstructure, elemental composition and distribution of voids in dissimilar metal-alloy welds of ∼10 mm thickness. All these characteristics are measured simultaneously in one experiment with a few hundred micrometre spatial resolution., A nondestructive study of the internal structure and compositional gradient of dissimilar metal-alloy welds through energy-resolved neutron imaging is described in this paper. The ability of neutrons to penetrate thick metal objects (up to several cm) provides a unique possibility to examine samples which are opaque to other conventional techniques. The presence of Bragg edges in the measured neutron transmission spectra can be used to characterize the internal residual strain within the samples and some microstructural features, e.g. texture within the grains, while neutron resonance absorption provides the possibility to map the degree of uniformity in mixing of the participating alloys and intermetallic formation within the welds. In addition, voids and other defects can be revealed by the variation of neutron attenuation across the samples. This paper demonstrates the potential of neutron energy-resolved imaging to measure all these characteristics simultaneously in a single experiment with sub-mm spatial resolution. Two dissimilar alloy welds are used in this study: Al autogenously laser welded to steel, and Ti gas metal arc welded (GMAW) to stainless steel using Cu as a filler alloy. The cold metal transfer variant of the GMAW process was used in joining the Ti to the stainless steel in order to minimize the heat input. The distributions of the lattice parameter and texture variation in these welds as well as the presence of voids and defects in the melt region are mapped across the welds. The depth of the thermal front in the Al–steel weld is clearly resolved and could be used to optimize the welding process. A highly textured structure is revealed in the Ti to stainless steel joint where copper was used as a filler wire. The limited diffusion of Ti into the weld region is also verified by the resonance absorption.

The unique possibilities enabled by neutron imaging for in situ remote diagnostics of microstructural characteristics during crystal growth are demonstrated, even when the materials and surrounding structures are opaque to other more conventional interrogation techniques. Neutron radiography is implemented to image remotely the uniformity of elemental distribution (e.g. dopant concentration) during crystal growth, the location of the liquid/solid interface and the presence of macroscopic crystal defects (e.g. cracks), all with a temporal resolution of 5–7 s., Neutrons are known to be unique probes in situations where other types of radiation fail to penetrate samples and their surrounding structures. In this paper it is demonstrated how thermal and cold neutron radiography can provide time-resolved imaging of materials while they are being processed (e.g. while growing single crystals). The processing equipment, in this case furnaces, and the scintillator materials are opaque to conventional X-ray interrogation techniques. The distribution of the europium activator within a BaBrCl:Eu scintillator (0.1 and 0.5% nominal doping concentrations per mole) is studied in situ during the melting and solidification processes with a temporal resolution of 5–7 s. The strong tendency of the Eu dopant to segregate during the solidification process is observed in repeated cycles, with Eu forming clusters on multiple length scales (only for clusters larger than ∼50 µm, as limited by the resolution of the present experiments). It is also demonstrated that the dopant concentration can be quantified even for very low concentration levels (∼0.1%) in 10 mm thick samples. The interface between the solid and liquid phases can also be imaged, provided there is a sufficient change in concentration of one of the elements with a sufficient neutron attenuation cross section. Tomographic imaging of the BaBrCl:0.1%Eu sample reveals a strong correlation between crystal fractures and Eu-deficient clusters. The results of these experiments demonstrate the unique capabilities of neutron imaging for in situ diagnostics and the optimization of crystal-growth procedures.

Deep penetration of neutrons into most engineering materials enables non-destructive studies of their bulk properties. The existence of sharp resonances in neutron absorption spectra enables isotopically-resolved imaging of elements present in a sample, as demonstrated by previous studies. At the same time the Doppler broadening of resonance peaks provides a method of remote measurement of temperature distributions within the same sample. This technique can be implemented at a pulsed neutron source with a short initial pulse allowing for the measurement of the energy of each registered neutron by the time of flight technique. A neutron counting detector with relatively high timing and spatial resolution is used to demonstrate the possibility to obtain temperature distributions across a 100 µm Ta foil with ~millimeter spatial resolution. Moreover, a neutron transmission measurement over a wide energy range can provide spatially resolved sample information such as temperature, elemental composition and microstructure properties simultaneously.

Computational models are employed to analyze residual chemical stresses arising from compositional variations in europium-doped BaBrCl crystals grown by the vertical Bridgman method. We find that significant chemical stress is produced by radial segregation of Eu in this system. In particular, the distribution of normal stresses is set by the radial concentration gradient, whose changing sign produces surface states in tension or compression. Crack opening from surface flaws will be promoted or suppressed by tensile or compressive surface stresses, respectively. Thus, crystal growth processing strategies that change the radial dopant concentration gradients are posited to affect the propensity for cracking. For this system, surface stresses are changed from states of tension to compression when the growth rate is increased, thus improving the chances to avoid cracking—a strategy that defies classical wisdom that dictates slower growth to improve outcomes. Similar strategies affecting segregation may prove beneficial to tailor chemical stress fields to reduce cracking in other crystal growth systems.

We apply continuum models to analyze phase change, heat transfer, fluid flow, solute transport, and segregation in order to understand prior neutron imaging observations of the vertical gradient freeze growth of Eu-doped BaBrCl. The models provide a rigorous framework in which to understand the mechanisms that are responsible for the complicated evolution of interface shape and dopant distribution in the growth experiment. We explain how a transition in the solid/liquid interface shape from concave to convex is driven by changes in radial heat transfer caused by furnace design. We also provide a mechanistic explanation of how dynamic growth conditions and changes of the flow structure in the melt result in complicated segregation patterns in this system. A growth pause caused by controller lock-up is shown to result in a band of solute depletion in accordance with classical theory. However, changing flow patterns during growth result in a non-monotonic axial distribution of solute that cannot be explained by simple application of classical segregation models. We assert that the approach presented here, namely the use of rigorous models in conjunction advanced diagnostics, such as neutron imaging, provides an exciting path forward for process optimization and control, accelerating the incremental advances that have, in the past, typically relied on empiricism, experience, and intuition.

The development of highly pixelated fast Medipix/Timepix readout enabled a large number of novel hybrid detectors, where incoming particles/photons are initially converted into a measureable charge, typically exceeding 1000 electrons. Among those possible converters are Microchannel plate (MCP) electron multipliers, which provide some unique capabilities when combined with bare Medipix/Timepix readout. Fast event multiplication with ~10–20 ps time jitter, localized within the MCP pore (typically 5–10 μm in diameter) and low dark currents make MCP/Medipix/Timepix hybrid devices very attractive for the high spatial and temporal resolution detection of low energy photons (soft X-ray, UV and visible), alpha particles, ions and also neutrons. In this review we present a brief overview of the capabilities of MCP/Medipix/Timepix detectors and show the results of experiments conducted with such hybrid detectors in various applications including neutron imaging, astronomy, mass spectroscopy and others.

In this study we compare conventional X-ray imaging with neutron radiography and neutron tomography of several crowned teeth and teeth with amalgam fillings. While X-ray images provide detailed information about the root area, they cannot probe anything in the area where metals are present, e.g. underneath the crown or behind the metal filling. Our results demonstrate that neutron radiography and neutron tomographic imaging can reveal the internal structure of treated teeth underneath the crown. These neutron imaging methods can be used to investigate the interaction of various fillings and crown attachment paste with the crown and the tooth itself. The integrity of tooth and fillings underneath the crown, the location of cavities and voids can be visualized in three dimensions and provide means for the improvements of dental filling and crown attachment materials and methods.

Microchannel plate sensors are widely used as photon counting imagers in many applications, including, astronomy, high energy physics, and remote sensing. Potential future NASA observatories with ultraviolet instruments, such as LUVOIR and HABEX, will require large area detectors (8k × 8k pixels) with large dynamic range (≥1 kHz/resel), high quantum efficiency (75% peak), and very low backgrounds (≤0.1 cts/sec/cm2 ). New microchannel plate technology combining borosilicate glass microcapillary arrays with high efficiency materials applied by atomic layer deposition are being developed with these goals in mind. Detectors with these microchannel plates can be made in large formats (up to 400 cm2 ) with focal plane matching, have high spatial resolution ( 110 nm range). New photocathodes, such as GaN and hybrid bialkali/alkali halide, have high quantum efficiencies over broadband wavelengths. Cross-strip anodes are well suited for large format detectors with high spatial resolution and high dynamic range requirements. Improvements to detector anodes and readout electronics have resulted in better spatial resolution (10×), output event rate (100×), and temporal resolution (1000×), all the while operating at lower gain (10×). Combining these developments can have a significant impact to potential future NASA sub-orbital and satellite instruments.

The mobility of magnetic domains forms the link between the basic physical properties of a magnetic material and its global characteristics such as permeability and saturation field. Most commonly, surface domain structure are studied using magneto-optical Kerr microscopy. The limited information depth of approx. 20 nanometers, however, allows only for an indirect interpretation of the internal volume domain structures. Here we show how accumulative high-frame rate dynamic neutron dark-field imaging is able for the first time to visualize the dynamic of the volume magnetic domain structures in grain oriented electrical steel laminations at power frequencies. In particular we studied the volume domain structures with a spatial resolution of ∼100 μm and successfully quantified domain sizes, wall velocities, domain annihilation and its duration and domain wall multiplication in real time recordings at power frequencies of 10, 25 and 50 Hz with ±262.5 A/m and ±525 A/m (peak to peak) applied field.

Energy-resolved neutron transmission imaging is used to reconstruct maps of residual strains in drilled and cold-expanded holes in 5-mm and 6.4-mm-thick aluminum plates. The possibility of measuring the positions of Bragg edges in the transmission spectrum in each 55 × 55 µm2 pixel is utilized in the reconstruction of the strain distribution within the entire imaged area of the sample, all from a single measurement. Although the reconstructed strain is averaged through the sample thickness, this technique reveals strain asymmetries within the sample and thus provides information complementary to other well-established non-destructive testing methods.

Through the use of Time-of-Flight Three Dimensional Polarimetric Neutron Tomography (ToF 3DPNT) we have for the first time successfully demonstrated a technique capable of measuring and reconstructing three dimensional magnetic field strengths and directions unobtrusively and non-destructively with the potential to probe the interior of bulk samples which is not amenable otherwise. Using a pioneering polarimetric set-up for ToF neutron instrumentation in combination with a newly developed tailored reconstruction algorithm, the magnetic field generated by a current carrying solenoid has been measured and reconstructed, thereby providing the proof-of-principle of a technique able to reveal hitherto unobtainable information on the magnetic fields in the bulk of materials and devices, due to a high degree of penetration into many materials, including metals, and the sensitivity of neutron polarisation to magnetic fields. The technique puts the potential of the ToF time structure of pulsed neutron sources to full use in order to optimise the recorded information quality and reduce measurement time., Comment: 12 pages, 4 figures

Spatially resolved neutron diffractive imaging with a microchannel plate collimator is used to directly map the nucleation of the skyrmion lattice (SkL) of the B20 compound MnSi at the conical to SkL transition as a function of magnetic field. Our study shows a macroscopic phase separation of the SkL and the conical phase at the border of the SkL phase pocket, reveals that the nucleation of the SkL starts at the edges of the sample, and quantifies the bending of the SkL due to demagnetization. Our study highlights the importance of geometric and demagnetizing effects for the SkL formation regarding the unambiguous interpretation of measurements of bulk properties such as possible phase coexistence and crossover regimes.

We present the concept of a single-photon imager capable of detecting up to 109 photons per second with simultaneous measurements of position (5–10 μm resolution) and time (few tens of picosecond resolution) for each individual photon over an active area of 7 cm2. The detector is based on a "hybrid" concept: a vacuum tube, with a transparent input window on which a suitable photocathode material is deposited, a micro-channel plate and a pixelated read-out anode designed in 65 nm CMOS technology. These figures of merit will open many important applications allowing significant advances in particle physics, life sciences or other emerging fields where the detection of single photons with excellent timing and position resolutions are simultaneously required.

Many high resolution event counting devices process one event at a time and cannot register simultaneous events. In this article a frame-based readout event counting detector consisting of a pair of Microchannel Plates and a quad Timepix readout is described. More than 10 4 simultaneous events can be detected with a spatial resolution of ~55 µm, while >103 simultaneous events can be detected with 1200 frames/sec, while the global count rate of the detector can exceed 5×108 particles/s when no timing information on every particle is required. For the first generation Timepix readout, the timing resolution is limited by the Timepix clock to 10–20 ns. Optimization of the MCP gain, rear field voltage and Timepix threshold levels are crucial for the device performance and that is the main subject of this article. These devices can be very attractive for applications where the photon/electron/ion/neutron counting with high spatial and temporal resolution is required, such as energy resolved neutron imaging, Time of Flight experiments in lidar applications, experiments on photoelectron spectroscopy and many others.

Novel microchannel plates have been constructed using borosilicate glass micro-capillary array substrates with 20 µm and 10 µm pores and coated with resistive, and secondary electron emissive, layers by atomic layer deposition. Microchannel plates in 33 mm, 50 mm and 20 cm square formats have been made and tested. Although their amplification, imaging, and timing properties are comparable to standard glass microchannel plates, the background rates and lifetime characteristics are considerably improved. Sealed tube detectors based on the Planacon tube, and a 25 mm cross delay line readout tube with a GaN(Mg) opaque photocathode deposited on borosilicate microchannel plates have been fabricated. Considerable progress has also been made with 20 cm microchannel plates for a 20 cm format sealed tube sensor with strip-line readout that is being developed for Cherenkov light detection.