Neutron Imaging System Research Articles

Redifining RADEN's high-resolution neutron imaging capabilities.

This study presents a significant development in the Energy-Resolved Neutron Imaging System RADEN, in the Japan Proton Accelerator Research Complex, Japan. Through a systematic study, the collimation power of the facility was reevaluated. What was initially considered to be values of 230, 420, and 760 have been proven to be much higher. To perform the calculation of the L/D factor of the beam, a state-of-the-art method has been used, along with a standard reference sample to measure the resolution of neutron images. To add robustness to the results, the study compares five different scintillators of different composition and thickness [6LiF:ZnS(Ag) of 50, 100, 200, and 300μm and Gd2O2S of 50 μm]. The calculated collimating power of the beam ranges between 470 and 1520. These results place a spotlight on an existing discrepancy between the geometrically calculated L/D and the actual measurable quantity, as well as highlight the superior performance of the RADEN beamline.

Optimization study on the compact thermal neutron imaging system based on portable D-T neutron generator

The distribution of lithium in lithium-ion batteries is critical to their performance and lifespan. Studies have shown that thermal neutron imaging can effectively perform non-destructive testing of this distribution. However, most current thermal neutron imaging systems rely on expensive, bulky, and immobile nuclear reactors or spallation neutron sources, significantly limiting their application scenarios. Therefore, this study adopts a portable D-T neutron generator as the neutron source and uses Monte Carlo simulation methods to optimize the design of key components in the thermal neutron imaging system. Simulation results indicate that the optimized thermal neutron imaging system achieves a thermal neutron flux of 8.36 × 104 n·cm−2·s−1 and a thermal neutron content (TNC) of 14.4% at a collimation ratio of 50. Within an imaging range of Φ100 mm, the non-uniformity of the thermal neutron fluence is approximately 2.07%. Simulation imaging results of lithium and iron step wedges demonstrate that the compact thermal neutron imaging system designed in this study can effectively distinguish between iron and lithium of the same thickness. This finding provides an important reference for subsequent detection of lithium distribution within lithium-ion batteries.

Source shape estimation for neutron imaging systems using convolutional neural networks.

Neutron imaging systems are important diagnostic tools for characterizing the physics of inertial confinement fusion reactions at the National Ignition Facility (NIF). In particular, neutron images give diagnostic information on the size, symmetry, and shape of the fusion hot spot and surrounding cold fuel. Images are formed via collection of neutron flux from the source using a system of aperture arrays and scintillator-based detectors. Currently, reconstruction of fusion source geometry from the collected neutron images is accomplished by solving a computationally intensive maximum likelihood estimation problem via expectation maximization. In contrast, it is often useful to have simple representations of the overall source geometry that can be computed quickly. In this work, we develop convolutional neural networks (CNNs) to reconstruct the outer contours of simple source geometries. We compare the performance of the CNN for penumbral and pinhole data and provide experimental demonstrations of our methods on both non-noisy and noisy data.

3D reconstruction of an inertial-confinement fusion implosion with neural networks using multiple heterogeneous data sources.

3D asymmetries are major degradation mechanisms in inertial-confinement fusion implosions at the National Ignition Facility (NIF). These asymmetries can be diagnosed and reconstructed with the neutron imaging system (NIS) on three lines of sight around the NIF target chamber. Conventional tomographic reconstructions are used to reconstruct the 3D morphology of the implosion using NIS [Volegov et al., J. Appl. Phys. 127, 083301 (2020)], but the problem is ill-posed with only three imaging lines of sight. Asymmetries can also be diagnosed with the real-time neutron activation diagnostics (RTNAD) and the neutron time-of-flight (nToF) suite. Since the NIS, RTNAD, and nToF each sample a different part of the implosion using different physical principles, we propose that it is possible to overcome the limitations of too few imaging lines of sight by performing 3D reconstructions that combine information from all three heterogeneous data sources. This work presents a new machine learning-based reconstruction technique to do just this. By using a simple physics model and group of neural networks to map 3D morphologies to data, this technique can easily account for data of multiple different types. A simple proof-of-principle is presented, demonstrating that this technique can accurately reconstruct a hot-spot shape using synthetic primary neutron images and a hot-spot velocity vector. In particular, the hot-spot's asymmetry, quantified as spherical harmonic coefficients, is reconstructed to within ±4% of the radius in 90% of test cases. In the future, this technique will be applied to actual NIS, RTNAD, and nToF data to better understand 3D asymmetries at the NIF.

Design and Initial Tests of a Fast Neutron Radiography Detector Prototype with Silicon Photomultiplier Readouts

Among non-destructive testing (NDT) techniques, fast neutron radiography with a higher penetration capability has achieved rapid advancements. However, the application of the radiography detector in many fast neutron imaging systems is limited by unfavorable detection efficiency and imaging spatial resolution. In this paper, a fast neutron radiography detector was designed, which was composed of a pixelated EJ200 scintillator array, a 16 × 16 silicon photomultiplier (SiPM) array, and capacitive multiplexing network readout electronics. The main parameters of the detector were optimized using Monte Carlo simulations. In addition, the prototype of the detector was fabricated and tested under a 14 MeV D-T neutron source. The preliminary test results demonstrated that the spatial resolution of the prototype reached 1.2 mm. Moreover, the conflict between spatial resolution and detection efficiency could be mitigated by using a pixelated scintillator structure. Overall, SiPMs enabled the extensive application of the imaging system because of their excellent photon detection performance, relatively low price, and joint possibility for large areas.

Source localization for neutron imaging systems using convolutional neural networks.

The nuclear imaging system at the National Ignition Facility (NIF) is a crucial diagnostic for determining the geometry of inertial confinement fusion implosions. The geometry is reconstructed from a neutron aperture image via a set of reconstruction algorithms using an iterative Bayesian inference approach. An important step in these reconstruction algorithms is finding the fusion source location within the camera field-of-view. Currently, source localization is achieved via an iterative optimization algorithm. In this paper, we introduce a machine learning approach for source localization. Specifically, we train a convolutional neural network to predict source locations given a neutron aperture image. We show that this approach decreases computation time by several orders of magnitude compared to the current optimization-based source localization while achieving similar accuracy on both synthetic data and a collection of recent NIF deuterium-tritium shots.

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.

Characterization of a SiPM-based monolithic neutron scatter camera using dark counts

The Single Volume Scatter Camera (SVSC) Collaboration aims to develop portable neutron imaging systems for a variety of applications in nuclear non-proliferation. Conventional double-scatter neutron imagers are composed of several separate detector volumes organized in at least two planes. A neutron must scatter in two of these detector volumes for its initial trajectory to be reconstructed. As such, these systems typically have a large footprint and poor geometric efficiency. We report on the design and characterization of a prototype monolithic neutron scatter camera that is intended to significantly improve upon the geometrical shortcomings of conventional neutron cameras. The detector consists of a 50 mm×56 mm× 60 mm monolithic block of EJ-204 plastic scintillator instrumented on two faces with arrays of 64 Hamamatsu S13360-6075PE silicon photomultipliers (SiPMs). The electronic crosstalk is limited to < 5% between adjacent channels and < 0.1% between all other channel pairs. SiPMs introduce a significantly elevated dark count rate over PMTs, as well as correlated noise from after-pulsing and optical crosstalk. In this article, we characterize the dark count rate and optical crosstalk and present a modified event reconstruction likelihood function that accounts for them. We find that the average dark count rate per SiPM is 4.3 MHz with a standard deviation of 1.5 MHz among devices. The analysis method we employ to measure internal optical crosstalk also naturally yields the mean and width of the single-electron pulse height. We calculate separate contributions to the width of the single-electron pulse-height from electronic noise and avalanche fluctuations. We demonstrate a timing resolution for a single-photon pulse to be (128 ± 4) ps. Finally, coincidence analysis is employed to measure external (pixel-to-pixel) optical crosstalk. We present a map of the average external crosstalk probability between 2×4 groups of SiPMs, as well as the in-situ timing characteristics extracted from the coincidence analysis. Further work is needed to characterize the performance of the camera at reconstructing single- and double-site interactions, as well as image reconstruction.

Development of a phoswich imaging detector to simultaneously acquire neutron and gamma photon images

A neutron detector requires selectively detecting neutrons not detecting gamma photons. However, since gamma photons are commonly emitted from a neutron source, the separation of neutrons and gamma photons is sometimes difficult. To solve this problem, we developed a phoswich imaging detector that can simultaneously but independently acquire images of neutrons and gamma photons. The developed neutron imaging system consists of a lithium-containing silver doped zinc sulfide (Li–ZnS(Ag)) plate stacked on a cerium doped yttrium aluminum perovskite (YAP(Ce)) plate to form a phoswich detector, which is optically coupled to a position sensitive photomultiplier tube (PSPMT). Neutrons are detected by the Li–ZnS(Ag) plate while gamma photons are detected by the YAP(Ce) plate. The scintillation light from the Li–ZnS(Ag) and YAP(Ce) plates is detected by the PSPMT, and the position of interaction is calculated. Pulse shape discrimination is used to separate the images of neutrons and gamma photons. The spatial resolution of the imaging detector for neutrons is ∼2.6 mm FWHM, and the sensitivity for 252Cf neutrons at 2 cm from the source enclosed in 2-cm-thick polystyrene plates is 5.2 cps/MBq. The spatial resolution of the imaging detector for gamma photons is also ∼2.6 mm FWHM, and the sensitivity for 60Co gamma photons at 2 cm from the source within 2-cm-thick polystyrene plates is 375 cps/MBq. The contamination by gamma photons in the neutron window was negligible. The developed phoswich imaging detector can simultaneously and independently acquire the images of neutrons and gamma photons, and thus it will impact the relevant research and applications of neutron detectors and imaging.

A lithium ore grade measurement based on the neutron & X-ray bi-modal imaging system

The grades of the minerals significantly affects the energy consumption and chemical pollution along with the beneficiation process for extracting lithium element from the ores. Based on the large neutrons’ macro cross section of the Li2O cluster inside the ores, the grades of lithium ores could be analyzed by the thermal neutron penetrating information. In this work, a bimodal imaging method, which utilizes both the information of penetrating neutrons and X-rays delivered by the same electron linear accelerator driven photoneutron system, was proposed to investigate the lithium concentration of each ore. A linearity R-square value of 0.991 between the results obtained with this method and those from the chemical method has been achieved. The average error in lithium concentration estimation is approximately 0.2 wt percent (wt%). The underlying principles and the experimental results will be elaborated on in this study.

Spot size measurement of a deuterium-tritium dense plasma focus using neutron radiography.

Neutron radiography is a technique uniquely suited to applications in nuclear diagnostics, non-destructive testing, and subcritical experiments. The spatial resolution of neutron radiographs is degraded by optical blur in the imaging system and the neutron source size, where the ideal source is point-like to optimize the point-spread function. A potential neutron source for radiography is the dense plasma focus (DPF), a coaxial Z-pinch that produces thermonuclear and beam-target neutrons. To assess if the source size is suitable for radiography, a neutron imaging system was used to measure the source size of the 4 MA Sodium DPF at the Nevada National Security Site operating with deuterium-tritium gas-fill. The source size was measured using the edge-spread function of tungsten objects, each having a rolled (convex) edge. The spot size was found to be 7-12mm full-width at half-max(FWHM) assuming a Gaussian source, though comparison is presented for Lorentzian and Bennett distributions. The average FWHM was found to be 8.6 ± 1.2mm vertically and 10.8 ± 1.2mm horizontally with respect to the image plane, averaging over varied edges and alignments. The results were sensitive to source alignment and edge metrology, which introduced notable uncertainties. These results are consistent with separate experimental measurements as well as magnetohydrodynamics simulations of this DPF, which suggest that neutron production can originate from pinches ∼5-7mm off-axis. These results suggest that the DPF should be used for radiography at low magnification (M < 1) where spot size does not dominate spatial blur.

Gamma-ray imaging of Np-237 metal using an organic glass imager

Neutron and gamma-ray imaging systems are deployed within the field of nuclear safeguards for the detection and localization of special nuclear materials and other materials of interest. 237Np is one of these materials of interest due its presence in spent nuclear fuel and potential for use in nuclear weapons when purified. Here, for the first time, a 6 kg neptunium sphere (98.8 wt% 237Np) was measured using a dual-particle imager, from the University of Michigan, consisting of organic glass and inorganic scintillators. The novel composition of organic glass scintillator was recently developed at Sandia National Labs and has been used in particle imaging systems due to its time resolution and particle discrimination capabilities. Gamma-ray energy spectra from single and coincident events were extracted and the sequencing of Compton scatter and photoelectric absorption gamma-ray events was used to generate images using simple backprojection. The emissions of interest in this work are the 312 keV and 416 keV gamma rays from 233Pa, a daughter isotope from the neptunium decay series. The results of this work show that there is close agreement between the true source location in angular space and the converged location from the gamma ray images created using the system. The gamma spectroscopy from single and coincident events also identified the characteristic emission from the daughter isotope and could be used to assist with the identification of 237Np. Furthermore, successful localization of the source with 5 s of data demonstrates the practical application of the imaging system for imaging and detection of material in weapons-useable quantities.

Quantifying neutron scintillator screens with X-ray computed tomography

As nondestructive imaging techniques become more commonplace, imaging systems must be improved and optimized to meet the growing demand. One key aspect of neutron imaging systems is the scintillator, which determines the time necessary to acquire an image and can also limit the spatial resolution achievable by a system. In this work, X-ray computed tomography was coupled with image processing to measure parameters of a boron-based neutron scintillator screen. The screen's surface and subsurface were examined for defects and a thickness measurement as a function of position was also successfully implemented. Higher resolution scans of a sub-volume of the scintillator coating enabled visualization of the packing of the converter and phosphor powders while also revealing microscopic porosity within the scintillator material. The converter-to-phosphor ratio was quantified with the examined area showing a ZnS:Ag phosphor volume of approximately 60.70% of the entire scintillator volume, while Na10B5O8 converter accounted for approximately 38.76% of the volume.

Simultaneous Gamma-Neutron Vision device: a portable and versatile tool for nuclear inspections

This work presents GN-Vision, a novel dual γ-ray and neutron imaging system, which aims at simultaneously obtaining information about the spatial origin of γ-ray and neutron sources. The proposed device is based on two position sensitive detection planes and exploits the Compton imaging technique for the imaging of γ-rays. In addition, spatial distributions of slow- and thermal-neutron sources (<100 eV) are reconstructed by using a passive neutron pin-hole collimator attached to the first detection plane. The proposed gamma-neutron imaging device could be of prime interest for nuclear safety and security applications. The two main advantages of this imaging system are its high efficiency and portability, making it well suited for nuclear applications were compactness and real-time imaging is important. This work presents the working principle and conceptual design of the GN-Vision system and explores, on the basis of Monte Carlo simulations, its simultaneous γ-ray and neutron detection and imaging capabilities for a realistic scenario where a 252Cf source is hidden in a neutron moderating container.

Design of moderator and collimator based on compact D-T neutron source for neutron imaging

ABSTRACT The Compact thermal neutron imaging systems have become an important development trend. Compact thermal neutron imaging systems can use neutron tubes as neutron sources because of their low cost, small size, and portability. In order to achieve optimal imaging quality, the moderator and collimator in a neutron imaging system based on neutron tubes source are designed and simulated using SuperMC. The entrance position of the collimator is determined by analyzing the performance of different materials with their parameter’s optimization. It is suggested that a cylinder-shaped structure made of carbon, hydrogenous materials, and heavy metals can significantly increase the thermal neutron flux at the sample location. Further dimension optimization is carried out to improve the neutron beam collimation performance. A vertically divergent collimator with a divergence angle of 5° and an air gap is selected, along with the addition of a B4C lining material. According to the modelling results, the sample position can be as far away from the neutron source as 77 cm with a neutron source yield of 1.14 × 109 n/s. The collimation ratio is 29, the thermal neutron flux is 1.08 × 103 n/cm2·s, and n/γ is 1.44 × 1012 n/cm2·Sv, which is suitable for thermal neutron radiography.

Prototype design and experimental study of fission neutron imaging at China Mianyang research reactor

Taking advantage of the special reaction principle between neutrons and elements, the neutron imaging method is better suited to testing the structure and the defects of light materials that are covered by heavy materials. In contrast to thermal or cold neutron imaging, the fast neutrons are best suited to testing the large samples composed of various materials. Fission neutrons emitted from the reactor are more stable than those from other neutron sources. Based on the beamline for thermal neutron imaging at China Mianyang Research Reactor (CMRR), a combination of filters could increase the proportion of the fission neutrons. Various fission neutron collimators were calculated, designed and measured. The flux of fission neutrons was close to 3 × 105 cm−2 s−1, when the collimation ratio was about 172. A fission neutron imaging system was built as well. The field of view (FOV) was as large as 400 mm × 400 mm, and the experimental spatial resolution was better than 0.5 mm. The fission neutron tomography was tested. The reconstruction results were comparable to the tomography performed in NECTAR at FRM-II. In addition, a serial time acquisition method was applied in the tomography to reduce the white noise. The ART reconstruction method may improve the quality of the reconstruction images.

Development of an event-by-event based Li–ZnS(Ag) neutron imaging detector with selective neutron detection capability

High sensitivity and high resolution is desired in such technologies as neutron radiography. However, the contamination of gamma photons in neutron images decreases the accuracy of neutron radiography. To solve this problem, we developed an event-by-event based neutron imaging system that can selectively detect neutrons. The developed neutron imaging system consists of an Li–ZnS(Ag) scintillator plate optically coupled to a flat panel photomultiplier tube (FP-PMT) with a light guide. Scintillation light emitted from the Li–ZnS(Ag) by the interaction with neutron-induced particles is used to calculate the position based on the center of mass calculations. The spatial resolution of the neutron imaging detector is ∼2.3 mm FWHM, and the sensitivity for 252Cf at 2 cm from the source with 2-cm-thick polystyrene is 20 cps/MBq. Background count fraction from 60Co gamma photons was 0.2 %. For various types of neutron absorption phantoms, high-contrast and high-resolution neutron images are obtained with the developed neutron imaging detector using a252Cf neutron source.

Neutron spectroscopy with TENIS using an artificial neural network

In this research, a ThErmal Neutron Imaging System (TENIS) consisting of two perpendicular sets of plastic scintillator arrays for boron neutron capture therapy (BNCT) application has been investigated in a completely different approach for neutron energy spectrum unfolding. TENIS provides a thermal neutron map based on the detection of 2.22 MeV gamma-rays resulting from 1H(nth, γ)2D reactions, but in the present study, the 70-pixel thermal neutron images have been used as input data for unfolding the energy spectrum of incident neutrons. Having generated the thermal neutron images for 109 incident mono-energetic neutrons, a 70 × 109 response matrix has been generated using the MCNPX2.6 code for feeding into the artificial neural network tools of MATLAB. The errors of the final results for mono-energetic neutron sources are less than 10% and the root mean square error (RMSE) for the unfolded neutron spectrum of 252Cf is about 0.01. The agreement of the unfolding results for mono-energetic and 252Cf neutron sources confirms the performance of the TENIS system as a neutron spectrometer.

Upgrading Neutron Imaging Facility at TRR-1/M1 Reactor

The neutron imaging system at Thai Research Reactor 1/Modification 1 (TRR-1/M1) is located at Thailand Institute of Nuclear Technology, Bangkok. It has been developed for research on non-destructive investigation using radiography and tomography techniques. The main instruments of the system consist of a neutron camera, a sample rotation system operated by in-house developed software, and a neutron shutter remotely controlled via a WIFI switch. In the first half of 2022, the TRR-1/M1 was temporarily shut down for annual maintenance. Meanwhile, additional shielding was installed in the imaging room for improving radiation safety. The current neutron camera was replaced with a new one to improve the imaging system performance. The camera performance, such as brightness and image quality, were investigated by varying the exposure time and imaging of the resolution test pattern, Siemens star, respectively. The results showed that the new camera set has higher brightness than the old one about 41% when compared to 70% grey value of 16 bit-format. The spatial resolution of 2D and 3D images were 0.59 mm and 1.37 mm, respectively. Moreover, the additional shields could reduce the radiation level around the reactor hall about 6-7 times. Then, neutron tomography using a new camera will be conducted to present the 3D visualization of various samples.

Update of Bragg edge analysis software “GUI-RITS”

The energy-resolved neutron imaging system, RADEN at J-PARC, has been providing to users a Rietveld-type analysis code, RITS, for pulsed neutron Bragg-edge transmission (BET) imaging with a graphical user interface (GUI) version, for fitting spectral data obtained with this instrument. In the last year, we updated the computational platform of GUI-RITS software from Scientific Linux 6 (SL6) and Python 2 to Windows 10 and Python 3, and added some functions to improve usability. The license agreement for this updated software is the Berkeley Software Distribution (BSD) 2-Clause License (non-copyleft) and is currently available for download from the RADEN website.