Scatter rejection in quantitative thermal and cold neutron imaging

The accuracy of quantitative neutron transmission radiography can be substantially decreased if highly scattering materials, such as water or plastics, exist in the sample. There are currently two main solutions to this problem: either performing experiments at a large distance between the detector and the sample or employ some numerical correction techniques. In the former case, the spatial resolution is substantially reduced by the limited beam divergence, while the latter correction requires a priori information about the sample and is limited to distances of above ∼2 cm. We demonstrate the feasibility of another technique, namely the possibility to remove the scattered neutron component from the transmitted neutron beam by a very compact polycapillary collimator. These ∼1 mm thick devices can be placed between the sample and the detector and remove most of the neutrons scattered at angles larger than the acceptance angle of the collimator (typically 1°). No image distortions above ∼10 μm scales are introduced by these collimators. The neutron transmission of highly scattering samples (water and plexiglass) is measured in our experiments with and without scatter rejection. In the latter case, the accuracy of measured transmission coefficient was substantially improved by our collimators.     

Cryogenic neutron detector by InSb semiconductor detector with high-density helium-3 gas converter

The neutron-detection characteristics of a cryogenic neutron detector comprising an InSb semiconductor detector and a helium-3 gas converter were evaluated at a gas pressure up to 12.4 atm at 4.2 K. The detector successfully detected stable neutrons under these conditions, where the density of the helium-3 gas is a few-hundred times higher than that at room temperature. It was found that the neutron detection efficiency was correlated with the gas pressure—even in a backward-detection configuration—in low-temperature, high-pressure helium-3.     

LiF crystals as high spatial resolution neutron imaging detectors

Neutron imaging by color center formation in LiF crystals was applied to a sensitivity indicator (SI) as a standard samples for neutron radiography. The SI was exposed to a 5 mm pinhole-collimated thermal neutron beam with an LiF crystal and a neutron imaging plate (NIP) for 120 min in the JRR-3M thermal neutron radiography facility. The image in the LiF crystal was read out using a laser confocal microscope. All gaps were clearly observed in images for both the LiF crystal and the NIP. The experimental results showed that LiF crystals have excellent characteristics as neutron imaging detectors in areas such as high spatial resolution.     

Evaluation of water distribution in a small operating fuel cell using neutron color image intensifier

Neutron radiography is one of the useful tools for visualizing water behavior in operating fuel cells. In order to observe the detailed information about the water distribution in membrane electrode assembly (MEA) and gas diffusion layer (GDL) in fuel cells, a high performance neutron imaging system is required. A neutron color image intensifier (NCII) is a high spatial resolution and high sensitivity neutron image detector. We have developed an imaging system using an NCII for visualizing the behavior of water in fuel cells. The pixel size of the imaging system is around 4.7 μm in the small view field. By using this system, water distribution of a small sized fuel cell was observed continuously every 20 s at the Thermal Neutron Radiography Facility (TNRF). In the results, the water area appears from the GDL and MEA regions, and expanded to the cathode side channel with time. However, the voltage was gradually reduced with time, and steeply dropped. It is considered that the reduction and the drop of voltage were caused by a blockage of gas flow due to accumulation of water in the GDL and the gas flow channel in the cathode side.     

Evaluations of the new LiF-scintillator and optional brightness enhancement films for neutron imaging

Japan Atomic Energy Agency has developed the neutron scintillator jointly with Chichibu Fuji Co., Ltd. In this study, we evaluated the new ZnS(Ag):Al/⁶Li scintillator developed for neutron imaging. It was confirmed that the brightness increased by about double while maintaining equal performance for the spatial resolution as compared with a conventional scintillator. High frame-rate imaging using a high-speed video camera system and this new scintillator made it possible to image beyond 10 000 frames per second while still having enough brightness. This technique allowed us to obtain a high-frame-rate visualization of oil flow in a running car engine. Furthermore, we devised a technique to increase the light intensity of reception for a camera by adding brightness enhancement films on the output surface of the scintillator. It was confirmed that the spatial resolution degraded more than double, but the brightness increased by about three times.     

On coherence in neutron imaging

The variety of imaging signals in neutron radiography and tomography became quite large compared to the pure absorption and scattering contrast in neutron radiographies and topographies in the early sixties or seventies of the last century. The diversity of absorption based techniques for neutron radiography and tomography is comparable to coherence based imaging techniques such as phase contrast, differential phase contrast, dark field imaging, diffraction enhanced contrast, refraction contrast, ultra small angle scattering contrast, grating interferometry and crystal interferometry, also the spin of the neutron was successfully used for imaging [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11] and [12]. We show which effects (total reflection, diffraction, refraction) contribute to e.g. a step boundary or a phase boundary. Taking this simple object, one can learn to understand the imaging procedure and what is displayed in a radiograph.     

A highly adaptive detector system for high resolution neutron imaging

An optimized detector system that allows high-resolution neutron imaging with desired flexibility is described. The presented system can be adapted and integrated with standard CCD-based neutron detectors. Novel neutron scintillating materials with good photon discrimination and optical lens components are tested and optimized for high-resolution neutron tomographic purposes. The presented detector system provides variable field of view and can be used in combination with different techniques, including dark-field, energy-selective, and neutron spin polarized imaging.     

Energy selective neutron imaging in solid state materials science

In neutron radiography and tomography, the image contrast is caused by a variation of the effective macroscopic cross-section over the sample volume. Narrowing the energy band of the polychromatic neutron beam in the cold energy range increases the image contrast significantly and opens an access to the crystallographic structure of the sample. Here, we show that crystallographic microstructures of welded stainless steel samples can be visualized and quantified in two and three dimensions by the energy selective neutron imaging. The energy selective neutron radiography maps preferred crystallite orientations over the sample and provides energy values of the highest image contrast. Furthermore, a high contrast neutron tomography visualizes preferred crystallite orientations over the whole macroscopic sample volume.     

MuhRec—A new tomography reconstructor

A new tool for computed tomography reconstruction is presented. The tool was developed to support the needs of users at neutron imaging beamlines and as a platform for algorithm development. It includes methods to handle large samples and artifact removal. The design is modular and allows tests of new concepts for preprocessing and back-projection. The reconstructor is tuned to provide the results fast even on a laptop computer. The reconstructor also has a graphical user interface which can be operated by new users after a short instruction.     

Neutron diffraction measurements at the INES diffractometer using a neutron radiative capture based counting technique

The global shortage of ³He gas is an issue to be addressed in neutron detection. In the context of the research and development activity related to the replacement of ³He for neutron counting systems, neutron diffraction measurements performed on the INES beam line at the ISIS pulsed spallation neutron source are presented. For these measurements two different neutron counting devices have been used: a 20 bar pressure squashed ³He tube and a Yttrium-Aluminum-Perovskite scintillation detector. The scintillation detector was coupled to a cadmium sheet that registers the prompt radiative capture gamma rays generated by the (n,γ) nuclear reactions occurring in cadmium. The assessment of the scintillator based counting system was done by performing a Rietveld refinement analysis on the diffraction pattern from an ancient Japanese blade and comparing the results with those obtained by a ³He tube placed at the same angular position. The results obtained demonstrate the considerable potential of the proposed counting approach based on the radiative capture gamma rays at spallation neutron sources.     

Fusion neutron detector calibration using a table-top laser generated plasma neutron source

Using a high intensity, femtosecond laser driven neutron source, a high-sensitivity neutron detector was calibrated. This detector is designed for observing fusion neutrons at the Z accelerator in Sandia National Laboratories. Nuclear fusion from laser driven deuterium cluster explosions was used to generate a clean source of nearly monoenergetic 2.45 MeV neutrons at a well-defined time. This source can run at 10 Hz and was used to build up a clean pulse-height spectrum on scintillating neutron detectors giving a very accurate calibration for neutron yields at 2.45 MeV.     

Dehydration of moulding sand in simulated casting process examined with neutron radiography

Natural bentonites are an important material in the casting industry. Smectites as the main component of bentonites plasticize and stabilise sand moulds. Pore water as well as interlayer water within the smectites are lost as a function of time, location and temperature. Although rehydration of the smectites should be a reversible process, the industrially dehydrated smectites lose their capability to reabsorb water. This limits the number of possible process cycles of the mould material. A full understanding of the dehydration process would help to optimise the amount of fresh material to be added and thus save resources. A simulated metal casting was investigated with neutron radiography at the ANTARES neutron imaging facility of the FRM II reactor of Technische Universität München, Germany.     

Coded source neutron imaging with a MURA mask

In coded source neutron imaging the single aperture commonly used in neutron radiography is replaced with a coded mask. Using a coded source can improve the neutron flux at the sample plane when a very high L/D ratio is needed. The coded source imaging is a possible way to reduce the exposure time to get a neutron image with very high L/D ratio. A 17×17 modified uniformly redundant array coded source was tested in this work. There are 144 holes of 0.8 mm diameter on the coded source. The neutron flux from the coded source is as high as from a single 9.6 mm aperture, while its effective L/D is the same as in the case of a 0.8 mm aperture. The Richardson-Lucy maximum likelihood algorithm was used for image reconstruction. Compared to an in-line phase contrast neutron image taken with a 1 mm aperture, it takes much less time for the coded source to get an image of similar quality.     

A comparison of neutron spectrum unfolding codes used with a miniature NE213 detector

The effects of unfolding technique on neutron spectra measured with a miniature NE-213 spectrometer are investigated. The codes used were FORIST, FERDOR and RADAK, a differential code FLYSPEC and one developed by the authors based on Neural Networks. The characteristics required of experimental test spectra were that they be structured, well known and have a significant component above 10 MeV. Four different test spectra were employed. It is found that all the codes performed well with the test spectra used, producing generally consistent results.     

First study of macroscopic neutron dark field imaging using scattering grids

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

Beryllium neutron activation detector for pulsed DD fusion sources

A compact fast neutron detector based on beryllium activation has been developed to perform accurate neutron fluence measurements on pulsed DD fusion sources. It is especially well suited to moderate repetition-rate (<0.2 Hz) devices, such as the plasma focus or Z-pinch. The detector comprises a beryllium metal sheet sandwiched between two large-area xenon-filled proportional counters. A methodology for calculating the absolute response function of the detector using a “first principles” approach is described. This calibration methodology is based on the ⁹Be(n,α)⁶He cross-section, energy calibration of the proportional counters, and numerical simulations of neutron interactions and beta-particle paths using MCNP5. The response function R(E_n) is determined over the neutron energy range 2-4 MeV. The count rate capability of the detector has been studied and the corrections required for high neutron fluence measurements are discussed. For pulsed DD neutron fluencies >3×10⁴ cm⁻², the statistical uncertainty in the fluence measurement is better than 1%. A small plasma focus device has been employed as a pulsed neutron source to test two of these new detectors, and their responses are found to be practically identical. Also the level of interfering activation is found to be sufficiently low as to be negligible.     

Development of a neutron tomography system using a low flux reactor

A neutron tomography instrument was designed and developed at the Royal Military College (RMC) of Canada with Queen's University to enhance these institutions' non-destructive evaluation capabilities. The neutron imaging system was built around a Safe Low-Power C(K)ritical Experiment (SLOWPOKE-2) nuclear research reactor. The low power and physical geometry of the reactor required that a novel design be developed to facilitate tomography. A unique rotisserie style rotary stage and clamping apparatus was developed. Furthermore, the low flux at the image plane (3×10⁴ n cm⁻² s⁻¹), necessitated that the image acquisition and reconstruction processes be optimized. Tomographs of numerous samples were obtained using the new tomography instrument at RMC.     

Study of hydrogen effusion in austenitic stainless steel by time-resolved in-situ measurements using neutron radiography

The purpose of the present study was to show the feasibility of measuring hydrogen effusion in austenitic stainless steel (1.4301) using neutron radiography at the facility ANTARES of the research reactor FRM II of the Technische Universität München. This method is appropriate to measure in-situ hydrogen effusion for hydrogen concentrations as small as 20 ppm_H. Experiments were carried out in the temperature range from room temperature up to 533 K. The measurement principle is based on the parallel comparison of electrochemically hydrogen charged specimen with hydrogen-free reference specimen at the same temperature. This allows the determination of the hydrogen concentration in the specimens as a function of time and temperature. Separate hot carrier gas extraction experiments using the same temperature-time profiles as the radiography experiments have been used to calibrate the grey values of the neutron transmission images into hydrogen concentrations. It can be stated that the hydrogen effusion correlates with the specimen temperature.