High Resolution Neutron Imaging with Li-glass Multicore Scintillating Fiber and Diffusion Studies to Enable Improved Neutron Imaging

High resolution neutron imaging is an essential tool used for fundamental characterization of novel x-ray opaque microstructures. Currently, advanced neutron scattering facilities enable users to image materials with state-of-the-art neutron radiography spatial resolutions of approximately 10-15 microns. Continued progress towards micron resolution is limited by the intensity and the linearity of available thermal neutron fluxes. This places an emphasis on increasing neutron conversion/detection efficiency while maintaining the spatial accuracy of the projected radiograph.This dissertation reports the results of experimental fabrication and characterization of a microstructured multicore 6-lithium-glass scintillating fiber as a proof-of-concept high resolution neutron imager. The approach towards micron–level thermal neutron imaging and fundamental scintillator materials research for relevant imaging technologies are presented. Fabrication trials and neutron/gamma discrimination observations for an initial square-packed multicore design are described first. Then the fabrication process used for a proof-of-concept hexagonal-packed multicore design,and an evaluation of its radioluminescence and chemical stability is presented. Scintillation characteristics of a neutron imaging face plate were estimated, and its spatial resolution was experimentally measured. The ultimate resolving power of the proof-of-concept multicore was comparable to the state-of-the-art. The impact of even higher resolution designs, with potential to track neutron conversion particles using smaller core pitch or different cladding material, is discussed. Neutron imaging often requires nonlinear detection systems that can accurately represent the spatial features of an irradiated object. While thin film and microchannel plate detectors have been heavily researched for this application, little effort has been made to create selective scintillating regions within structured silicate glass detectors. This dissertation presents the continued research of diffusing trivalent cerium in lithium loaded glass. The creation of near surface regions of scintillation with thermal diffusion of the Ce3+ activator into 6Li glass is presented, and its use for neutron imaging with a bent optical fiber taper is discussed. The activation energy of Ce within the silicate is calculated and its valance state is observed as a function of diffusion depth. The diffusion process is then adopted for use with YAP (YAlO3:Ce) for associated particle imaging applications

Neutron resonance spectroscopy for the characterisation of materials and objects

The use of neutron resonance spectroscopy to investigate and study properties of materials and objects is the basis of neutron resonance transmission analysis (NRTA) and neutron resonance capture analysis (NRCA). NRTA and NRCA are non-destructive methods to determine the elemental and isotopic composition without the need of any sample preparation and resulting in a negligible residual activity. The basic principles of NRTA and NRCA are explained. The use of NRTA and NRCA to determine the elemental composition of archaeological objects and to characterise nuclear materials is reviewed. Other applications of neutron resonance spectroscopy such as imaging, detection of explosives and drugs and thermometry are briefly discussed. A combination of NRTA and NRCA, referred to as Neutron Resonance Densitometry (NRD), is presented as a non-destructive method to quantify nuclear material, in particular the amount of special nuclear material in particle-like debris of melted fuel that is formed in severe nuclear accidents. Finally the importance of accurate nuclear resonance parameters for these applications is discussed and the performance of NRTA for the characterization of nuclear material in the presence of matrix material is assessed.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Application of Grating-Based Interferometry to Additive Manufacturing, Lithium-ion Batteries, and Crystals

X-ray and neutron imaging are convenient ways to non-destructively observe novel materials. X-rays provide advantages of low cost and high brilliance while neutrons show bulk and isotopic sensitivity. Imaging provides a way for observing chemical and physical properties of materials without the need for destruction. The way of the imaging future is utilizing imaging with grating-based interferometry. In comparison to traditional radiography and tomography, by using absorption and phase gratings in the beam path, the absorption, phase, and scattering of a sample can be detected. In essence, three image datasets can be obtained in one experiment, saving substantially on costs (especially at expensive neutron facilities), time and materials. With several methods of interferometry available, the focus in this work is Talbot-Lau interferometry and newer designs referred to as near-field and far-field interferometry. X-ray Talbot-Lau interferometry experiments were performed at the LSU synchrotron, Center for Advanced Microstructures and Devices (CAMD), using a microfocus X-ray tube and synchrotron X-rays (38 keV). Neutron Talbot-Lau experiments were performed at the CONRAD2 beamline (HZB, Berlin, Germany) and far-field experiments at the NG6 beamline (NIST, Gaithersburg, USA). Neutron imaging of the additive manufactured samples revealed pore structures and evi- dence of fracture as a function of fatigue. Battery imaging shows the migration of lithium across battery layers on a visual and quantitative level. X-ray and neutron imaging of potentially twinned crystals revealed the importance of preserving data in the 2D projection images that was lost in volume reconstruction. A comparison of Talbot-Lau, near-field, and far-field interferometry with application to additively manufactured samples, lithium-ion batteries, and geometrically twinned crystals is presented

Energy-selective neutron imaging for materials science

Common neutron imaging techniques study the attenuation of a neutron beam penetrating a sample of interest. The recorded radiograph shows a contrast depending on traversed material and its thickness. Tomography allows separating both and obtaining 3D spatial information about the material distribution, solving problems in numerous fields ranging from virtually separating fossils from surrounding rock to water management in fuel cells. It is nowadays routinely performed at PSI¿s neutron imaging facilities. Energy-selective neutron imaging studies the wavelength-dependency of the cross-section by using a beam of reduced wavelength bandwidth instead of averaging out the cross-section over the incident beam spectrum. The range of observed contrasts/image information is than extended and can largely be understood in the context of the Bragg law. Different types of monochromator (mechanical neutron velocity selector, double crystal monochromator, filter materials) are characterized for use in neutron imaging. In polycrystalline samples, sharp Bragg edges are observed as coherent elastic scattering at the (hkl) plane can occur for all wavelengths up to 2dhkl, after which a sharp increase in transmission intensity is observed. Much like diffraction peaks, they contain information on e.g. crystal phase or projected strain. The absence of coherent elastic scattering past the last Bragg edge (Bragg cut-off) allows for quantification. In samples with few grains or even single crystals, all orientations w.r.t. the beam are no longer present and rather than Bragg edges, the cross section now exhibits distinct peaks, the ensemble of which holds information on the crystallite¿s phase, orientation and shape. A spatial variation in contrast appears across the sample, between those grains fulfilling the Bragg condition ¿ scattering and decreasing the transmitted beam intensity ¿ and those that do not. After initial qualitative assessments, recent advances on the quantitative grain orientation mapping are made based on time-of-flight measurements of high energy resolution recorded at the ISIS pulsed neutron source. But where do these scattered neutrons go to? A new set-up was developed to permit simultaneous transmission and diffractive neutron imaging. Capturing the neutrons diffracted by a grain also yields a projection of that grain, with the position on the detector indicative of the orientation. These projections can in turn be used for algebraic reconstruction, which yields a grain volume as well. After feasibility studies on an iron single crystal cube the recent push towards polycrystalline samples will is illustrated with a neutron diffraction contrast tomography (nDCT) of a coarse-grained aluminium strain sample

Event Centroiding Applied to Energy-Resolved Neutron Imaging at LANSCE

The energy-dependence of the neutron cross section provides vastly different contrast mechanisms than polychromatic neutron radiography if neutron energies can be selected for imaging applications. In recent years, energy-resolved neutron imaging (ERNI) with epi-thermal neutrons, utilizing neutron absorption resonances for contrast as well as for quantitative density measurements, was pioneered at the Flight Path 5 beam line at LANSCE and continues to be refined. Here we present event centroiding, i.e., the determination of the center-of-gravity of a detection event on an imaging detector to allow sub-pixel spatial resolution and apply it to the many frames collected for energy-resolved neutron imaging at a pulsed neutron source. While event centroiding was demonstrated at thermal neutron sources, it has not been applied to energy-resolved neutron imaging, where the energy resolution requires to be preserved, and we present a quantification of the possible resolution as a function of neutron energy. For the 55 μm pixel size of the detector used for this study, we found a resolution improvement from ~80 μm to ~22 μm using pixel centroiding while fully preserving the energy resolution