Low-Information Radiation Imaging using Rotating Scatter Mask Systems and Neural Network Algorithms

While recent studies have demonstrated the directional capabilities of the single-detector rotating scatter mask (RSM) system for discrete, dual-particle environments, there has been little progress towards adapting it as a true imaging device. In this research, two algorithms were developed and tested using an RSM mask design previously optimized for directional detection and simulated 137Cs signals from a variety of source distributions. The first, maximum-likelihood expectation-maximization (ML-EM), was shown to generate noisy images, with relatively low accuracy (145% average relative error) and signal-to-noise ratio (0.27) for most source distributions simulated. The second, a novel regenerative neural network (ReGeNN), performed exceptionally well, with significantly higher accuracy (33\% average relative error) over all source types compared to ML-EM and drastically improved signal-to-noise ratio (0.85) in the reconstructed images. The imaging capabilities of ReGeNN were then experimentally validated using an additively-manufactured mask. Measuring two point and one ring 22Na source distributions, a modified ReGeNN was able to successfully train on simulated noisy signals and accurately predict the relative size and direction of the three sources. To support future design optimizations to overcome current limitations of the current mask design, a ray tracing algorithm was also developed as an alternative to more rigorous Monte Carlo RSM simulations. This ray tracing code was shown to significantly improve computational efficiency, at a slight cost to the simulated signal accuracy for more complex mask designs

Adaptive Imaging with a Cylindrical, Time-Encoded Imaging System

Most imaging systems for terrestrial nuclear imaging are static in that the design of the system and the data acquisition protocol are defined prior to the experiment. Often, these systems are designed for general use and not optimized for any specific task. The core concept of adaptive imaging is to modify the imaging system during a measurement based on collected data. This enables scenario specific adaptation of the imaging system which leads to better performance for a given task. This dissertation presents the first adaptive, cylindrical, time-encoded imaging (c-TEI) system and evaluates its performance on tasks relevant to nuclear non-proliferation and international safeguards. We explore two methods of adaptation of a c-TEI system, adaptive detector movements and adaptive mask movements, and apply these methods to three tasks, improving angular resolution, detecting a weak source in the vicinity of a strong source, and reconstructing complex source scenes. The results indicate that adaptive imaging significantly improves performance in each case. For the MATADOR imager, we find that adaptive detector movements improve the angular resolution of a point source by 20% and improve the angular resolution of two point sources by up to 50%. For the problem of detecting a weak source in the vicinity of a strong source, we find that adaptive mask movements achieve the same detection performance as a similar, non-adaptive system in 20%-40% less time, depending on the relative position of the weak source. Additionally, we developed an adaptive detection algorithm that doubles the probability of detection of the weak source at a 5% false-alarm rate. Finally, we applied adaptive imaging concepts to reconstruct complex arrangements of special nuclear material at Idaho National Laboratory. We find that combining data from multiple detector positions improves image uniformity of extended sources by 38% and reduces the background noise by 50%. We also demonstrate 2D (azimuthal and radial) imaging in a crowded source scene. These promising experimental results highlight the potential for adaptive imaging using a c-TEI system and motivate further research toward specific, real-world applications.PHDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/163009/1/nirpshah_1.pd

Coded Aperture Imaging: novel approaches to high-energy high-resolution laboratory imaging

Rapid advancement is being made in laser driven x-ray and particle sources, pushing the boundaries in temporal duration, spatial and spectral distribution, and maximum energy. These advancements need to be complimented with development of imaging capabilities, in order to fully characterise and utilise the new source potential. Here, coded apertures are used to investigate novel approaches to high-energy high-resolution aperture based imaging. Firstly, coded aperture theory is applied to high-energy x-ray sources such as those generated using laser wakefield techniques. The coded aperture is compared to a single pinhole aperture, to discuss whether the prior assumption of highly attenuating substrates is required when using coded apertures. The coded aperture with scatter and partial attenuation included, dubbed a `CASPA', is demonstrated with a 511 keV source simulation, showing that the fully attenuating 18~mm thick tungsten substrate for a single pinhole can be replaced with a 250 um thick tungsten CASPA. Furthermore, the thin CASPA is not mechanism specific, and the physical processes behind the scatter and partial attenuation is found to be inconsequential as long as the combined result yields adequate hologram contrast for image decoding to occur. Secondly, an investigation is conducted into imaging with spectral and spatial information for applications such as laser-solid interaction hotspots. Combing coded apertures with Ross pair filters, a banded spectrally-resolving coded aperture is discussed, dubbed a `BaSCA', using multiple non-redundant array designs on a single aperture and single non-spectrally resolving detector. Finally, the application of a CASPA for imaging high-resolution high-energy neutron sources from inertial confinement fusion experiments is discussed. Using the National Ignition Facility at Lawrence Livermore National Laboratory as an example, a CASPA is designed for the 14.1 MeV neutrons, and reconstruction techniques discussed. In comparison to the currently implemented 20 cm thick gold grand array, it is suggested here that a 10 mm tungsten CASPA would suffice - potentially reducing manufacturing costs, increasing ease of implementation and field of view

Coherent methods in the X-ray sciences

X-ray sources are developing rapidly and their coherent output is growing extremely rapidly. The increased coherent flux from modern X-ray sources is being matched with an associated rapid development in experimental methods. This article reviews the literature describing the ideas that utilise the increased brilliance from modern X-ray sources. It explores how ideas in coherent X-ray science are leading to developments in other areas, and vice versa. The article describes measurements of coherence properties and uses this discussion as a base from which to describe partially-coherent diffraction and X-ray phase contrast imaging, with its applications in materials science, engineering and medicine. Coherent diffraction imaging methods are reviewed along with associated experiments in materials science. Proposals for experiments to be performed with the new X-ray free-electron-lasers are briefly discussed. The literature on X-ray photon correlation spectroscopy is described and the features it has in common with other coherent X-ray methods are identified. Many of the ideas used in the coherent X-ray literature have their origins in the optical and electron communities and these connections are explored. A review of the areas in which ideas from coherent X-ray methods are contributing to methods for the neutron, electron and optical communities is presented.Comment: A review articel accepted by Advances in Physics. 158 pages, 29 figures, 3 table

Digital Image Processing

This book presents several recent advances that are related or fall under the umbrella of 'digital image processing', with the purpose of providing an insight into the possibilities offered by digital image processing algorithms in various fields. The presented mathematical algorithms are accompanied by graphical representations and illustrative examples for an enhanced readability. The chapters are written in a manner that allows even a reader with basic experience and knowledge in the digital image processing field to properly understand the presented algorithms. Concurrently, the structure of the information in this book is such that fellow scientists will be able to use it to push the development of the presented subjects even further

DUAL-MODALITY (NEUTRON AND X-RAY) IMAGING FOR CHARACTERIZATION OF PARTIALLY SATURATED GRANULAR MATERIALS AND FLOW THROUGH POROUS MEDIA

Problems involving mechanics of partially saturated soil and physics of flow through porous media are complex and largely unresolved based on using continuum approach. Recent advances in radiation based imaging techniques provide unique access to simultaneously observe continuum scale response while probing corresponding microstructure for developing predictive science and engineering tools in place of phenomenological approach used to date. Recent developments with X-ray/Synchrotron and neutron imaging techniques provided tools to visualize the interior of soil specimen at pore/grain level. X-ray and neutron radiation often presents complementary contrast for given condensed matter in the images due to different fundamental interaction mechanisms. While X-rays mainly interact with the electron clouds, neutrons directly interact with the nucleus of an atom. The dual-modal contrasts are well suited for probing the three phases (silica, air and water) of partially saturated sand since neutrons provide high penetration through large sample size and are very sensitive to water and X-rays of high energy can penetrate moderate sample sizes and clearly show the particle and void phases. Both neutron and X-ray imaging techniques are used to study microstructure of partially saturated compacted sand and water flow behavior through sand with different initial structures. Water distribution in compacted sand with different water contents for different grain shapes of sand was visualized with relatively coarse resolution neutron radiographs and tomograms. Dual-modal contrast of partially saturated sand was presented by using high spatial resolution neutron and X-ray imaging. Advanced image registration technique was used to combine the dual modality data for a more complete quantitative analysis. Quantitative analysis such as grain size distribution, pore size distribution, coordination number, and water saturation along the height were obtained from the image data. Predictive simulations were performed to obtain capillary pressure – saturation curves and simulated two fluid phase (water and air) distribution based image data. In-situ water flow experiments were performed to investigate the effect of initial microstructure. Flow patterns for dense and loose states of Ottawa sand specimens were compared. Flow patterns and water distribution of dense Ottawa and Q-ROK sand specimens was visualized with high resolution neutron and X-ray image data

Uncertainty Quantification in Emission Quantitative Imaging

Imaging detectors have potential to improve the reliability of plutonium holdup measurements. Holdup measurement is a significant challenge for nuclear safeguards and criticality safety. To infer holdup mass today, inspectors must combine data from counting (non-imaging) detectors with spatial measurements, process knowledge, and survey estimates. This process results in limited certainty about the holdup mass. Imaging detectors provide more information about the spatial distribution of the source, increasing certainty. In this dissertation we focus on the emission quantitative imaging problem using a fast-neutron coded aperture detector. We seek a reliable way to infer the total intensity of a neutron source with an unknown spatial distribution. The source intensity can be combined with other measurements to infer the holdup mass. To do this we first create and validate a model of the imager. This model solves the forward problem of estimating data given a known source distribution. We use cross-validation to show that the model reliably predicts new measurements (with predictable residuals). We then demonstrate a non-Bayesian approach to process new imager data. The approach solves the inverse problem of inferring source intensity, given various sources of information (imager data, physical constraints) and uncertainty (measurement noise, modeling error, absence of information, etc). Bayesian approaches are also considered, but preliminary findings indicate the need for advanced Markov chain algorithms beyond the scope of this dissertation. The non-Bayesian results reliably provide confidence intervals for medium-scale problems, as demonstrated using a blind-inspector measurement. However, the confidence interval is quite large, due chiefly to modeling error.PHDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/136929/1/ambevill_1.pd

GEANT4 : a simulation toolkit

Abstract Geant4 is a toolkit for simulating the passage of particles through matter. It includes a complete range of functionality including tracking, geometry, physics models and hits. The physics processes offered cover a comprehensive range, including electromagnetic, hadronic and optical processes, a large set of long-lived particles, materials and elements, over a wide energy range starting, in some cases, from 250 eV and extending in others to the TeV energy range. It has been designed and constructed to expose the physics models utilised, to handle complex geometries, and to enable its easy adaptation for optimal use in different sets of applications. The toolkit is the result of a worldwide collaboration of physicists and software engineers. It has been created exploiting software engineering and object-oriented technology and implemented in the C++ programming language. It has been used in applications in particle physics, nuclear physics, accelerator design, space engineering and medical physics. PACS: 07.05.Tp; 13; 2

An iterative method to deconvolve coded-mask images

Efficent astronomical imaging at energy greater than 20 keV is mainly achieved through modulation, either time (i.e. HXMT) or spatial (i.e. IBIS/INTEGRAL), techniques. Currently, the coded mask technique is widely used with the true spatial intensity distribution reconstructed from the data by the cross-correlation (CC) method. As the sensitivity of instruments increases, so must the angular resolution in order to avoid problems with source confusion. The IBIS 12’ angular resolution is clearly not sufficient to distinguish all the sources in the crowded field of the Galactic Centre. One possibility to overcome this problem is to change the deconvolution method. The objective of this thesis is to evaluate the real imaging capability of the Direct Demodulation (DD) method. It deconvolves incomplete and noisy data by iteratively solving the image formation equation under physical constraints. With the goal of exploiting the DD technique, in the early of the 1990s the HXMT mission was designed, where the imaging capability is obtained through the temporal modulation of the detected counts by a set of mechanical collimators. To achieve this goal, we developed the Lucy-Richardson (LR) code to reconstruct directly hard-X/soft- ray images. It assumes that the data and the noise follow a Poisson distribution and it guarantees the non-negativity of the restored images. For the moment, any kind of regularization or constraint was implemented in the underlying optimization problem, so this will be ill-posed yet. Due to the general nature of the DD and the fact that HXMT has still to fly, the IBIS/INTEGRAL data and its PSF were used to check our own code. The pure geometrical PSF considering only the effects due to the photon propagation from the mask to the detector was created. Our CC code implements the same balanced cross-correlation as the standard software for IBIS/INTEGRAL analysis. The CC deconvolved images are the reference for the image quality obtained with the LR. The great improvement in the theoretical angular resolution and location precision is evident. It is independent on the source position in the total FOV, the iteration number and the source flux. Within the parameters of the simulations used, the LR statistical uncertainty was found to be a factor of 10 smaller than that obtained with the CC. Furthermore, the LR deconvolved images have less fluctuating reconstructed background. The main LR drawback is the flux evaluation of the reconstructed source. It is mainly due to the choice of the correct iteration number. The use of a-priori information about the unknown object allows a complete regularization of the problem, so probably solving the problem with the flux estimation. Keywords: Coded-mask, Lucy, Richardson, INTEGRAL, IBI

Paraxial diffusion-field retrieval

Unresolved spatially-random microstructure, in an illuminated sample, can lead to position-dependent blur when an image of that sample is taken using an incoherent imaging system. For a small propagation distance, between the exit surface of the sample and the entrance surface of a position-sensitive detector, the paraxial approximation implies that the blurring influence of the sample may be modeled using an anomalous-diffusion field. This diffusion field may have a scalar or tensor character, depending on whether the random microstructure has an autocorrelation function that is rotationally isotropic or anisotropic, respectively. Partial differential equations are written down and then solved, in a closed-form manner, for several variants of the inverse problem of diffusion-field retrieval given suitable intensity images. Both uniform-illumination and structured-illumination schemes are considered. Links are made, between the recovered diffusion field and certain statistical properties of the unresolved microstructure. The developed theory -- which may be viewed as a crudely parallel form of small-angle scattering under the Guinier approximation -- is applicable to a range of paraxial radiation and matter fields, such as visible light, x rays, neutrons, and electrons