Quantification of Fast-Neutron Sources with Coded Aperture Imaging

Quantification of the mass of plutonium in facilities that process plutonium is important for both nuclear safeguards concerns and safety concerns, and multiple methods to nondestructively quantify plutonium sample characteristics have been proposed, particularly when the sample is located directly adjacent to or within the measurement device. In prior work, coded-aperture fast neutron imaging has been developed to demonstrate the imaging of neutron emitting radiation sources in a qualitative fashion, where the sources may be located meters to tens of meters away. Building upon prior work, this work develops the use of a Maximum Likelihood Expectation Maximization (MLEM) reconstruction technique to simultaneously reconstruct neutron sources measured from different detector positions. Moreover, a modified system response model is developed to accurately but quickly perform forward projections in order to accurately reconstruct and quantify neutron source characteristics including source intensity and location. The system response model incorporates mask transmission, a heterogeneous detector pixel array, scattering within the mask, and scattering within the detector, allowing for the expected detector data from a single source position to be generated in less than a second. The behavior of the MLEM reconstruction technique is discussed, and measurements of Cf-252 sources, acting as a surrogate Pu material, are reconstructed and analyzed. Using the methods developed here, a single 74 µCi Cf-252 point source placed at a distance of 200 cm is reconstructed within 2% of the known position and within 3% of known intensity at distances up to 300 cm. Measurements of more than one source and implications for Pu measurements in facilities are also discussed

MOXE: An X-ray all-sky monitor for Soviet Spectrum-X-Gamma Mission

A Monitoring Monitoring X-Ray Equipment (MOXE) is being developed for the Soviet Spectrum-X-Gamma Mission. MOXE is an X-ray all-sky monitor based on array of pinhole cameras, to be provided via a collaboration between Goddard Space Flight Center and Los Alamos National Laboratory. The objectives are to alert other observers on Spectrum-X-Gamma and other platforms of interesting transient activity, and to synoptically monitor the X-ray sky and study long-term changes in X-ray binaries. MOXE will be sensitive to sources as faint as 2 milliCrab (5 sigma) in 1 day, and cover the 2 to 20 KeV band

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

Neutron and Photon Imaging Capabilities of Bismuth-loaded Plastic

Plastic scintillators utilizing iridium complex fluorophores offer substantial improvements in light yield, and their light yield is not significantly quenched in compositions with bismuth metalorganic loading at 21% weight. These advances may resolve significant capability gaps for low-cost, portable, and durable dual-particle imaging (DPI) systems for nuclear safety, security, and safeguard purposes. However, all candidate materials should first undergo investigation utilizing industry standards to quantify and evaluate their capabilities. As such, a 21% bismuth-loaded polyvinyl toluene (BiPVT) scintillator fabricated by Lawrence Livermore National Laboratory (LLNL) is computationally and experimentally evaluated as a small, pixelated radiographic array, with individual pixel dimensions of 2×2×19 mm. To facilitate direct comparisons, the same evaluations are conducted for two same-sized arrays made from EJ-200 and EJ-256 scintillator, respectively. ASTM standard test methods and practices are utilized to calculate the modulation transfer function and basic spatial resolution for each array, both from measured and simulated data. Measurements are recorded by pressure coupling all three arrays to a commercial a-Si digital radiographic panel, and the computational model replicates the experimental design. Computational and experimental results are compared for all three arrays in the x-ray and fast neutron environments. The x-ray results demonstrate equivalent performance between the evaluated BiPVT array and the more ideally manufactured EJ-200 array, while the BiPVT array outperforms a similar array made from EJ-256. The agreement between simulated and experimental x-ray results validates the applied computational methodology and suggests more ideally manufactured BiPVT arrays may significantly outperform similar arrays made from EJ-200. Experimental results in a fast neutron environment demonstrate superior performance of the BiPVT array compared to the EJ-256 array, while the EJ-200 array is found to outperform both. Additionally, the performance of a second array made from a separate 21% bismuth-loaded plastic (Ir-Bi-Plastic) is evaluated experimentally in both x-ray and neutron environments using the same radiographic panel and methodology. The Ir-Bi-Plastic array consists of 64 pixels with individual dimensions of 5×5×20 mm, and the results suggest it will outperform similar arrays made from EJ-200 in both x-ray and neutron environments. These findings suggest plastic scintillators with iridium complex fluorophores and 21% weight bismuth-loading hold promise over more traditional material alternatives for DPI applications supporting nuclear safety, security, and safeguard missions

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

Use of Gas Electron Multiplier (GEM) Detectors for an Advanced X-ray Monitor

We describe a concept for a NASA SMEX Mission in which Gas Electron Multiplier (GEM) detectors, developed at CERN, are adapted for use in X-ray astronomy. These detectors can be used to obtain moderately large detector area and two-dimensional photon positions with sub mm accuracy in the range of 1.5 to 15 keV. We describe an application of GEMs with xenon gas, coded mask cameras, and simple circuits for measuring event positions and for anticoincidence rejection of particle events. The cameras are arranged to cover most of the celestial sphere, providing high sensitivity and throughput for a wide variety of cosmic explosions. At longer timescales, persistent X-ray sources would be monitored with unprecedented levels of coverage. The sensitivity to faint X-ray sources on a one-day timescale would be improved by a factor of 6 over the capability of the RXTE All Sky Monitor.Comment: 10 pages, 5 figs., in X-Ray and Gamma Ray Instrumentation for Astronomy XI, SPIE conference, San Diego, Aug. 200

Research in particles and fields

Research activities in Cosmic Rays Gamma Rays, and Astrophysical Plasmas are covered The investigation of the astrophysical aspects of cosmic rays and gamma rays and of the radiation and electromagnetic field environment of the Earth and other planets are studied. These investigations are carried ut by means of energetic particle and photon detector systems flown on spacecraft and balloons. The emphasis is on precision measurements with high resolution in charge mass and energy. An extensive bibliography is given

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