Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Douglas S. McGregor / The low natural abundance and supply shortage of ³He has resulted in an increase in the cost of ³He. The increase in cost of ³He proportional counters has motived the development of low cost, high efficiency, low gamma-ray sensitivity alternative technologies. A recently developed alternative technology is the ⁶Li-based suspended foil microstrip neutron detector (SFMND) that combines the neutron-conversion and gamma-ray discrimination capabilities of ⁶Li foils with the mechanical robustness and electrical capabilities of microstrip electrodes. SFMNDs differ from Li-foil multi-wire proportional counters because the anode wires are replaced by a single microstrip electrode that improves the mechanical robustness, reduces the microphonic sensitivity, and allows for more ⁶Li foils to be incorporated within a smaller form factor. The first-ever SFMNDs containing one and five 96%-enriched, 75-µm thick ⁶Li foils were fabricated using a silicon microstrip electrode. Neutron-sensitivity testing was performed yielding measured intrinsic thermal-neutron detection efficiencies, εth, of 4.02 ± 0.04% and 14.58 ± 0.11%, respectively. High electrode capacitance and gain instability were exhibited by the silicon microstrip electrode during neutron-sensitivity testing that led to the search for an electrically-stable microstrip-electrode substrate. Schott Borofloat® 33 glass was identified as an electrically-stable substrate and microstrip electrodes were fabricated and characterized. The Schott Borofloat® 33 microstrip electrodes were electrically-stable for a minimum duration of time of approximately 23 hours and had capacitances over an order of magnitude less than the identically sized silicon microstrip electrodes. One- and five-foil SFMNDs were fabricated with a Schott Borofloat® 33 microstrip electrode. Using 96%-enriched, 75-µm thick ⁶Li foils, the one- and five-foil devices had maximum measured εth of 12.58 ± 0.15% and 29.75 ± 0.26%, respectively, with measured gamma-ray rejection ratios of 6.46 x 10⁻⁵ ± 4.32 x 10⁻⁷ and 7.96 x 10-5 ± 4.65 x 10-7 for a ¹³⁷Cs exposure rate of 50 mR hr⁻¹. Devices containing one, five, ten, and twenty 96%-enriched, 75-µm thick ⁶Li foils were simulated using MCNP6 and are theoretically capable of having εth of 18.36%, 54.08%, 65.43%, and 68.36%, respectively. The deviation between measured and simulated εth is suspected to occur due to the electric field strength distribution, electron attachment, microstrip-electrode capacitance, or any combination thereof and solutions for each of these suspected concerns are described.

Master of Science / Department of Chemical Engineering / James H. Edgar / Hexagonal boron nitride has been used as an inert, refractory material with excellent resistance to thermal decomposition and oxidation for more than fifty years. In the past few years, hBN has been targeted for potential electrical and optical devices such as neutron detectors, ultraviolet light emitters, deep ultraviolet light detectors, and substrates for graphene and other atomically-thin two-dimensional materials. All of these potential applications benefit from high quality, single crystals, with thicknesses varying from nanometers to microns. This research was undertaken to investigate four aspects of hBN crystal growth and recovery. (1) In an effort to optimize hBN crystal growth from a nickel-chromium flux, a series of stepped cooling experiments were undertaken. The temperature profile was stepped in a way as to promote growth in both the a and c directions, at their optimal growth conditions. Crystals were found to be typically 100-500 µm across and thickness of approximately 20-30 µm with a pyramid-like crystal habit. (2) A method for the removal of hBN crystals prior to freezing of the metal flux was demonstrated using a specialized hot pressed boron nitride crucible capable of removing hBN crystals from the flux in situ. (3) Growth of isotopically pure hBN crystals was undertaken. By modifying the crucible material for solution growth, enrichment of hBN crystals over 90% was accomplished. (4) Exfoliation of hBN has many potential applications, specifically as graphene-hBN heterostructures where layers approaching thicknesses of single atoms are most effective surface to interact with graphene as an electronic device. Several methods were tested toward exfoliating a single crystal resulting in few-layered hexagonal boron nitride nanosheets. As a result of these investigations a greater understanding of hBN bulk growth, its isotopic enrichment, its recovery, and its exfoliation was obtained.

abstract: Due to diminishing availability of 3He, which is the critical component of neutron detecting proportional counters, large area flexible arrays are being considered as a potential replacement for neutron detection. A large area flexible array, utilizing semiconductors for both charged particle detection and pixel readout, ensures a large detection surface area in a light weight rugged form. Such a neutron detector could be suitable for deployment at ports of entry. The specific approach used in this research, uses a neutron converter layer which captures incident thermal neutrons, and then emits ionizing charged particles. These ionizing particles cause electron-hole pair generation within a single pixel's integrated sensing diode. The resulting charge is then amplified via a low-noise amplifier. This document begins by discussing the current state of the art in neutron detection and the associated challenges. Then, for the purpose of resolving some of these issues, recent design and modeling efforts towards developing an improved neutron detection system are described. Also presented is a low-noise active pixel sensor (APS) design capable of being implemented in low temperature indium gallium zinc oxide (InGaZnO) or amorphous silicon (a-Si:H) thin film transistor process compatible with plastic substrates. The low gain and limited scalability of this design are improved upon by implementing a new multi-stage self-resetting APS. For each APS design, successful radiation measurements are also presented using PiN diodes for charged particle detection. Next, detection array readout methodologies are modeled and analyzed, and use of a matched filter readout circuit is described as well. Finally, this document discusses detection diode integration with the designed TFT-based APSs. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2014

Nous avons développé un détecteur intégré à base d'une diode pour être embarqué dans un ballon stratosphérique afin de caractériser l'environnement radiatif atmosphérique. Le détecteur a été calibré avec une source Californium, et il a été pleinement caractérisé lors de tests sous faisceaux de neutrons qui produisent diverses particules ionisantes secondaires. Les sections efficaces différentielles de détection pour différentes énergies de faisceaux de neutrons sont avérées être en bon accord avec les simulations effectuées avec le code MC-Oracle. Nous avons effectué un certain nombre de vols en ballon stratosphériques (avec l'ESA et le CNES) et confirmé la corrélation entre le taux de comptage et de l'altitude. En outre, nous avons observé que l'environnement radiatif n'est pas isotrope et démontré le potentiel de notre outil pour étudier l'environnement radiatif atmosphérique. Ces résultats sont utiles pour estimer le flux de particules qui affecte appareils et systèmes électroniques à bord des appareils. / We developed an integrated silicon detector to be embedded in a stratospheric balloon in order to investigate the radiative atmospheric environment. The detector was calibrated with a Californium source, and it was fully characterized under neutron beams which produced various secondary ionizing particles. Differential detection cross sections for different neutron beam energies were shown to be in good agreement with simulations performed with the MC-Oracle code. We performed four stratospheric balloon flights (with ESA and CNES) and confirmed the correlation between the count rate and the altitude. Moreover, we observed that the radiative environment is not isotropic and demonstrated the potential of our tool for investigating the radiative atmospheric environment. These results are useful for estimating the particle flux that affects electronic devices and onboard aircraft systems.

Large amounts of money are being applied to the construction of the next generation of spallation sources for neutron scattering. Neutron powder diffraction instruments will be an important element of these facilities and the incorporation of detectors into these instruments with a high neutron capture efficiency is desirable. A new detector design named the Flexible Embedded Fiber Detector (FEFD) has been developed and tested for this thesis. This detector is based on wavelength shifting fibers embedded in a zinc-sulfide lithium-fluoride based scintillator. The virtue of this design is that the detecting surface can be curved around the Debye-Scherrer rings. This virtue is lacking in other detector designs, making them more complex and poorer in performance than our FEFD detectors. Monte Carlo calculations were performed to determine the neutron capture efficiencies of our FEFD detectors, which proved to be much higher than those of the proposed powder diffractometer design for the Spallation Neutron Source and about equal with the efficiency for the ISIS powder diffractometer design. Four FEFD detector prototypes were then fabricated and tested at the Intense Pulsed Neutron Source at Argonne National Laboratory. We find that our measured and calculated relative efficiencies are in good agreement.

The neutron-neutron (n-n) scattering length is a fundamental parameter in nuclear physics; however, measurements are plagued with large uncertainties caused by neutron detector cross talk. Many experimentalists also rely upon computer code to calibrate their neutron detectors. Experiments give one of two different numbers but there is still no adequate explanation for this discrepancy. We have developed a new neutron detector expressly for the purpose of improving the n-n scattering length measurement. It offers two important advantages: 1) minimal cross talk and 2) high counting efficiency. We calibrated the detector from 1 MeV to 6 MeV at 1 MeV increments. We have shown that the computer code, MCNP, does not always give the correct detector efficiency, and that reliance upon this code for calibration could be a large factor for error in previous experiments. Preliminary tests show no cross talk between two like detectors and suggest that these detectors in a n-n scattering length measurement.

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Master of Science / Department of Chemical Engineering / James H. Edgar / Pure boron exists in two main polymorphs, the common β-rhombohedral boron and the relatively rare α-rhombohedral boron. α-rhombohedral boron (α-B) possesses several extraordinary properties: self-healing from radiation damage and a high hole mobility. In addition, the [superscript]10B isotope has a large thermal neutron capture cross section. Such properties make it an excellent candidate for novel electronic device, such as direct energy conversion devices (alphacells and betacells) and neutron detectors. However, research on the properties and applications of α-B has been limited due to the difficulty to produce high quality α-B crystals of significant size. The preparation of α-rhombohedral boron is challenging for several reasons: first, α-rhombohedral boron has a low thermodynamic stability; it is only stable below 1100°C, at higher temperature β-rhombohedral boron is the stable polymorph. In addition, at elevated temperatures, boron is highly reactive, which make it is difficult to produce pure boron crystals. The primary goal of this research was to produce high quality α-B crystals of significant size. The main focus of this study was to explore the feasibility of producing α-B from a copper flux. Copper is a promising solvent for α-B crystal growth: the eutectic temperature of copper-boron is low, 996°C, and the phase diagram of copper-boron is relatively simple, and there are not many intermediate boride-copper compounds. In addition, copper is easily removed from crystals by etching with concentrated nitric acid. Last but not least, copper is less expensive than other metal solvents such as platinum. Boron crystal growth from a platinum solvent and vapor-liquid-solid growth by chemical vapor deposition were also performed for comparison. A series of crystals were grown over a range of initial boron concentrations (9.9 to 27.7 mole %) and cooling rates. Small irregular-shaped black crystals (>100μm) and well-faceted red crystals in various shapes, as large as 500 microns were produced. The crystals were characterized by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, x-ray diffraction analysis, and Raman spectroscopy. The correlation between experiment results and experimental parameters (source materials, the purity of growth atmosphere, and crucible materials, etc.) are reported. Suggestions about further investigation for α-B crystal growth are proposed.