Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Investigation of Delayed Ettringite Formation Damage Process Using Simultaneous Neutron and X-ray Tomography(2019) Feuze Lekem, serge alain; Amde, Amde M; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Delayed ettringite formation (DEF) is a significant deterioration process in concrete which involves the growth of ettringite [Ca6Al2(SO4)3(OH)12 ·26H2O] crystals leading to cracking and reduction of compressive strength. Conditions leading to DEF are well known and include among others cement chemistry, presence of humidity, heat curing of concrete structures, and the presence of cracks. The mechanisms and kinetics by which deterioration occur is still not well understood despite numerous investigations. Understanding the mechanism and kinetics of concrete deterioration due to DEF is important in order to prevent such costly deterioration and to improve concrete durability. In this research, concrete specimens were prepared with type III Portland cement and under different conditions that were designed to either promote or inhibit DEF. These consisted of a control set, a set subjected to a heat cycle and a third set made with elevated potassium content of 1.72% and also thermally cycled. They were tested periodically up to 380 days by conventional methods such as expansion and weight change measurements and compressive strength testing. Scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDAX) confirmed the presence and the morphology of ettringite in voids at different ages. Simultaneous neutron and X-ray tomography, a new nondestructive microscopic method was used to scan the specimens at regular intervals in order to assess the feasibility of the method in monitoring the progress and characterizing DEF induced damages. The linear regression analysis of the correlation of expansion with weight change data revealed that expansion and deterioration process occurred in three distinct successive stages. In the first stage, the ettringite fills the pores with little or no expansion; in the second, the expansion appears to be creep due to expansive stresses in the filled pores and in the third stage, crack propagation leads to significant expansion and loss of compressive strength. The results of the linear regression also revealed that the mechanism of DEF is the replacement of pre-existing calcium hydroxide crystals. Through non-linear curve fitting, the kinetic of deterioration was modeled using the Kolmogorov-Avrami-Johnson-Miehl model. The simultaneous neutron and X-ray tomography allowed visualization of the interior of the specimen due to enhance phase segmentation. MATLAB routines were developed to allow for correction for beam hardening and to enhance phase segmentation. The study showed that with improved resolution, proper sample sizing, the method can be effectively used to characterize concrete damage due to expansive phases.Item Neutron Detection by Scintillation of Noble-Gas Excimers(2012) McComb, Jacob Collin; al-Sheikhly, Mohamad; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Neutron detection is a technique essential to homeland security, nuclear reactor instrumentation, neutron diffraction science, oil-well logging, particle physics and radiation safety. The current shortage of helium-3, the neutron absorber used in most gas-filled proportional counters, has created a strong incentive to develop alternate methods of neutron detection. Excimer-based neutron detection (END) provides an alternative with many attractive properties. Like proportional counters, END relies on the conversion of a neutron into energetic charged particles, through an exothermic capture reaction with a neutron absorbing nucleus (10B, 6Li,3He). As charged particles from these reactions lose energy in a surrounding gas, they cause electron excitation and ionization. Whereas most gas-filled detectors collect ionized charge to form a signal, END depends on the formation of diatomic noble-gas excimers (Ar2*, Kr2*, Xe2*). Upon decaying, excimers emit far-ultraviolet (FUV) photons, which may be collected by a photomultiplier tube or other photon detector. This phenomenon provides a means of neutron detection with a number of advantages over traditional methods. This thesis investigates excimer scintillation yield from the heavy noble gases following the boron-neutron capture reaction in 10B thin-film targets. Additionally, the thesis examines noble-gas excimer lifetimes with relationship to gas type and gas pressure. Experimental data were collected both at the National Institute of Standards and Technology (NIST) Center for Neutron Research, and on a newly developed neutron beamline at the Maryland University Training Reactor. The components of the experiment were calibrated at NIST and the University of Maryland, using FUV synchrotron radiation, neutron imaging, and foil activation techniques, among others. Computer modeling was employed to simulate charged-particle transport and excimer photon emission within the experimental apparatus. The observed excimer scintillation yields from the 10B(n,α)7Li reaction are comparable to the yields of many liquid and solid neutron scintillators. Additionally, the observed slow triplet-state decay of neutron-capture-induced excimers may be used in a practical detector to discriminate neutron interactions from gamma-ray interactions. The results of these measurements and simulations will contribute to the development and optimization of a deployable neutron detector based on noble-gas excimer scintillation.Item Characterization and Modeling of Off-Specular Neutron Scattering for Analysis of Two Dimentionally Ordered Structures(2011) Metting, Christopher Jason; Briber, Robert; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Off-specular neutron reflectometry is an instrumental technique which can be utilized for the characterization of thin-film systems in the depth and in-plane directions simultaneously. Currently, its use is limited both experimentally by the available neutron flux at modern neutron facilities and theoretically by a lack of widely available, user friendly, and open-source modeling software. This thesis describes work carried out on the development of a software package which utilizes currently available mathematical approximations to characterize model systems and evaluates the abilities and deficiencies of each algorithm. The evaluation will be carried out within the framework of a well-structured, object oriented, Python software package which is versatile and extendable. As new approximations and mathematical treatments are developed, they can be incorporated into the software infrastructure and tested with minimal effort. We show that, at high q, the Born approximation can be used to qualitatively model off-specular scattering data; however, it does not capture any of the dynamic effects observed in real data. Some dynamical effects can be captured by perturbing the wavefunction by interactions with the substrate/incident media interface; however, low q scattering as well as scattering at the 'horizons' is still inaccurately represented. Currently, the best interpretation of the off-specular scattering can be accomplished with the complete distorted wave Born approximation. This is shown to produce theory functions which match quite well with scattering data. Neutron coherence length is an important parameter in off-specular reflectometry as it dictates the number of feature periods being probed by the neutron beam. To determine the coherence length, a series of magnetic gratings were fabricated. Specular and off-specular measurements were used to evaluate the shape of the neutron wave packet work is still on going for a complete interpretation of these results.