A. James Clark School of Engineering
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Item Neutron Detection by Noble Gas Excimer Scintillation(2013) Beasten, Amy Elizabeth; Al-Sheikhly, Mohamad; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The field of neutron detection has many essential applications, from nuclear reactor instrumentation, oil-well logging, radiation safety, and, in recent years, homeland security. Due to the shortage and increasing cost of the neutron absorber used in most conventional gas-filled proportional counters, there has been an increased motivation for the development of alternative methods of neutron detection that do not rely on 3He. Excimer-based neutron detection (END) is a potential alternative with many advantages, notably the lack of dependence on 3He. Similar to traditional proportional counters, END operates on the interaction of a neutron with a neutron absorbing nucleus (10B, 6Li, or 3He). The energetic charged particles produced in these reactions lose energy in the surrounding gas background and cause ionization and excitation of the noble gas molecules. The difference between END and traditional gas-filled detectors, which collect the ionized charge to produce a detectable signal, is the formation of noble gas excimers (Ar2*, Kr2*, or Xe2*). These excited dimers decay from an excited state back to ground level and emit far-ultraviolet (FUV) radiation in the form of photons which can be collected using a photomultiplier tube (PMT) or other photon detector. The most important advantage to these potential detectors is the fact that they do not rely on the use of 3He. The excimer scintillation yield from rare noble gases following the 10B neutron capture reaction in both 10B enriched BF3 gas and reticulated vitreous carbon foam (RVC) coated with a layer of B4C is the focus of this thesis. Experimental data were collected at the National Institute of Standards and Technology (NIST) and on a recently established thermal neutron beamline at the Maryland University Training Reactor (MUTR). The comparison of these data to data from previous thin-film experiments provides the groundwork for the continuation of future END work using these materials, which will be used to develop and optimize a deployable neutron detector based on excimer emission.Item THERMODYNMICS AND STRUCTURE OF POLY(ETHYLENE OXIDE) IN MIXTURES OF WATER AND ETHANOL(2011) SHIN, SANG HAK; Briber, Robert M; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Poly(ethylene oxide) (PEO) is one of the most researched synthetic polymers due to the complex behavior which arises from the interplay of the hydrophilic and hydrophobic sites on the polymer chain. PEO in ethanol forms an opaque gel-like mixture with a partially crystalline structure. Addition of a small amount of water disrupts the gel: 5 wt % PEO in ethanol becomes a transparent solution with the addition of 4 vol % water. The phase behavior of PEO in mixed solvents have been studied using small-angle neutron scattering (SANS). PEO solutions (5 wt % PEO) which contain 4 vol % - 10 vol % (and higher) water behave as an athermal polymer solution and the phase behavior changes from UCST to LCST rapidly as the fraction of water is increased. 2 wt % PEO in water and 10 wt % PEO in ethanol/ water mixtures are examined to assess the role of hydration. The observed phase behavior is consistent with a hydration layer forming upon the addition of water as the system shifts from UCST to LCST behavior. At the molecular level, two or three water molecules can hydrate one PEO monomer (water molecules form a sheath around the PEO macromolecule) which is consistent with the suppression of crystallization and change in the mentioned phase behavior as observed by SANS. The clustering effect of aqueous PEO solution (M.W of PEO = 90,000 g/mol) is monitored as an excess scattering intensity at low-Q. Clustering intensity at Q = 0.004 Å^-1 is used for evaluating the clustering effect. The clustering intensity is proportional to the inverse temperature and levels off when the temperature is less than 50 ˚C. When the temperature is increased over 50 ˚C, the clustering intensity starts decreasing. The clustering of PEO is monitored in ethanol/ water mixtures. The clustering intensity increases as the fraction of water is increased. Based on the solvation intensity behavior, we confirmed that the ethanol/ water mixtures obey a random solvent mixing rule, whereby solvent mixtures are better at solvating the polymer that any of the two solvents. The solution behavior of PEO in ethanol was investigated in the presence of salt (CaCl2) using SANS. Binding of Ca2+ ions to the PEO oxygens transforms the neutral polymer to a weakly charged polyelectrolyte. We observed that the PEO/ethanol solution is better solvated at higher salt concentration due to the electrostatic repulsion of weakly charged monomers. The association of the Ca2+ ions with the PEO oxygen atoms transforms the neutral polymer to a weakly charged polyelectrolyte and gives rise to repulsive interactions between the PEO/Ca2+ complexes. Addition of salt disrupts the gel, which is consistent with better solvation as the salt concentration is increased. Moreover, SANS shows that the phase behavior of PEO/ethanol changes from UCST to LCST as the salt concentration is increased.