Dissertations / Theses: 'Neutron scattering'

1

Klug, Joakim. "Elastic Neutron Scattering at 96 MeV." Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis : Univ.-bibl. [distributör], 2003. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-3453.

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2

Grundy, Michael J. "Neutron scattering from interfaces." Thesis, University of Bristol, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.357011.

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3

Huxley, Andrew David. "Neutron scattering from superconductors." Thesis, University of Cambridge, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239626.

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4

Buffler, Andy. "Fast neutron scattering analysis." Doctoral thesis, University of Cape Town, 1998. http://hdl.handle.net/11427/17137.

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Bibliography: pages 217-225.
The scattering of a beam of fast monoenergetic neutrons is used to determine elemental compositions of small (0.2-1 kg) samples of materials. Particular emphasis is placed on the measurement of concentrations of the elements H, C, N and O, which are the principal constituents of contraband materials, such as explosives and narcotics. Scattered neutrons are detected by liquid scintillators located at forward and at backward angles, and different elements are identified by their characteristic scattering signatures derived either from a combination of time-of-flight and pulse height measurements or from pulse height measurements alone. Atom fractions for H, C, N, O and other elements are derived from unfolding analyses based on these scattering signatures and used to identify materials. Effects of neutron interactions in surrounding materials, either in the neutron beam or between the scatterer and the detectors, can be detected and allowed for in such a way as not to interfere significantly in the identification of the scatterer. The Fast Neutron Scattering Analysis technique provides a non- intrusive method for detecting and identifying sub-kilogram quantities of contraband materials. Methods for locating the positions of small contraband items in packages of volume up to about 0.5m³ are described and a two-stage screening system for detecting contraband hidden in small packages is proposed.

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5

Moore, Brian Randolph. "A neutronic study of an intense epithermal neutron source based on the ⁹BE(P,N) ⁹B reaction for neutron capture therapy." Thesis, Georgia Institute of Technology, 1992. http://hdl.handle.net/1853/16364.

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6

Ott, Frédéric. "Neutron scattering on magnetic nanostructures." Habilitation à diriger des recherches, Université Paris Sud - Paris XI, 2009. http://tel.archives-ouvertes.fr/tel-00429509.

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7

Öhrn, Angelica. "Neutron Scattering at 96 MeV." Doctoral thesis, Uppsala universitet, Tillämpad kärnfysik, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-8425.

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Data on elastic scattering of 96 MeV neutrons from 56Fe, 89Y and 208Pb in the angular interval 10-70° are presented. The previously published data on 208Pb have been extended, as a new method has been developed to obtain additional information at the most forward angles. The results are compared with phenomenological and microscopic optical potentials. The theory predictions are in general in good agreement with the experimental data. A study of the deviation of the zero-degree cross section from Wick's limit has been performed. The data on 208Pb are in agreement with Wick's limit, while those on lighter nuclei overshoot the limit significantly. A novel analysis method has been developed to obtain the inelastic neutron emission cross sections from the existing 56Fe data. The method is based on folding a trial spectrum with the response of the detector setup. The data cover the angular interval 26-65° and an excitation energy range of 0-45 MeV, ranges hitherto not studied. The results are compared with nuclear model predictions and found to be in good agreement with the experimental data.

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8

Nield, V. M. "Neutron scattering studies of disorder." Thesis, University of Oxford, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.316918.

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9

An, Shuwang. "Neutron scattering from adsorbed species." Thesis, University of Oxford, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.297936.

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10

Tait, Kimberly. "Inelastic Neutron Scattering and Neutron Diffraction Studies of Gas Hydrates." Diss., The University of Arizona, 2007. http://hdl.handle.net/10150/194926.

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Gas hydrates (clathrates) are elevated-pressure (P) and low-temperature (T) solid phases in which gas molecule guests are physically incorporated into hydrogen-bonded, cage-like ice host frameworks. Natural clathrates have been found worldwide in permafrost and in ocean floor sediments, as well as in the outer solar system (comets, Mars, satellites of the gas giant planets). Diffraction patterns have been collected of gas hydrates at various methane and ethane compositions by preparing samples in an ex situ gas hydrate synthesis apparatus, and CO₂ gas hydrates were prepared in situ to look at the kinetics of formation. Storage of hydrogen in molecular form within a clathrate framework has been one of the suggested methods for storing hydrogen fuel safely, but pure hydrogen clathrates H₂(H₂O)₂ form at high pressures. It has been found that mixed clathrates (a stabilizer molecule in the large cage) and hydrogen gas together can reduce the pressures and temperatures at which these materials form. In situ neutron inelastic scattering experiments on hydrogen adsorbed into a fully deuterated tetrahydrofuran water ice clathrate show that the adsorbed hydrogen has three rotational excitations (transitions between J = 0 and 1 states) at approximately 14 meV in both energy gain and loss. These transitions could be unequivocally assigned the expected slow conversion from ortho- to para-hydrogen resulted in a neutron energy gain signal at 14 meV, at a temperature of 5 K (kT= 0.48 meV). A doublet in neutron energy loss at approximately 28.5 meV are interpreted as J = 1 → 2 transitions. In situ neutron inelastic scattering experiments on hydrogen adsorbed into ethylene oxide, a structure I former, were also carried out at the Los Alamos Neutron Scattering Center (LANSCE). There is convincing evidence (shifted rotational mode of molecular hydrogen) that hydrogen is capable of diffusing in the small cages of ethylene oxide clathrate. Values are also obtained for the librational modes of enclathrated ethylene oxide and several water translation modes. Also reported for the first time are the internal modes (higher frequencies) of ethylene oxide in ethylene oxide clathrate as measured by inelastic neutron scattering.

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11

Horton, J. C. "Neutron scattering from star-branched polymers." Thesis, University of Cambridge, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.233980.

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12

Goff, Jonathan Peter. "Neutron scattering studies of fluorite compounds." Thesis, University of Oxford, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.305411.

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13

Howe, M. A. "Neutron scattering studies of molten salts." Thesis, University of Oxford, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.382644.

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14

Hackett, M. A. "Neutron scattering studies of fluorite compounds." Thesis, University of Oxford, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.379963.

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15

Brunier, Thierry Marcel. "Neutron scattering studies of amorphous materials." Thesis, University of Reading, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.254171.

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16

Stead, William John. "Neutron scattering from molecular metallic compounds." Thesis, University of Oxford, 1986. http://ora.ox.ac.uk/objects/uuid:a1978bc4-b4ff-42c4-855a-7103b39a75c1.

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The alkali metal graphite intercalates and the conducting polymers polyacetylene and polypyrrole are reviewed and compared. Polyacetylene was synthesized by Luttinger's method and doped by exposure to caesium vapour. The doped and undoped polyacetylene were characterized by neutron powder diffraction. Hydrogen absorption and subsequent pyrolysis of caesium doped polyacetylene were studied. Mass spectrum analysis of the gases evolved at 300°C showed methane and other alkanes, indicating H₂ addition to the polymer chain followed by methane elimination. The graphite intercalates C₈Cs and C₂₄Cs reacted irreversibly with acetylene, neutron powder diffraction indicating graphite and two new substances. Mass spectrum analysis of gases evolved on heating showed ethylene, butene and butadiene, suggesting a polymerization. Polypyrrole was synthesized electrochemically and characterized by cyclic voltammetry. Gram quantities of polypyrrole doped with BF^-₄ and C10^-₄ were prepared electrochemically under high vacuum conditions to achieve high purity and avoid oxygen doping. Perchlorate doped polypyrrole prepared in an oxygen-free environment proved dangerously unstable, especially when dry. The neutron scattering law for a harmonic oscillator and the energy level splitting pattern of a 1-D rotor in a cos3θ hindering potential are reviewed and applied to solid acetonitrile. Intense lines in the neutron spectrum at 140 and 190 cm^-1 are assigned to methyl group torsions with barriers to rotation of 400 and 734 cm^-1. Hydrogen is reversibly physisorbed by intercalates C₂₄K, C₂₄Rb and C₂₄Cs at 100K to form compounds C₂₄M(H₂)₂. Neutron spectra of H₂, HD and D₂ physisorbed by C₂₄Rb and C₂₄Cs at energy transfers between 0.3-3 meV and 10-200 meV (1 meV = 8.065 cm^-1) were measured and assigned to rotational tunnelling and librations of hydrogen molecules in two sites of different dimensions. The cos2θ barriers to rotation were 57 and 78 meV respectively in C₂₄Rb(H₂)_x and 51 and 78 meV in C₂₄Cs(H₂)_x. A structural model is proposed for the metal layers with the hydrogen molecules in the layers and perpendicularly oriented.

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17

Allen, Peter James. "Small angle neutron scattering from superconductors." Thesis, Durham University, 1991. http://etheses.dur.ac.uk/5994/.

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Small angle neutron scattering is a well known technique for studying the properties of solids at small but finite energies and momenta. In this work it has been applied to the study of the low energy properties of superconductors. The low transition temperature superconductors niobium and lead and the high transition temperature superconductor Y(_1)Ba(_2)Cu(_3)O(_7) have been investigated. The results indicate a temperature dependent excess scattering of around 150 mbam/sr/atom from Y(_1)Ba(_2)Cu(_3)O(_7) at wavevectors q of around 0.03 Å and an energy of less than 30 µeV. No such effect was observable, even at much lower wavevectors (q around 0.004 Å (^-1) in the low transition temperature superconductor niobium. However, a drop in transmission of a niobium sample at T(_c) indicates an increase in cross section of 71 ± 15 mbarn/atom. No simple explanation for these effects has been found.

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18

Poland, G. A. "Neutron scattering experiments analysis and modelling." Thesis, University of Portsmouth, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.328178.

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19

Fennell, Tom. "Neutron scattering investigation of frustrated magnets." Thesis, University College London (University of London), 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.408152.

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20

Ohlson, Alice Elisabeth. "Neutron-deuteron breakup and quasielastic scattering." Thesis, Massachusetts Institute of Technology, 2009. http://hdl.handle.net/1721.1/51589.

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Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2009.
Includes bibliographical references (p. 89).
Quasielastic scattering and deuteron breakup in the 200 MeV region is studied by impinging a pulsed neutron beam on a deuterium target at the Weapons Neutron Research facility at the Los Alamos Neutron Science Center. The scattered neutrons from the d(n,np)n reaction are detected by a wall of neutron time-of-flight scintillators, and scattered protons are detected by two scintillators and a wire chamber. This setup allows for measurement of the incident neutron energy, scattered neutron energy, and scattered proton energy, as well as the scattering angle and position of the scattered neutron. The results of the experiment are compared with a Monte Carlo simulation of neutron-proton scattering, to observe the differences between two-body and three-body quasielastic collisions. The four-dimensional differential cross section for the d(n,np)n reaction is determined from the data.
by Alice Elisabeth Ohlson.
S.B.

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21

Aldus, R. "Neutron scattering studies of frustrated magnets." Thesis, University College London (University of London), 2010. http://discovery.ucl.ac.uk/19623/.

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Here is presented an experimental study of the low-temperature magnetic behaviour of the spin ice holmium titanate, Ho_2Ti_2O_7; stuffed spin ice, Ho_{2+x}Ti_{2-x}O_{7-x/2}; and neodymium zirconate, Nd_2Zr_2O_7. The main techniques utilised here are based on neutron scattering which gives information on the static and dynamic behaviour of materials. Stuffed spin ice represents the addition of holmium magnetic moments into holmium titanate. This addition of the Ho^3^+ affects the oxygen lattice which is responsible for the considerable magnetocrystalline anisotropy in these materials. Inelastic scattering measurements reveal that a crystal of Ho_{2.3}Ti_{1.7}O_{6.85} is a static magnetic system at the lowest energies. Elastic neutron scattering on the same crystal reveals short-range order that distinguishes it from spin ice. Neodymium zirconate, Nd_2Zr_2O_7, is a stoichiometric pyrochlore material isomorphous to holmium titanate in structure. Here a single crystal and a powder sample have been investigated by both elastic and inelastic scattering. The powder and crystals exhibit low temperature intensity in the same temperature range indicating a low temperature transition which has been shown to be second order. A field induced transition has also been studied, but its origin has not been established. Holmium titanate is a stoichiometric material unlike stuffed spin ice. Here a study has been conducted into scattering in the hhl plane under application of a magnetic field at a temperature of 60 mK. The results indicate a breaking of the ordered \alpha-chain structure within the material as the field is removed.

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22

Babkevich, Peter. "Quantum materials explored by neutron scattering." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:5f6ef05d-e846-47e1-b59f-864ea4fa2f3f.

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This thesis describes neutron scattering experiments on strongly correlated systems exhibiting a range of emergent phenomena: antiferromagnetism, charge order, superconductivity and multiferroicity. I have examined the La_{2}CoO_{4} compound which is a Mott insulator and orders antiferromagnetically near room temperature. The La_{2}CoO_{4} sample was studied using spherical neutron polarimetry and I present magnetic structure models to describe the two antiferromagnetic phases of the compound. Furthermore, the magnetic fluctuations have been investigated using neutron time-of-flight technique. This has allowed us to extract the dominant exchange interactions in the system. More interestingly, the work on La_{2}CoO_{4} presented in this thesis provides a basis for the experimental evidence of an hourglass dispersion in La_{5/3}Sr_{1/3}CoO_{4}, previously only observed in the copper oxide based superconductors. This dispersion has been understood in terms of a stripe ordered magnetic phase and was found to be well described by a linear spin-wave model. Neutron scattering experiments were also carried out on the new iron-based high-temperature superconductors, FeSe_{x}Te_{1−x}. A range of compositions were studied, including both antiferromagnetically ordered and superconducting. Below the superconducting phase transition temperature, a spin resonance mode was found centred on the antiferromagnetic wavevector. This is an important feature shared by many unconventional superconductors. The spin resonance intensity was found to reflect the order parameter of the superconducting state. Polarised inelastic neutron scattering experiments have revealed a small anisotropy between the in-plane and out-of-plane magnetic fluctuations at the resonance. This anisotropy cannot be readily explained by the usual anisotropic terms in the Hamiltonian. This could be evidence of new physics in the FeSe_{x}Te_{1−x} superconductors. Finally, I have studied CuO – a high-temperature multiferroic. Analysis of polarised neutron diffraction experiments shows that the magnetic domain population can be varied using an externally applied electric field. This unambiguously demonstrates coupling between the magnetic and ferroelectric degrees of freedom. Using representation analysis I derive the incommensurate magnetic structure in the multiferroic phase. The origin of the magnetoelectric coupling is consistent with models based on the inverse Dzyaloshinskii-Moriya interaction.

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23

Criswell, Leah. "Neutron diffraction and quasielastic neutron scattering studies of films of N-alkanes and a branched alkane absorbed on graphite." Diss., Columbia, Mo. : University of Missouri-Columbia, 2007. http://hdl.handle.net/10355/6010.

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Thesis (Ph. D.)--University of Missouri-Columbia, 2007.
The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on March 11, 2008) Includes bibliographical references.

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24

Mermod, Philippe. "Neutron-Deuteron Scattering and Three-Body Interactions." Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-6739.

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25

Helme, Lucy. "Neutron Scattering Studies of Correlated Electron Systems." Thesis, University of Oxford, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.491531.

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This thesis presents neutron scattering studies of three correlated electron systems, each of which exhibit different competing interactions. These include charge order, magnetic order and lattice degrees of freedom. The main focus is on magnetic excitations within the systems. In all cases the experimental data have been analysed through comparison with theoretical models. Chapter 3 presents an investigation into the Jahn-Teller effect in the rare earth oxide Pr02, through inelastic neutron scattering studies of the crystal field transitions above and below a static structural distortion temperature. The data are compared with a point-charge model of the crystal field levels. \Ve conclude that the observed temperature evolution of the crystal field levels originates from the structural distortion due to the Jahn-Teller effect. Chapter 4 describes studies of magnetic excitations in the layered charge-ordered transition-metal oxide La1.5Sro.5Co04 through inelastic neutron scattering studies, and subsequent comparison with spin-wave dispersion models. It was found that the spin-wave excitations were decoupled from the charge order. Inclusion of the strong crystal anisotropy was necessary in order to successfully describe the data. Chapters 5 and 6 present studies of the magnetically ordered phase of Nax Co02 with x rv 0.75, a metallic layered transition-metal oxide. Chapter 5 describes investigations into the magnetic excitations in the compound, which were successfully modelled by linear spin-wave theory, including terms for the anisotropy. The excitations were found to be highly three dimensional despite the layered nature of the crystal structure. Chapter 6 presents a diffraction study of a spin-flop transition in an applied magnetic field, which confirmed the magnetic order. The transition field was found to be in excellent agreement with the exchange and anisotropy parameters extracted in chapter 5.

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26

Su, Tsueu-Ju. "Neutron and X-ray scattering from interfaces." Thesis, University of Oxford, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.319060.

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27

Lake, Alysia C. I. "Neutron scattering studies of alternating chain antiferromagnets." Thesis, University of Oxford, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.360395.

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28

Moy, David. "Study of ionomeric membranes by neutron scattering." Thesis, University of Oxford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.297306.

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29

Longmore, A. "Neutron scattering and praseodymium suppression of superconductivity." Thesis, University of Oxford, 1995. http://ora.ox.ac.uk/objects/uuid:46caa612-2ff0-40ae-8cdc-898a9bb9c639.

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PrBa₂Cu₃O₆₊_x is anomalous among the compounds which can be made by substituting different rare-earth ions for yttrium (Y) in YBa₂Cu₃O₆₊_x:it is the only compound which has the same structure as YBa₂Cu₃O₆₊_x, and yet does not superconduct. This unusual property makes it an important system to study, since the differences between the two compounds could produce theoretical insights into the mechanisms of cuprate, or high-temperature, superconductivity. This thesis describes neutron scattering investigations of the magnetic properties of PrBa₂Cu₃O₆₊_x. I have investigated the magnetic ordering in well-characterised, single-crystal samples, both with and without Al-contamination. In the data analysis, proper account is taken of the sample mosaicity and the different intrinsic peak shapes encountered. Some of the most striking new characteristics reported here are that (i), there is a clear interaction between the Pr ions and the Cu-O₂ planes, (ii), the ordered Pr moments tilt away from the c-axis (as suggested previously by Mossbauer spectroscopy), and (iii), there is two-dimensional ordering in the Al-containing crystals. I show how observed trends in the magnetic ordering, as O- and Al-contents vary, may be understood in terms of charge redistribution involving the hybridisation of the Pr ions. I have also examined the crystal field transitions of the praseodymium ion. Because of the difficulties previously encountered in measuring these in PrBa₂Cu₃O₆₊_x, I have used instead the compounds PrO₂ and Y_0.3Pr_0.7Ba₂Cu₄O₈, which contain praseodymium in a chemically similar environment. Y_0.3Pr_0.7Ba₂Cu₄O₈ shows the same broadening effects as PrBa₂Cu₃O₆₊_x; PrO₂ shows signs of mixed-valency, with temperature-dependent lifetime effects. The application of symmetry considerations and the point-charge model to the crystal field measurements is considered in some detail.

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30

Welsh, Gary. "Morphology of block copolymers from neutron scattering." Thesis, Durham University, 1992. http://etheses.dur.ac.uk/5606/.

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Block copolymers are widely used commercially and so a complete understanding of these systems requires that the nature of the polymer interface, the effect of deformation and the interaction between polymer substituents are known. Triblock copolymers of poly(styrene-isoprene-styrene) were synthesised and used in a Small Angle Neutron Scattering (SANS) study of deformation (Ch. 3) . Three sets of samples were made and, of these, two sets were found to have degraded giving unexpected results (Ch. 3.2). The third set, SPH150 series, however, did exhibit the expected anisotropy on deformation (Ch. 3.3). In these samples, the extension parallel to the stretch direction was affine for all elongation ratios studied and, perpendicular to the stretch direction, the extension was non-affine. The interaction between polymer substituents was examined using SANS on an isotopic diblock copolymer of polystyrene to ascertain if the mixing of hydrogenous and deuterated species was ideal (Ch. 4) . The results obtained showed the mixing between these species could be assumed to be ideal. The nature of the polymer-polymer interface was studied on the CRISP reflectometer using poly(styrene- isoprene) diblock samples (Ch. 5) . The results obtained for unannealed samples showed that a 'pseudo- equilibrium' had been achieved in these samples which was lost on annealing.

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31

Garawi, M. S. "Thermal neutron scattering by some molecular liquids." Thesis, University of Kent, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.378356.

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32

Guy, Carlo Alessandro. "Neutron scattering investigations of some disordered systems." Thesis, University of Reading, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.240699.

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33

Hulme, Robert Alexander. "Studies of glass structure by neutron scattering." Thesis, University of Reading, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.293601.

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34

Lindley, E. J. "Neutron scattering studies of transition metal alloys." Thesis, University of Southampton, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.355332.

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35

Ansbro, Simon. "Molecular nanomagnets probed by inelastic neutron scattering." Thesis, University of Manchester, 2017. https://www.research.manchester.ac.uk/portal/en/theses/molecular-nanomagnets-probed-by-inelastic-neutron-scattering(8206c00e-50bf-48e4-888d-6161791e8159).html.

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Molecular nanomagnets have the potential to address many technological challenges whilst also giving insights into fundamental quantum behaviours. In order to accomplish this it is crucial that a determination of the magnetic properties of these nanoscopic objects is established. Inelastic neutron scattering (INS) is at the forefront of these investigations and is a technique capable of unravelling vast amounts of information that can enable a detailed understanding and tailoring of the properties of these molecules. This thesis demonstrates the application of this state-of-the-art technique to a number of different yet important nanomagnets in the field, pushing the technique to the limit to yield new insights into the underlying physics in these systems. The potential of this technique is finely displayed in the unravelling of the spin dynamics in a supramolecular (Cr7Ni)2 dimer, where the intermolecular entanglement is portrayed in the vast 4D phase space extracted. This work also harnesses the full power of the 4D-INS technique to resolve the long standing issue of the spin Hamiltonian in the archetypal single molecule magnet (SMM) Mn12, which after hundreds of papers since the realisation of its magnetic properties some 25 years ago had still not been convincingly settled. A little utilised avenue to building on Mn12 and generating better SMMs is also suggested and the evolution of the technique demonstrated by 8 characterising the magnetic properties in a tiny quantity of nanomagnet dimers.

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36

Baker, Michael Lloyd. "Antiferromagnetic wheels probed by inelastic neutron scattering." Thesis, University of Manchester, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.542789.

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37

Higgins, Stewart A. "Neutron scattering studies of disordered magnetic systems." Thesis, University of Edinburgh, 1985. http://hdl.handle.net/1842/14065.

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38

Wheeler, Elisa Maria da Silva. "Neutron scattering from low-dimensional quantum magnets." Thesis, University of Oxford, 2007. http://ora.ox.ac.uk/objects/uuid:a8411774-4a3e-4fc3-80a1-d7e8612cba71.

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Neutron scattering measurements were used to investigate the magnetic and crystal structure and magnetic excitations of three compounds characterized as low-dimensional quantum magnets. The materials are frustrated systems with low spin quantum number. The first was a powder sample of AgNiO₂. The Ni ions form a triangular lattice antiferromagnet in which, according to the published crystal structure, both the orbital order and magnetic couplings are frustrated. However, it is shown here that there was a small distortion of the crystal structure at 365 K, which is proposed to result from charge disproportionation and this relieves the orbital frustration. The magnetic structure was investigated and, below 20 K, the triangular lattice of electron-rich Ni sites was observed to order into antiferromagnetic stripes. Investigations of the magnetic excitations showed that the main dispersions were within the triangular plane, indicating a strong two-dimensionality. The dispersion was larger along the stripes than between the stripes of collinear spins. The second material investigated was CoNb₂O₆, a quasi Ising-like ferromagnet. It was studied with a magnetic field applied transverse to the Ising direction. The magnetic field introduced quantum fluctuations which drove a phase transition at a field comparable to the main exchange interaction. The phase diagram of the magnetic order was mapped outs and a transition from an ordered phase to a paramagnetic phase was identified at high field. This low-temperature high-field phase transition was further investigated by inelastic neutron scattering measurements to observe the change in the energy gap and magnetic excitation spectrum on either side of the transition. The spectrum had two components in the ordered phase and had sharp magnon modes in the paramagnetic phase. The third material was the spin-half layered antiferromagnet CuSb₂O₆. It has a square lattice of Cu²⁺ ions in which the main interaction is across only one diagonal of the square. The magnetic structure was studied by neutron scattering with a field applied along the direction of the zero-field ordered moment. A spin-flop was observed at low field and there was evidence for a high-field transition. The magnetic excitation spectrum was unusual in that it had an intense resonance at 13 meV at the magnetic Brillouin zone boundary.

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39

Bryan, Matthew S. "Inelastic Neutron Scattering of Nanoconfined Superfluid Helium." Thesis, Indiana University, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10842052.

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The dynamics of liquid ⁴He confined in a mesoporous powder FSM-16 are reported in this dissertation, including the roton linewidth, excitation spectrum, and Compton profile. With an ordered triangular lattice structure, FSM-16 is a high surface area porous glass with hexagonal pores a few nanometers in diameter. Neutron backscattering results examined the roton linewidth as a function of temperature. Observed linewidths in confinement are consistent with the theoretical and experimental results of the bulk liquid. The temperature-filling phase diagram was explored at intermediate fillings and low temperatures. The maxon and roton excitations are used as indicators of density for a thin-film that transitions into a three dimensional confined fluid. The resulting excitation spectrum at low fillings does not correspond to the bulk liquid at any pressure. The deep inelastic neutron scattering results found an enhanced single particle kinetic energy, with full pore and thin film liquid deviating from the bulk momentum distribution in shape.

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40

SUKUMARAN, SATHISH KUMAR. "SMALL ANGLE NEUTRON SCATTERING FROM COMPLEX SYSTEMS." University of Cincinnati / OhioLINK, 2002. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1028818463.

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41

Rahighi, Javad. "High resolution neutron spectrometer development and 14 MeV neutron scattering by Bi." Thesis, University of Edinburgh, 1985. http://hdl.handle.net/1842/16916.

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Han, Sang-Wook. "Spin-polarized neutron reflectivity and x-ray scattering studies on thin film superconductors /." free to MU campus, to others for purchase, 1999. http://wwwlib.umi.com/cr/mo/fullcit?p9962527.

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43

Resler, David Alan. "Structure of ¹⁴C via Elastic and Inelastic Neutron Scattering from ¹³C: Measurement, R-matrix Analysis, and Shell Model Calculations." Ohio University / OhioLINK, 1987. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1472558171.

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44

Abdelrahman, Magdy Shehata. "Scattering correction and image restoration in neutron radiography and computed tomography." Access restricted to users with UT Austin EID Full text (PDF) from UMI/Dissertation Abstracts International, 2000. http://wwwlib.umi.com/cr/utexas/fullcit?p3025066.

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45

Cope, Elizabeth Ruth. "Dynamic properties of materials : phonons from neutron scattering." Thesis, University of Cambridge, 2010. https://www.repository.cam.ac.uk/handle/1810/226116.

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Abstract:

A detailed understanding of fundamental material properties can be obtained through the study of atomic vibrations, performed experimentally with neutron scattering techniques and coupled with the two powerful new computational methodologies I have developed. The first approach involves phonon-based simulations of the pair distribution function - a histogram of localised atomic positions generated experimentally from total scattering data. This is used to reveal ordering behaviour, to validate interatomic models and localised structure, and to give insights into how far dynamic behaviour can be studied using total scattering techniques. Most importantly, the long-standing controversy over dynamic disorder in β-cristobalite is resolved using this technique. Inelastic neutron spectroscopy (INS) allows \emph{direct} study of vibrational modes through their interaction with the neutron beam, and is the experimental basis for the second strand of the new methodology. I have developed new simulation and refinement tools based on the next generation of spectrometers currently being commissioned at the ISIS pulsed neutron source. This allows a detailed powder spectroscopy study of cristobalite and vitreous silica demonstrating that the Bose peak and so-called 'fast sound' features can be derived from standard lattice dynamics in both the crystal and the amorphous counterpart, and allowing discussion of their origins. Given the controversy in the literature, this is a key result. The new methodology also encompasses refinement of interatomic models against powder INS data, with aluminium providing a successful test-case. A more complex example is seen in calcite, with experimental data collected during the commissioning of the new MERLIN spectrometer. Simulated one-phonon coherent INS spectra for the single crystal and powder (the latter including approximations to multi-phonon and multiple scatter) are fully convolved with experimental resolution functions. These are used in the analysis of the experimental data, yielding previously unpublished dispersion curves and soft mode information, as well as allowing the effectiveness of powder refinement of more complex materials to be assessed. Finally, I present further applications with technologically important materials - relaxor ferroelectrics and high temperature pnictide superconductors. The conclusions draw together the different strands of the work, discussing the importance of these new advances together with future developments and scientific applications.

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46

Van, Den Eynde Gert. "Neutron transport with anisotropic scattering: theory and applications." Doctoral thesis, Universite Libre de Bruxelles, 2005. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/211048.

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Abstract:

This thesis is a blend of neutron transport theory and numerical analysis. We start with the study of the problem of the Mika/Case eigenexpansion used in the solution process of the homogeneous one-speed Boltzmann neutron transport equation with anisotropic scattering for plane symmetry. The anisotropic scattering is expressed as a finite Legendre series in which the coefficients are the ``scattering coefficients'. This eigenexpansion consists of a discrete spectrum of eigenvalues with its corresponding eigenfunctions and the continuous spectrum [-1,+1] with its corresponding eigendistributions. In the general case where the anisotropic scattering can be of any (finite) order, multiple discrete eigenvalues exist and these have to be located to have the complete spectrum. We have devised a stable and robust method that locates all these discrete eigenvalues. The method is a two-step process: first the number of discrete eigenvalues is calculated and this is followed by the calculation of the discrete eigenvalues themselves, now being able to count them down and make sure none are forgotten.

During our numerical experiments, we came across what we called near-singular eigenvalues: discrete eigenvalues that are located extremely close to the continuum and hence lead to near-singular behaviour in the eigenfunction. Our solution method has been adapted and allows for the automatic detection of such a near-singular eigenvalue.

For the elements of the continuous spectrum [-1,+1], there is no non-zero function satisfying the associated eigenequation but there is a non-zero distribution that does satisfy it. It is not feasible to compute a distribution as such but one can evaluate integrals in which this distribution appears. The continuum part of the eigenexpansion can hence only be characterised by its (angular) moments. Accurate and fast numerical quadrature is needed to evaluate these integrals. Several quadrature methods have been evaluated on a representative test function.

The eigenexpansion was proved to be orthogonal and complete and hence can be used to represent the infinite medium Green's function. The latter is the building block of the Boundary Sources Method, an integral solution method for the neutron transport equation. Using angular and angular/spatial moments of the Green's function, it is possible to solve with high accuracy slab problems. We have written a one-dimensional slab code implementing this Boundary Sources Method allowing for media with arbitrary order anisotropic scattering. Our results are very good and the code can be considered as a benchmark code for others.

As a final application, we have used our code to study the discrete spectrum of a well-known scattering kernel in radiative transfer, the Henyey-Greenstein kernel. This kernel has one free parameter which is used to fit the kernel to experimental data. Since the kernel is a continuous function, a finite Legendre approximation needs to be adopted. Depending on the free parameter, the approximation order and the number of secondaries per collision, the number of discrete eigenvalues ranges from two to thirty and even more. Bounds for the minimum approximation order are derived for different requirements on the approximation: non-negativity, an absolute and relative error tolerance.

Doctorat en sciences appliquées
info:eu-repo/semantics/nonPublished

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Mukherjee, Chiron Jeeb. "Neutron scattering studies on low-dimensional quantum magnets." Thesis, University of Oxford, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.418479.

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Tennant, David Alan. "Neutron scattering studies of spin waves and spinons." Thesis, University of Oxford, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.259852.

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Kenzelmann, Michel. "Neutron scattering studies of three one-dimensional antiferromagnets." Thesis, University of Oxford, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.393244.

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Fairbanks, M. "Neutron and light scattering studies of molten salts." Thesis, University of Oxford, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.233457.

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Dissertations / Theses on the topic 'Neutron scattering'

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