943 research outputs found

    Properties of axicon systems for collecting foil-excited accelerator beam spectra

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    Axicon system properties for collecting foil-excited accelerator beam spectr

    Bayesian Hierarchical Modelling Frameworks for Flawed Data in Environment and Health

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    In the fields of environment and health, available data is usually not a perfect representation of the quantity we are interested in, such as the number of people contracting a disease or the number of environmental hazards occurring in a given area or time period. Instead, data often suffer from a number of flaws, some of which can pose serious problems. For example, counts of disease cases or environmental hazards may suffer from under-reporting, such that the recorded count is less than or equal to the true count. In some cases, we will never know the true number. This inevitably convolutes our understanding of the risk the disease or natural hazard poses to society. A similar example is delayed reporting of counts, where we may eventually know the true count or something trivially close to it after a period of time. However, we often need to make important decisions, such as how to respond to a disease outbreak, before this certainty is available to us and based on any partial information we may instead have at our disposal. In this thesis we discuss different ways in which data may be flawed, which we refer to as flawed observation mechanisms, and the risks they pose to practitioners if ignored. Moving beyond previous approaches to tackling this issue, which mostly constitute bespoke solutions to individual problems, we present a conceptual framework for simultaneously modelling quantities we are interested in and any flawed observation mechanisms. We argue that the key strengths of this framework are its ability to rigorously quantify uncertainty, its flexibility and its interpretability. We spend the rest of the thesis demonstrating the power this framework offers to practitioners, with chapters dedicated to the general problems of under-reporting and delayed reporting, as well as a chapter dedicated to the exposition of a model which informs global health policy. Each of these chapters is broadly self contained, with individual discussions of the problems addressed. The thesis concludes with an overview of the effectiveness of our approach and some suggestions for future research.Natural Environment Research Council (NERC

    Covalent bonding and the nature of band gaps in some half-Heusler compounds

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    Half-Heusler compounds \textit{XYZ}, also called semi-Heusler compounds, crystallize in the MgAgAs structure, in the space group F4ˉ3mF\bar43m. We report a systematic examination of band gaps and the nature (covalent or ionic) of bonding in semiconducting 8- and 18- electron half-Heusler compounds through first-principles density functional calculations. We find the most appropriate description of these compounds from the viewpoint of electronic structures is one of a \textit{YZ} zinc blende lattice stuffed by the \textit{X} ion. Simple valence rules are obeyed for bonding in the 8-electron compound. For example, LiMgN can be written Li+^+ + (MgN)−^-, and (MgN)−^-, which is isoelectronic with (SiSi), forms a zinc blende lattice. The 18-electron compounds can similarly be considered as obeying valence rules. A semiconductor such as TiCoSb can be written Ti4+^{4+} + (CoSb)4−^{4-}; the latter unit is isoelectronic and isostructural with zinc-blende GaSb. For both the 8- and 18-electron compounds, when \textit{X} is fixed as some electropositive cation, the computed band gap varies approximately as the difference in Pauling electronegativities of \textit{Y} and \textit{Z}. What is particularly exciting is that this simple idea of a covalently bonded \textit{YZ} lattice can also be extended to the very important \textit{magnetic} half-Heusler phases; we describe these as valence compounds \textit{ie.} possessing a band gap at the Fermi energy albeit only in one spin direction. The \textit{local} moment in these magnetic compounds resides on the \textit{X} site.Comment: 18 pages and 14 figures (many in color

    Synthesis of new, single-isomer quaternary ammonium derivatives of beta-cyclodextrin for electrophoretic enantiomer separations

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    The isolation of individual enantiomers of drugs is an important subject of interest in the pharmaceutical and medical fields, because stereochemistry can have a significant effect on the biological activity of the drug. Therefore, it is important to develop enantiomeric separation methods for the determination of the optical purity of drugs, since the undesired enantiomer is regarded as one of the impurities. The available single isomer anionic cyclodextrins (CD) can resolve the enantiomers of only a few weakly acidic analytes. To rectify this problem, the chloride salts of heptakis(6-deoxy-6-morpholinio)-cyclomaltoheptaose (HMBCD), and mono(6- deoxy-6-N,N,N r,N r,N r-pentamethylethylenediammonio)-cyclomaltoheptaose (PEMEDA-BCD), the first members of the permanently charged, single-isomer cationic cyclodextrin family, have been synthesized. The purity of process intermediates and final products was determined by HPLC-ELSD and indirect UV-detection capillary electrophoresis. Structural identity was verified by 1D and 2D NMR and massBoth cationic CD derivatives have been used for the separation of the enantiomers of strong acid, weak acid, weak base, ampholytic, and neutral analytes by capillary electrophoresis. Because the charge state of these cationic chiral resolving agents is independent of the pH of the buffer, separation could be performed in both low and high pH buffers without compromising the charge density of the resolving agent. Contrary to expectation, the multiply charged HMBCD showed poor complexation with the newly synthesized strong electrolyte test analytes. The weak binding between the analytes and HMBCD resulted in separation of enantiomers of only three strong electrolyte analytes. Strong complexation was observed between PEMEDA-BCD and the anionic and nonionic analytes in both low and high pH buffers, though complexation was stronger in the high pH buffer. Due to strong complexation between the anions and PEMEDA-BCD, only low concentrations of the resolving agent were required to effect good enantiomer resolutions. spectrometry

    Pacioli\u27s forgotten book: The Merchant\u27s Ricordanze

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    Double entry bookkeeping emerged by the end of the 13th century and was adopted by, for example, the Datini of Prato during the 1380s. In the transition from single to double entry evident in the Datini Archives, initially accounting records were kept in an account book called a Ricordanze. Record books of this name were typical of Tuscany and, when such books were first used in Tuscany, businessmen began to use them also as a form of personal diary and autobiographical record. Others not in business followed suit and maintained purely personal biographical diaries of the same name. For those in business, the Ricordanze thus developed into a hybrid: partly autobiography and personal and, partly, a place to record matters relating to his business, including details of transactions and of other matters he did not wish to forget, such as promises, obligations, and conditional agreements. As revealed in the Datini archives for the 14th and 15th centuries, use of a Ricordanze for this purpose was discontinued in the accounting system and the book was replaced with another called a Memoriale, which contained details of all business transactions. By the time Pacioli wrote the first published description of double entry bookkeeping, the Memoriale was identified as one of the three principal account books of that system. The others were the Giornale [journal] and the Quaderno [ledger]. However, largely unnoticed by accounting scholars, towards the end of his treatise, Pacioli also describes another book that merchants would be wise to keep\u27: a Ricordanze. Not a personal Ricordanze nor a hybrid personal plus business Ricordanze, nor a version of a Memoriale. Pacioli,s Ricordanze was intended to serve a very specific purpose: it was a book dedicated to maintaining a record of things that should not be forgotten. As such, it was intended to provide an extra layer of managerial control over the affairs of the merchant beyond that provided by the double entry system. This paper considers the role of Pacioli\u27s Ricordanze, of the records that may be maintained within it, discusses the merits of maintaining a record book of this type, and questions why such a clearly useful device does not appear to have been adopted even though it was described in the same treatise which led to the universal adoption of double entry bookkeeping

    Magnetic tight-binding and the iron-chromium enthalpy anomaly

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    We describe a self consistent magnetic tight-binding theory based in an expansion of the Hohenberg-Kohn density functional to second order, about a non spin polarised reference density. We show how a first order expansion about a density having a trial input magnetic moment leads to the Stoner--Slater rigid band model. We employ a simple set of tight-binding parameters that accurately describes electronic structure and energetics, and show these to be transferable between first row transition metals and their alloys. We make a number of calculations of the electronic structure of dilute Cr impurities in Fe which we compare with results using the local spin density approximation. The rigid band model provides a powerful means for interpreting complex magnetic configurations in alloys; using this approach we are able to advance a simple and readily understood explanation for the observed anomaly in the enthalpy of mixing.Comment: Submitted to Phys Rev

    Environmental Differences between Migratory and Resident Ungulates—Predicting Movement Strategies in Rocky Mountain Mule Deer (Odocoileus hemionus) with Remotely Sensed Plant Phenology, Snow, and Land Cover

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    Migration is a valuable life history strategy for many species because it enables individuals to exploit spatially and temporally variable resources. Globally, the prevalence of species’ migratory behavior is decreasing as individuals forgo migration to remain resident year-round, an effect hypothesized to result from anthropogenic changes to landscape dynamics. Efforts to conserve and restore migrations require an understanding of the ecological characteristics driving the behavioral tradeoff between migration and residence. We identified migratory and resident behaviors of 42 mule deer (Odocoileus hemionus) based on GPS locations and correlated their locations to remotely sensed indicators of forage quality, land cover, snow cover, and human land use. The model classified mule deer seasonal migratory and resident niches with an overall accuracy of 97.8% and cross-validated accuracy of 81.2%. The distance to development was the most important variable in discriminating in which environments these behaviors occur, with resident niche space most often closer to developed areas than migratory niches. Additionally, snow cover in December was important for discriminating summer migratory niches. This approach demonstrates the utility of niche analysis based on remotely sensed environmental datasets and provides empirical evidence of human land use impacts on large-scale wildlife migrations

    C/O white dwarfs of very low mass: 0.33-0.5 Mo

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    The standard lower limit for the mass of white dwarfs (WDs) with a C/O core is roughly 0.5 Mo. In the present work we investigated the possibility to form C/O WDs with mass as low as 0.33 Mo. Both the pre-WD and the cooling evolution of such nonstandard models will be described.Comment: Submitted to the "Proceedings of the 16th European White Dwarf Workshop" (to be published JPCS). 7 pages including 13 figure

    Landau-Zener-Stueckelberg effect in a model of interacting tunneling systems

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    The Landau-Zener-Stueckelberg (LZS) effect in a model system of interacting tunneling particles is studied numerically and analytically. Each of N tunneling particles interacts with each of the others with the same coupling J. This problem maps onto that of the LZS effect for a large spin S=N/2. The mean-field limit N=>\infty corresponds to the classical limit S=>\infty for the effective spin. It is shown that the ferromagnetic coupling J>0 tends to suppress the LZS transitions. For N=>\infty there is a critical value of J above which the staying probability P does not go to zero in the slow sweep limit, unlike the standard LZS effect. In the same limit for J>0 LZS transitions are boosted and P=0 for a set of finite values of the sweep rate. Various limiting cases such as strong and weak interaction, slow and fast sweep are considered analytically. It is shown that the mean-field approach works well for arbitrary N if the interaction J is weak.Comment: 13 PR pages, 15 Fig

    Magnetic hysteresis in Ising-like dipole-dipole model

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    Using zero temperature Monte Carlo simulations we have studied the magnetic hysteresis in a three-dimensional Ising model with nearest neighbor exchange and dipolar interaction. The average magnetization of spins located inside a sphere on a cubic lattice is determined as a function of magnetic field varied periodically. The simulations have justified the appearance of hysteresis and allowed us to have a deeper insight into the series of metastable states developed during this process.Comment: REVTEX, 10 pages including 4 figure
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