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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Magnetism in uranium and praseodymium intermetallic compounds studied by inelastic neutron scattering

Allen, Jonathan Paul January 2004 (has links)
The dispersion of magnetic excitations in PrNiSn has been studied using inelastic neutron scattering. The dispersion relations along [0 0 Q] and [Q 0 0] have been determined for the excitations observed at 1.5, 2.5 and 5.2 meV. The two lower excitations are proposed to be modes originating from the 2 meV level predicted by earlier inelastic measurements. Additional measurements of the 3.5 meV excitation already studied were performed to provide higher resolution data and improved statistics. Preliminary measurements on NdNiSn were also made. Searches have been made using inelastic neutron scattering to look for crystalline electric field (CEF) excitations in the uranium-based intermetallics UPdSn, UCu2Sn and U3Pd20Si6. In UPdSn, a distinct CEF excitation has been observed at 50 meV in the paramagnetic phase. On cooling, the excitation shifts to lower energies and its temperature dependence exhibits marked changes at the antiferromagnetic transition temperatures of TN =25 and 40 K. The quasielastic scattering below 20 meV is found to increase significantly below the lower transition temperature. A set of Stevens parameters have been proposed to account for the observed data. The magnetic scattering observed in UCu2Sn is a broad, asymmetric peak centred at 7 meV which disappears above the quadrupolar ordering temperature of TQ = 16.5 K. The temperature dependence of the peak has been determined and comparisons with two different energy level schemes have been made. Three clear CEF excitations, which are visible in both the paramagnetic and ordered phase, have been observed at energies of 14.3, 23.9 and 31.6 meV in U3Pd20Si6. The temperature dependence of the excitations has been followed and a tentative level scheme proposed. A sharp, intense peak at 3 meV is visible at low temperatures and has been attributed to the exchange induced splitting of the 8c ground state below TN =19 K.
2

A measurement of the recently discovered Higgs Boson in the decay into two photons with associated jets, using the ATLAS detector at the LHC

Cantrill, Robert J. B. January 2014 (has links)
A new boson has been discovered and measurements lire under way using the 7TcV and 8TeV proton-proton collision data from the L1rge Hadron Collider to determine whether or not this is the Higgs boson as predicted by the Standard Model of Particle Physics (SM). Experimentally measuring the nature of this particle's couplings to other particles will help determine this. The Standard Model Higgs boson is expected to be produced by a variety of production mechanisms. The SM prediction is that the gluon-gluon fusion (ggF) and vector boson fusion (VRF) production mechanisms are the two production processes with the highest and second-highest, rates respectively. This thesis concentrates on the study of the Higgs boson via its decay into two photons, which was one of the key discovery channels. Part of this analysis is to mensure the ratio (9l) of these rates using J3fb I of ATLAS .fi = 8TeV proton-proton collision data and determine if ~ is consistent with the SM prediction. Using the diphoton decay channel, events were selected to fonn a category 01" data events which is enriched in VBF events with little gluon-gluon fusion contamination. The selection procedure was optimised using a boosted decision tree (BOT) multi variate classifier. The distinguishing feature of this analysis was that the ~DT was trained using hackground events from the data sample, so as to reduce the dependency 0 11 the modelling of the background processes. It was shown that using a BDT classifier, the VBF signal significance improves by 24.0% relative to the standard cut·based analysis and suffers from 12.0% less ggF signal contamination. Using this event classification ~ was measured as ~ = OvnF /(a 881' + O"vBd = 0.037 ± 0.067(stat) ± O.035(syst) where O"VBF and OggF are the respecti ve cross .~ec tions of the vector hoson fusion process and the gluon-gluon fusion process. The SM prediction is 9l ;.;;; 0.075 Although the uncertainty on the currcllI measurement is large. it is shown using pseudodata, that this choice of categorisation will help reduce the uncertainty on 9l when more data are available.
3

A study of the formation, dissociation and reactivity of molecular dications

Harper, Sarah Margaret January 2005 (has links)
This thesis describes the commissioning of a new position-sensitive coincidence (PSCO) time-of-flight (TOF) mass spectrometer, which has been designed and constructed for studying the dynamics and kinematics of dication-neutral reactions at low collision energies (4-25 eV). These reactions commonly form two singly charged ions that the PSCO experiment detects in coincidence on an event-by-event basis, allowing definitive partner ion associations to be made. Flight time and positional data are recorded for each ion allowing the calculation of their initial velocity vectors from which the complete dynamics and kinematics for each reactive event, including determination of reactant and product states, may be determined. The "simple" Ar2+-He collision system was investigated for the purposes of commissioning and quantifying the energy resolution of the new PSCO experiment, since it has been previously studied in the literature. Following the successful commissioning of the PSCO experiment, three more complex systems were studied: Ne2+-Ar, Ne2+-N2, CF22+-H 2O, in order to obtain an in-depth understanding of the dynamics and energetics of dication-neutral reactions. The PSCO investigation of the 2_ _ , electron transfer reaction in the Ne-Ar collision system revealed bimodal angular distributions of both products, Ne+ and Ar+, indicating the presence of two different electron transfer channels. The extraction of such detailed information for a simple "two-body" reaction (a reaction where two products are formed) shows that the PSCO experiment is a powerful tool for determining detailed dication-neutral reaction mechanisms. The Ne-N2 collision system indicated the presence of three reaction channels forming Ne+ + N+ + N in coincidence. Two of these channels were investigated revealing distinctly different mechanistic pathways. One channel involves the formation of a transitory collision complex, and the other appears consistent with a sequential process involving the fast dissociation of N2+*. Five reaction channels were observed for the CF22+-H2O collision system: three electron transfer channels and two bond-forming channels including a previously unobserved hydride transfer reaction.
4

The spectroscopy of neutron-rich phosphorus nuclei populated in grazing reactions

Hodsdon, Andrew January 2007 (has links)
No description available.
5

A study of Higgs production in association wirh squark pairs for the CMS experiment

Lynch, Clare January 2007 (has links)
No description available.
6

A theory of electron-hole pair excitation in the adsorption of simple atoms on metal surfaces

Mizielinski, Matthew January 2007 (has links)
Recent experiments have provided direct evidence for the excitation of electronhole pairs during the adsorption of atoms on metal surfaces. The excitation of electron-hole pairs is an inherently non-adiabatic process which is often ignored in standard theoretical treatments of surface phenomena, using tools such as density functional theory (DFT), as the Born-Oppenheimer approximation cannot be used. To obtain a theoretical model for the electronic excitation process it is therefore necessary to go beyond conventional methods. Previous theoretical descriptions have used a nearly-adiabatic approximation to describe electronic excitations. However, these methods have been found to fail in situations where an adsorbing atom undergoes a transition between a spin-polarised and unpolarised state. In this thesis we develop a fully non-adiabatic theory using a simple description of the adsorbate-metal interaction; the time-dependent, mean-field Newns- Anderson model. This model describes a simple electronic system in which a band of metal states interacts with a single atomic orbital, which can undergo a ‘spin-transition’. We derive expressions describing the time-dependent transfer of charge and energy between the adsorbate and surface, as well as the spectrum of electronic excitations generated. Each of these results describe the evolution of the electronic system in terms of a simple set of parameters. These results are demonstrated using a set of example parameter variations to explore the impact of variables such as adsorbate speed and the temperature of the system. A set of parameter variations describing the interaction of hydrogen isotopes with copper and silver surfaces are obtained from DFT calculations. These parameters are used to drive our model through a single approach of the adsorbate to the surface. We find the results of these calculations to be in good agreement with reported experimental results. Our conclusions, and some possible directions for further work, are summarised in the final chapter.
7

Experimental measurement of N = 8 shell breaking in the 12Be ground state

Pain, Steven D. January 2004 (has links)
An experiment has been performed to measure exclusive single neutron knockout cross sections from 12Be to study its ground state structure. The yield to the first (0.32 MeV, ½-) and second (1.78 MeV, 5/2+, unbound) excited states in 11Be gives information on the admixture of (1p½)2 and (1d5/2)2 components in the ground state of 12Be. A fragmentation beam of 12Be of ~10000 pps (95% pure) was incident on a carbon target at 41 MeV/u. The beam particles were tracked onto the target, and their energies were measured from their time of flight. The beam-like residues 10,11Be were measured in a position sensitive telescope mounted at zero degrees, and neutrons were measured in the DeMoN array. The ½- state of 11Be was identified by measuring coincident 320 keV gamma-rays, using four Nal detectors. Full kinematic reconstruction of unbound states in 11Be was performed using coincident neutrons and 10Be ions, which showed clear evidence for the 1.78 MeV state in 11Be. A strong background, due to reactions occurring in the zero degree telescope, was measured and subtracted by acquiring data with no target present. Neutron angular distributions in the laboratory frame were measured in coincidence with 10,11Be, and momentum distributions of neutrons and beam-like particles were measured for exclusive reaction channels. Detailed simulations were performed in order to quantitatively interpret the relative energy spectrum for the reconstructed 11Be, using the measurements of momentum distributions as constraints. The cross sections for the production of 11Be in its first (1/2-) and second (5/2+) excited states were measured to be 33.5(5.6) mb and 22.4(4.4) mb respectively. This is the first strong evidence for a (1d5/2)2 component in the 12Be ground state. Data were also acquired for the Coulomb excitation and breakup of 11Be on Pb and C targets.
8

50 Jahre Nukleare Analytik in Rossendorf – interdisziplinäre Forschung und Dienstleistungen

Niese, Siegfried 01 October 2013 (has links)
Die Nukleare Analytik begann in Rossendorf im Jahr 1957 mit der Anwendung der Indikatormethode für Verteilungsuntersuchungen im Zusammenhang mit der Verarbeitung bestrahlter Kernbrennstoffe. Nach Inbetriebnahme des Forschungsreaktors stand die Entwicklung und Anwendung der Aktivierungsanalyse zur Untersuchung von Halbleitermaterialen sowie geologischer und medizinischer Proben im Mittelpunkt. Zur Verbesserung der Nachweisgrenze von Radionukliden wurden Koinzidenzverfahren entwickelt und eine unterirdische Messkammer eingerichtet. Nach Stilllegung des Forschungsreaktors wurde die Radioaktivität in der Umgebung des ehemaligen Uranbergbaues, in Materialien aus dem Rückbau von Reaktoren und anderen Kernanlagen und anderen natürlichen und technischen Proben bestimmt. / In 1957 the work in radioanalytical chemistry has been started in Rossendorf with the application of the tracer method for investigation of the distribution of nuclides in reprocessing of nuclear fuels. After put into operation of the research reactor the development and application of activation analysis became the main topic. For improvement of the detection limits of radionuclides coincidence methods has been developed and an underground counting room was build. After shut down of the research reactor the radioactivity in the environment of the former uranium mining, and in materials from the decommissioning of reactors and other nuclear equipments and further natural and industrial samples.
9

Étude expérimentale d'interactions entre antennes HF et plasma périphérique d'un Tokamak / Experimental study of the interaction between RF antennas and the edge plasma of a tokamak

Kubic, Martin 23 October 2013 (has links)
Les antennes en opération dans la gamme de fréquence cyclotron ionique représentent un moyen utile pour chauffer du plasma dans les tokamaks et autres plasmas de fusion. Ces systèmes de chauffage sont amenés à jouer un rôle important dans le projet ITER. Conjointement avec le chauffage souhaité, les interactions parasites avec le bord du plasma et de la limite des matériaux apparaissent. Plusieurs de ces effets délétères sont causés par la formation de la radio-fréquence des gaines. L'objectif de cette thèse est d'étudier, principalement de façon expérimentale, les modifications du plasma de bord «scrape-off layer» causées par les effets non-linéaires des gaines RF. Cela se fait en utilisant les sondes électrostatiques (de Langmuir, Retarded Field Analyser, tunnel) pour différentes configurations du plasma et des antennes: avec des études paramétriques en fonction du déséquilibre entre les émetteurs de l'antenne, de la puissance injectée et de la densité SOL. De plus, l'influence des gaines RF sur les mesures du potentiel de la gaine avec le RFA sont analysées. Cette étude s'effectue à l'aide d'un code 1D basé sur le modèle cinétique «particle-in-cell». Ces simulations ont montré que la RFA est capable de mesurer de manière fiable le potentiel gaine, toutefois cela reste limité pour les fréquences de plasma ionique wpi proche de la fréquence injectée wrf. Par contre, pour des conditions réelles du SOL (wpi>wrf), quand RFA est magnétiquement connectée à la structure de l'antenne RF, il est fortement sous-estimé. Enfin, les mesures de RFA dans Tore Supra indiquent que les potentiels RF se propagent au moins jusqu'à de 12m de l'antenne le long de lignes de champ magnétiques / Antennas operating in the ion cyclotron range of frequency (ICRF) provide a useful tool for plasma heating in many tokamaks and are foreseen to play an important role in ITER. However, in addition to the desired heating in the core plasma, spurious interactions with the plasma edge and material boundary are known to occur. Many of these deleterious effects are caused by the formation of radio-frequency (RF) sheaths. The aim of this thesis is to study, mainly experimentally, scrape-off layer (SOL) modifications caused by RF sheaths effects by means of Langmuir probes that are magnetically connected to a powered ICRH antenna. Effects of the two types of Faraday screens' operation on RF-induced SOL modifications are studied for different plasma and antenna configurations - scans of strap power ratio imbalance, injected power and SOL density. In addition to experimental work, the influence of RF sheaths on retarding field analyzer (RFA) measurements of sheath potential is investigated with one-dimensional particle-in-cell code. One-dimensional particle-in-cell simulations show that the RFA is able to measure reliably the sheath potential only for ion plasma frequencies wpi similar to RF cyclotron frequency wrf, while for the real SOL conditions (wpi > wrf), when the RFA is magnetically connected to RF region, it is strongly underestimated. An alternative method to investigate RF sheaths effects is proposed by using broadening of the ion distribution function as an evidence of the RF electric fields in the sheath. RFA measurements in Tore Supra indicate that RF potentials do indeed propagate from the antenna 12m along magnetic field lines
10

On-ground characterization of the cold atoms space clock PHARAO / Caractérisation et recherche des performances ultimes du système embarqué Pharao/Aces

Moric, Igor 19 December 2014 (has links)
La thèse présente les résultats expérimentaux obtenus au cours du développement et des essais au sol du modèle de vol de l'horloge à atomes froids PHARAO. PHARAO est le premier étalon primaire de fréquence dédié à des applications spatiales. Il est développé par l'agence spatiale française CNES. PHARAO est un des principaux instruments de la mission spatiale de l'ESA: ACES (Atomic Clock Ensemble in Space). Le lancement est prévu en 2016. La mission est basée sur des comparaisons de très hautes performances en temps et en fréquence, entre PHARAO et un ensemble d’horloges basées au sol, pour effectuer des tests en physique fondamentale. La charge utile sera installée sur une palette extérieure de la Station spatiale internationale. Après une introduction sur les horloges atomiques et un résumé de la mission ACES, l'architecture de PHARAO optimisée pour la microgravité et son fonctionnement sont décrits. Ensuite nous présentons les mesures et l'analyse de la stabilité de fréquence. Au sol la stabilité de fréquence est mesurée à un niveau de 3,1x10-13 t-1/2. Cette valeur est en accord avec les différentes sources de bruit. En microgravité la stabilité atteindra 10-13 t-1/2. Pour terminer les principaux déplacements de fréquence sont analysés. Une étude détaillée est donnée sur les propriétés des blindages magnétiques, leurs hystérésis et la conception d’une compensation magnétique active. L'objectif est de réduire l'incertitude sur l’effet Zeeman du second ordre au niveau de quelques 10-17. La détermination de la température de l’environnement des atomes est également analysée avec l'objectif d'atteindre une incertitude sur le déplacement de fréquence par le rayonnement du corps noir dans la gamme de 10-17. Un budget préliminaire sur l’incertitude de fréquence de l’horloge au sol s’établit à 1,1x 10-15. Ce budget est compatible avec un objectif de 3x10-16 en microgravité. La prochaine étape verra l’assemblage tous les autres instruments ACES pour un lancement prévu en 2016. / This thesis presents the experimental results obtained during the development and the ground tests of the flight model of the cold atoms space clock PHARAO. PHARAO, the first Primary Frequency Standard (PFS) for space applications, is developed by the French space agency CNES. It is a main instrument of the ESA space mission ACES: Atomic Clock Ensemble in Space with a launch scheduled on 2016. The mission is based on high performances time and frequency comparisons between a payload including PHARAO and ground based clocks to perform tests in fundamental physics. The payload will be installed on an external pallet of the International Space Station. After an introduction on atomic clocks and a summary on the ACES mission, the PHARAO architecture, optimized for microgravity environment, and its operation is described. It is followed by the measurements and the analysis of the frequency stability. On ground the frequency stability is measured at a level of 3.1 10-13 t-1/2. This value is in agreement with the different sources of noise. In space the frequency stability will reach 10-13 t-1/2. Finally the main frequency shifts are analyzed. A detailed study is given on magnetic shield properties, hysteresis and the design of the active magnetic compensation. The objective is to reduce the uncertainty of the second order Zeeman effect within few 10-17. The temperature determination of the atomic environment is also detailed and the goal is to reach an uncertainty on the blackbody frequency shift in the 10-17 range. A preliminary budget on the frequency accuracy of PHARAO on ground is evaluated at 1.1 10-15. This value is compatible with the expected accuracy budget of 3x10-16 when the clock will operate in microgravity. In the next step all the ACES instruments will be assembled for a launch scheduled on 2016.

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