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1
Piegay, Nathalie. "DYNAMIQUE DE L'AIMANTATION DANS LES SOLUTIONS HELIUM3-HELIUM4 HYPERPOLARISEES." Phd thesis, Université Pierre et Marie Curie - Paris VI, 2002. http://tel.archives-ouvertes.fr/tel-00003086.
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Nous présentons une étude détaillée de l'évolution de l'aimantation nucléaire au sein d'une solution d'3He polarisé dans l'4He à pression de vapeur saturante à des températures de l'ordre de 1 K. En champ magnétique très faible (~mT), des mesures RMN précises sont réalisées sur des solutions 3He-4He hyperpolarisées, obtenues par liquéfaction de gaz polarisé par pompage optique. Nous avons mesuré deux quantités intrinsèques: le temps de relaxation longitudinale et le coefficient de diffusion de spin, susceptibles d'apporter des informations sur les interactions atomiques effectives dans les solutions non-dégénérées. Les précautions expérimentales prises (en particulier l'utilisation d'enduit de césium) garantissent une relaxation très peu influencée par les termes de surface et permettent d'extraire la relaxation due aux interactions dipolaires entre les noyaux des atomes d'3He. Nous avons mesuré des temps de relaxation allant jusqu'à 8 heures pour les solutions les plus diluées. Nous avons trouvé qu'à température fixée, le temps de relaxation dipolaire est inversement proportionnel à la densité en 3He, en accord avec la théorie des solutions quantiques diluées. Par contre, la mesure de la dépendance en température ne reflète pas clairement les prédictions théoriques existantes qui devront être revues. Nous avons mesuré des valeurs du coefficient de diffusion avec une grande précision et déterminé les conditions expérimentales où cette mesure n'est pas faussée par l'effet du champ dipolaire sur l'évolution de l'aimantation transverse. Nous avons systématiquement étudié l'évolution de cette aimantation après une simple impulsion 90° en fonction de la densité d'aimantation de l'échantillon et d'un gradient appliqué. Nous avons observé le développement d'instabilités dans l'évolution de l'aimantation dues au champ dipolaire. Il est également apparu que la refocalistion de l'aimantation transverse après une seconde impulsion 180° est profondément perturbée par le champ dipolaire.
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Dutay, Jean-Claude. "Influence du melange vertical et de la couche de melange sur la ventilation de l'ocean. Simulations numeriques des traceurs transitoires tritium-helium3 et cfcs avec le modele opa." Paris 6, 1998. http://www.theses.fr/1998PA066112.
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Les traceurs transitoires tritium-helium3 et cfcs sont utilises pour la validation du modele de circulation generale oceanique global opa (lodyc). Ces traceurs anthropiques sont particulierement adaptes pour valider des processus de ventilation d'echelle de temps de quelques decennies, tels que la ventilation de la thermocline et la formation des eaux profondes et intermediaires. La validation du modele et l'analyse des simulations est realisee par comparaison avec des mesures obtenues lors de campagnes oceanographiques (geosecs, tto, ajax). Les simulations ont permis de tester la nouvelle parametrisation du melange vertical (tke) basee sur une equation d'evolution de l'energie cinetique turbulente, dont le principal avantage consiste a reproduire une couche melangee a la surface de l'ocean. La validation avec les traceurs transitoires anthropiques a permis de mettre en evidence le role essentiel de cette couche de melange et de son cycle saisonnier sur les processus de ventilation de l'ocean. La couche de melange a pour effet de renforcer l'injection et la subduction des traceurs dans la thermocline, et les simulations ont montre l'importance du cycle saisonnier et de la subduction hivernale dans le modele.
3
Lea, Ramona. "Produzione di ipernuclei leggeri in collisioni Pb-Pb con l'esperimento ALICE a LHC." Doctoral thesis, Università degli studi di Trieste, 2013. http://hdl.handle.net/10077/8582.
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2011/2012The subject of the present PhD thesis is the study of the production of light hypernuclei in ultra-relativistic Pb-Pb collisions with ALICE (A Large Ion Collider Experiment), one of the four major experiments at the LHC (Large Hadron Collider). The main physics goal of the ALICE experiment is the investigation of the properties of the strongly interacting matter at high energy density ($>$ 10 GeV/fm$^3$) and high temperature ($\approx$ 0.2 GeV) conditions. According to the lattice Quantum Chromo Dynamics (QCD) calculations, under these conditions (i.e. high temperature and large energy density) hadronic matter undergoes a phase transition to a ``plasma'' of deconfined quarks and gluons (Quark Gluon Plasma, QGP). In the first chapter of the thesis a general introduction to the heavy-ion physics will be given. Then the main quantities related to QGP formation (i.e. \textit{probes}) will be described. Finally the most important results obtained at SPS, RHIC and LHC experiments will be shown and discussed. In the second chapter a short description of the LHC and its experimental conditions will be reported and an overview of the ALICE experiment will be given. A description of the different detectors and their performances during data taking will be described; in addition a description of the computing framework will be given. The third chapter will be devoted to an introduction of the (anti)(hyper)nuclei production in heavy-ion collisions. The two main approaches which are believed to govern nuclei production (i.e. coalescence and thermal models) will be described, and an overview on the results at different energies will be shown. A comparison of the theoretical results will be also shown, with particular regards to the energies at the LHC. The fourth chapter is devoted to the description of the analysis method used to get (anti)hypertriton production yield in \PbPb~collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76~TeV with the ALICE experiment via its mesonic decay \hyp~$\rightarrow$ \he + \pim (\antihyp $\rightarrow$ \antihe + \pip). In the beginning of the chapter the analysis technique used for particle identification and for the determination of secondary vertices will be described. The analysis will be divided into two distinct parts: the first one based on the data sample collected by the ALICE experiment during the first LHC heavy-ion run held at the end of 2010, while the second one based on data collected at the end of 2011. A detailed description of the study on efficiency evaluation and signal extraction will be shown for both analysis, together with a study of the systematic uncertainties. The results on the production yield of (anti)hypertriton will also be shown. The estimation of the hypertriton lifetime will be provided in the final section of the chapter.\\ In the fifth chapter the method used to obtain the \pt~spectrum of \he~will be presented. The raw spectra, the efficiency evaluation, systematic errors and feed-down from \hyp~will be presented. The final spectrum will be used to evaluate the production yield of \he(\antihe) in the whole \pt~region, from 0 to $\infty$. \\ Finally, in the last chapter, the present experimental results will be compared with published relevant results and with the most recent theoretical findings. Moreover, the measurement of the ``Strangeness Population Factor'' [S$_{3}$= \hyp/\he/($\Lambda$/p)] at the LHC energies will be provided. This quantity is a valuable tool to probe the nature of dense matter created in high-energy heavy-ion collisions and to validate theoretical models.
-------------------------------------------------------------------------------------------------- Questa tesi è dedicata allo studio della produzione di ipernuclei leggeri in collisioni ultra-relativistiche di ioni piombo (Pb) con l'esperimento ALICE (A Large Ion Collider Experiment), uno dei quattro grandi esperimenti del Large Hadron Collider (LHC) del CERN. Il principale obiettivo scientifico dell'esperimenento ALICE è lo studio delle proprietà della materia in condizioni estreme di energia (> 10 GeV/fm^3) e di temperatura (~ 0.2 GeV) mediante lo studio di collisioni di ioni piombo. Calcoli di Cromo Dinamica Quantistica (QCD) su reticolo prevedono, infatti, che in condizioni di alta temparatura e grande energia la materia adronica subisca un transizione di fase verso un ``plasma'' di quark e gluoni deconfinati (Quark Gluon Plasma, QGP). Nel primo capitolo della tesi verranno descritte in maniera generale la fisica degli ioni pesanti e le grandezze caratterische usate per provare la formazione del QGP (probes). Verranno quindi mostrati e discussi i risultati sperimentali che possono provare l'esistenza di uno stato deconfinato della materia nucleare ottenuti agli esperimenti a SPS, RHIC e LHC. Nel secondo capitolo saranno brevemente presentati il Large Hadron Collider (LHC) e le condizioni sperimentali di lavoro durante i primi tre anni di presa dati; in seguito verrà data un'ampia panoramica dell'esperimento ALICE. Saranno descritti i differenti sotto-rivelatori che formano l'esperimento e verranno inoltre mostrate le loro performance durante l'acquisione dati; inoltre verrà fornita una descrizione del framework di calcolo utilizzato nell'analisi dei dati. Il terzo capitolo sarà dedicato alla descrizione dei maccanismi di produzione di (anti)(iper)nuclei in collisioni di ioni pesanti: verranno descritti i due meccanismi di produzione che si ritiene governino la loro produzione (coalescenza e modello termico) e verrà mostrata una panoramica sui risultati ottenuti a diverse energie. Inotre saranno presentati diversi calcoli teorici, ponendo particolare attenzione ai risultati aspettati all'energia di LHC. Il quarto capitolo contiene la descrizione del metodo di analisi utilizzato per valutare lo yield di pruduzione dell'(anti)ipertritone attraverso il suo canale di decadimento mesonico \hyp~$\rightarrow$ \he + \pim (\antihyp $\rightarrow$ \antihe + \pip) in collisioni \PbPb~con energia nel centro di massa $\sqrt{s_{\rm{NN}}}$ = 2.76~TeV. Inizialmente verrà descritta la tecnica di analisi utilizzata per l'identificazione di particelle e dei vertici secondari, quindi sarà fornita la descrizione dettagliata della tecnica di analisi. L'analisi dei dati è stata siddivisa in due distinte parti: la prima è dedicata alla descrizione della procedura utilizzata per l'analisi dei dati raccolti da ALICE durante la prima acquisizione di collisioni Pb--Pb alla fine del 2010; nella seconda parte, invece, verrà descritta la procedura di analisi dei dati raccolti durante la seconda presa dati nel Dicembre 2011. Verranno quindi descritte in modo dettagliato l'estrazione del segnale, lo studio del fondo combinatoriale e gli errori sistematici. Infine, nella parte finale del capitolo, varrà fornita una stima della vita media dell'ipertritone.\\ Nel quinto capitolo sarà presentato il metodo usato per ottenere lo spettro in pT di (anti-3He)3He. Verranno descritti: la procedura di estrazione del segnale, la stima dell'efficienza in funzione del momento trasverso, la valutazione degli errori sistematici e la procedure usata per sottrarre il feed-down dovuto al decadimento dell'ipertitone. Lo spettro verrà quindi utilizzato per valutare lo yield di produzione di (anti-3He) 3He. Infine, nel sesto e ultimo capitolo, i risultati sperimentali ottenuti verranno confrontati con i risultati teorici discussi nel Capitolo 3.
XXV Ciclo
1985
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Yu, Nan. "The infrared spectrum of the helium molecular ion helium-3 positive helium-4." Diss., The University of Arizona, 1988. http://hdl.handle.net/10150/184419.
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This dissertation is concerned with the first experimental observation of the vibration-rotation spectrum of the helium molecular ion, ³He⁴He⁺. The Doppler-tuned fast-beam laser-resonance technique has been used. The ions produced in an electron-impact ion source form an ion beam of a few KeV and then intersect nearly colinearly with a CO IR laser beam of fixed frequency. The velocity of the ions is scanned to tune the ions into resonance with the laser frequency. Subsequent resonance detection is realized by using charge exchange processes of the ion with a target gas. Twelve spectral lines have been measured which correspond to nine different vibration-rotation transitions of the R branch in the fundamental band of the ground state of ³He⁴He⁺. The combination of the kinematic compression of the fast-beam method and the special frequency stabilization of the CO laser render an experimental accuracy of 0.0006 cm⁻¹ or 0.4 ppm. The accuracy exceeds that of the available theory by three orders of magnitude. The narrow spectral linewidth also allows partial resolution of the magnetic hyperfine structure due to helium-3. A hyperfine structure calculation has been carried out using a numerical multi-configuration self-consistent-field method. The agreement between the calculation and the observation is very satisfying.
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Hjort, Hans H. "Helium-Helium scattering in vacuum and in liquid ³HE /." The Ohio State University, 1999. http://rave.ohiolink.edu/etdc/view?acc_num=osu1488187049539918.
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Jörder, Felix [Verfasser], and Andreas [Akademischer Betreuer] Buchleitner. "Dynamical localization in driven helium = Dynamische Lokalisierung in getriebenem Helium." Freiburg : Universität, 2015. http://d-nb.info/1119805643/34.
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Todd, Allan. "Helium-Antihydrogen Interactions." Thesis, University of Nottingham, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.485533.
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Sawkey, Daren Lewis. "Thermomechanical effect in normal liquid helium-3 with helium-4 pre-plating." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp04/mq22392.pdf.
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Porter, Ryan Lucian. "THEORY AND APPLICATION OF HELIUM AND HELIUM-LIKE IONS IN ASTROPHYSICAL ENVIRONMENTS." UKnowledge, 2006. http://uknowledge.uky.edu/gradschool_diss/430.
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A complete model of helium-like line and continuum emission in astrophysical plasmas has been incorporated into the plasma simulation code CLOUDY. All elements between He and Zn are treated, any number of levels can be considered, and a full treatment of radiative and collisional processes is included. This includes photoionization from all levels, line transfer including continuum pumping and destruction by background opacities, scattering, and collisional processes. The model is calculated self-consistently with the ionization and thermal structure of the surrounding nebula. The result is a complete line and continuum spectrum of the nebula. The model helium atom is described and compared to a second standalone helium atom in the low-density case. The effects of the mixing of singlet and triplet terms, the truncation of the physical system, and the convergence of the predicted line intensities as a function of the number of quantum levels explicitly included are considered. New Case-B emissivities are calculated for the helium atom at a range of electron temperatures and densities common in planetary nebulae. Observations of the Orion Nebula are analyzed and compared with predictions of the model helium atom. Observations of low-metallicity extragalactic objects by other authors are analyzed. The methods and details of the model helium-like ions are described. The standard X-ray diagnostics of these ions are revisited and augmented with semi-analytical and numerical calculations of ultraviolet line diagnostics. Finally, a new interface between CLOUDY and the X-ray spectral analysis tool XSPEC is discussed.
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Batey, G. J. "Thermal measurements in helium." Thesis, University of Nottingham, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.376489.
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Steer, A. P. "Liquid helium acoustic microscope." Thesis, University of Exeter, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.381340.
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Turner, Robert Paul. "Pumping helium 3 in a helium mixture with a magnetic field : thermal boundary resistance between liquid helium mixtures and silver sinters." Thesis, Lancaster University, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.242945.
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Petluri, Raghuram. "Angular dependent interaction of metastable helium molecules on the surfaces of helium nanodroplets /." View online ; access limited to URI, 2005. http://0-wwwlib.umi.com.helin.uri.edu/dissertations/dlnow/3186917.
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Gspann, J. [Verfasser], and R. [Verfasser] Ries. "Wechselwirkung eines Caesiumatomstrahls mit Helium-3- und Helium-4-Clusterstrahlen / J. Gspann, R. Ries." Karlsruhe : KIT-Bibliothek, 2014. http://d-nb.info/1196577234/34.
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Mendoza, Luna Luis Guillermo. "Free and hindered-rotation of helium excimers in liquid helium via a bulk experiment." Thesis, University of Leicester, 2015. http://hdl.handle.net/2381/35982.
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Superfluidity is a many-body quantum effect observed for the first time in liquid helium. In the context of modern nanoscience, a natural question is whether superfluidity exists at the nanoscale and if so, under what conditions it occurs. Superfluidity can be probed by means of a torsional pendulum immersed in liquid helium: a decrease in the moment of inertia of the pendulum was observed during the superfluid transition. By replacing the torsional pendulum with a carbonyl sulfide molecule embedded in helium droplets, Grebenev and coworkers explored superfluidity at the nanoscale. They established that 60 4He atoms is the threshold to observe superfluidity. The thermodynamic conditions necessary for this transition could not be ascertained in Grebenev's work since in the helium droplet technique the transition into the superfluid state is impossible to control. One possible way around this experimental limitation is to perform a bulk experiment and embed short-lived helium excimers because all other molecules would freeze. The excimers are in Rydberg states and emit fluorescence sensitive to their environment. In this work, helium excimers have been produced in bulk liquid helium using corona discharges. A wide range of the phase diagram of helium has been probed via fluorescence spectroscopy of Rydberg excimers for the first time: molecular transitions in gaseous, supercritical, vapor and normal liquid phases have been studied systematically. Depending on the thermodynamic conditions, sharp as well as broadened spectra have been observed. The linewidths and lineshifts of a transition of interest have been interpreted on the basis of a model that considers emission from two kinds of excimer: on the one hand, excimers embedded in voids and fully solvated in liquid helium exhibiting hindered rotation, and on the other hand, excimers residing in larger gas pockets within the liquid helium, exhibiting free rotation. The relative contributions of each species of excimer have been estimated in a ratio of approximately 1:5. Hindered transitions were identified for pressures and temperatures in the vapor phase, before helium liquefies. These points in the phase diagram show the formation of clusters between excimers and ground-state helium atoms, indicating that the He_2 -He interaction is stronger than that of He-He.
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Zhu, Jinzhen [Verfasser], and Armin [Akademischer Betreuer] Scrinzi. "Photoionization of the helium hydride ions and helium atoms / Jinzhen Zhu ; Betreuer: Armin Scrinzi." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2020. http://d-nb.info/1223849813/34.
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Claas, Patrick. "Femtosekundenspektroskopie an alkalidotierten Helium-Nanotröpfchen." [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=978886488.
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Wendler, Martin. "Helium- und Sauerstoffdiffusion in Glasschmelzen /." Clausthal-Zellerfeld : Papierflieger, 1996. http://bvbr.bib-bvb.de:8991/F?func=service&doc_library=BVB01&doc_number=007293193&line_number=0001&func_code=DB_RECORDS&service_type=MEDIA.
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Matthias, John Robert. "Quantum evaporation from superfluid helium." Thesis, University of Exeter, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.390167.
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Klingner, Nico. "Ionenstrahlanalytik im Helium-Ionen-Mikroskop." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2017. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-220036.
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Die vorliegende Arbeit beschreibt die Implementierung ionenstrahlanalytischer Methoden zur Charakterisierung der Probenzusammensetzung in einem Helium-Ionen-Mikroskop mit einem auf unter einen Nanometer fokussierten Ionenstrahl. Zur Bildgebung wird dieser im Mikroskop über Probenoberflächen gerastert und die lokale Ausbeute an Sekundärelektronen gemessen. Obwohl sich damit ein hoher topografischer Kontrast erzeugen lässt, lassen sich weder aus der Ausbeute noch aus der Energieverteilung der Sekundärelektronen verlässliche Aussagen zur chemischen Zusammensetzung der Probe treffen. Daher wurden in dieser Arbeit verschiedene ionenstrahlinduzierte Sekundärteilchen hinsichtlich ihrer Eignung für die Elementanalytik im Helium-Ionen-Mikroskop verglichen. Zur Evaluation standen der Informationsgehalt der Teilchen, deren Analysierbarkeit sowie deren verwertbare Ausbeute. Die Spektrometrie rückgestreuter Teilchen sowie die Sekundärionen-Massenspektrometrie wurden dabei als die geeignetsten Methoden identifiziert und im Detail untersucht. Gegenstand der Untersuchung waren physikalische Limitierungen und Nachweisgrenzen der Methoden sowie deren Eignung zum Einbau in ein Helium-Ionen-Mikroskop. Dazu wurden verschiedene Konzepte von Spektrometern evaluiert, erprobt und hinsichtlich ihrer Effizienz, Energieauflösung und Umsetzbarkeit im Mikroskop bewertet. Die Flugzeitspektrometrie durch Pulsen des primären Ionenstrahls konnte als die geeignetste Technik identifiziert werden und wurde erfolgreich in einem Helium-Ionen-Mikroskop implementiert. Der Messaufbau, die Signal- und Datenverarbeitung sowie vergleichende Simulationen werden detailliert beschrieben. Das Spektrometer wurde weiterhin ausführlich hinsichtlich Zeit-, Energie- und Massenauflösung charakterisiert. Es werden ortsaufgelöste Rückstreuspektren vorgestellt und damit erstmalig die Möglichkeit zur Ionenstrahlanalytik im Helium-Ionen-Mikroskop auf einer Größenskala von ≤ 60 nm aufgezeigt. Das Pulsen des primären Ionenstrahls erlaubt es zudem, die Technik der Sekundärionen-Massenspektrometrie anzuwenden. Diese Methode bietet Informationen zur molekularen Probenzusammensetzung und erreicht für einige Elemente niedrigere Nachweisgrenzen als die Rückstreuspektrometrie. Damit konnten erstmalig im Helium-Ionen-Mikroskop gemessene Sekundärionen-Massenspektren sowie die ortsaufgelöste Elementanalyse durch spektrometrierte Sekundärionen demonstriert werden. Die Ergebnisse dieser Arbeit sind in der Fachzeitschrift Ultramicroscopy Band 162 (2016) S. 91–97 veröffentlicht. Ab Oktober 2016 werden diese auch in Form eines Buchkapitels in dem Buch „Helium Ion Microscopy“, Springer Verlag Heidelberg zur Verfügung stehenThe present work describes the implementation of ion beam analysis methods in a helium-ion-microscope for the determination of sample compositions with a focused ion beam of < 1 nm size. Imaging in the microscope is realized by scanning the focused ion beam over the sample surface while measuring the local secondary electron yield. Although this procedure leads to a high topographical contrast, neither the yield nor the energy distribution of the secondary electrons deliver reliable information on the chemical composition of the sample. For this purpose, in this work different ion beam induced secondary particles were compared with respect to their suitability for the analysis of the chemical composition in the helium-ion-microscope. In particular the information content of the particles, their analysability and their yield were evaluated. As a result, the spectrometry of backscattered particles and the mass spectrometry of sputtered secondary ions were identified as the most promising methods and regarded in detail. The investigation focused on physical limitations and detection limits of the methods as well as their implementability into a helium-ion-microscope. Therefor various concepts of spectrometers were evaluated, tested and validated in terms of their efficiency, energy resolution and practicability in the microscope. Time-of-flight spectrometry by pulsing the primary ion beam could be identified as the most suitable technique and has been successfully implemented in a helium-ion-microscope. The measurement setup, signal processing and data handling as well as comparative simulations are described in detail. Further the spectrometer was characterized explicitly in terms of time, energy and mass resolution. Spatially resolved backscattering spectra will be shown demonstrating the feasibility of performing ion beam analysis in a helium-ion-microscope for the first time on a size scale of ≤ 60 nm. By pulsing the primary ion beam the technique of secondary ion mass spectrometry becomes automatically accessible. This method provides information on the molecular composition of samples and can reach higher detection limits than those from backscattering spectrometry. For the first time, in a helium-ion-microscope measured secondary ion mass spectra and spatially resolved elemental analysis by spectrometry of secondary ions, could be demonstrated. The results of this work are published 2016 in the scientific journal Ultramicroscopy, volume 162 on pages 91 to 971. In October 2016 there will be another publication as a book chapter in „Helium Ion Microscopy“ (publisher: Springer Verlag Heidelberg)
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Haynes, S. R. "Condensed phases of confined helium." Thesis, University of Sussex, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.372719.
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Rorai, Cecilia. "Vortex reconnection in superfluid helium." Doctoral thesis, Università degli studi di Trieste, 2012. http://hdl.handle.net/10077/7868.
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2010/2011La presente tesi concerne la modellizzazione e simulazione numerica, attraverso l'equazione di Gross-Pitaevskii (chiamata anche equazione di Schroedinger non lineare), della dinamica dei vortici quantistici nell'elio superfluido e in particolare del fenomeno della riconnessione. La riconnessione si verifica qualora due vortici approssimativamente antiparalleli, si intersecano e si scambiano le estremità. Questo fenomeno è stato osservato sperimentalmente e risulta essere una caratteristica essenziale della turbolenza quantistica.
XXIII Ciclo
1983
23
Ludwig, Thorsten. "Anticrossing-Spektroskopie schnell bewegter Heliumatome nach Elektroneneinfang in 10 bis 50 keV HE+-He-Stössen." [S.l.] : [s.n.], 2005. http://deposit.ddb.de/cgi-bin/dokserv?idn=974015822.
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Schmidt, Lothar. "Effekte molekularer Symmetrien in der Elektronenemission bei langsamen He2+-He-Stössen eine kinematisch vollständige experimentelle Untersuchung /." [S.l. : s.n.], 2000. http://deposit.ddb.de/cgi-bin/dokserv?idn=961247037.
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Walter, Michael. "Photo(doppel)ionisation von Helium und einfachen Molekülen." [S.l. : s.n.], 2000. http://deposit.ddb.de/cgi-bin/dokserv?idn=961295074.
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Czasch, Achim. "Doppelt hoch angeregte Zustände von Helium partielle Wirkungsquerschnitte und Winkelverteilungen der Elektronenemission /." [S.l. : s.n.], 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=972184260.
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Gaiser, Christof. "Properties of helium and gas thermometry." Aachen Shaker, 2008. http://d-nb.info/990735125/04.
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Gerlach, Stefan. "Dipol-Dipol-Wechselwirkungen ultrakalter Helium-Rydbergatome." [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=981536980.
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Bussiahn, René. "Laserabsorptionsspektroskopische Untersuchungen in Helium-Xenon-Glimmentladungen." [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=98444811X.
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Guilleumas, Montserrat. "Nucleation and cavitation in liquid helium." Doctoral thesis, Universitat de Barcelona, 1995. http://hdl.handle.net/10803/667593.
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Las transiciones de fase no siempre ocurren bajo condiciones de equilibrio. Un fluido puede ser conducido a un estado metastable. A pesar de ser internamente estable, existe otra configuración con un potencial termodinámico menor, por tanto energéticamente más favorable y a la cual tenderá el sistema. Esta transición está bloqueada por una barrera termodinámica que puede ser superada con resultado de la formación y crecimiento de pequeños nucleos de la nueva fase estable en la fase metastable. Este proceso se llama nucleación (formación de gotas en un vapor sub-enfriado) o cavitacion (burbujas en un líquido sobre-calentado).
En esta tesis hemos estudiado los fenómenos de cavitación y nucleación en helio dentro del marco de la teoria del funcional de la densidad. Primero hemos estudiado la activación térmica en 3He y 4He puros y en las mezclas 3He-4He, estimando la presión a partir de la cual tiene lugar la transición de fase. Para los sistemas puros hemos estudiado también la activación cuántica a muy bajas temperaturas, estimando la temperatura de transición entre ambos regímenes.
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Cai, Zhiming. "A study of global helium resources." Thesis, University of Cambridge, 2010. https://www.repository.cam.ac.uk/handle/1810/252193.
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Helium investment is fundamentally driven by the expectations of future helium demand of both oil & gas companies and the major industrial gas companies. When they conduct helium business, the relationship between the two groups of companies is governed by long-term contracts with take-or-pay clauses. System dynamics has been applied as a main research methodology, complemented by critical review, time series data analysis, interviews and private communications with helium industry experts and data collection. A system dynamics model of helium resources and the market has been created on the basis of the fundamental principles of economic theory, to examine the likely dynamic behaviour of the future helium market (i.e. the path of prices and resources extraction rate). The following issues have been thoroughly investigated: the nature of helium resources, supply chain and production technologies; helium market structure and production policies of competing firms; and the dynamics of the interaction among demand, supply, Government policy and technology. Monte Carlo simulation was conducted to examine the main uncertainties in connection with the dynamics of the future helium market: future supply from natural gas and the length of time that supply will be available; costs of obtaining helium from lean sources, including the atmosphere; future demand and how sensitive the demand will be to unprecedented prices. Key findings: (1) helium depletion is not imminent, and there is at least 30-50 years to allow users to develop new technologies to avoid or lessen their dependency on helium molecules; panic about helium depletion is unwarranted; (2) the periodical occurrences of helium shortages is fundamentally dependent on the investment policies of producers and their expectations of future helium demand, the lead time of investment, and realisation of demand; any helium supply shortage is not necessarily an indication of the imminence of helium depletion; (3) helium production as a by-product of the cryogenic separation of oxygen and nitrogen from air could be a potential solution for particular types of helium user when the cost of extraction from natural gas resources rises to a point at which backstop resources (e.g. helium from the atmosphere) comes into play; (4) increase in inventory in trans-fill facilities could be another way to smooth out the irregularities of supply and demand balance, especially after the BLM reserve, which has served as a crude helium inventory, is depleted; (5) potential big users of helium, such as fusion technology, need to take sustained action to reduce the potential demand for helium.
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Paetkau, Mark Jeffrey. "Helium mixtures in 87% porous aerogel." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/nq29088.pdf.
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Huang, Jiachang. "A helium high-temperature drift chamber." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape9/PQDD_0013/NQ38900.pdf.
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Miller, Jonathan I. "The design of robust helium aerostats /." Thesis, McGill University, 2005. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=99000.
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Tethered helium aerostats are receiving renewed attention in the scientific and surveillance communities. However, conventional aerostats cannot consistently survive high winds. The goal of this research was to design an aerostat that could be deployed for very long periods, thus reducing operating costs and interruptions in data acquisition. Existing designs and fabrication techniques were first reviewed and replicated in the construction of a 2.5 m diameter spherical aerostat. The constructed balloon was then flown outdoors to observe its operational qualities. A low-cost data acquisition system was assembled to characterize the balloon's dynamics. The results were used to inform a Finite Element Analysis model evaluating the critical stresses in a 10.15 m diameter balloon's envelope and its tendency to "dimple" when subjected to high wind speeds. A second model was created to appraise the performance of an aerostat with a partially hard shell, made of carbon fiber, in highly loaded areas.
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Lee, Richard Albert Marlor. "A cosmological experiment in liquid helium." Thesis, Lancaster University, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239061.
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Buckland, J. R. "New methods in helium atom scattering." Thesis, University of Cambridge, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.597056.
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This thesis presents developments in both the experimental techniques and in the theoretical understanding of the helium atom scattering. These contributions open the way for more effective experimental investigations and more quantitative analysis of helium - surface scattering data. The introduction starts with an outline of the contents of the thesis, followed by a description of the diffraction geometry of the Cambridge helium scattering apparatus and a method of measuring diffraction intensities at constant incident condition. The next two chapters describe two experimental improvements in the field of helium scattering. In chapter 2 the characterisation of a molecular beam source shows that high performance (velocity spread < 1%) can be achieved with a simple and inexpensive design. Chapter 3 describes the development, testing and characterisation of a 2-dimensional multichannel scalar data acquisition device for use with two experimental methods under development, namely an imaging helium detector and time-of-flight measurements using a double chopper gate helium beam. Chapter 4 describes the investigation of helium scattering from a lattice gas of Pb adsorbed on the Cu(001) surface. Experimental helium diffraction intensities for the Pb/Cu(001) system were previously accounted for by invoking an adsorbate - induced corrugation in the Cu(001) substrate. In this work, a more detailed analysis shows that a substrate corrugation is not necessary to reproduce the experimental results. The investigation includes a review of a range of approximations and methods available for calculating scattered intensities within the hard wall approximation of the helium-surface potential. The remainder of the thesis presents a detailed experimental and theoretical investigation of elastic helium scattering from the Si(111)-(1x1)H surface.
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Lusher, C. P. "Nuclear magnetic relaxation in gaseous helium." Thesis, University of Sussex, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.356513.
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Longitudinal relaxation times T1 have been measured in 'He gas, using pulsed NMR, for number densities between 3x 1023 and 6x 102' spins/m' and temperatures between 0.6 and 15K. Relaxation takes place on or near the walls of the pyrex sample cells and measurements of Tl give information about the surface phases. A low temperature amplifier containing GaAs FET devices was developed to improve the spectrometer sensitivity. An amplifier noise temperature of 0.9 ± 0.5K was obtained at 1.16 MHz and an NMR signal was observed at 4.2K with the sensitivity being mainly limited by Johnson noise in the receiver coil. Baking the pyrex cells under vacuum and discharge cleaning the walls before sealing in the sample gas were found to increase the bulk gas Tl's by two or three orders of magnitude. A cryogenic wall coating of solid molecular hydrogen was found to delay the formation of a 'He monolayer on cooling and T, measurements were consistent with a binding energy of %, 13K for a 3He atom to a hydrogen surface. Once a 3He monolayer has formed the dipolar interaction between adsorbed spins is thought to be the dominant source of relaxation in the sealed cells. The presence of "He generally causes T, to rise on cooling below 2K due to preferential adsorption of "He at the surface. However, 'He atoms which dissolve in quasiparticle states in the superfluid helium film can be an extra source of relaxation. In the dirty cells relaxation probably takes place in quasiparticle states at the free surface of the saturated helium film, which are bound with an energy of 5.1 t 0.3K. In a cleaned, sealed cell a T, of ti 8 hours was measured at 7.7 MHz and 0.8K. In this case relaxation is probably occurring 2 or 3 helium layers away from the helium-hydrogen interface
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Marchalant, Pascale J. "(e, 2e) excitation-ionization of helium." Thesis, Queen's University Belfast, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.337019.
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Boatwright, Adrian Lional. "Molecular behaviour in superfluid helium clusters." Thesis, University of Nottingham, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.478961.
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Klier, Jurgen. "Liquid helium on weak binding substrates." Thesis, University of Exeter, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.337745.
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Christie, S. "Contrast in the helium acoustic microscope." Thesis, University of Exeter, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.381299.
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Tanner, Marc D. (Marc David) 1974. "Helium breath : an updated 6.111 curriculum." Thesis, Massachusetts Institute of Technology, 1998. http://hdl.handle.net/1721.1/47585.
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Thesis (S.B. and M.Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1998.Includes bibliographical references (p. 85).
by Marc D. Tanner.
S.B.and M.Eng.
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Hofmans, Marlous. "Experimental characterization of helium plasma jets." Thesis, Institut polytechnique de Paris, 2020. http://www.theses.fr/2020IPPAX062.
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Cette thèse porte sur l’étude d’un jet de plasma d'hélium à pression atmosphérique alimenté par des impulsions positives unipolaires à une fréquence de l’ordre du kHz. Des expériences sont effectuées pour caractériser la dynamique de propagation, la structure de l'écoulement et la température dans un jet en expansion libre, ainsi que l'influence d'une cible métallique sur le plasma.La spectroscopie à polarisation Stark indique un champ électrique axial d'environ 10 kV/cm dans le capillaire du jet et une augmentation jusqu'à 20 kV/cm dans le panache, qui est constante pour différentes amplitudes et durées de l'impulsion de tension appliquée. La diffusion Thomson et la diffusion Raman rotationnelle sont utilisées pour déterminer la densité électronique et la température électronique, à différentes positions axiales et radiales, ainsi que la température du gaz et la densité de N2 et O2 de l'air environnant qui sont mélangés dans le flux d’hélium.La comparaison quantitative de ces résultats expérimentaux avec les résultats d'un modèle fluide 2D montre une bonne concordance et permet une meilleure compréhension des résultats obtenus, à savoir que le champ électrique dans le front d'ionisation augmente avec la quantité d’air intégré au flux d’hélium au lond de la propagation. L'imagerie Schlieren révèle l'apparition de structures turbulentes à des débits élevés et lors de l'application des impulsions de tension. On constate que la température du gaz, mesurée par une sonde de température, augmente d'environ 12 C quand le plasma est allumé et d'environ 25 C lorsqu'une cible métallique est placée devant le jetThis thesis studies an atmospheric pressure helium plasma jet that is powered by positive, unipolar pulses at a kHz frequency. Experiments are performed that focus on the propagation dynamics, flow structure and temperature in a freely expanding jet, as well as the influence of a metallic target on the plasma.Stark polarization spectroscopy yields an axial electric field of around 10 kV/cm in the capillary of the jet and an increase up to 20 kV/cm in the plume, which is constant for different amplitudes and durations of the applied voltage pulse. Thomson and rotational Raman scattering are used to determine the electron density and electron temperature, at different axial and radial positions, as well as the gas temperature and the density of N2 and O2 that are mixed into the helium from the surrounding air.Quantitative comparison of these experimental results with results from a 2D fluid model show a good agreement and allow for a better understanding of the obtained results, namely that the electric field in the ionization front depends linearly on the flow composition at that location. Schlieren imaging shows the onset of turbulent structures at high applied flow rates and at the application of the voltage pulses. The gas temperature, as measured by a temperature probe, is found to increase by around 12 C when the plasma is ignited and by around 25 C when a metallic target is placed in front of the jet
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Masuhara, Naoto. "Spin waves in liquid helium-3 /." The Ohio State University, 1985. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487259580261871.
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Agarwal, Shradha. "Helium mobility in advanced nuclear ceramics." Thesis, Paris 11, 2014. http://www.theses.fr/2014PA112197/document.
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Cette thèse a pour objectif d’apporter des informations quantitatives sur la mobilité de l’hélium dans des céramiques nucléaires avancées comme TiC, TiN et ZrC, soumises à des traitements thermiques ou bien en présence de dommages d’irradiation. L’approche expérimentale développée au cours de ce travail est basée sur l’implantation ionique d’ions d’hélium-3 de 3 MeV en profondeur dans les trois matériaux précédemment cités et sur la mesure du profil de concentration en profondeur de l’isotope 3He au moyen d’une réaction nucléaire spécifique induite par des deutérons, 3He(d, p0)4He. La microscopie électronique à transmission et la spectrométrie Raman sont couplées à l’analyse par réaction nucléaire.Parmi les principaux résultats obtenus :- aucun relâchement d’hélium n’est observé à température ambiante pour les trois composés. Les valeurs d’énergie d’activation associée au relâchement d’hélium après un recuit thermique dans l’intervalle 1100 – 1600°C sont comprises entre 0,77 et 1,2 eV et semblent étroitement liées à la microstructure initiale du composé (stoéchiométrie et taille de grains). La capacité de rétention de l’hélium-3 dans des carbures ou nitrures de métaux de transition soumis à des traitements thermiques en conditions contrôlées croît dans l’ordre ZrC < TiC < TiN.- la formation de blisters n’et observée qu’à la surface de ZrC.- les profils d’implantation d’hélium présentent deux composantes pour les trois matériaux, l’une située au voisinage de la fin de parcours des ions et la seconde plus proche de la surface. Cette dernière résulte probablement du piégeage d’atomes d’hélium par les lacunes natives présentes.- les valeurs obtenues pour le coefficient apparent de diffusion de l’hélium varient dans l’intervalle 3,58E-19 – 5,296E-18 m^2s^-1 pour TiN et 4,20E-18 – 2,59E-17 m^2s^-1 pour TiC.Les valeurs correspondantes obtenues pour l’énergie d’activation sont respectivement de 2,50 eV pour TiC et de 1,05 eV pour TiN. Le mécanisme impliqué repose sur une dissociation des amas atomes d’hélium – lacunes au voisinage de la fin de parcours des ions. Plus en surface, la diffusion est plutôt du type substitutionnel.- l’observation au MET de sections transverses de TiN préparées par la technique FIB révèlent la présence de bulles d’hélium dès recuit à 1100°C et montrent la croissance des bulles avec la température. L’énergie d’activation de croissance des bulles a été estimée à 0,38 eV. A partir de 1400°C, cette croissance résulte vraisemblablement de l’absorption de lacunes par les amas.- la pression interne des bulles a été calculée à l’aide du modèle de Trinkaus, et nous avons montré qu’à partir de 1500°C, cette pression tendait à s’approcher de la valeur du module de cisaillement de TiN (240 GPa) et qu’elle atteignait la pression d’équilibre de 2 GPa à 1600°C.- à 1100°C, il semble que la densité des bulles présentes dans TiN varie linéairement avec la fluence d’implantation. A 1500°C, la taille des bulles est d’autant plus grande que la fluence est faible.- pour ZrC, l’effet de la fluence sur la mobilité de l’hélium est comparable à celui observé pour TiN. A la plus basse fluence, le relâchement d’hélium est très faible. Il croît avec la température de recuit et avec la fluence d’implantation.- la pré-Irradiation des trois composés par des auto-Ions avant implantation d’hélium provoque une augmentation de la dureté au moins =jusqu’à une dose de 27 dpa. Une très faible augmentation du paramètre de maille est alors détectée (≤ 0.5%).- dans le cas de ces matériaux non amorphisables sous irradiation aux ions, le recuit par perte d’énergie électronique ou bien le pré-Endommagement balistique ne jouent a priori aucun rôle sur la mobilité de l’hélium, étudiée sous l’angle d’une activation thermiqueWhile the current second and third generation nuclear plant designs provides an economically, technically, and publicly acceptable electricity supply in many markets, further advances in nuclear energy system design can broaden the opportunities for the use of nuclear energy. The fourth generation of nuclear reactors is under development. These new reactors are designed with the following objective in mind: sustainability, safety and reliability, economics, proliferation resistance. Out of six Generation IV systems namely, Gas-Cooled Fast Reactor (GFR), Lead-Cooled fast reactor (LFR), Molten Salt Reactor (MSR), Sodium-Cooled Fast Reactor (SFR), Supercritical-Water-Cooled Reactor (SCWR), Very-High-Temperature Reactor (VHTR), this work is dedicated to identify specific fuel type that is compatible with gas-Cooled fast reactor (GFR) in-Core service conditions and could be extended to diagnose potential cladding material for SFR. The French strategy is mainly oriented towards the development of sodium-Cooled fast reactors (SFR) and very slightly focused on GFR. This dissertation is focused on the study of transition metal ceramics which are candidates for fuel coatings in GFR and have been considered as potential cladding materials for SFR. The specific fuel type in GFR should consists of spherical fuel particle made up of UC or UN, surrounded by a ceramic coating which provides structural integrity and containment of fission products. The most promising candidates for ceramic coatings are ZrN, ZrC, TiN, TiC & SiC due to a combination of neutronic performance, thermal properties, chemical behavior, crystal structure, and physical properties. It is obvious that these ceramics would be exposed to energetic fission products from fuel such as heavy ions and neutrons. These high-Energy neutron will knock the atoms in the surrounding materials and can induce (n, α) reactions, thus producing high concentration of helium atoms during and after reactor operation. The helium atoms produced are energetic and can easily penetrate into the surrounding material. Helium atoms are considered to be highly insoluble in previously studied structural nuclear materials. The accumulation of helium into solid matrix, can lead to the formation of bubbles, cavity, swelling, embrittlement etc. Helium can strongly induce grain boundary cavitation that can produce formation of inter-Granular channels, which may serve as pathways for release of radioactive elements to the environment or lead to grain-Boundary weakening and de-Cohesion. Particularly in ceramics, large quantities of helium can also lead to dimensional changes and cracks due to over-Pressurized helium bubbles. Therefore, study of helium behavior in advanced nuclear ceramics under high operating temperatures and extreme radiation conditions predicted for GFRs is viewed as crucial. In this thesis, ion-Implantation technique and material characterization techniques are used to study diffusion of helium in transition metal ceramics under thermal and extreme irradiation environments. Our main aim during this thesis is: 1) To calculate diffusion and migration energies of helium under different experimental conditions by applying theoretical models on experimental data.2) To investigate the role of microstructure such as grain boundaries, native vacancies and porosity on helium accumulation and its evolution after helium accumulation.3) To know the role of helium introduction conditions on helium diffusion. 4) To establish and validate an approach to calculate pressure built by helium gas inside the bubbles and to verify if the pressure approaches mechanical stability limit
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Hoendervanger, Lynn. "A New Metastable Helium Machine : An Investigation into the Attributes of Trapping, Cooling and Detecting Metastable Helium." Thesis, Palaiseau, Institut d'optique théorique et appliquée, 2014. http://www.theses.fr/2014IOTA0006/document.
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Cette thèse décrit le travail accompli au cours des trois dernières années sur la nouvelle expérience d’Hélium métastable de l'Institut d'Optique à Palaiseau.Le premier chapitre décrit une étude visant à améliorer à la fois l'efficacité et la précision du système de détection par galettes à micro-canaux (MCP). Nous avons fait des mesures avec des galettes recouvertes d'une couche d'or sur la face avant, obtenant un accroissement de l'efficacité de détection mais également une réduction de la précision. L'ajout d'une tension intermédiaires entre les deux galettes empilées a au contraire améliorer à la fois l'efficacité et la précision des MCP, en l’absence d’une couche d’or.Le deuxième chapitre est consacré à la construction de l'appareil expérimental pour le refroidissement et le piégeage d’atomes. L’excitation de l'état fondamental de l’Hélium à l'état métastable est décrite, ainsi que la collimation et le refroidissement ultérieur par Zeeman lent du faisceau atomique chaud résultant. Le faisceau ralenti est alors capturé dans un piège magnéto-optique (PMO), dans lequel nous avons capturé 8x108 atomes.Dans le troisième chapitre une étude originale du refroidissement Doppler tridimensionnel dans un PMO et une mélasse désaccordée vers le rouge de la transition atomique est discutée. L’atome d’Hélium métastable est unique et ses propriétés ont permis une telle étude. En effet, les faibles densités atomiques impliquent qu’il n'y a pas de diffusion multiple de photons d’une part, et la faible masse et la faible largeur de la transition 23S1 -> 23P2 rend inefficace les processus de refroidissement sous la limite Doppler. Ces conditions nous ont permis d’observer pour la première fois à trois dimensions un gaz refroidit dans le régime Doppler.Le quatrième chapitre présente une étude sur les collisions dans un piège magnéto-optique d’Hélium métastable. Les collisions Penning induites par la lumière, en particulier à des intensités élevées et à des fréquences proches de la fréquence de transition, sont responsables de pertes élevées d’atomes piégés. Nous mesurons le coefficient de taux associé à ces pertes, Ksp = 2,8 ± 0,4 x 10-7cm3/ sThis thesis describes the work done over the past three and a half years on the new metastable helium experiment at the Institut d'Optique in Palaiseau. In the first chapter it describes a study to improve both the efficiency and the accuracy of the Microchannel Plate (MCP) detection system. We have experimented with adding a gold layer on the top of the input plate, something that we have found increases the efficiency but also decreases the accuracy. The addition of a voltage between the two stacked plates has been shown to both raise the efficiency and improve the accuracy in non-coated MCPs.The second chapter is devoted to the construction of the experimental apparatus. Here the excitation of ground state helium to its metastable state is described, as well as the subsequent collimation and cooling by Zeeman slower of the resulting hot atomic beam. The slowed beam is then captured in a Magneto-Optical Trap, in which we have captured 8x108 atoms.In the third chapter an original study on three-dimensional Doppler cooling in a red-detuned molasses and in the Magneto-Optical trap is presented. The metastable helium system is unique as there is no multiple scattering of photons and there are no sub-Doppler effects. This allows for a never before seen experimental realisation of pure Doppler cooling theory. The fourth chapter describes a study on collisions in a magneto-optical trap of metastable helium. Light-induced Penning collisions are responsible for high trap losses at high intensities and at frequencies close to the transition frequency. We measure the constant rate coefficient to Ksp = 2.8 ± 0.4 x 10-7cm3/s
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Alahmade, Walaa. "EXPERIMENTAL INVESTIGATION AND MONTE CARLO SIMULATION OF QUASIELASTIC ELECTRON SCATTERING FROM HELIUM-3 CLUSTERS IN HELIUM-4." Kent State University / OhioLINK, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=kent1619697731858548.
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Wu, Huayou. "Convergent close-coupling calculations of positron-helium collisions." Thesis, Wu, Huayou (2004) Convergent close-coupling calculations of positron-helium collisions. PhD thesis, Murdoch University, 2004. https://researchrepository.murdoch.edu.au/id/eprint/416/.
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The Convergent Close Coupling (CCC) method is applied, for the first time, to the scattering of positrons on helium. The helium target wave functions are obtained within various configuration interaction (CI) expansions. In the full CI expansion the two electrons are treated equally and thus all electron-electron correlations are taken into account. In the frozen-core (FC) approximation the CI expansion fixes one of the electrons to be described by a pure 1s orbital of He+, while maintaining the required singlet and triplet symmetries. Lastly, the multi-configuration (MC) approximation relaxes the FC approximation to allow the description of the inner electron to include several low-lying orbitals and is therefore more accurate than the FC approximation. The accuracy of the target wave functions is tested by comparing the calculated energy levels with the experimental data.
Based on positron-hydrogen scattering, comprehensive close-coupling formulas for positron-helium scattering are developed. The reduced two-centre V-matrix elements are derived in momentum space for various channels. These include direct, excitation and rearrangement channels, i.e. positronium formation.
We first consider low energy positron-helium elastic scattering for energies below the positronium formation threshold of 17.8 eV. Utilizing a single-centre expansion the elastic cross section and phase shifts have been calculated as a function of the positron incident energy. The calculations agree very well with the experimental data and the variational calculations, but not previous single- or double-centre close-coupling calculations.
We then consider energies above the first ionization threshold (24.6 eV) and calculate helium elastic, excitation, fragmentation and total cross sections within the single-centre expansion approach. Good agreement with the available experimental and other theoretical results has been obtained.
The studies have proved that a single-centre expansion, with accurate target state description, can deliver accurate data of practical value over a broad range of energies. However in the low-energy region, between the positronium formation threshold of 17.8 eV and the ionization threshold of 24.6 eV, implementation of the two-centre expansion is required. We expect this work to be undertaken in the near future, based on the derivations presented in this thesis.
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Wu, Huayou. "Convergent Close-Coupling Calculations of Positron-Helium Collisions." Murdoch University, 2004. http://wwwlib.murdoch.edu.au/adt/browse/view/adt-MU20050107.101745.
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The Convergent Close Coupling (CCC) method is applied, for the first time, to the scattering of positrons on helium. The helium target wave functions are obtained within various configuration interaction (CI) expansions. In the full CI expansion the two electrons are treated equally and thus all electron-electron correlations are taken into account. In the frozen-core (FC) approximation the CI expansion fixes one of the electrons to be described by a pure 1s orbital of He+, while maintaining the required singlet and triplet symmetries. Lastly, the multi-configuration (MC) approximation relaxes the FC approximation to allow the description of the inner electron to include several low-lying orbitals and is therefore more accurate than the FC approximation. The accuracy of the target wave functions is tested by comparing the calculated energy levels with the experimental data.
Based on positron-hydrogen scattering, comprehensive close-coupling formulas for positron-helium scattering are developed. The reduced two-centre V-matrix elements are derived in momentum space for various channels. These include direct, excitation and rearrangement channels, i.e. positronium formation.
We first consider low energy positron-helium elastic scattering for energies below the positronium formation threshold of 17.8 eV. Utilizing a single-centre expansion the elastic cross section and phase shifts have been calculated as a function of the positron incident energy. The calculations agree very well with the experimental data and the variational calculations, but not previous single- or double-centre close-coupling calculations.
We then consider energies above the first ionization threshold (24.6 eV) and calculate helium elastic, excitation, fragmentation and total cross sections within the single-centre expansion approach. Good agreement with the available experimental and other theoretical results has been obtained.
The studies have proved that a single-centre expansion, with accurate target state description, can deliver accurate data of practical value over a broad range of energies. However in the low-energy region, between the positronium formation threshold of 17.8 eV and the ionization threshold of 24.6 eV, implementation of the two-centre expansion is required. We expect this work to be undertaken in the near future, based on the derivations presented in this thesis.
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Schmiedeskamp, Jörg. "Weiterentwicklung einer Produktionsanlage und der Speicherungs- bzw. Transportkonzepte für hochpolarisiertes 3He Anwendungen in Kernspintomographie und physikalischer Grundlagenforschung /." [S.l.] : [s.n.], 2005. http://deposit.ddb.de/cgi-bin/dokserv?idn=975909355.
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