Thèses sur le sujet « Nuclear resonant scattering »

The neutron-rich nucleus 28Mg has been studied for the first time above the particle decay threshold using α-particle resonant scattering with a beam of radioactive 24Ne ions from the SPIRAL facility at GANIL. The thick target inverse kinematics technique was used to permit measurement of the differential cross section at 180◦ in the centre-of-mass frame, for the excitation energy region from 15 to 21 MeV, with a single beam energy. Since no previous experimental data exist with which to compare the current work, data were also taken for 24Mg using a 20Ne beam with the same experimental set-up. Comparison of these data with previous work yields excellent agreement and so validates the data collection method. In 28Mg, energies and widths of thirteen new states are reported with relative strengths given for ten of these. For two states, spin-parity assignments are made and it is found, by calculation of the α-decay branching ratios and comparison to a theoretical model, that these two states do not appear to be strongly α clustered. The underlying structure of 28Mg in this region is therefore not clear, and further experimental work is required in order to establish a full understanding, with a particular focus on the energy resolution of the measurement. The experimental technique is a powerful tool for the study of α elastic scattering cross sections, and combined with new radioactive beam facilities will prove an effective method of investigating α clustering in a wide range of unstable nuclei.

In the framework of the exotic clustering in light neutron rich nuclei, an experimental study for the search of 8Li-alpha cluster configurations on 12B was performed. The 8Li-4He elastic scattering excitation function was measured by using the thick target inverse kinematics scattering method implemented by time of flight measurements that allows the elastic channel from the inelastic one discrimination. The 8Li beam was provided by the LNS radioactive beam facility EXCYT at the energy of 30 MeV. The absolute cross-section was measured for CM energy between 13 MeV and 19.5 MeV. The procedure that has been used to extract energies and angles in CM system from the detected alpha energies and 4He stopping power for 8Li and 4He is shown in details. A measurements of the 4He stopping power for 7Li ions used to cross-check the stopping power codes data include in our calculations is discussed.

When a liquid is cooled to produce a glass, its dynamics, dominated by the structural relaxation, slows down dramatically, becoming of ≈100 s at the glass-transition temperature, Tg. At a slightly higher temperature (≈1.2Tg), a second, faster process, known as Johari-Goldstein (βJG) decouples from the structural one and remains active even in the glassy-state. It is nowadays established that the βJG -process is deeply connected to the structural one, though its microscopic origin and its role in the glass transition are still under debate. In this Thesis the spatial and temporal properties of the atomic motions within the JG-relaxation are investigated in two mono-hydroxyl alcohols using the technique known as nuclear γ-resonance time-domain interferometry (TDI). The results here obtained show that, within the βJ-relaxation, about one molecule out of four undergoes a restricted dynamics characterized by displacements of the order of 10% of the average inter-molecular distance. These re-arrangements correspond to local cage-breaking events and such un-caged molecules form a percolating cluster within the sample. At the same time we also found evidences of larger re-arrangements, occurring at a longer timescale with respect to the cage-breaking events and reaching out at least to the inter-molecular length-scale.

Thesis: S.B., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2016.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 58-60).
Simulations are vital to the safe design and operation of nuclear reactors. It is therefore important that they accurately treat the physics of nuclear interactions. This work investigates the phenomenon of neutron resonance scattering with a moving target, which can affect the post-collision properties of the neutron and macroscopic values such as temperature reactivity coefficients. First, this research validates a faster computational treatment for resonance scattering--the accelerated resonance elastic scattering (ARES) kernel sampling method--against the already verified Doppler broadening rejection correction (DBRC) treatment in the open-source OpenMC Monte Carlo neutron transport code being developed at the Massachusetts Institute of Technology. In an effort to improve computational efficiency, the optimal energy limits where this phenomenon should be treated are determined and compared to the less costly but inaccurate approximation of assuming a constant constant cross section for determining the reaction kinematics. To reduce memory requirements and facilitate coupling with heat transfer, a new data representation was recently adopted in OpenMC based on the multipole formalism. However, this new approach invalidates the previous implementations of DBRC and ARES. This thesis thus developed a modified DBRC algorithm compatible with the new data representation. This new method is also validated against the previous DBRC method. While more computationally costly, the use of the multipole representation in treating resonance scattering reduces memory requirements by a hundred-fold and facilitates the representation of temperature dependent cross sections.
by Vivian Y. Tran.
S.B.

Giant resonances are fundamental modes of excitations at high frequency, corresponding to shape oscillations around the equilibrium of the nuclear system. Their study over the years has provided useful information on nuclear structure and on the effective nucleon–nucleon interaction. In particular, the understanding of the electric-dipole response around the binding energy is presently attracting considerable interest, since the dipole strength distribution in that region affects the reaction rates in astrophysical scenarios. Moreover, it provides information on the neutron skin and thus on the symmetry energy of the equation of state. This concentration of electric dipole strength around the particle threshold is commonly denoted as Pygmy Dipole Resonance (PDR) and it is caused by the oscillation of the neutron skin against the inert proton-neutron core. From the experimental point of view, by comparing results of photon-scattering and a scattering experiments, a clear selectivity in the population of these “pygmy” states has been observed. These findings have called for additional high resolution investigation, by studying the γ-decay with advanced Ge array (such as AGATA), making use of different probes to excite the resonance states, as heavy ions. For this reason, an experiment aimed at the study of the γ-decay from high-lying bound and unbound states in several nuclei (124Sn, 208Pb and 140Ce) has been performed at Legnaro National Laboratories (LNL - INFN). The giant resonance modes have been excited by inelastic scattering of 17O beam at 20 MeV/u. The analysis described in this thesis has been focused on the studies of the Pygmy Dipole Resonance in the 124Sn nucleus.

Physics
Ph.D.
Non-spherical components of the nucleon wave function are measured through p(e,e'p)π experiment at the Δ+(1232) resonance for Q2 = 0.04, 0.09, and 0.13 (GeV/c)2 utilizing the Jefferson National Accelerator Facility (JLab) pulsed beam and Hall A spectrometers. The new data extend the measurements of the Coulomb quadrupole amplitude to the lowest momentum transfer ever reached. The results disagree with predictions of constituent quark models and are in reasonable agreement with dynamical calculations that include pion cloud effects, chiral effective field theory and lattice calculations. The reported measurements indicate that improvement is required to the theoretical calculations and provide valuable input that will allow their refinements. The Coulomb to magnetic multipole ratio (CMR) and generalized polarizability (GP) of the nucleon are also measured through virtual Compton scattering (VCS) for Q² = 0.2 (GeV/c)2 utilizing the Mainz Microtron (MAMI) continuous beam and A1 spectrometers. This data represents the first low Q² GP measurement at the Δ+(1232) resonance. The GP measurement explores a region where previous data and theoretical calculations disagree. The CMR measurement will be the first VCS extraction to compare with world data generated through pion electroproduction. The Dispersion Relation (DR) model used for the VCS extraction provides a new theoretical framework for the data signal and backgrounds that is largely independent from the pion electroproduction models. The independence of the DR from the traditional models provides a strong crosscheck on the ability of the models to isolate the data signal.
Temple University--Theses

The result of work investigating hydrogen diffusion in a variety of Metal- Hydrogen systems using both the nmr and qns techniques is presented. Chapter 1 provides a description of the uses and general character of MeH systems and is intended as a pragmatic introduction. In addition, diffusion theory is discussed and again general theory relevant to later work is presented regarding the properties of hydrogen diffusion within metal lattices. Chapter 2 gives an exposition on both the fundamental theory of the techniques used and detailed theoretical results which describe the effect of hydrogen diffusion on various observable parameters; primarily the nuclear spin-lattice relaxation time Ti and the neutron scattering function S(Q,ώ). Chapter 3 describes the experimental apparatus and procedures used for both the nmr and neutron scattering studies as well as for sample preparation. Chapter 4 sets out the results of an nmr study of single crystal samples of β-NbHx, β-VHx and -YHx. Individual theoretical treatments of the expected anisotropy of the hydrogen Knight shift and spin-lattice relaxation rate due to the dipole-dipole interaction are presented for each of the systems. The T1 results obtained for a single domain sample of β-NbHx 75 are in excellent agreement with theory, however it is not possible to determine the exact nature of the diffusion mechanism except to predict that it is likely to be an isotropic mechanism rather than a one dimensional mechanism such as intra-chain. A Ti anisotropy is also observed in two single crystal samples of β-VHx. In the case of a single domain sample the orientational dependence of the results is in good agreement with theory and the anisotropy of T1 [001]//Bo I T1 [001]!Bo=17% is in excellent agreement with the maximum predicted 16%. The results tend to confirm the theoretical methods used and the diffusion mechanism suggested by earlier qns studies. For a single crystal of -YHx0.17, again excellent agreement with theory is observed in the form of the Ti angular dependence. An anisotropy of T1 [001]//Bo / Ti min=85% is observed which greater than predicted by three theoretical diffusion mechanisms. The magnitude of and errors in the results do not enable any determination of the true diffusion mechanism in the -YHx system. Chapter 5 presents a study of the high temperature T1 anomaly which is an unexpected reduction in the relaxation time at temperatures where the Korringa (or conduction electron) mechanism is expected to dominate. Richards’ model of large anharmonic vibration of hydrogen at neighbouring interstitial sites which wras suggested as an explanation of the anomaly is described. The results of a qns study of Nb0.75V0.25H0.20 and YH1,97 powdered samples, which display the T1 phenomenon, are then presented. The study measures hydrogen diffusion coefficients for both samples in the temperature regime of the anomaly, which finds that hydrogen diffusion in both samples exhibit the usual Arrhenius temperature dependence. For the  -phase alloy system Ea -0.16 ± 0.02 eV and D0 -1.76 (±0.4) x 10-4 cms-1, and for the sub-stoichiometric dihydride sample Ea =0.33 ± 0.09 eV and D0 -0.70 (±0.4) x 10-4 cms-1. The conclusion is that Richards’ model does not hold and other possible models are discussed including sub-lattice melting and hydrogen pairing.

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Tese (Doutoramento)
IPEN/T
Instituto de Fisica, Universidade de Sao Paulo - IF/USP

L’étude des noyaux non liés permet de tester notre compréhension du noyau atom- ique dans des conditions extrêmes. Dans cette thèse nous avons analysé deux expériences distinctes réalisées au GANIL au cours desquels nous avons étudié le fluor 15 situé deux neutrons au delà de la drip-line proton. Une des particularités intéressante de ce noyau est la présence d’états de parité négative particulièrement étroits et situés au dessus des barrières Coulombiennes et centrifuges. Nous nous sommes intéressés dans ce travail à ces états, nous avons confirmé l’existence du second état excité 1/2- et avons observé deux nouveaux états, un état 5/2- et un état 3/2- en mesurant leur décroissance par émission d’un ou de deux protons. Nous avons discuté la structure de ces états et avons tenté d’expliquer l’asymétrie observée entre l’état 3/2- du fluor 15 et son analogue dans le carbone 15. Nous avons détaillé aussi la possibilité d’observer des décroissances gammas entre états non-liés
The study of unbound nuclei beyond the drip-lines allows us to test our understand- ing of the atomic nucleus under extreme conditions. In this thesis, we have analyzed two separate GANIL experiments in which fluorine 15, located two proton beyond the proton drip-line, was studied. One of the interesting particularity of this nucleus is the presence of particularly narrow negative parity states, above the Coulomb and centrifugal barriers. In this work, we were interested in theses states. We have confirmed the existence of the second excited state 1/2- and have observed two new states, a 5/2- state and a 3/2- state by measuring their decay by emitting either one or two protons. The structure of these states has been discussed and we have tried to explain the origin of the asymmetry observed between the 3/2- state of fluorine 15 and its analogue state in carbon 15. We have also detailed the possibility of gamma transitions between unbound states

Neutron scattering and nuclear magnetic resonance are excellent probes of the physical properties of the high Tc Superconductors. Both techniques have been used in this work. La2-xBaxCu04 [214] has a maximum Tc of ~30K and was one of the first of the high Tc materials to be identified. The structure of the un-doped parent compound has been known for some time so any structural changes as a result of Ba doping might also be associated with the onset of superconductivity. A high resolution neutron powder diffraction study is reported for a range of doping and temperatures (especially in the region of Tc). The parent compound is confirmed to be tetragonal at room temperature; space group I4/mmm. For Ba doping near to x=0.15 a sequence of phase transitions is observed: tetragonal (I4/mmm) at room temperature, orthorhombic (Abma) below ~250K and returning to tetragonal (P42/ncm) below -100K, but co-existing with the orthorhombic phase to low temperatures. Refined structural cell parameters are given for each doping at selected temperatures. The reasons for these changes and their possible link with the onset of superconductivity is discussed. Nd2-xCexCu04 has been reported to be an n-type superconductor (in contrast to other high Tc compounds) with a maximum Tc of ~24K. It has a similar structure to La[214] and has also been the subject of a high resolution neutron powder diffraction study. The range of doping was more restricted than in the Ba doped La[214] so as to match the range for which superconductivity occurs. The temperature dependence of the structure was looked at for each x in the vicinity of Tc. No structural phase transitions were resolved in any of the samples — all remained tetragonal (I4/mmm). A smooth decrease in lattice parameters a and c was observed as a function of temperature. Refined cell parameters are given at selected temperatures for each x. The contrast with La[214] and the possibility of phase separation are discussed. The only magnetic rare-earth (R) to suppress superconductivity upon substitution for Y in YBa2Cu3CO7 [123] (Tc -90K) is Pr. It was initially suggested that this could be explained by a 4+ Pr valence resulting in Cu-O plane hole filling. A high resolution inelastic neutron scattering study was undertaken to determine the crystal electric field (CEF) spectra of Pr[123] and mixed Y/Pr[123]. A detailed theoretical analysis has determined that the true valence is Pr3+ and the reasons for the loss of superconductivity are considered in light of this. The refined CEF parameters are listed and are used to calculate both the magnetic susceptibility and specific heat of Pr[123]. - Zn/Ga have been shown to substitute onto the Cu-O planes/chains of Y[123]. Tc reduces more rapidly with doping for Zn than for Ga. High resolution NMR was used to determine the changes in shift and relaxation time of the 89Y resonance for a range of doping of each element. The two sets of data are in sharp contrast and the analysis of the results indicates that Zn doping on the planes produces fluctuating, localised magnetic moments which may be linked to the marked suppression of superconductivity.

O presente trabalho aborda o estudo de correlações entre estrutura e propriedade de uma nova composição de vidros fluorofosfato. O trabalho compreende na síntese e caracterização de vidros fluorofosfatos com a seguinte composição: [80Ba(PO3)2 - 20Al(PO3)3]1-x[80BaF2 - 20AlF3]x(0 ≤ x ≤ 40 - mol%).Ressonância Magnética Nuclear e Espalhamento Raman são técnicas indicadas no estudo estrutural de materiais amorfos, como por exemplo, os vidros. Através dos resultados obtidos por difração de raios-X (DRX) e análise térmica diferencial (DTA) confirmam a formação de um sólido amorfo com concentrações de íons fluoretos de até 30 mol%. No entanto, a partir dos resultados da quantificação dos íons fluoretos via 19F RMN sugerem que houve uma quantidade significativa de perda de fluoreto ou ainda troca dos íons fluoretos por óxidos. Os espectros de 31P e Raman indicam uma gradual conversão das unidades Q2m em Q1m com o aumento de x. 19F RMN indica que os átomos de flúor estão em dois ambientes químicos distintos, onde os íons fluoretos estão ligados com átomos de fósforo ou distribuídos em um ambiente misto de Al/Ba. Resultados obtidos via 19F{31P}-REDOR confirmam a formação da ligação P-F. Os espectros de 27Al mostram a predominância dos átomos de alumínio com geometria octaédrica e ainda os resultados de 27Al{31P}-REDOR sugerem uma coordenação preferencial dos Al com unidades fosfato. Através dos resultados dos acoplamentos dipolares homonucleares via 31P{31P}-DRENAR corroboram com a deconvolução dos espectros de 31P na atribuição das espécies Q1m e Q2m.
In this work was studied the development of structure/property correlations for new fluorophosphate glasses compositions: [80Ba(PO3)2 - 20Al(PO3)3]1-x[80BaF2 - 20AlF3]x(0 ≤ x ≤ 40 - mol%). Most suitable techniques for the structural analysis in case of disordered and amorphous material are Nuclear Magnetic Resonance (NMR) Spectroscopy and Raman scattering. Results from X-ray diffraction (XRD) and differential thermal analysis (DTA) reveal that amorphous samples were obtained for all glass compositions up to x ≤ 30. However, the fluorine quantification via 19F MAS NMR suggests significant fluoride loss and/or fluoride/oxide replacement. Both 31P and Raman results indicate gradual conversion of Q2m units to Q1m units with increasing the fluorine concentration. 19F MAS NMR spectra indicate fluorine atoms in two different chemical environments bonded wither covalently to P-atoms or located in a mixed Al/Ba environment. The P-F assignment is confirmed by 19F{31P}-REDOR experiments. 27Al NMR spectra shows dominantly six-coordinated aluminium, and the 27Al{31P}-REDOR data suggest nearly exclusive coordination of Al with phosphate species. The dipolar coupling constant obtained by 31P{31P}-DRENAR are roughly consistent with 31P spectral deconvolution suggesting the presence of Q1and Q2 species.

Over the last years, the evolution of nuclear structure as a function of the N/Z ratio has become a key question in nuclear physics. Special attention has been given to the vibrational modes of dipole nature: nuclei with a neutron excess show an increase of dipole strength close to the neutron separation energy, commonly associated to a new type of collective motion called the Pygmy Dipole Resonance (PDR). Besides being intrinsically interesting as new structure phenomena, the pygmy resonance is also expected to play an important role in nuclear astrophysics, for the effect it has on neutron capture rates along the r-process path. From the experimental point of view, the PDR has been investigated systematically in a large number of stable nuclei with the photon scattering technique, but to gain more insight into its structure different experiments, with different probes, are needed. For this reason, an experiment was performed in June 2010 at Legnaro National Laboratories, aimed at the study of highly excited states in the target nuclei, including the pygmy region and up to the giant resonance region. The resonance states were populated by the inelastic scattering of a 17O beam at the energy of 20 MeV/u, and their subsequent gamma decay was measured with the AGATA (Advanced GAmma-ray Tracking Array) Demonstrator, the new generation HPGe array based on the techniques of pulse shape analysis and gamma-ray tracking. The Demonstrator was coupled to an array of 3 large volume LaBr3:Ce scintillators to increase the total efficiency; the remaining solid angle was covered by 20 BaF2 scintillators. Two segmented ΔE-E Si telescopes were used to identify of the scattered beam ions and to measure the excitation energy transferred to the target nucleus. The experiment measured the gamma decay from two targets: the doubly magic 208Pb, which had been previously measured with the same technique but a worse energy resolution, and the semimagic 90Zr. This thesis work details the analysis performed on this experiment and presents the first preliminary results on the measurements of the PDR in 208Pb and 90Zr with the inelastic scattering of 17O at 20 MeV/u.

La Chromodynamique Quantique (QCD) sur réseau permet d'étudier de façon ab-initio et non-perturbative les processus d'interaction forte. Ce formalisme, qui permet une régularisation covariante de la théorie de l'interaction forte, fournit aussi un cadre naturel pour le calcul et la simulation numérique de la Chromodynamique Quantique. Dans cette thèse, après un tour d'horizon des principales propriétés de la QCD et une présentation détaillée de notre discrétisation de cette théorie sur un réseau, nous étudions de façon approfondie deux problèmes de physique hadronique : le phénomène de diffusion résonante et la structure du nucléon. Les calculs sont réalisés avec les configurations de jauge de la Collaboration Budapest-Marseille-Wuppertal, générées avec une action de Wilson améliorée avec 2+1 saveurs de quarks dynamiques. Elles couvrent une large gamme de pas de réseau, de volumes et de masses des quarks différents, permettant ainsi une étude fine de la sensibilité de nos résultats à ces paramètres, et fournissant un bon contrôle sur l'extrapolation au continu. Notre étude de la diffusion de particules sur le réseau est menée grâce à une méthode proposée par M. Lüscher. Nous avons choisi le cas particulier de la diffusion pion-pion dans le canal résonant du méson rho, et analysé nos données avec une méthode variationnelle aux valeurs propres généralisées. Nous présentons les déphasages pion-pion ainsi que les paramètres de la résonance obtenus de façon détaillée, tout en garantissant un bon contrôle de nos erreurs systématiques. Nos résultats apportent une avancée importante dans le panorama des études de diffusion sur le réseau car ce sont les premiers réalisés à la masse physique du pion, pour laquelle la désintégration du rho en deux pions peut effectivement avoir lieu. Les valeurs obtenues pour les paramètres de la résonance du méson rho sont accord avec l'expérience, et confirment la faible dépendance du couplage entre le rho et les deux pions à la masse du pion. L'exploration de la structure du nucléon se fait à travers un calcul complet des facteurs de forme électrofaibles isovectoriels, avec une étude approfondie du rayon de charge électrique et de la charge axiale. Notre analyse présente aussi des données à la masse physique du pion, ce qui s'avère crucial pour maîtriser les extrapolations au point physique, étant données les variations violentes prédites par la perturbation chirale de ces quantités. Notre calcul utilise une projection sur les états du nucléon à la source et au puits, et une méthode de fit combinant les fonctions de corrélation à deux et trois points afin de réduire et de contrôler au maximum les contaminations pouvant venir des états excités. Bien que davantage de données seraient nécessaires pour déterminer très précisément le rayon et la charge axiale au point physique avec une évaluation pertinente des erreurs systématiques, les valeurs que nous obtenons sont en bon accord avec l'expérience, et suggèrent que les effets dus aux états excités sont faibles et sous contrôle. Notre analyse souligne aussi que l'utilisation de configurations de jauge avec des masses de pion proches de la valeur physique et avec des grands volumes semble indispensable à une étude précise de la structure du nucléon sur réseau
The formalism of Quantum Chromodynamics on the lattice (or Lattice QCD) allows to perform ab-initio non-perturbative studies of strong-interaction driven processes, as it provides both a covariant regularisation of the theory of QCD and a natural framework for numerical computations. In this work, after a review of the main features of QCD and a step-by-step presentation of our discretization of QCD on a lattice, we undertake detailed studies of two problems of hadronic physics: the phenomenon of resonant scattering and the structure of the nucleon. The lattice calculations are performed with the Budapest-Marseille-Wuppertal Collaboration's 2+1-flavour gauge configurations, which give access to a wide range of lattice spacings, volumes and quarks masses, thereby allowing to study the sensibility of our results on these parameters, and to perform a complete continuum extrapolation. These configurations include dynamical quarks, and use a clover-improved Wilson QCD action. To investigate the scattering of particles on the lattice, we set up a Lüscher analysis for the emblematic case of pion-pion scattering in the channel of the rho meson resonance. We analyse our data with a variational generalized eigenvalue method, and give an in-depth calculation of the scattering phase-shifts and the corresponding resonance parameters, with a full control of the systematic errors. Our results provide an important step for lattice studies of scattering states, as they are the first to be performed at the physical pion mass, where one can see the actual decay of the rho into two pions. The obtained rho meson parameters are in good agreement with the experimental values, and consistent with a weak pion mass dependence of the coupling between the rho and two pions. As for our probe of the structure of the nucleon, we present a complete extraction of the electroweak isovector form factors, with a comprehensive study of the electric charge squared radius and of the axial charge. Our analysis also feature data at the physical pion mass, which turns out to be crucial in order to perform safe extrapolations to the physical point, as the chiral perturbation theory predicts violent variations of these quantities near the massless-quarks point. Our calculation includes source and sink projections onto the nucleon state, as well as a combined fit method between the two-point and three-point correlation functions to control the contamination of our data by excited states. Although one would need more data to perform a high-accuracy determination of the nucleon radius and axial charge at the physical point with a relevant estimation of the systematic errors, the results we obtain are in good agreement with the experiment and suggest that the excited-state effects are under control. Our analysis also highlights that gauge configurations ensembles near the physical pion mass and with large volumes must be used in order to extract accurate information about the nucleon structure from lattice calculations

Thesis (Ph.D.); Submitted to the UNIVERSITY OF CALIFORNIA, BERKELEY, CA (US); 9 Dec 2004.
Published through the Information Bridge: DOE Scientific and Technical Information. "LBNL--56744" Guo, Fanqing. USDOE Director. Office of Science. Office of Nuclear Physics (US) 12/09/2004. Report is also available in paper and microfiche from NTIS.

Optical trapping of metal nanoparticles investigates phenomena at the interface of plasmonics and optical micromanipulation. This thesis combines ideas of optical properties of metals originating from solid state physics with force mechanism resulting from optical trapping. We explore the influence of the particle plasmon resonance of gold and silver nanospheres on their trapping properties. We aspire to predict the force mechanisms of resonant metal particles with sizes in the Mie regime, beyond the Rayleigh limit. Optical trapping of metal nanoparticles is still considered difficult, yet it provides an excellent tool to investigate their plasmonic properties away from any interface and offers opportunities to investigate interaction processes between light and nanoparticles. Due to their intrinsic plasmon resonance, metal nanoparticles show intriguing optical responses upon interaction with laser light. These differ greatly from the well-known bulk properties of the same material. A given metal nanoparticle may either be attracted or repelled by laser light, only depending on the wavelength of the latter. The optical forces acting on the particle depend directly on its polarisability and scattering cross section. These parameters vary drastically around the plasmon resonance and thus not only change the magnitude but also the direction and entire nature of the acting forces. We distinguish between red-detuned and blue-detuned trapping, that is using a trapping wavelength shorter or longer than the plasmon resonance of the particle. So far optical trapping of metal nanoparticles has focussed on a wavelength regime far from the particle’s resonance in the infrared. We experiment with laser wavelengths close to the plasmon resonance and expand the knowledge of metal nanoparticle trapping available to date. Existing theoretical models are put to the test when we compare these with our real experimental situations.

Prolyl oligopeptidase (POP) is an 81-KDa bidomain enzyme which hydrolyses short proline-containing peptides. This enzyme is involved in mnemonic and cognitive processes, and the dysregulation of POP activity is related to mental diseases. Probably, POP modulates the phosphoinositide signalling pathway through protein-protein interactions (PPI). Hence, the development of POP inhibitors is an area of great interest for the treatment of cognitive deficits associated with mental and neurodegenerative diseases. Recently, it has been found that the administration of POP inhibitors increases the clearance of a-synuclein aggregates in vivo, indicating that POP can be related to some extend with the pathogenic conditions of Parkinson’s disease (PD). Probably, this increase in the a-synuclein catabolism might be a consequence of a direct interaction between the two proteins. For this reason, POP inhibitors might be drug candidates for the preventive treatment of PD. Although the X-ray structure of POP is well studied, it is not clear which are the conformational fluctuations responsible for the circulation of substrates and products during the catalytic cycle. Several studies suggest that loops surrounding the active site are involved in a gating mechanism, while others postulate that interdomain separation might expose the active site. Moreover, such conformational transitions might be essential for the recognition events of POP. The elucidation of the conformational landscape of POP is a challenging task due to the high molecular weight of the enzyme. In this PhD thesis we have used a combination of robust biophysical tools (in particular, NMR and SAXS) together with molecular dynamics simulations (MD) in order to decipher the conformational dynamics of POP in solution. In addition, POP was also analysed by ion mobility mass spectrometry, an emerging biophysical tool in structural biology. Finally, we performed preliminary studies of the interaction between POP and a-synuclein by NMR. The results obtained in this PhD thesis demonstrated that POP exists in solution in a slow conformational exchange between open and closed conformations. The conformational transitions involved the periodic separation of the two domains in a hinge-type motion. Relaxation dispersion experiments showed that this long range conformational transition was better described by several independent motions of different amplitudes, stressing the highly dynamic behaviour of POP. Moreover, the analysis of SAXS data complemented by MD simulations found that the interdomain separation caused the inactive arrangement of the active site. This suggests that the separation between domains might be critical for substrate recruitment and product release. Of interest, inhibitors caused the total displacement of this equilibrium towards the stabilized closed conformation, therefore quenching dynamics and the catalytic activity. The study of the interaction between POP and a-synuclein by NMR disclosed that both proteins might be involved in a weak and transient interaction. Of interest, this interaction showed more affinity in the case of POP bound to inhibitors. In this case, interaction specially affected a broad segment of the C-terminal region of a-synuclein. This result suggested that the recognition between the two proteins depends on the conformational state of POP. Therefore, modulating the conformational landscape of POP by inhibitors might control this interaction. In summary, the results obtained in this PhD thesis demonstrated that POP undergo slow exchange between open and closed conformations in solution, and found that inhibitors have deep effects in the native conformational landscape of POP. Of interest, these conformational transitions might be essential for regulating the PPI necessary for the biological function of POP. Hence, the in vivo effects of POP inhibitors might result as a consequence of the alterations in the recognition events of POP.
La prolil oligopeptidasa (POP) es un enzim de 81 KDa que hidrolitza pèptids curts amb contingut en prolina. La POP actua en el sistema nerviós central mitjançant interaccions proteïna-proteïna (IPP), i la seva funció biològica està relacionada amb la memòria i els processos cognitius. Per aquesta raó, els inhibidors de la POP són compostos d’interès terapèutic per al tractament dels dèficits cognitius. Recentment, s’ha descobert que els inhibidors de la POP poden prevenir la patogènesis de la malaltia de Pàrkinson, probablement a través d’una interacció directa entre la POP i l’a-sinucleïna (la principal proteïna causant dels processos neurodegeneratius de la malaltia de Parkinson). Tot i que l’estructura cristal·logràfica de la POP està ben definida, no es sap quines són les transicions conformacionals que permeten completar el cicle catalític de la POP. Probablement, aquesta riquesa conformacional també té un paper rellevant en el control de les IPP. Malauradament, l’estudi conformacional complet de la POP és tot un repte degut al seu elevat pes molecular. En aquesta tesis doctoral s’ha emprat una combinació de tècniques biofísiques avançades (en concret, la resonància magnètica nuclear, la dispersió de raigs X de baix angle, i l’espectrometria de masses de mobilitat iònica) conjuntament amb simulacions de dinàmica molecular, per tal d’analitzar la dinàmica conformacional de la POP en solució. A més, s’ha estudiat la possible interacció entre la POP i l’a-sinucleïna mitjançant experiments de RMN. Els resultats obtinguts en aquesta tesi doctoral han demostrat que la POP es troba en solució en un equilibri conformacional lent entre conformacions obertes i tancades, originades a partir de la separació entre dos dominis. Els inhibidors de la POP causen una gran estabilització de la conformació tancada, amb la qual cosa l’equilibri dinàmic es desplaça totalment cap a aquesta conformació. A més, es va poder detectar una interacció dèbil i transitòria entre la POP i l’a-sinucleïna, que esdevenia especialment afavorida en la presència d’inhibidors. Així, els nostres resultats suggereixen que la diversitat conformacional de la POP es necessària per a la seva funció, i que els inhibidors poden desencadenar la seva funció biològica desplaçant l’equilibri conformacional.

Eukaryotic cells rely on a complex system of membrane-enclosed compartments that are maintained by the trafficking of shuttling vesicles. The fusion of these vesicles with the target compartment relies on multiprotein complexes that have been conserved throughout eukaryotic evolution. SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins are considered the engine of membrane fusion in all trafficking pathways of the cell. Upon specific protein-protein interactions, SNARE proteins that are localized in opposing membranes form a four helix bundle that releases free energy and induces membrane fusion. The SNARE motif is the elementary unit of this bundle and defines all SNARE proteins. SNARE proteins possess other regulatory domains that contribute in modulating the specificity of the fusion event. One of these accessory elements is the Longin Domain (LD). Other than being well conserved among all eukaryotes, the LD is not limited to SNAREs only and is present in many molecular processes related to the life cycle of vesicles. LD-containing SNAREs are called Longins and are divided in three main subfamilies: Ykt6p, Sec22b, VAMP7. The Longin Domain (LD) is usually composed of about 120 amino acids arranged in a globular structural fold that consists of five ? strands (?1- ?5) sandwiched by one ? helix (?1) on one side and two helices (?2 and ?3) on the other side. The LD can fold back onto the SNARE motif in Ykt6p and Sec22b but not in Nyv1p – a fourth minor longin subfamily. This intramolecular interaction involves a surface-exposed hydrophobic pocket contributed by the ?1-?3 structural elements, which is bound by the SNARE motif. This mechanism eventually inhibits and prevents unspecific formation of the SNARE complex, thus regulating the vesicle fusion process. However, very little is known about the dynamic properties of such mechanism. The present study uses VAMP7 as a model system to reveal these characteristics. Our interest in VAMP7 relies on the fact that not only VAMP7 lacks any direct evidence of the LD-SNARE interaction, but it also offers a system of natural variations to the usual LD-SNARE domain arrangement that can prove extremely useful in our study. The present work reveals unknown dynamic properties of the LD-SNARE interaction supporting a dominantly “closed conformation” for Longins, with heterogeneous characteristics. The results shown in this research complement well with what we already know about a similar auto-inhibitory mechanism observed in the Syntaxin subfamily of SNAREs. Therefore, we provide here new bases for a better understanding of the regulatory mechanisms involved in vesicle fusion.
Le cellule eucariote sono caratterizzate da un complesso sistema di membrane, che offre svariate compartimentazioni con diverse condizioni chimico-fisiche. Se da una parte tale sistema permette la realizzazione di un’ampia gamma di processi biochimici, dall’altra richiede un altrettanto complesso sistema di interscambio atto al suo mantenimento. Tale interscambio è assicurato dal trafficking di vescicole che originano da un compartimento donatore e riversano il loro contenuto in un compartimento accettore attraverso un processo che richiede la fusione delle membrane lipidiche. Tale processo si fonda sull’organizzazione di complessi macromolecolari a cui contribuiscono varie famiglie proteiche ben conservate attraverso l’evoluzione eucariotica. La famiglia delle SNARE è una di queste. Le SNAREs sono considerate i motori della fusione di membrane. La loro capacità di formare complessi specifici in trans tra le due memrane su cui risiedono fornisce il contributo energetico necessario a indurre la fusione degli strati lipidici. Tali complessi consistono in un intreccio di quattro eliche chiamate SNARE motifs, domini di circa 60-70 amino acidi che definiscono tutte le SNAREs. Oltre allo SNARE motif, le SNAREs contengono spesso domini accessori a funzione regolativa. Uno di questi è il Longin Domain (LD). Il LD non è limitato alle sole SNAREs e anzi si ritrova in altre famiglie proteiche tutte coinvolte in processi molecolari riguardanti il ciclo vitale di una vescicola. Nelle SNAREs, il LD definisce una famiglia chiamata Longins, suddivisa a sua volta nelle proteine Ykt6, Sec22b e VAMP7. Il LD consiste di circa 120 aminoacidi organizzati in una struttura spaziale globulare che comprende un piano di cinque foglietti ? (?1- ?5), complessati da un’alfa elica (?1) su un lato e da altre due eliche (?2-?3) sull’altro. In Ykt6 e Sec22b si è dimostrata la possibilità che il LD si ripieghi sullo SNARE motif e lo coordini su una sua superficie idrofobica compresa tra ?1 e ?3. Questo meccanismo si è dimostrato in grado di prevenire la formazione di complessi SNARE non specifici. Tuttavia ben poco si conosce ad oggi sulla natura di questa interazione in termini dinamici, a differenza di quanto invece si sa per un analogo meccanismo osservato nella famiglia SNARE delle Sintaxine. In altri temrini non è dato sapere se nelle Longine questo meccanismo implica una conformazione stabilmente “chiusa” di LD e SNARE, o se piuttosto esso si realizza come un equilibrio dinamico tra conformazioni aperte e chiuse. Una serie di motivi, tra cui l’assenza di dati diretti per questo fenomeno in VAMP7 e la possibilità di usufruire di sue varianti naturali, ci hanno spinto a scegliere VAMP7 come sistema modello per fornire le risposte ai suddetti interrogativi. I nostri dati suggeriscono per le Longine una conformazione stabilmente chiusa, ma non omogenea e capace di cambi conformazionali molto rapidi. Questo lavoro complementa bene quanto già noto per le sintaxine e fornisce dunque la possibilità di comprendere meglio i meccanismi regolativi gneralmente adottati nella fusione vescicolare.

L'étude des résonances géantes monopolaires isoscalaires (ISGMR) et des résonances géantes quadrupolaires isoscalaires (ISGQR) dans les noyaux stables, a permis d'obtenir ces dernières décennies des informations fondamentales sur la structure et la matière nucléaire. En particulier, le centroïde de la ISGMR peut être relié au module d'incompressibilité de la matière nucléaire infinie. Des données dans les noyaux exotiques nous aideraient à contraindre ce module d'incompressibilité. Dans les noyaux instables, une seule mesure a, à l'heure actuelle, été réalisée (56Ni). Afin d'étudier l'évolution de la ISGMR et de la ISGQR le long d'une chaîne isotopique, des mesures dans un noyau exotique riche en neutrons sont donc nécessaires.L'expérience étudiée dans cette thèse a été réalisée au Grand Accélérateur National d'Ions Lourds (GANIL) à Caen en septembre 2010. Un faisceau de 68Ni à 50 AMeV et d'une intensité de 10^4 pps a été produit et purifié sur la ligne LISE. Les réactions de diffusion inélastique de particules alpha (alpha, alpha') et de deutons (d,d') sur 68Ni en cinématique inverse ont été étudiées avec la cible active Maya. Il s'agit de la première mesure de la ISGMR et de la ISGQR dans un noyau instable riche en neutrons.Pour chaque expérience, le spectre en énergie d'excitation a été reconstruit et les distributions angulaires étudiées par deux méthodes indépendantes. Les sections efficaces expérimentales ont été comparées à celles issues de calculs DWBA utilisant des densités de transition RPA. L'analyse en (alpha, alpha') a permis l'observation d'une ISGMR fragmentée avec un épaulement à 21.1+/-0.6 MeV, d'une ISGQR concentrée à 16.9+/-0.8 MeV qui épuise 61+/-17% de la règle de somme pondérée en énergie (EWSR). De plus, un mode " soft GMR ", prédit mais jamais observé, a été identifié à 13.4+/-0.5 MeV. Tous ces résultats sont confirmés par l'analyse en (d,d'), à l'exception de l'observation de la ISGQR pour laquelle les conditions de fonctionnement n'étaient pas favorables.

Les génomes eucaryotes sont très complexes et peuvent être très grands. Par exemple, le génome humain contient environ de 20 000 à 25 000 gènes codant pour des protéines. L'expression de ces gènes doit être strictement régulée à de nombreux niveaux (tels que l'organisation de la chromatine, la transcription des gènes, le traitement et l'exportation de l'ARN messager ainsi que la traduction) pour le bon fonctionnement de la machinerie cellulaire. De nombreuses protéines et complexes protéiques sont impliqués dans ces processus essentiels de régulation, tels que les remodeleurs de la chromatine, les activateurs, co-activateurs et répresseurs de la transcription et particulièrement la machinerie générale de transcription. Chez les eucaryotes, la transcription de gènes codant pour des protéines est appelée transcription génique de classe II, elle est catalysée par l'ARN polymérase II (Pol II). La transcription des gènes par la polymérase II nécessite l'interaction coopérative de plusieurs protéines et complexes protéiques afin de faciliter l'assemblage d'un complexe de pré-initiation (PIC) au promoteur de base. Le complexe de pré-initiation comprend l'ARN polymérase II et les facteurs de transcription généraux (GTFs) - TFIIA, TFIIB, TFIID, TFIIE, TFIIF et TFIIH ainsi que le complexe de Médiateur et une grande variété de co-activateurs transcriptionnels.Une étape fondamentale dans l'assemblage d'un complexe de pré-initiation est la reconnaissance du promoteur de base par le facteur de transcription général TFIID. TFIID est un complexe multi protéique d'environ 1,6 MDa. Chez l'homme, il comprend une vingtaine de sous-unités constituées de 14 protéines différentes - la protéine de liaison à la boite tata (TBP) et ses facteurs associés (TAFs 1 à 13). Une série d'études sur la TFIID humaine et ses sous-ensembles ont été réalisés depuis sa découverte il y a plus de 20 ans, cherchant à comprendre la structure et le mécanisme de ces facteurs de transcription général essentiel, cependant l'architecture de TFIID, ses activités, ses fonctions, ses rouages et ses mécanismes d'assemblage cellulaire reste largement incompris à ce jour.Cette thèse décrit les études biochimiques que nous avons effectuées sur trois sous-ensembles distincts de TFIID humain. Nous avons utilisé un certain nombre de techniques de biologie structurale : la cristallographie, la spectroscopie à résonance magnétique nucléaire (RMN) et la diffusion des rayons X aux petits angles (SAXs), pour étudier le complexe formé par les facteurs humains, associés à la protéine de liaison à la boite tata, TAF1 et TAF7. Ces études structurelles fournissent un aperçu détaillé sur l'interface d'interaction complexe de TAF1/TAF7, misent de concert avec des données disponibles dans la littérature, elles mettent en évidence la nature dynamique de l'interaction TAF1/TAF7 dans le complexe de TFIID humain.Dans une deuxième étude, nous avons analysé un complexe formé par TAF11, TAF13 et TBP en utilisant un panel de méthodes biophysiques et biochimiques : l'analyse électrophorétique de retard sur gel (EMSA), l'ultracentrifugation analytique (AUC), la chromatographie d'exclusion stérique (SEC) analyse, le pull-down, la spectrométrie de masse native et la spectrométrie de masse chimique à réticulation (CLMS). Ce complexe fait penser au complexe TAF1/TBP qui imite la boite tata.De plus, dans le cadre des efforts en cours au sein du laboratoire du Pr Imre Berger afin de déterminer la structure de l'holo-TFIID humaine, nous avons reconstitué un grand sous-ensemble de TFIID (900 KDa) appelé 9TAF, qui est composé de neuf différents facteurs associés de TBP. Nous avons effectué des études d'électro-microscopie par coloration négative sur le complexe 9TAF qui nous ont fourni des informations à faible résolution. Ces études ouvrent la voie à de futures études de cryo-EM sur le complexe 9TAF pour obtenir un modèle de plus haute resolution
Eukaryotic genomes are highly complex and can be very large. For example, the human genome contains approximately 20,000-25,000 protein coding genes. Expression of these genes needs to be tightly regulated at many levels, including chromatin organization, gene transcription, mRNA processing and export and translation, for proper functioning of cellular machinery. Many proteins and protein complexes are involved in these essential regulatory processes, examples include chromatin remodelers, transcriptional activators and coactivators, transcriptional repressors and notably the general transcription machinery. Transcription of protein coding genes in eukaryotes is called Class II gene transcription, and is catalyzed by RNA polymerase II (Pol II). Gene transcription by Pol II requires the cooperative interaction of multiple proteins and protein complexes to facilitate the assembly of a preinitiation complex (PIC) at the core promoter. The PIC comprises Pol II and the General Transcription Factors (GTFs)- TFIIA, TFIIB, TFIID, TFIIE, TFIIF, and TFIIH, together with the Mediator complex and a large variety of transcriptional coactivators.A fundamental step in PIC assembly is recognition of the core promoter by GTF TFIID, a magdalton sized multiprotein complex. In humans, TFIID comprises about twenty subunits made up of 14 different proteins – the TATA box binding protein (TBP) and its associated factors (TAFs, numbered 1 to 13). A range of studies on human TFIID and its subassemblies have been carried out since its discovery more than two decades ago, to understand the structure and mechanism of this essential GTF, but the architecture of TFIID, its activities, its functions, its inner workings and the mechanisms of its cellular assembly have eluded detailed understanding to date.This thesis describes biochemical, biophysical, structural and functional studies carried out on three distinct human TFIID subassemblies. We used a number of structural biology techniques, including crystallization, nuclear magnetic resonance (NMR) spectroscopy and small angle X-ray scattering (SAXS) to analyse a complex formed by the human TBP associated factors TAF1 and TAF7. These structural studies provide detailed insights into the intricate interaction interface formed by TAF1 and TAF7, and, together with other data available from the literature, highlight the dynamic nature of the TAF1/TAF7 interaction in the human TFIID complex.In a second study, we analyzed a novel complex formed by TAF11, TAF13 and TBP using a range of biophysical and biochemical methods including electrophoretic mobility shift assay (EMSA), analytical ultracentrifugation (AUC), size exclusion chromatography (SEC) analysis, pull-down assay, native mass-spectroscopy and chemical cross-linking mass spectroscopy (CLMS). This complex is reminiscent of a so-called TATA-box mimicry discovered previously in a TAF1/TBP complex.As part of the ongoing efforts in the Berger laboratory to determine the structure of human holo-TFIID, we furthermore produced and purified a large (~900 kDa) TFIID subassembly called 9TAF, which is composed of nine different TBP associated factors. We carried out negative stain EM studies and random conical tilt (RCT) analysis on 9TAF to obtain low resolution structural information. These studies set the stage for future cryo-EM studies of this 9TAF complex to obtain a high(er) resolution model to decipher the inner workings of human TFIID

Les interactions protéine-proteine sont considérées comme un domaine de recherche important puisqu’elles contrôlent la plupart des processus cellulaires. Chez les cellules eucaryotes, les protéines multi-domaines (MDP), constituées d’au moins deux domaines, représentent plus de 70 % des protéines. Au sein d’une MDP, ces domaines peuvent être identiques ou différents et sont reliés par un segment intrinsèquement désordonné de longueur et de flexibilité variable. Ces protéines peuvent alors adopter de multiples conformations dans l’espace et interagir de manière spécifique avec leurs partenaires biologiques. Malgré de nombreux efforts de recherche dans le domaine, certaines questions restent encore non résolues ou nécessitent une étude approfondie. Mon projet de recherche est d’étudier et de définir le rôle des segments intrinsèquement désordonnés de la protéine STAM2 (Signal transducing adapter molecule 2) impliquée dans la machinerie ESCRT (Endosomal Sorting Complexe Required for Transport) , première étape dans le processus de dégradation lysosomale. Plus précisément, l’étude se focalise sur les effets de la flexibilité et la dynamique de ces segments dans le cas du processus de reconnaissance moléculaire entre STAM2 et l’ubiquitine ou di-ubiquitine. Différents mutants ont alors été conçus : soit avec un domaine totalement ou partiellement supprimé, soit avec un raccourcissement ou une suppression complète du segment ou soit avec de multiples mutations dans la séquence peptidique du segment. Ces différents construits ont été analysés en utilisant une combinaison de techniques biophysiques telles que la relaxation de spin par résonance magnétique nucléaire (RMN), la diffusion des rayons X aux petits angles (SAXS) et le dichroïsme circulaire (CD). Il a alors été démontré qu’une altération du segment désordonné peut entraîner un changement de la dynamique de la protéine et/ou un changement conformationnel. La modification de ce segment influe sur le mouvement inter-domaine et modifie l’affinité entre les construits de STAM2 et la di-ubiquitine sans modifier l’intégrité de chaque domaine et de leur site de liaison. En résumé, les segments intrinsèquement désordonnés procurent une certaine plasticité à la protéine ce qui lui permet de s’adapter et de remplir sa fonction biologique. Il est alors possible d’imaginer dans un futur proche que ces segments soient la nouvelle génération de cibles thérapeutiques pouvant réduire ou supprimer certaines interactions nocives
Protein-protein interaction is considered as an important field of research, as it is the key to control variable cell processes and pathways. In eucaryotic cells, multidomain proteins (MDPs), which consist of more than one domain, take up over 70 % of the pool. Those identical or different domains of a MDP are connected to each other by a linker of variable length and flexibility. For long flexible linker, it allows the protein to sample a wide range of conformation and to adjust interaction in a subtle way. Despite numerous efforts of research on the field, some issues remain unanswered or require further investigation. As part of this thesis, my work aims to define the role taken by the intrinsically disordered linker within MDPs. For that purpose, the STAM2 (Signal transducing adapter molecule 2) protein of the ESCRT (Endosomal Sorting Complexes Required for Transport) machinery was chosen to examine the effect of the flexibility and dynamics of the linker regions on the molecular recognition with ubiquitin and Lysine63-linked di-ubiquitin (K63-Ub2). Such efforts were carried out by designing specific mutants altering the linker regions in different ways. The various truncated versions undergo half or complete deletion of a domain or have their linker either shortened, deleted or modified in the amino acid composition. With a combination of the several biophysical methods namely NMR (Nuclear Magnetic Resonance) spin relaxation, SAXS (Small Angle X-ray Scattering) and CD (Circular Dichroism), the study has demonstrated that the alteration in the linker region modifies the flexibility and the dynamics of the protein, one among them possibly introduces slight change in conformation. Furthermore, the modification of the linker has an impact on the inter-domain motion and alter binding affinities between STAM2 constructs and di-ubiquitin without affecting domains integrity or binding sites. In brief, disordered linkers provide plasticity to the protein, which allow adaptability and specificity to molecular recognition process. As a further application, the linkers included in multidomain proteins could also be the next generation of druggable target as their modification may reduce or completely abolish interactions

Une contribution à l'étude de la dynamique des polymères non enchevêtrés à l'état fondu est apportée, en se focalisant sur les aspects de dynamique locale d'un polymère particulier déjà fort étudié expérimentalement et par simulation, à savoir le polyéthylène.

Néanmoins, plusieurs nouveaux résultats spécifiques à cette macromolécule linéaire ou ayant une portée sur les fondus de polymères en général sont obtenus grâce à une approche présentant plusieurs aspects originaux.

Notre étude par simulation de la phase liquide du polyéthylène est menée sur une large gamme de températures. La nouveauté principale à ce sujet est l'attention particulière portée à la qualité d'équilibration des échantillons à chaque température. A cette fin, des techniques sophistiquées d'échantillonnage Monte Carlo, mises au point récemment, ont été utilisées pour générer des configurations initiales, la phase de polyéthylène à l'état fondu ainsi obtenue pouvant être stable ou métastable. Un programme original de Dynamique Moléculaire a par ailleurs été écrit, en incorporant diverses procédures d'optimisation adaptées au cas du polyéthylène représenté par un modèle atomistique.

Des observables de diffusion quasi-élastique de neutrons et de résonance magnétique nucléaire sont analysées sur base d'une combinaison linéaire continue d'exponentielles, dont les poids sont donnés par une distribution des temps de relaxation. Cette méthode permet de mieux mettre en évidence les différents processus de relaxation sondés, en évitant les biais induits par un choix particulier de forme analytique servant à une procédure d'ajustement.

Ayant participé à une expérience de spectroscopie par temps de vol de neutrons, un schéma commun d'analyse est adopté pour les données expérimentales et de simulation concernant le polyéthylène à 450 K. D'autre part, une étude très fouillée de l'évolution de la fonction de diffusion intermédiaire incohérente vers les températures plus basses, suivie par simulation, a permis de distinguer différents processus dynamiques et de déterminer parfois leur origine moléculaire.

Ces résultats sont combinés à une analyse de la fonction d'auto corrélation d'orientation d'un vecteur C-H en termes d'une description microscopique des processus dynamiques, proposée lors d'une étude récente de fondus de polyéthylène par résonance magnétique nucléaire du 13C. Deux approches complémentaires sont exploitées afin de révéler les caractéristiques essentielles des deux types de relaxation impliqués.

Doctorat en Sciences
info:eu-repo/semantics/nonPublished

Ce travail presente l'etude, principalement par diffusion aux petits angles (dpa) de neutrons et de rayons-x, de solutions d'ionomeres. Les differentes analyses de l'intensite dpa (variation de contraste, analyse du comportement asymptotique, positions du pic d'interferences), appliquees aux solutions d'ionomeres perfluores (ipf), menent a la conclusion de la presence de particules colloidales de symetrie cylindrique. Cette geometrie est obtenue pour des solutions de trois ipf dans des solvants varies couvrant une gamme importante de constante dielectrique. La valeur du rayon depend de l'ipf et du solvant. En particulier les solutions dans l'eau se trouvent avoir les particules aux rayons les plus importants. Les etudes complementaires de solutions d'ipf par viscosimetrie font apparaitre des comportements dependant fortement du solvant (presence ou non d'effet polyelectrolyte suivant le solvant). Quelques mesures rmn mettent en evidence le comportement atypique des solutions aqueuses. L'etude par dpa de solutions aqueuses d'autres ionomeres, copolymeres d'ethylene et d'acide methacrylique, met la encore en evidence l'existence de particules colloidales, de geometrie ellipsoidale, dont la taille depend du taux de neutralisation

Mesures, par la méthode piezothermique, du coefficient de dilatation thermique et du coefficient de compressibilité isotherme, dans l'intervalle 200-450 k, sous des pressions atteignant 4000 bars; ajustement des résultats, établissement d'une équation phénoménologique du coefficient de dilatation comme fonction de la pression et de la température, permettant la détermination des propriétés thermodynamiques. Discussion de l'organisation moléculaire à la lumière des résultats obtenus et de données spectroscopiques (RMN, diffusion de la lumière).

Valence fluctuations of Fe²⁺ and Fe³⁺ were studied in a solid solution of Li_xFePO₄ by nuclear resonant forward scattering of synchrotron x rays while the sample was heated in a diamond-anvil pressure cell. The spectra acquired at different temperatures and pressures were analyzed for the frequencies of valence changes using the Blume-Tjon model of a system with a fluctuating Hamil- tonian. These frequencies were analyzed to obtain activation energies and an activation volume for polaron hopping. There was a large suppression of hopping frequency with pressure, giving an anomalously large activation volume. This large, positive value is typical of ion diffusion, which indicates correlated motions of polarons, and Li⁺ ions that alter the dynamics of both.

In a parallel study of Na_xFePO₄, the interplay between sodium ordering and electron mobility was investigated using a combination of synchrotron x-ray diffraction and nuclear resonant scattering. Conventional Mossbauer spectra were collected while the sample was heated in a resistive furnace. An analysis of the temperature evolution of the spectral shapes was used to identify the onset of fast electron hopping and determine the polaron hopping rate. Synchrotron x-ray diffraction measurements were carried out in the same temperature range. Reitveld analysis of the diffraction patterns was used to determine the temperature of sodium redistribution on the lattice. The diffraction analysis also provides new information about the phase stability of the system. The temperature evolution of the iron site occupancies from the Mossbauer measurements, combined with the synchrotron diffraction results give strong evidence for a relationship between the onset of fast electron dynamics and the redistribution of sodium in the lattice.

Measurements of activation barriers for polaron hopping gave fundamental insights about the correlation between electronic carriers and mobile ions. This work established that polaron-ion interactions can alter the local dynamics of electron and ion transport. These types of coupled processes may be common in many materials used for battery electrodes, and new details concerning the influence of polaron-ion interactions on the charge dynamics are relevant to optimizing their electrochemical performance.

The elastic and the thermodynamic properties of minerals under extreme P-T conditions are of general importance to the geodynamic modeling and the interpretation of seismic-wave observations. An accurate laboratory determination of these properties provides constraints to determine the composition and temperature of Earth's interior. In this thesis, I focus on the application of nuclear resonant scattering, an advanced synchrotron based spectroscopic technique, to further our understanding of candidate materials in Earth's interior. Specific examples include enstatite, which is an abundant mineral in the mantle, and iron-nickel alloy, which is believed to be the major component of the core. Our nuclear resonant scattering experiments is complemented with other synchrotron based techniques, such as diffraction.

Nuclear resonant scattering is capable of detecting subtle changes in the mineral's hyperfine parameters, and can therefore be sensitive to the transitions occurring in minerals under pressure. For example, we explore the site-specific hyperfine behavior of iron in a ⁵⁷Fe-enriched powdered enstatite sample using nuclear resonant scattering and diamond-anvil cells in two independent experiments. The (Mg_0.980Fe_0.020(5) )(Mg_0.760Fe_0.240)Si₂O₆ sample is pressurized up to 36 GPa at ambient temperature. In one experiment, NaCl is used as the pressure-transmitting medium, and in the other experiment, Ne surround the sample. Analyses of both data sets reveal a change in the trend or discontinuity in the hyperfine parameters around 10 GPa, indicative of a structural transformation in enstatite. However, the detailed behaviors of the iron sites with pressure appear to depend on the local stress conditions provided by the different pressure media.

Nuclear resonant scattering is also used to measure the elastic properties of iron-bearing enstatite at high pressures. The behavior of synthetic powdered ⁵⁷Fe-enriched (Mg_0.980Fe_0.020(5) )(Mg_0.760Fe_0.240)Si₂O₆ is explored by X-ray diffraction (XRD) and nuclear resonance inelastic X-ray scattering (NRIXS). The Pbca-structured enstatite sample is compressed in fine pressure increments for our XRD measurements. One structural transition between 10.1 and 12.2 GPa is identified from the XRD data. The XRD reflections observed for the high-pressure phase are best matched with space group P2₁/c. The partial phonon density of states (DOS) is derived from the raw NRIXS data up to 17 GPa, and from the low-energy region of the DOS, the Debye sound velocity is determined. We use the equation of state determined from XRD and Debye sound velocity to compute the isotropic compressional and shear wave velocities of enstatite at different pressures. We combine density-functional theory with nuclear resonant scattering to understand the local site symmetry of the Fe atoms in our sample. We compare our experimental results with seismic observations to understand large lateral variations in shear wave velocities in the upper mantle.

Recently, nuclear resonant scattering has been shown to be a powerful probe in determining the solid-liquid boundary of iron-bearing materials. To capture the sample's transient temperature fluctuations and reduce uncertainties in melting temperatures, we have developed a Fast Temperature Readout (FasTeR) spectrometer in-line with nuclear resonant scattering measurements under extreme conditions at Sector 3-ID-B of the Advanced Photon Source at Argonne National Laboratory. Dedicated to determining the sample's temperature near its melting point, FasTeR features a fast readout rate (about 100 Hz), high sensitivity, large dynamic range and well-constrained focus. FasTeR is capable of reading out temperatures about 1 to 2 magnitudes faster than the conventional CCD spectrometer, without sacrificing accuracy, and is especially suitable for dynamic measurements at extreme conditions.

By combining nuclear resonant scattering with the laser heated diamond anvil cell and the FasTeR spectrometer, we have determined the melting temperatures of fcc-structured iron and iron-nickel alloy at high pressures. We find that the melting curve of Fe is slightly higher than the melting curve of Fe_0.9Ni_0.1, but the difference is smaller than the uncertainty. We calculate the fcc-hcp-l triple point of Fe_0.9Ni_0.1 to be 117±3 GPa and 3285±200 K, and 111±3 GPa and 3390±200 K for Fe. With the fcc-hcp-l triple points of Fe and Fe_0.9Ni_0.1, and the thermophysical parameters of hcp-Fe determined from a NRIXS measurement, we compute the high pressure melting curves of hcp-structured Fe and Fe_0.9Ni_0.1. We estimate the upper bound of Earth's inner core-outer core boundary temperature as ~5600±200 K, and we compute the upper bound of outer core temperature with an adiabatic model. We discuss the implications of these temperatures on the phase relations of deep Earth materials.

Angular distributions for photon scattering from 12C have been measured at the Saskatchewan Accelerator Laboratory (SAL) high duty factor bremsstrahlung facility. Data were taken at 160, 200, 250 and 290 MeV endpoint energies and at angles ranging from 20 degrees to 150 degrees with the 53 cm diameter x 56 cm long Boston University Sodium Iodide (BUNI) detector. Coherently scattered photons were easily distinguished from neutral pion decay photons in detected energy spectra. The excellent resolution of the detector and the high duty cycle at SAL have made it possible to obtain new data on the angular distributions for elastic and, in some cases, for inelastic scattering as well. It was essential to distinguish between elastic scattering and inelastic scattering to the 4.44 MeV state in 12C. This was further complicated by the continuous nature of bremsstrahlung such that only a region within about 4 MeV of the endpoint could be used. Hence, precise energy calibration and stability of the calibration were required. Since elastic cross sections can be small, pileup transferring inelastic events into the elastic region was a concern and required careful attention. The angular distributions for elastic scattering were compared with a simple calculation of a dipole distribution weighted by the charge form factor and scaled by forward cross sections derived from total absorption cross sections with dispersion relations. This calculation badly underestimates cross sections at back angles. The more serious calculation with the Δ-hole model at 200 MeV overestimates the measurements at all angles. Some cross sections for inelastic scattering leading to the 4.44 MeV first excited state in carbon have been extracted. These cross sections are small compared to the elastic channel at forward angles, but are dominant at backward angles. Since this experiment was the first to separate elastic from inelastic photon scattering at these energies, these new data should stimulate further theoretical study.



Giant multipole resonances are a fundamental property of nuclei and

arise from the collective motion of the nucleons inside

the nucleus. Careful studies of these resonances and their properties provides

insight into the nature of nuclear matter and constraints

which can be used to test our theories.

An investigation of the Isovector Giant Quadrupole Resonance (IVGQR)

in ²⁰⁹Bi has been preformed using the High Intensity γ-ray

Source (HIγS) facility. Intense nearly monochromatic

polarized γ-rays were incident upon a ²⁰⁹Bi target producing

nuclear Compton scattered γ-rays that were detected using the HIγS

NaI(Tl) Detector Array (HINDA). The HINDA array consists of six

large (10''x10'') NaI(Tl) core crystals, each surrounded by an

optically segmented 3'' thick NaI(Tl) annulus. The scattered γ-rays

both parallel and perpendicular to the plane of polarization were

detected at scattering angles of 55° and 125° with

respect to the beam axis. This was motivated by the realization that

the term representing the interference between the electric dipole

(E1) and electric quadrupole (E2) amplitudes, which appears in the

theoretical expression for the ratio of the polarized cross sections,

has a sign difference between the forward and backward angles and also

changes sign as the incident γ-ray energy is scanned over the E2

resonance energy. The ratio of cross sections perpendicular and

parallel to the plane of polarization of the incident γ-ray were

measured for thirteen different incident γ-ray energies between 15 and

26 MeV at these two angles and used to extract the parameters of the

IVGQR in ²⁰⁹Bi.

The polarization ratio was calculated at 55° and

125° using a model consisting of E1 and E2 giant resonances as

well as a modified Thomson scattering amplitude. The parameters of the E1 giant

resonance came from previous measurements of the Giant Dipole

Resonance (GDR)

in ²⁰⁹Bi. The finite size of the nucleus was

accounted for by introducing a charge form factor in the (modified)

Thomson amplitude. This form factor was obtained from

measurements of the charge density in inelastic electron scattering

experiments.

The resulting curves were fit to the data by varying the

E2 parameters until a minimum value of the χ² was found.

The resulting parameters from the fit yield an IVGQR in ²⁰⁹Bi

located at E_res=23.0±0.13(stat)±0.25(sys) MeV

with a width of Γ=3.9±0.7(stat)±1.3(sys) MeV and a

strength of 0.56±0.04(stat)±0.10(sys) Isovector Giant

Quadrupole Energy Weighted Sum Rules (IVQEWSRs).

The ability to make precise measurements of the parameters of the

IVGQR demonstrated by this work opens up new challenges to both

experimental and theoretical work in nuclear structure. A detailed

search for the missing sum rule strength in the case of ²⁰⁹Bi should

be performed. In addition, a systematic study of a number of nuclei

should be studied with this technique in order to carefully examine

the A dependence of the energy, width and sum rule strength of the

IVGQR as a function of the mass number A. The unique properties of

the HIγS facility makes it the ideal laboratory at which to perform

these studies.

Such a data base will provide more stringent tests of nuclear

theory. The effective parameters of collective models can be fine

tuned to account for such precision data. This should lead to new

insights into the underlying interactions responsible for the nature

of the IVGQR. Furthermore, with the recent advances in computational

power and techniques, microscopic shell model based calculations

should be possible and could lead to new insights into the underlying

properties of nuclear matter which are responsible for the collective

behavior evidenced by the existence and properties of the IVGQR.

Dissertation

Tesis (Dr. en Física)--Universidad Nacional de Córdoba, Facultad de Matemática, Astronomía, Física y Computación, 2019.
Actualmente, se cuenta con modernas técnicas para el tratamiento del cáncer incluidas dentro del marco de la radioterapia, conocidas como radioterapias no-convencionales. Estas técnicas fueron investigadas bajo el mismo precepto que todas las terapias de cáncer que utilizan radiaciones ionizantes: realizar la conformación de la dosis en el interior del paciente de la forma más precisa posible, sin dañar tejidos sanos o radio-sensibles. Entre estas técnicas no-convencionales, se encuentra la hadronterapia, que consiste básicamente en la utilización de partı́culas cargadas pesadas (protones, partı́culas alfa, iones de carbono, etc) para la conformación de dosis en pequeños volumenes. Las principales ventajas de la hadronterapia, se deben a la naturaleza del depósito de energı́a descripto por la fı́sica del problema. Dada la alta transferencia lineal de energı́a que poseen los iones utilizados en hadronterapia, los daños biológicos resultan superiores a los de la radioterapia convencional, y por ende es necesario contar con herramientas que permitan establecer el “control / verificación del rango de los iones” en el interior del paciente durante el procedimiento de irradiación. Para este fin, es necesario identificar procesos fı́sicos que produzcan alguna señal interpretable desde el exterior del paciente que sea potencialmente útil para monitorear el haz terapéutico. Una posibilidad consiste en aprovechar las propiedades del agua lı́quida equivalente a las del tejido blando, y analizar los procesos nucleares que den lugar a estados excitados del núcleo de oxı́geno con consecuente emisión foto-nuclear. En este trabajo de tesis, se investigó un nuevo enfoque basado en la posibilidad de monitorear el haz de terapia por medio de algún elemento no presente, en forma natural, en el tejido biológico; que al interactuar de alguna forma con la radiación incidente produzca alguna consecuencia correlacionada con los procesos de interés. Asimismo, se analiza la potencial detección de fotones de aniquilación en la radioterapia convencional debido a la inclusión de nanopartı́culas de oro en el material tejido-equivalente a modo de aproximación experimental a la detección, y correlación con el proceso de irradiación. En términos de hadronterapia, y particularmente la protonterapia, se estudia la factibilidad de aprovechar la fusión nuclear entre protones de energı́as bajas (respecto de las energı́as incidentes de terapia) y el isótopo de boro 11, dando lugar a dos significativas ventajas: la mejora dosimétrica debido a la emisión de partı́culas alfa resultantes del proceso de fusión; y la verificación online e in-situ del rango de los protones, con la consecuente herramienta de diagnóstico por imágenes médicas, debido a la emisión de fotones gamma prompt (“rápidos”).
Nowadays, different techniques are being used for the treatment of cancer in the framework of radiotherapy, thoroughly known as non-conventional radiotherapies. These techniques were researched with the same goal than most cancer therapies utilizing ionizing radiation: conforming the dose inside the patient in the more accurate way possible, sparing healthy or radiosensitive tissues. Among these non-conventional techniques is the hadrontherapy, consisting mainly in using heavy charged particles (protons, alpha particles, carbon ions, etc) to perform the dose conformation in small volumes. The principal advantages of hadrontherapy are due to the nature of the energy deposition mechanism of the charged particles in matter, as described by the physics of the problem. Given the high linear energy transfer (LET) the ions used in hadrontherapy possess, the biological damage is higher than the conventional radiotherapy, and for this reason it is necessary to provide tools that allows the medical physicists to establish the “control / verification of the range of the ions” inside the patient during the irradiation procedure. To this aim, it is necessary to identify physical processes that leads to an interpretable signal outside the patient that are potentially useful for the therapeutic beam monitoring. A posibility consists of taking advantage of the water-equivalent properties of the biological tissue, and analyze the nuclear processes with excited states of the oxygen nuclei with the consecuent emission of a photo-nuclear photon. In this thesis work, a new approach is studied, based on the possibility of monitoring the therapeutic beam using an element not naturally present in the biological tissue, and whose interaction with the ionizing radiation used in the therapy produces a consequence correlated with the process of interest. These are called the “theranostics” methods. Likewise, the potential detection of annihilation photons in conventional radiotherapy is analyzed, using gold nanoparticles inside a tissue-equivalent material, mostly as an experimental approach to the detection and correlation with the irradiation process. In terms of hadrontherapy, particularly regarding protontherapy, this work shows the feasibility of taking advantage of the so-called proton-boron fusion, consisting of using the low-energy protons (respect therapy incident energies) interaction with boron 11 isotopes, which leads to two significant advantages: dosimetric improvement due to the alpha particles produced in the reaction; and an online and in-situ verification of the range of the protons, with the consequent tool for medical imaging, due to the emission of prompt-gamma photons.
Fil: Geser, Federico Alejandro. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía, Física y Computación; Argentina.

The Isoscalar Giant Quadrupole Resonance (ISGQR) in low-mass nuclei has been investigated in high energy-resolution experiments using proton inelastic scattering at Ep equals to 200 MeV. The measurements extend and complete the investigation already undertaken for medium and heavy-mass nuclei using the K600 magnetic spectrometer and a proton beam from the K200 Separated Sector Cyclotron (SSC) facility of iThemba LABS, South Africa.

Tunneling phenomenon has been studied since the time of Sir Isaac Newton. In the case of neutron tunneling phenomenon, it is the quantum mechanics wave-particle duality which manifests itself. In this case, particularly, the neutron wave-packet under total reflection condition suffers the so-called frustrated total reflection as known in standard optics. More accurately, this tunneling phenomenon shows itself via sharp dips in the plateau of total reflection. The prerequisite to observe such quantum mechanics phenomenon lies within a thin film Fabry-Perot resonator configuration. This thin film Fabry-Perot resonator geometry consists of two reflecting mirrors separated by a transparent material from a neutron optics viewpoint. In view of the specific neutron scattering properties related to the spin of the neutron wave-packet. As a direct proof, isotopic nickel based thin films Fabry-Perot resonator have been fabricated by depositing thin film of nickel by ion beam sputtering. The vacuum chamber was pumped down to the pressure of 10-8 mbar and deposition was performed at pressure of 2x10-4 mbar. The deposition rate was kept at 1.5 nm / minute and thickness layers were monitored by a calibrated quartz microbalance. Unpolarized neutron reflectometry measurements were carried out at the ORPHEE reactor using the time-of-flight EROS reflectometer. The incidence neutron wavelength varied between 3 – 25 Å. The grazing angle and angular resolution were of the order of 0.8˚ and 0.05 respectively. The software program, a Matlab routine for the simulation of specular X-ray and neutron reflectivity data with matrix technique, was employed to simulate the phenomenon and thereafter the experimentally obtained data and calculated (theoretical) data were compared. From the analysis of the comparison, a conclusion was drawn about the agreement between experimental data and theoretical data. The tunneling phenomenon has been observed in nanostructured isotopic nickel based thin film Fabry-Perot resonator. It manifested itself by the existence of dips, tunneling resonances, in the total reflection plateau due to quasi-bound states in the isotopic nickel based thin film Fabry-Perot resonator. In total, there were 7 tunneling resonances. The full widths at half maximum of these dips were found to decrease with increasing momentum wave vector transfer (Q) and this correlated to the neutron lifetime in the nanostructured isotopic nickel based thin film Fabry-Perot resonator.
Physics
M. Sc. (Physics)